14.1 ecology and environment

Today we hear people from all walks of life using the terms ecology and environment. Students, homeowners, politicians, planners, and union leaders speak of “environmental issues” and “ecological concerns”. Often these terms are interpreted in different ways, so we need to establish some basic definitions.

Ecology is the branch of biology that studies the relationships between organisms and their environments. This is a very simple definition for a very complex branch of science. Most ecologists define the word environment very broadly as anything that affects an organism during its lifetime. These environmental influences can be divided into two categories. Other living things that affect an organism are called biotic factors, and nonliving influences are called abiotic factors. If we consider a fish in a stream, we can identify many environmental factors that are important to its life. The temperature of the water is extremely important as an abiotic factor, but it may be influenced by the presence of trees (biotic factor) along the stream bank that shade the stream and prevent the Sun from heating it. obviously, the kind and number of food organisms in the stream are important biotic factors as well. The type of material that makes up the stream bottom and the amount of oxygen dissolved in the water are other important abiotic factors, both of which are related to how rapidly the water is flowing.

As you can see, characterizing the environment of an organism is a complex and challenging process; everything seems to be influenced or modified by other factors. A plant is influenced by many different factors during its lifetime: the types and amounts of materials in the soil; the amount of sunlight hitting the plant; the animals that eat the plant; and the wind, water, and temperature. Each item on this list can be further subdivided into other areas of study. For instance, water is important in the life of plants, so rainfall is studied in plant ecology. But even the study of rainfall is not simple. The rain could come during one part of the year, or it could be evenly distributed throughout the year. The rainfall could be hard and driving, or it could come as gentle, misty showers of long duration. The water could soak into the soil for later use, or it could run off into streams and be carried away.

Temperature is also very important to the life of a plant. For example, two areas of the world can have the same average daily temperature of 10 C but not have the same plants because of different temperature extremes. In one area, the temperature may be 13 C during the day and 7 C at night, for a 10 C average. In another area, the temperature may be 20 C in the daytime and only 0 C at night, for a 10 C average. Plants react to extremes in temperature as well as to the daily average. Furthermore, difference parts of a plant may respond differently to temperature. Tomato plants will grow at temperature below 13 C but will not begin to develop fruit below 13 C.

The animals in an area are influenced as much by abiotic factors as are the plants. If noliving factors do not favor the growth of plants, there will be little food and few hiding places for animal life. Two types of areas that support only small numbers of living animals are deserts and Polar Regions. Near the polar regions of the earth, the low temperature and short growing season inhibits growth; therefore, there are relatively few species of animals with relatively small numbers of individuals. Deserts receive little rainfall and therefore have poor plant growth and low concentrations of animals. On the other hand, tropical rainfalls have high rates of growth and large numbers of animals of many kinds.

As you can see, living things are themselves part of the environment of other living things. If there are too many animals in an area, they can demand such large amounts of food that they destroy the plant life, and the animals themselves will die. So far we have discussed how organisms interact with their environments in rather general terms. Ecologists have developed several concepts that help us understand how biotic and abiotic factors interrelate in a complex system.

14.2 the organization of ecological system

Ecologists can study ecological relationships at several different levels of organization. The smallest living unit is the individual organism. Groups of organisms of the same species are called populations. Interacting populations of different species are called communities. And an ecosystem consists of all the interacting organisms in an area and their interactions with their abiotic surroundings. Figure 14.2 shows how these different levels of organization are related to one another.

All living things require continuous supplies of energy to maintain life. Therefore, many people like to organize living systems by the energy relationships that exist among the different kinds of organisms present. An ecosystem contains several different kinds of organisms. Those that trap sunlight for photosynthesis, resulting in the production of organic material from inorganic material, are called producers. Green plants and other photosynthetic organisms such as algae and cyanobacteria are , in effect, converting sunlight energy into the energy contained within the chemical bonds of organic compounds. There is a flow of energy from the Sun into the living matter of plants.

The energy that plants trap can be transferred through a number of other organisms in the ecosystem. Because all of these organisms must obtain energy in the form of organic matter, they are called consumers. Consumers cannot capture energy from the sun as plants do. All animals are consumers. They either eat plants directly or eat other sources of organic matter derived from plants. Each time the energy enters a different organisms, it is said to enter a different trophic level, which is a step, or stage, in the flow of energy through an ecosystem. The plants receive their energy directly from the Sun and are said to occupy the first trophic level.

Various kinds of consumers can be divided into several categories, depending on how they fit into the flow of energy through an ecosystem. Animals that feed directly on plants are called herbivores, or primary consumers, and occupy the second trophic level. Animals that eat other animals are called carnivores, or secondary consumers, and can be subdivided into different trophic levels depending on what animals they eat. Animals that feed on herbivores occupy the third trophic level and are known as primary carnivores. Animals that feed on the primary carnivores are known as secondary carnivores and occupy the fourth trophic level. For example, a human may eat a fish that ate a frog that ate a spider that ate an insect that consumed plants for food.

This sequence of organisms feeding on one another is known as a food chain. Figure 14.4 shows the six different trophic levels in this food chain. Obviously, there can be higher categories, and some organisms don’t fit neatly into this theoretical scheme. Some animals are carnivores at some times and herbivores at others; they are called ominivores. They are classified into different trophic levels depending on what they happen to be eating at the moment. Of an organism dies, the energy contained within the organic compound of its body is finally released to the environment as heat by organisms that decompose the dead body into carbon dioxide, water, ammonia, and other simple inorganic molecules. Organisms of decay, called decomposers, are things such as bacteria, fungi, and other organisms that use dead organisms as sources of energy.

This group of organisms efficiently converts nonliving organic matter into simple inorganic molecules that can be used by producers in the process of trapping energy. Decomposers are thus very important components of ecosystems that cause materials to be recycled. As long as the sun supplies the energy, elements are cycled through ecosystems repeatedly. Table 14.1 summarizes the various categories of organisms within an ecosystem. Now that we have a better idea of how ecosystems are organized, we can look more closely at energy flow through ecosystem.

14.3 the great pyramids: energy, numbers, biomass

The ancient Egyptians constructed elaborate tombs we call pyramids. The broad base of the pyramid is necessary to support the upper levels of the structure, which narrows to a point at the top. This same kind of relationship exists when we look at how the various trophic levels of ecosystems are related to one another.

The pyramid of energy

A constant source of energy is needed by any living thing. There are two fundamental physical laws of energy that are important when looking at ecological systems from an energy point of view. First of all, the first law of thermodynamics states that energy is neither created nor destroyed. That means that we should be able to describe the amounts in each trophic level and follow energy as it flows through successive trophic levels. The second law of thermodynamics states that when energy is converted from one form to another some energy escapes as heat. This means that as energy passes from one trophic level to the next there will be a reduction in the amount of energy in living things and an increase in the amount of heat.

   At the base of the energy pyramid is the producer trophic level, which contains the largest amount of energy of any of the trophic levels within an ecosystem. In an ecosystem, the total energy can be measured in several ways. The total producer trophic level can be harvested and burned. The number of calories of heat energy produced by burning is equivalent to the energy content of the organic material of the plants. Another way of determining the energy present is to measure the rate of photosynthesis and respiration and calculate the amount of energy being trapped in the living material of the plants.

Because only the plants, algae, and cyanobacteria in the producer trophic level are capable of capturing energy from the Sun, all other organisms are directly or indirectly dependent on the producer trophic level. The second trophic level consists of herbivores that eat the producers. This trophic level has significantly less energy in it for several reasons. In general, there is about a 90% loss of energy as we proceed from one trophic level to the next higher level. Actual measurements will vary from one ecosystem to another. Some may lose as much as 99%, while other more efficient systems may lose only 70%, but 90% is a good rule of thumb. This loss in energy content at the second and subsequent trophic levels is primarily due to the second law of thermodynamics. Think of any energy-converting machine; it probably releases a great deal of heat energy. For example, an automobile engine must have a cooling system to get rid of the heat energy produced. An incandescent light bulb also produces large amounts of heat. Although living systems are somewhat different, they must follow the same energy rules.

In addition to the loss of energy as a result of the second law of thermodynamics, there is an additional loss involved in the capture and processing of food material by herbivores. Although herbivores don’t need to chase their food, they do need to travel to where food is available, then gather, chew, digest, and bolize it. all these processes require energy.

Just as the herbivore trophic level experiences a 90% loss in energy content, the higher trophic levels of primary carnivores, secondary carnivores, and tertiary carnivores also experience a reduction in the energy available to them.

Figure 14.5 shows the flow of energy through an ecosystem. At each level, the energy content decreases by about 90%.

The pyramid of numbers

Because it may be difficult to measure the amount of energy in any one trophic level of an ecosystem, people often use other methods to quantify the different trophic levels. One method is to simply count the number of organisms at each trophic level. This generally gives the same pyramid relationship, called a pyramid of numbers. Obviously this is not a very good method to use if the organisms at the different trophic levels are of greatly differing sizes. For example, if you count all the small insects feeding on the  leaves of one large tree, you would actually get an inverted pyramid.

The pyramid of biomass

Because of the size-difference problem, many  people like to use biomass as a way of measuring ecosystems. Biomass is usually determined by collecting all the organisms at one trophic level and measuring their dry weight. This eliminates the size-difference problem because all the organisms at each trophic level are weighed. This pyramid of biomass also shows the typical 90% loss at each trophic level. Although a biomass pyramid is better than a pyramid of numbers in measuring some ecosystems, it has some shortcomings.

Some organisms tend to accumulate biomass over long periods of time, whereas others do not. Many trees live for hundreds of years; their primary consumers, insects, generally live only one year. Likewise, a whale is a long-lived animal, whereas its food organisms are relatively short-lived. Figure 14.7 shows two biomass pyramids.

14.4 community interactions

In the previous section we looked at ecological relationships from the point of view of ecosystems and the way energy flows through them. But we can also study relationships at the community level and focus on the kinds of interactions that take place among organisms.

As you know from the discussion in the previous section, one of the ways that organisms interact is by feeding on one another. A community includes many different food chains and many organisms may be involved in several of the food chains at the same time, so the food chains become interwoven into a food web. In a community, the interacting food chains usually result in a relatively stable combination of populations.

Although communities are relatively stable we need to recognize that they are also dynamic collections of organisms: as one population increases, another decreases, this might occur over several years, or even in the period of one year. This happens because most ecosystems are not constant. There may be differences in the rainfall throughout the year or changes in the amount of sunlight and in the average temperature. We should expect populations to fluctuate as abiotic factors change. A change in the size of one population will trigger changes in other populations as well. Figure 14.9 shows what happens to the size of a population of deer as the seasons change. The area can support 100 deer from January through February, when plant food for deer is least available. As spring arrives, plant growth increases. It is no accident that deer breed in the fall and give birth in the spring. During the spring producers are increasing, and the area has more available food to support a large deer population. It is also no accident that wolves and other carnivores that feed on deer give birth in the spring. The increased available energy in the form of plants means more food for deer, which, in turn, means more energy for the wolves at the next trophic level.

If numbers of a particular kind of organism in a community increase or decrease significantly, some adjustment usually occurs in the populations of other organisms within the community. For example, the populations of many kinds of small mammals fluctuate from year to year. This results in changes in the numbers of their predators or the predators must switch to other prey species and impact other parts of the community. As another example, human have used insecticides to control the populations of many kinds of insects. Reduced insect populations may result in lower numbers of insect-eating birds and affect the predators that use these birds as food. Furthermore the indiscriminate use of insecticides often increases the populations of herbivorous, pest insects because insecticides often kill many beneficial predator insects that normally feed on the pest, rather than just the one or two target pest species.

Because communities are complex and interrelated, it is helpful if we set artificial boundaries that allow us to focus our study on a definite collection of organisms. An example of a community with easily determined natural boundaries is a small pond. The water’s edge naturally defines the limits of this community. You would expect to find certain animals and plants living in the pond, such as fish, frogs, snails, insects, algae, pondweeds, bacteria, and fungi. But you might ask at this point, what about the plants and animals that live right at the water’s edge? That leads us to think about the animals that spend only part of their lives in the water. That awkward-looking, long-legged bird wadding in the shallows and darting its long beak down to spear a fish has its nest atop some tall trees away from the water. Should it be considered part of the pond community? Should we also include the deer that comes to drink at dusk and then wanders away? Small parasites could enter the body of the deer as it drinks. The immature parasite will develop into an adult within the deer’s body. That same parasite must spend part of its life cycle in the body of a certain snail. Are these parasites part of the pond community? Several animals are members of more than one community. What originally seemed to be a clear example of a community has become less clear-cut. Although the general outlines of a community can be arbitrarily set for the purpose of a study, we must realize that the boundaries of a community, or any ecosystem for that matter, must be considered somewhat artificial.

14.5 types of communities

Ponds and other small communities are parts of large regional terrestrial communities known as biomes. Biomes are particular communities of organisms that are adapted to particular climate conditions. The primary climatic factors that determine the kinds of organisms that can live in an area are the amount and pattern of precipitation and the temperature ranges typical for the region. The map in figure 14.11 shows the distribution of the major biomes of the world. Each biomes can be characterized by special climate conditions, particular kinds of organisms, and characteristic activities of the organisms of the region.

Temperate deciduous forest

The temperate deciduous forest covers a large area from the Mississippi River to the Atlantic Coast, and from Florida to southern Canada. This type of biome is also found in parts of Europe and Asia. Temperate deciduous forests exist in parts of the world that have moderate rainfall (75-130 centimeters per year) spread over the entire year and a relatively long summer growing season ( 130-260 days without frost).this biome, like other land-based biomes, is named for major feature of the ecosystem, which in this case happens to be the dominant vegetation. The predominant plants are large trees that lose their leaves more or less completely during the fall of the year and are therefore called deciduous. The trees typical of this biome are adapted to conditions with significant precipitation and short mild winters. Since the trees are the major producers and new leaves are produced each spring, one of the primary consumers in this biome consists of leaf-eating insects. These insects then become food for a variety of birds that typically raise their young in the forest during the summer and migrate to more moderate climates in the fall. Many other animals like squirrels, some birds, and deer use the fruits of the trees as food. Carnivores such as foxes, hawks, and owls eat many of the small mammals and birds typical of the region. Another feature typical of the temperate deciduous forest is an abundance of spring woodland wildflowers that emerge early in the spring before the trees have leafed out. Of course, because the region is so large and has somewhat different climatic conditions in various areas, we can find some differences in the particular species of trees (and other organisms) in this biome. For instance, in Maryland the tulip tree is one unusual that people plant it in lawns and parks as a decorative tree. Aspen, birch, cottonwood, oak, hickory, beech, and maple are typical trees found in this geographic region. Typical animals of this biomes are many kinds of leaf-eating insects, wood-boring beetles, migratory birds, skunks, porcupines, deer, frogs, opossums, owls, and mosquitoes. In much of this region, the natural vegetation has been removed to allow for agriculture, so the original character of the biome is gone except where farming is not practical or the original forest has been preserved.

Grassland

The biome located to the west of the temperate deciduous forest in North America is the grassland or prairie biome. This kind of biome is also common in parts of Eurasia, Australia, and South America. The rainfall (30-85 centimeters per year) in grasslands is not adequate to support the growth of trees and the dominant vegetation consists of various species of grasses. It is typical to have long periods during the year when there is no rainfall. Trees are common in this biome only among streams where they can obtain sufficient water. Interspersed among the grasses are many kinds of prairie wildfowlers. The dominant animals are those that use grasses as food; large grazing mammals (bison and pronghorn antelope); small insects (grasshoppers and ants); and rodents (mice and prairie dogs). A variety of carnivores (meadowlarks, coyotes, and snakes) feed on the herbivores. Most of the species of birds are seasonal visitors to the prairie. At one time fire was a common feature of the prairie during the dry part of the year.

     Today most of original grasslands, like the temperate deciduous forest, have been converted to agricultural uses. Breaking the sod (the thick layer of grass roots) so that wheat, corn, and other grains can be grown exposes the soil to the wind, which may cause excessive drying and result in soil erosion that depletes the fertility of the soil. Grasslands that are too dry to allow for farming typically have been used as grazing land for cattle and sheep. The grazing of these domesticated animals has modified the natural vegetation as has farming in the moister grassland regions.

Savanna

A biome that is similar to a prairie is a savanna. Savannas are tropical biome of central Africa, northern Australia, and parts of South America that have distinct wet and dry seasons. Although these regions may receive 100 centimeters of rainfall per year there is an extended dry season of three months or more. Because of the extended period of dryness the dominant vegetation consists of grasses. In addition, a few thorny, widely spaced drought-resistant trees dot the landscape. Many kinds of grazing mammals are found in this biome—various species of antelope, wildbeest, and zebras in Africa; various kinds of kangaroos in Australia; and a large rodent, the capybara, in South America. Another animal typical of savanna is the termite, colonial insects that typically build mounds above ground.

During the wet part of the season the trees produce leaves, the grass grows rapidly, and most of the animals raise their young. In the Africa savanna, a seasonal migration of the grazing animals is typical. Many of these tropical grasslands have been converted to grazing for cattle and other domesticated animals.

Desert

Very dry areas are known as deserts and are found throughout the world wherever rainfall is low and irregular. Typically the rainfall is less than 25 centimeters per year. Some deserts are extremely hot; others can be quite cold during much of the year. The distinguishing characteristic of desert biome is low rainfall, not high temperature. Furthermore, deserts show large daily fluctuations in air temperature. When the Sun goes down at night, the land cools off very rapidly because there is no insulating blanket of clouds to keep the heat from radiating into space.

A desert biome is characterized by scattered, thorny plants that lack leaves or have reduced leaves. Because leaves tend to lose water rapidly, the lack of leaves is an adaptation to dry conditions. Under these conditions the stems are green and carry on photosynthesis. Many of the plants, like cacti, are capable of storing water in these freshy stems. Others store water in their roots. Although this is a very harsh environment, many kinds of flowering plants, insects, reptiles, and mammals can live in this biome. The animals usually avoid the hottest part of the day by staying in burrows or other shaded, cool areas. Staying underground on in the shade also allows the animal to conserve water.

There are also many annual plants but the seeds only germinate and grow following the infrequent rainfalls. When it does rain the desert blooms.

Boreal coniferous forest

Through parts of southern Canada, extending southward along the Appalachian and Rocky Mountains of the United States, and in much of northern Europe and Asia we find communities that are dominated by evergreen trees. This is the taiga, boreal coniferous forest, or northern coniferous forest biome. The evergreen trees are especially adapted to withstand long, cold winters with abundant snow fall. Typically the growing season is less than 120 days and rainfall ranges between 40 and 100 centimeters per year. However, because of the low average temperature, evaporation is low and the climate is humid. Most of the trees in the wetter, colder areas are spruces and firs, but some drier, warmer areas have pines. The wetter areas generally have dense stands of small trees intermingled with many other kinds of vegetation and many lakes and bogs. In the mountains of the western United States, pine trees are often widely scattered and very large, with few branches near the ground. The area has a parklike appearance because there is very little vegetation on the forest floor. Characteristic animals in this biome include mice, snowshoe hare, lynx, bears, wolves, squirrels, moose, midges, and flies. These animals can be dormant in winter (insects and bears); those that are specially adapted to withstand the severe winters (snowshoe hare, lynx); those that live in protected areas (mice under the snow); and those that migrate south in the fall (most birds).

Temperate rainforest

The coastal areas of northern California, Oregon, Washington, British Columbia, and Southern Alaska contain an unusual set of environmental conditions that support a temperate rainforest. The prevailing winds from the west bring moisture-laden air to the coast. As the air meets the coastal mountains and is forced to rise, it cools and the moisture falls as rain or snow. Most of these areas receive 200 centimeters or more precipitation per year. This abundance of water, along with fertile soil and mild temperatures, results in a lush growth of plants.

   Sitka spruce, Douglas fir, and western hemlock are typical evergreen coniferous trees in the temperate rainforest. Undisturbed forests of this region have trees as old as 800 years that are nearly 100 meters tall. Deciduous trees of various kinds (red alder, big leaf maple, black cottonwood) also exist in open areas where they can get enough light. All trees are covered with mosses, ferns, and other plants that grow on the surface of the trees. The dominant color is green because most surfaces have something photosynthetic growing on them.

When a tree dies and falls to the ground it rots in place and often serves as a site for the establishment of new trees. This is such a common feature of the forest that the fallen, rotting trees are called nurse trees. The fallen tree also serves as a food source for a variety of insects, which are food for a variety of larger animals.

Because of the rich resource of trees, 90% of the original temperate rainforest has already been logged. Many areas have been protected because they are home to the endangered northern spotted owl and marbled murrelet ( a sea bird).

Tundra

North of the coniferous forest biome is an area known as the tundra. It is characterized by extremely long, severe winters and short, cool summers. The growing season is less than 100 days and even during the short summer the nighttime temperature approach 0 C. rainfall is low (10- 25 centimeters per year). The deeper layers of the soil remain permanently frozen, forming a layer called the permafrost. Because the deeper layers of the soil are frozen, when the surface thaws the water forms puddles on the surface. Under these conditions of low temperature and short growing season, very few kinds of animals and plants can survive. No trees can live in this region. Typical plants and animals of the area are grasses, sedges, dwarf willow, and some other shrubs, reindeer moss (actually a  lichen), caribou, wolves, musk oxen, fox, snowy owls, mice, and many kinds of insects. Many kinds of birds are summer residents only. The tundra community is relatively simple, so any changes may have drastic and long-lasting effects. The tundra is easy to injure and slow to heal; therefore we must treat it gently. The construction of the Alaskan pipeline has left scars that could still be there 100 years from now.

Tropical rainforest

The tropical rainforest is at the other end of the climate spectrum from the tundra. Tropical rainforests are found primarily near the equator in Central and South America, Africa, parts of southern Asia, and Pacific Islands. The temperature is high (averaging about 27 C), rain falls nearly every day (typically 200-900 centimeters per year), and there are thousands of species of plants in a small area. Balsa ( a very light wood), teak (used in furniture), and ferns the size of trees are examples of plants from the tropical rainforest. Typically, every plant has other plants growing on it. tree trunks are likely to be covered with orchids, many kinds of vines, and mosses. Tree frogs, bats, lizards, birds, ms and an almost infinite variety of insects inhabit the rainforest. These forests are very dense, and little sunlight reaches the forest floor. When the forest is opened up (by a hurricane or the death of a large tree) and sunlight reaches the forest floor, the opened area is rapidly overgrown with vegetation.

Because plants grow so quickly in these forests, people assume the soils are fertile, and many attempts have been made to bring this land under cultivation. In reality, the soils are poor in nutrients. The nutrients are in the organisms, and as soon as an organism dies and decomposes its nutrients are reabsorbed by other organisms. Typical North American agricultural methods, which require the clearing of large areas, cannot be used with the soil and rainfall conditions of the tropical rainforest. The constant rain falling on these fields quickly removes the soil’s nutrients so that heavy applications of fertilizer are required. Often these soils become hardened when exposed in this way. Although most of these forests are not suitable for agriculture, large expanses of tropical rainforest are being cleared yearly because of the pressure for more farmland in the highly populated tropical countries and the desire for high-quality lumber from many of the forest trees.

The relationship between elevation and climate

The distribution of terrestrial ecosystems is primarily related to temperature and precipitation. Air temperatures are warmest near the equator and become cooler as the poles are approached. Similarly, air temperature decreases as elevation increases. This means that even at the equator it is possible to have cold temperatures on the peaks of tall mountains. Therefore, as one proceeds from sea level to the tops of mountains, it is possible to pass through a series of biomes that are similar to what one would encounter traveling from the equator to the North Pole.

14.6 succession

Each of the communities we have just discussed is relatively stable over long periods of time. A relatively stable, long-lasting community is called a climax community. The word climax implies the final step in a series of events. That is just what the word means in this context because communities can go through a series of predictable, temporary stages that eventually result in a long-lasting stable community. The process of changing from one type of community to another is called succession, and each intermediate stage leading to the climax community is known as a successional stage or successional community.

Two different kinds of succession are recognized: primary succession, in which a community of plants and animals develops where none existed previously, and secondary succession, in which a community of organisms is disturbed by a natural or human-related event (e.g. hurricane, volcano, fire, forest harvest) and returned to a previous stage in the succession. Primary succession is much more difficult to observe than secondary succession because there are relatively few places on earth that lack communities of organisms. The tops of mountains, newly formed volcanic rock, and rock newly exposed by erosion or glaciers can be said to lack life. However, bacteria, algae, fungi, and lichens quickly begin to grow on the bare rock surface, and the process of succession has begun. The first organisms to colonize an area are often referred to as pioneer organisms, and the community is called a pioneer community.

Lichens are frequently important in pioneer communities. They are unusual organisms that consist of a combination of algae cells and fungi cells—a combination that is very hardy and is able to grow on the surface of bare rock. Because algae cells are present, the lichen is capable of photosynthesis and can form new organic matter. Furthermore tiny consumer organisms can make use of the lichens as a source of food and a sheltered place to live. The action of the lichens also tends to break down the rock surface upon which they grow. This fragmentation of rock by lichens is aided by the physical weathering process of freezing and thawing, dissolution by water, and wind erosion. Lichens also trap dust particles, small rock particles, and the dead remains of lichens and other organisms that live in and on them. These processes of breaking down rock and trapping particles result in the formation of a thin layer of soil.

As the soil layer becomes thicker, small plants such as mosses may become established, increasing the rate at which energy is trapped and adding more organic matter to the soil. Eventually, the soil may be able to support larger plants that are even more efficient at trapping sunlight, and the soil-building process continues at a more rapid pace. Associated with each of the producers in each successional stage is a variety of small animals, fungi, and bacteria. Each change in the community makes it more difficult for the previous group of organisms to maintain itself. Tall plants shade the smaller ones they replaced; consequently, the smaller organisms become less common, and some may disappear entirely. Only shade-tolerant species will be able to grow and compete successfully in the shade of the taller plants. As this takes place we can recognize that one stage has succeeded the other.

Depending on the physical environment and the availability of new colonizing species, succession from this point can lead to different kinds of climax communities. If the area is dry, it might stop at a grassland stage, it is cold and wet, a coniferous forest might be the climax community. If it is warm and wet, it may be a tropical forest. The rate at which this successional process takes place is variable. In some warm, moist, fertile area the entire process might take place in less than 100 years. In harsh environments, like mountaintops or very dry areas, it may take thousands of years.

Primary succession can also be observed in the progression from the aquatic community to a terrestrial community. Lakes, ponds, and slow-moving parts of rivers accumulate organic matter. Where the water is shallow, this organic matter supports the development of rooted plants. In deeper water, we find only floating plants like water lilies that send their roots down to the mucky bottom. In shallower water, upright rooted plants like cattails and rushes develop. The cattail community contributes more organic matter, and the water level becomes more shallow. Eventually, a mat of mosses, grasses, and even small trees may develop on the surface along the edge of the water. If this continues for perhaps 100 to 200 years, an entire pond of lake will become filled in. more organic matter accumulates because of the large number of producers and because the depression that was originally filled with water becomes drier. this will usually result in a wet grassland, which in many area will be replaced by the climax forest community typical of the area.

Secondary succession occurs when a climax community or one of the successional stages leading to it is changed to an earlier stage. For example, when land is converted to agriculture the original climax vegetation is removed. When agricultural land is abandoned it returns to something like the original climax community. One obvious difference between primary succession and secondary succession is that in the latter there is no need to develop a soil layer. Another difference is that there is likely to be a reservoir of seeds from plants that were part of the original climax community. The seeds may have existed for years in a dormant state or they may be transported to the disturbed site from undisturbed sites that still hold the species typical of the climax community for the region. If we begin with the bare soil the first year, it is likely to be invaded by a pioneer community of weed species that are annual plants. Within a year or two, perennial plants like grasses become established. Because most of the weed species need bare soil for seed germination, they are replaced by the perennial grasses and other plants that live in association with grasses. The more permanent grassland community is able to support more insects, small mammals, and birds than the weed community could. If rainfall is low, succession is likely to stop at this grassland stage. If rainfall is adequate, several species of shrubs and fast-growing trees that require lots of sunlight (e.g., birch, aspen, juniper, hawthorn, sumac, pine, spruce, and dogwood) will become common. As the trees become larger, the grasses fail to get sufficient sunlight and die out. Eventually, shade-tolerant species of trees (e.g., beech, maple, hickory, oak, hemlock, and cedar) will replace the shade-intolerant species, and a climax community results.

14.7 human use of ecosystems

Most human use of ecosystems involves replacing the natural climax community with an artificial early successional stage. Agriculture involves replacing natural forest or prairie communities with specialized grasses such as wheat, corn, rice, and sorghum. This requires considerable effort on our part because the natural process of succession tends toward the original climax community. This is certainly true if remnants of the original natural community are still locally available to colonize agricultural land. Small woodlots in agricultural areas of the eastern United States serve this purpose. Much of the work and expense of farming is necessary to prevent succession to the natural climax community. It takes a lot of energy to fight nature.

     Forestry practices often seek to simplify the forest by planting single-species forest of the same age. This certainly makes management and harvest practices easier and more efficient, but these kinds of communities do not contain the variety of plants, animals, fungi, and other organisms typically found in natural ecosystems.

Human-constructed lakes or farm ponds often have weed problems because they are shallow and provide ideal conditions for the normal successional processes that lead to their being filled in. often we do not recognize what a powerful force succession is.

The extent to which humans use an ecosystem is often tied to its productivity. Productivity is the rate at which an ecosystem can accumulate new organic matter. Because plants are the producers, it is their activities that are most important. Ecosystems in which conditions are most favorable for plants growth are the most productive. Warm, moist, sunny areas with high levels of nutrients in the soil are ideal. Some areas have low productivity because of the essential factors is missing. Deserts have low productivity because water is scarce, arctic areas because temperature is low, and the open ocean because nutrients are in short supply. Some communities, such as coral reefs and tropical rainforest, have high productivity. Marshes and estuaries are especially productive because the waters running into them are rich in the nutrients that aquatic photosynthesizers need. Furthermore, these aquatic systems are usually shallow so that light can penetrate through most of the water column.

Humans have been able to make use of naturally productive ecosystems by harvesting food from them. However, in most cases, we have altered certain ecosystems substantially to increase productivity for our own purposes. In so doing, we have destroyed the original ecosystem and replaced it with an agricultural ecosystem. For example, nearly all of the Great Plains region of North America has been converted to agriculture. The original ecosystem included the Native Americans who used buffalo as a source of food. There was much grass, many buffalo, and few humans. Therefore, in the Native Amerians’ pyramid of energy, the base was more than ample. However, with the exploitation and settling of America, the population in North America increased at a rapid rate. The top of the pyramid became larger. The food chain (prairie grass—buffalo—human) could no longer supply the food needs of the growing population. As the top of the pyramid grew, it became necessary for the producer base to grow larger. Because wheat and corn yield more biomass for humans than the original prairie grasses could, the settlers’ domestic grain and cattle replaced the prairie grass and buffalo. This was fine for the settlers, but devastating for the buffalo and Native Americans.

In similar fashion the deciduous forests of the east were cut down and burned to provide land for crops. The crops were able to provide more food than did harvesting game and plants from the forest.

Anywhere in the world where the human population increases, natural ecosystems are replaced with agricultural ecosystems. In many parts of the world, the human demand for food is so large that it can be met only if humans occupy the herbivore trophic level rather than the carnivore trophic level. Humans are omnivores that can eat both plants and animals as food, so they have a choice. However, as the size of the human population increases, it cannot afford the 90% loss that occurs when plants are fed to animals that are in turn eaten by humans. In much of the less-developed world, the primary food is grain; therefore, the people are already at the herbivore level. It is only in the developed countries that people can afford to eat meat. This is true from both an energy point of view and a monetary point of view. Figure 14.23 shows a pyramid of biomass having a producer base of 100 kilograms of grain. The second trophic level only has 10 kilograms of cattle because of the 90% loss typical when energy is transferred from one trophic level to the next (90% of the corn raised in the United States is used as cattle feed). The consumers at the third trophic level, humans in this case, experience a similar 90% loss. Therefore, only 1 kilogram of humans can be sustained by the two-step energy transfer. There has been a 99% loss in energy: 100 kilograms of grain are necessary to sustain 1 kilogram of humans.

Because much of the world’s population is already feeding at the second trophic level, we cannot expect food production to increase to the extent that we could feed 10 times more people than exist today.

It is unlikely that most people will be able to fulfill all their nutritional needs by just eating grains. In addition to calories, people need a certain amount of protein in their diets and one of the best resources of protein is meat. Although protein is available from plants, the concentration is greater from animal sources. Major parts of Africa, Asia, and Latin America have diets that are deficient in both calories and protein. These people have very little food, and what food they do have is mainly from plant sources. These are also the parts of the world where human population growth is most rapid. In other words, these people are poorly nourished and, as the population increases, they will probably experience greater calories and protein deficiency. This example reveals that even when people live as consumers at the second trophic level, they may still not get enough food, and if they do, it may not have the protein necessary for good health. It is important to point out that there is currently enough food in the world to feed everyone. The primary reasons for starvation are political and economic. Wars and civil unrest disrupt the normal food-raising process. People leave their homes and migrate to areas unfamiliar to them. Poor people and poor countries cannot afford to buy food from the countries that have a surplus.

Many biomes, particularly the drier grasslands, cannot support the raising of crops. However, they can still be used as grazing land to raise livestock. Like the raising of crops, grazing often significantly alters the original grassland ecosystem. Some attempts have been made to harvest native species of animals from grasslands, but the species primarily involved are domesticated cattle, sheep, and goats. The substitution of the domesticated animals displaces the animals that are native to the area and also alters the plant community, particularly if too many animals are allowed to graze.

Even aquatic ecosystems have been significantly altered by human activity. Overfishing of many areas of the ocean has resulted in the loss of some important commercial species. For example, the codfishing industry along the east coast of North America has been destroyed by overfishing. Pacific salmon species are also heavily fished and disagreements among the countries that exploit these species may cause the decline of this fishery as well.

Boreal Coniferous Forest

Through parts of southern Canada, extending southward along the Appalachian and Rocky mountains of the united states, and in much of northern Europe and Asia we find communities that are dominated by evergreen trees. This is the taiga, boreal coniferous forest, or northern coniferous forest biome. The evergreen trees are especially adapted to withstand long, cold winters with abundant snowfall. Typically the growing season is less than 120 days and rainfall ranges between 40 and 100 centimeters per year. However, because of the low average temperature, evaporation is low and the climate is humid. Most of the trees in the wetter, colder areas are spruces and firs, but some drier, warmer areas have pines. The wetter areas generally have dense stands of small trees intermingled with many other kinds of vegetation and many small lakes and bogs. In the mountains of the western United States, pines trees are often widely scattered and very large, with few branches near the ground. The area has a parklike appearance because there is very little vegetation on the forest floor. Characteristic animals in this biome include mice, snowshoe hare, lynx, bears, wolves, squirrels, moose, midges, and flies. These animals can be divided into four general categories: those that become dormant in winter (insects and bears); those that are specially adapted to withstand the severe winters (snowshoe hare, lynx); those that live in protected areas (mice under the snow);and those that migrate south in the fall (most birds).

Temperate Rainforest

The coastal areas of northern California, Oregon, Washington, British Columbia, and southern Alaska contain an unusual set of environmental conditions that support a temperate rainforest. The prevailing winds from the west bring moisture-laden air to the coast. As the air meets the coastal mountains and is forced to rise, it cools and the moisture falls as rain and snow. Most of these areas receive 200 centimeters or more precipitation per year. This abundance of water, along with fertile soil and mild temperatures, results in a lush growth of plants.

Sitka spruce, Douglas fir, and western hemlock are typical evergreen coniferous trees in the temperate rainforest. Undisturbed (old growth) forests of this region have trees as old as 800 years that are nearly 100 meters tall. Deciduous trees of various kinds (red alder, big leaf maple, black cottonwood) also exist in open areas where they can get enough light. All trees are covered with mosses, ferns and other plants that grow on the surface of the trees. The dominant color is green because most surfaces have something photosynthetic growing on them.

When a tree dies and falls to the ground it rots in place and often serves as a site for the establishment of new trees. This is such common feature of the forest that the fallen, rotting trees are called nurse trees. The fallen tree also serves as a food source for a variety of insects, which are food for a variety of larger animals.

Because of the rich resource of trees, 90% of the original temperate forest has already been logged. Many areas have been protected because they are home to the endangered northern spotted owl and marbled murrelet (a sea bird).

Tundra

North of the coniferous forest biome is an area known as the tundra. It is characterized by extremely long, severe winters and short, cool summers. The growing season is less than 100 days and even during the short summer the nighttime temperatures approach 0 ⁰C. Rainfall is low (10-25 centimeters per year). The deeper layers of the soil remain permanently frozen, forming a layer called the permafrost. Because the deeper layers of the soil are frozen, when the surface thaws the water forms puddles on the surface. Under these conditions of low temperature and short growing season, very few kinds of animals and plants can survive. No trees can live in this region. Typical plants and animals of the area are grasses, sedges, dwarf willow, and some other shrubs, reindeer moss (actually a lichen), caribou, wolves, musk oxen, fox, snowy owls, mice, and many kinds of insects. Many kinds of birds are summer residents only. The tundra community is relatively simple, so any changes may have drastic and long-lasting effects. The tundra is easy to injure and slow to heal; therefore we must treat it gently. The construction of the Alaskan pipeline has left scars that could still be there 100 years from now.

Tropical rainforest

The tropical rainforest is at the other end of the climate spectrum from the tundra. Tropical rainforests are found primarily near the equator in Central and South America, Africa, parts of southern Asia, and some pacific islands. The temperature is high (averaging about 27 C), rain falls nearly every day (typically 200-1000 centimeters per year), and there are thousands of species of plants in a small area. Balsa (a very light wood), teak (used in furniture), and ferns are examples of plants from the tropical rainforest. Typically, every plant has other plants growing on it. Tree trunks are likely to be covered with orchids, many kinds of vines, and mosses. Tree frogs, bats, lizard, birds, ms, and an almost infinite variety of insects inhabit the rainforest. These forests are very dense, and little sunlight reaches the forest floor. When the forest is opened up (by a hurricane or the death of a large tree) and sunlight reaches the forest floor, the opened area is rapidly overgrown with vegetation.

Because plants grow so quickly in these forests, people assume the soils are fertile, and many attempts have been made to bring this land under cultivation. In reality, the soils are poor in nutrients. The nutrients are in the organisms, and as soon as an organism dies and decomposes its nutrients are reabsorbed by other organisms. Typical North American agricultural methods, which require the clearing of large areas, cannot be used with the soil and rainfall conditions of the tropical rainforest. The constant rain falling on these fields quickly removes the soil’s nutrients so that heavy applications of fertilizer are required. Often these soils become hardened when exposed in this way. Although most of these forests are not suitable for agriculture, large expanses of forests are being cleared yearly because of the pressure for more farmland in the highly populated tropical countries and the desire for high-quality lumber from many of the forest trees.

The relationship between elevation and climate

The distribution of terrestrial ecosystems is primarily related to temperature and precipitation. Air temperatures are warmest near the equator and become cooler as the poles are approached. Similarly, air temperature decreases as elevation increases. This means that even at the equator it is possible to have cold temperatures on the peaks of tall mountains. Therefore, as one proceed from sea level to the tops of mountains, it is possible to pass through a series of biomes that are similar to what one would encounter traveling from the equator to the North Pole.

14.6 succession

Each of the communities we have just discussed is relatively stable over long periods of time. A relatively stable, long-lasting community is called a climax community. The word climax implies the final step in a series of events. That is just what the word means in this context because communities can go through a series of predictable, temporary stages that eventually result in a long-lasting stable community. The process of changing from one type of community to another is called succession, and each intermediate stage leading to the climax community is known as a successional stage or successional community.

Two different kinds of succession are recognized: primary succession, in which a community of plants and animals develops where none existed previously, and secondary succession, in which a community of organisms is disturbed by a natural or human-related event (e.g., hurricane, volcano, fire, forest harvest) and returned to a previous stage in the succession. Primary succession is much more difficult to observe than secondary succession because there are relative few places on earth that lack communities of organisms. The tops of mountains, newly formed volcanic rock, and rock newly exposed by erosion or glaciers can be said to lack life. However, bacteria, algae, fungi, and lichens quickly begin to grow on the bare rock surface, and the process of succession has begun. The first organisms to colonize an area are often referred as pioneer organisms, and the community is called a pioneer community.

Lichens are frequently important in pioneer communities. They are unusual organisms that consist of a combination of algae cells and fungi cells—a combination that is very hardy and is able to grow on the surface of bare rock. Because algae cells are present, the lichen is capable of photosynthesis and can form new organic matter. Furthermore, many tiny consumer organisms can make use of the lichens as a source of food and a sheltered place to live. The action of the lichens also tends to beak down the rock surface upon which they grow. This fragmentation of rock by lichens is aided by the physical weathering processes of freezing and thawing, dissolution by water, and wind erosion. Lichens also trap dust particles, small rock particles, and the dead remains of lichens and other organisms that live in and on them. These processes of breaking down rock and trapping particles result in the formation of a thin layer of soil.

As the soil layer becomes thicker, small plants such as mosses may become established, increasing the rate at which energy is trapped and adding more organic matter to the soil. Eventually, the soil may be able to support larger plants that are even more efficient at trapping sunlight, and the soil-building process continues at a more rapid pace. Associated with each of the producers in each successional stage is a variety of small animals, fungi, and bacteria. Each change in the community makes it more difficult for the previous group of organisms to maintain itself. Tall plants shade the smaller ones they replaced; consequently, the smaller organisms become less common, and some may disappear entirely. Only shade-tolerant species will be able to grow and compete successfully in the shade of the taller plants. As this takes place we can recognize that one stage has succeeded the other.

Depending on the physical environment and availability of new colonizing species, succession from this point can lead to different kinds of climax communities. If the area is dry, it might stop at a grassland stage. If it is cold and wet, a coniferous forest might be the climax community. If it is warm and wet, it may be tropical rainforest. The rate at which this successional process takes place is variable. In some warm, moist, fertile areas the entire process might take place in less than 100 years. In harsh environments, like mountain tops or very dry areas, it may take thousands of years.

Primary succession can also be observed in the progression from an aquatic community to a terrestrial community. Lakes, ponds, and slow-moving parts of rivers accumulate organic matter. Where the water is shallow, this organic matter supports the development of rooted plants. In deeper water, we find only floating plants like lilies that send their roots down to the mucky bottom. In shallower water, upright rooted plants like cattails and rushes develop. The cattail community contributes more organic matter, and the water level becomes more shallow. Eventually, a mat of mosses, grasses, and even small trees may develop on the surface along the edge of the water. If this continues for perhaps 100 to 200 years, an entire pond or lake will become filled in. more organic matter accumulates because of the large number of producers and because the depression that was originally filled water becomes drier. This will be replaced by the climax forest community typical of the area.

Second succession occurs when a climax community or one of the successional stages leading to it is changed to an earlier stage. For example, when land is converted to agriculture the original climax vegetation is removed. When agricultural land is abandoned it returns to something like the original climax community. One obvious difference between primary and secondary succession is that in the latter there is no need to develop a soil layer. Another difference is that there is likely to be a reservoir of seeds from plants that were part of the original climax community. The seeds may have existed for years in a dormant state or they may be transported to the disturbed site from undisturbed sites that still hold the species typical of the climax community for the region. If we begin with bare soil the first year, it is likely to be invaded by a pioneer community of weed species that are annual plants. Within a year or two, perennial plants like grasses become established. Because most of the weed species need bare soil for seed germination, they are replaced by the perennial grasses and other plants that live in association with grasses. The more permanent grassland community is able to support more insects, small mammals, and birds than the weed community could. If rainfall is low, succession is likely to stop at this grassland stage. If rainfall is adequate, several species of shrubs and fast-growing trees that require lots of sunlight (e.g., birch, aspen, juniper, hawthorn, sumac, pine, spruce, and dogwood) will become common. As the trees become larger, the grasses fail to get sufficient sunlight and die out. Eventually, shade-tolerant species of trees (e.g., beech, maple, hickory, oak, hemlock, and cedar) will replace the shade-intolerant species, and a climax community results.

14.7 human use of ecosystems

Most human use of ecosystems involves replacing the natural climax community with an artificial early successional stage. Agriculture involves replacing natural forest or prairie communities with specialized grasses such as wheat, corn, rice, and sorghum. This requires considerable effort on our part because the natural process of succession tends toward the original climax community. This is certainly true if remnants of the original natural community are still locally available to colonize agricultural land. Small woodlots in agricultural areas of the eastern United States serve this purpose. Much of the work and expense of farming is necessary to prevent succession to the natural climax community. It takes a lot of energy to fight nature.

Forestry practices often seek to simplify the forest by planting single-species forests of the same age. This certainly makes management and harvest practices easier and more efficient, but these kinds of communities do not contain the variety of plants, animals, fungi, and other organisms typically found in natural ecosystems.

Human-constructed lakes or farm ponds often have weed problem because they are shallow and provide ideal conditions for the normal successional processes that lead to their filled in. often we do not recognize what a powerful succession is.

The extent to which humans use an ecosystem is often tied to its productivity. Productivity is the rate at which an ecosystem can accumulate new organic matter. Because plants are the producers, it is their activities that are most important. Ecosystems in which conditions are most favorable for plant growth are the most productive. Warm, moist, sunny areas with high levels of nutrients in the soil are ideal. Some areas have low productivity because one of the essential factors is missing. Deserts have low productivity because water is scarce, arctic areas because temperature is low, and the open ocean because nutrients are in short supply. Some communities, such as coral reefs and tropical rainforests, have high productivity. Marshes and estuaries are especially productive because the waters running into them are rich in the nutrients that aquatic photosynthesizers need. Furthermore, these aquatic systems are usually shallow so that light can penetrate through most of the water column.

Human have been able to make use of naturally productive ecosystems by harvesting the food from them. However, in most cases, we have altered certain ecosystems substantially to increase productivity for our own purposes. In so doing, we have destroyed the original ecosystem and replaced it with an agricultural ecosystem. For example, nearly all of the Great Plains region of North American has been converted to agriculture. The original ecosystem included the Native Americans who used buffalo as a source of food. There was much grass, many buffalo, and few humans. Therefore, in the Native Americans’ pyramid of energy, the base was more than ample. However, with the exploitation and settling of American, the population in North American increased at a rapid rate. The top of the pyramid became larger. The food chain (prairie grass—buffalo—human) could no longer supply the food needs of the growing population. As the top of the pyramid grew, it became necessary for the producer base to grow larger. Because wheat and corn yield more biomass for humans than the original prairie grasses could, the settlers’ domestic grain and cattle replaced the prairie grass and buffalo. This was fine for the settlers, but devastating for the buffalo and Native Americans.

In similar fashion the deciduous forests of the east were cud down and burned to provide land for crops. The crops were able to provide more food than did harvesting game and plants from the forest.

Anywhere in the world where the human population increases, natural ecosystems are replaced with agricultural ecosystems. In many part of the world, the human demand for food is so large that it can be met only if humans occupy the herbivore tropic level rather than the carnivore tropic level. Human are omnivores that can eat both plants and animals as food, so they have a choice. However, as the size of the human population increases, it can not afford the 90% loss that occurs when plants are fed to animals that are in turn eaten by humans. In much of the less-developed world, the primary food is grain; therefore, the people are already at the herbivore level. It is only in the developed countries that people can afford to eat meat. This is true from both an energy point of view and a monetary point of view. Figure 14.23 shows a pyramid of biomass having a producer base of 100 kilograms of grain. The second tropic level only has 10 kilograms of cattle because of the 90% loss typical when energy is transferred from one tropic level to the next (90% of the corn raised in the United States is used as cattle feed). The consumers at the third level, humans in this case, experience a similar 90% loss. Therefore, only 1 kilogram of humans can be sustained by the two-step energy transfer. There has been a 99% loss in energy: 100 kilograms of grain are necessary to sustain 1 kilogram of humans.

Because much of the world’s population is already feeding at the second tropic level, we can not expect food production to increase to the extent that we could feed 10 times more people than exist today.

It is unlikely that most people will be able to fulfill all their nutritional needs by just eating grains. In addition to calories, people need a certain amount of protein in their diets and one of the best sources of protein is meat. Although protein is available from plants, the concentration is greater from animal sources. Major parts of Africa, Asia and Latin American have diets that are deficient in both calories and protein. These people have very little food, and what food they do have is mainly from plant sources. These are also the parts of the world where human population growth is most rapid. In other word, these people are poorly nourished and, as the population increases, they will probably experience greater calories and protein deficiency. This example reveals that even when people live as consumers at the second tropical level, they may still not get enough food, and if they do, it may not have protein necessary for good health. It is important to point out that there is currently enough food in the world to feed every one. The primary reasons for starvation are political and economic. Wars and civil unrest disrupt the normal food raising process. People leave their homes and migrate to areas unfamiliar to them. Poor people and poor countries cannot afford to buy food form the countries that have a surplus.

Many biomes, particularly the drier grasslands, cannot support the raising of crops. However, they can still be used as grazing land to raise livestock. Like the raising of crops, grazing often significantly alters the original grassland ecosystem. Some attempts have been made to harvest native species of animals from grasslands, but the species primarily involved are domesticated cattle, sheep, and goats. The substitution of the domesticated animals displaces the animals that are native to the area and also alters the plant community, particularly if too many animals are allowed to graze.

Even aquatic ecosystems have been significantly altered by human activity. Overfishing of many areas of the ocean has resulted in the loss of some important commercial species. For example, the codfishing industry along the east coast of North American has been destroyed by overfishing. Pacific salmon species are also heavily fished and disagreements among the countries that exploit these species may cause the decline of this fishery as well.