Fundamentals of Ecology

Into Ecology [Chapter]

          The environment includes both living and nonliving parts. The study of the interrelationships between the living and non-living portions of the environment is the subject matter of ecology. Ecologists study the interactions between living organisms and between organisms and their nonliving environment. Understanding these interrelationships and interactions can aid in the development of useful conservation measures.

                              ORGANISMS AND THEIR ENVIRONMENTS

            You're already aware of some relationships among organisms and their environments. For example, you know pollution can harm environments and therefore, organisms. You also know about relationships between certain organisms. You know bacteria and fungi are organisms that cause diseases in other living things. Organisms and their environments interact in many ways. It has been suggested that the phrase, "Everything is connected to everything else," describes the relationships between organisms and their environments. Look for examples of these connections as you study this unit.                                                                    

The Biosphere

            The portion of Earth that supports life is called the biosphere. The biosphere extends from several kilometers up in the atmosphere to the deepest parts of the oceans. It includes the solid portion of the land where life is found. Sometimes the biosphere is called the ecosphere because the three regions of the biosphere--air, water, and land--are connected. If chemicals, such as pesticides, are sprayed into the air, they may eventually pass into water systems or may cover the land. Fertilizers spread on the surface of the land may get into the water or air. Care must be taken to protect all parts of the biosphere so that organisms in each part survive.

            Every organism in the biosphere depends on its environment for survival. The environment supplies organisms with energy and materials for growth and repair. Plants use sunlight, water, carbon dioxide, and inorganic nutrients for photosynthesis. Animals use plants and other organisms for their supply of energy and organic matter. Fungi and bacteria get their energy and materials by decomposing dead matter and wastes.

            Organisms depend on nonliving and living factors in the  environment. Nonliving factors in the environment are called abiotic  factors. Abiotic factors include water, soil, temperature, light, air, and minerals. Living factors in the environment are called biotic factors. All the living organisms in a pond represent the biotic factors. Plants, bacteria, fish, worms, and crayfish are typical pond organisms.

Populations and Communities

            Just as cells are grouped into tissues and tissues into organs and then systems, organisms also can be put within groups. A population is a group of organisms of the same species that live in one area during some specific time. A species is considered to be a group of organisms that are capable of breeding with each other under natural conditions and producing fertile offspring. For example, mosquitoes on the surface of a pond in the spring and maple trees in a Vermont forest in the fall make up two populations.

            Populations can be grouped together. All the populations of different species that interact with each other within an area make up a community. All the protists, plants, and animals that interact on a coral reef make up a reef community.

            Within a community, each organism is found in a specific location. The habitat is the environment of a particular type of organism. For example, ferns are found in a moist, shady floor habitat of a forest community. The habitat of some snails is the leaf litter on the forest floor. In a pond community, a frog's habitat is near the edge of the water and includes both water and land. A trout in the same community has its habitat in the deeper, cooler portion of the pond.

            All of the biological, chemical, and physical factors of a species environment are part of its niche. The niche includes what a species needs to survive and reproduce in its environment. What organisms eat, how they get food, how they attract mates, where they live and what they do in their environment make up the niche. The habitat is part of an organism's niche. A habitat is sometimes considered a species address. The niche is the lifestyle or occupation of a species.

            Habitats often overlap and different organisms can be found in the same location. However, no two species can occupy exactly the same niche at the same time for very long. If they do, they begin to compete for the same basic and essential requirements. You may think that birds within a tree have the same niche. Careful observation will reveal differences -- some birds eat insects while others eat seeds; some feed beneath the tree while others feed in the tree. Some birds even get their food away from the tree. The birds may also have different methods of reproduction. They may have different mating behaviors, and they may nest in different spots.


            The biotic community and its abiotic environment interact and function as a system. This interactive system is called an ecosystem. The interaction involves the transfer of energy and materials among the organisms. Ecosystems have no size limitations. They may be as large as a desert or as small as the drops of water on a plant leaf. Plants, soil bacteria, soil nutrients, air spaces, and light and temperature are part of the interactive system within a garden.

            An ecosystem is self-sustaining when three conditions are met. First, it must have a relatively constant source of energy. Sunlight supplies the energy to most ecosystems. Second, energy must be converted by a living system into chemical bond energy in organic molecules. Plants, algae, and certain groups of bacteria accomplish this through the process of photosynthesis. Third, organic matter and inorganic nutrients must be recycled for reuse. In most ecosystems, this recycling is carried out by decomposers.

            An ecosystem becomes unstable when any of these three conditions is affected.  For example, if the flow of energy from the sun is disrupted, photosynthesis if affected. Without the food of plants, other organisms and the plants themselves would die off. If essential nutrients are unavailable or if certain species die off, the ecosystem could lose its ability to sustain itself. To remain stable, an ecosystem needs to maintain a dynamic balance between its biotic and abiotic factors.

                                                 BIOTIC RELATIONSHIPS

            Suppose you are going to run in an important race tomorrow. You have heard that "carbohydrate loading" tonight will provide additional energy. Therefore, you carefully select high carbohydrate foods. Like other living things, you get energy from your foods. The foods that you and other animals consume are byproducts of other organisms.

            Not only do you consume other organisms, but you also compete with other organisms for food. Every summer insects take a large share of home gardens. As much as you might not like to think about, your body is the home of many parasites. Guess who is the source of their food?

Feeding Relationships

            Within an ecosystem, organisms that make food by photosynthesis are called producers. Recall that plants, certain protists and some monerans use energy from the sun in this process. Producers become the food and energy source for consumers. Consumers are organisms that feed on other living things. They include animals, fungi, bacteria, and some protists.

            Consumers that feed directly on the producers are called primary consumers. Primary consumers are the food for the secondary consumers. Animals that derive nearly all of their food resources from plant matter are called herbivores. Secondary and higher level consumers that get most of their food from eating the flesh of other animals are known as carnivores. Omnivores eat both plants and animals.

            Decomposers are consumers that break down plant and animal remains and wastes. They decay the organic matter, making its parts available for reuse. The most common decomposers are bacteria and fungi. Scavengers are animals that feed on the dead bodies of other animals. Saprobes are organisms that obtain their nutrition from plant and animal remains.

            Energy flows through an ecosystem when organisms feed. Organisms are consumers. Higher-level consumers are not required for an ecosystem to be self-sustaining.

            Producers are called autotrophs, which means "self-feeders" since they "feed themselves" by making food in the process of photosynthesis. Autotrophs, such as plants, convert inorganic sources of energy into organic forms. Consumers are called heterotrophs  which means "other-feeders" since they feed on other organisms. Heterotrophs require organic molecules to carry out their life functions.

Food Chains and Food Webs

            In a self-sustaining ecosystem very little is wasted. Herbivores, like the grasshopper, eat the leaves of plants. In turn, carnivores like snakes or bullfrogs eat the grasshoppers. Other animals such as hawks may eat these carnivores. When any of these organisms die, decomposers in turn, consume them. After the organisms are decomposed, their nutrients are eventually taken up and used by green plants. In this way, matter is transferred through the ecosystem. Nutrients are transferred from producers to consumers in a feeding relationship known as a food chain. Each organism that eats or decomposes another is thus a link in that chain.

            Food chains often are often unstable because a change in the population size of any species may affect the chain in either direction. For example, if a primary consumer depends on one plant species for its food, the loss of that species could result in the death of the consumer. As an example, giant pandas have nearly become extinct because they eat bamboo shoots almost exclusively. The supply of these plants is growing smaller because the pandas' habitat is being destroyed by humans.

            Simple food chains of this type are rare in nature. Food chains are often seen in ecosystems that are attempting to re-establish themselves after volcanic activity or fires. Food chains are also seen in newly formed areas such as new islands.  The various feeding levels of producers and consumers in a food chain are called trophic levels. Producers belong to the first trophic level, primary consumers the second, secondary consumers the third. In nature, most organisms rely on many different sources of food for their nutritional needs. Animals may feed on several different types of food at the same or different trophic level. Depending on the availability of the specific foods, foxes may eat mice, rabbits, berries, or insects. Sea otters eat clams, sea urchins, mussels, and abalone. Bears eat plant parts as well as fish.                                             

            Omnivores are both primary and secondary consumers dependent upon whether they are eating plant or animal matter. Complex interrelationships begin to develop involving different trophic levels. Food chains interconnect into a more complex feeding sequence known as a food web. Food webs represent a more diversified feeding sequence and provide greater stability to the ecosystem.

Energy Flow through an Ecosystem

            While matter passes from part to part within an ecosystem, energy flows through it. Energy from the sun is collected, concentrated and converted into chemical bond energy by producers.  As organisms at each succeeding trophic level break down their food, chemical bond energy in the food is released as chemical energy and heat. This process of energy transfer and conversion is not 100 percent efficient and is governed by the Second Law of Thermodynamics or Energetics.

            Green plants convert only a small percentage of the sun's energy into chemical bond energy of organic molecules. Producers, consumers, and decomposers use some of the energy to maintain their life functions. Consumers are unable to digest some of the materials they take in. The energy in those undigestible foods is released as metabolic wastes. The energy in the waste material is not available to the consumer that produced it. The waste may be used as an energy source by organisms like the dung beetles.

            Each time that energy is converted from one form to another, some of the energy is given off as heat. This energy is no longer available to the organism for work or growth. Ecosystems cannot sustain themselves without a constant input of energy. Organisms within the ecosystem require a constant supply of food to serve as fuel in maintaining their life processes. In both cases, a constant source of energy is needed because of the inefficiency of energy conversion.      

            As energy flows through the ecosystem, only a small amount is used to produce new growth materials for the organisms within each level. Actual percentages vary with each trophic level and type of organism, but most scientists have agreed to the ten percent rule. This rule suggests that only about 10 percent of the energy in the organic matter at each trophic level is converted to usable materials for growth and development at the next trophic level. The remaining 90 percent is unavailable for work or growth because it has been given off as heat or is bound in waste materials. If 1000 units of energy are available to a first level consumer, the second level consumer that eats it has about 100 units of energy available to it.

            Scientists use models to show the transfer of usable and unusable forms of energy in food chains. Pyramid models of this type are only representations of the natural world. They can be used to illustrate basic ideas and aid in predicting possible outcomes.

            The pyramid of energy shows that as the number of links in the food chain increases, the amount of usable energy available to the next trophic level decreases. The pyramid of numbers shows that fewer organisms are supported at higher levels of a food chain. Biomass is a measure of the amount of living matter. The pyramid of biomass is a model that shows that less living matter can be supported at higher trophic levels.

            Energy pyramids are always pyramid shaped, but this is not always true for the pyramids of numbers and biomass. The pyramid of numbers, for example, is affected the size of the organisms. One tree might provide food for thousands of insects. A single tuna provides tuna fish sandwiches for a larger number of humans. The pyramid of biomass might also be distorted. For example, the biomass of a whale is several times greater than the total biomass of the plankton it consumes. The whale survives because the plankton reproduce very rapidly and thus maintain their population size.

            The pyramid models emphasize four important ideas. 1) All food chains begin with producers. 2) Consumers depend, directly or indirectly, on producers for their energy. 3) The amount of usable energy available to each trophic level is directly related to the number of links in a food chain. 4) Solar energy is required at the producer level to begin the flow of energy through the ecosystem. These principles reinforce the basic idea of interconnectedness of biotic and abiotic environmental factors.

 Factors that Affect Population Size

            Interactions among organisms affect the sizes of populations and the way these populations are distributed. These interactions can affect energy flow in food chains and webs. The interactions also help maintain the size and vitality of populations and prevent the depletion of needed resources like food and water. The end result of these interactions is the maintenance of a dynamic balance or homeostasis within the ecosystem.

            Organisms may compete for food, water, light, space, mates, and nutrients. Organisms compete with each other to obtain the same essential resources. Competition between organisms of the same species is intraspecific competition. Two sand crabs may compete for food; two robins may compete for mates. Interspecific competition occurs between organisms of different species. For example, kelp and red algae may compete for space, nutrients, and light in the ocean. If organisms that are competing modify their requirements, the competition is reduced.

            If competition is severe enough, it may cause organisms to migrate in search of resources. It can even cause some organisms to die. Forests and prairies have been destroyed in favor of farmlands and housing development sites. Humans compete with other organisms within these environments for food and shelter and in the process cause the migration or death of many organisms. For example, the California condor is a species that is nearly extinct due to humans competing for its habitat. The nesting areas of these birds have been destroyed. Now humans are trying to save the condors by breeding them in captivity. Many other species, both plants and animals, are being threatened in the same way by competition with humans.

            Population sizes within communities are affected not just by competition, but also by the kinds of food each organism consumes. An organism that captures and eats another is a predator. The organism that a predator captures is the prey. Predators and prey regulate the population size of each other. When the number of prey increases, an increase in the number of predators results. With the increase in predators, more prey are captured and eaten. The result is that the prey population decreases. With a decrease in the prey population, less food is available for the predator population. Therefore, a decrease in the predator population follows.

            This type of relationship is not always obvious since most organisms rely on several food sources in a food web. Owls may eat mice, rats, rabbits, or other prey, depending on their availability. Usually the classic pattern of increase and decrease in predator-prey relationships is seen when the species are in a confined or restricted area with few additional food sources. Other factors such as sickness, over-hunting, and earthquakes can cause short-term imbalances in either a predator or a prey population.

                                                        ABIOTIC FACTORS

            Not many people live in Death Valley, especially in the summer. The sun scorches the land. Daytime temperatures top 50oC. Rainfall in Death Valley is usually less than 40 mm per year. Physical factors determine the types of organisms that live in this environment. Light, temperature, and moisture play major roles in the distribution of plant and animal communities.

Light, Temperature, and Water

            You may have tried to grow houseplants outdoors and found that the sun burned the leaves. Perhaps you have forgotten to water your garden plants and found that they had wilted or died in the summer heat. Different plant species have different light requirements. Ferns on a forest floor require shade or diffuse sunlight. Other plants, like the desert cacti, require bright light. The intensity and duration of light affects plant growth and distribution. At the equator, plants get 12 h of light each day. In Alaska, plants may get 22 h of light each day in the middle of summer and about 2 h of light each day in the middle of winter.

            Radiant energy of the sun is changed to heat as it is absorbed by air, land, and water. This energy conversion helps maintain the earth's temperature and affects the movement of water through the biosphere. The movement of water affects the movement of other essential chemical elements.

            Temperature affects the rate of metabolic processes, reproduction, and survival of plants. Differences in air temperature create air movements that carry moisture toward or away from plants. Air temperature determines the amount of water vapor and other gases that the air may hold. Soil temperature determines the rate of water absorption by the roots of plants and the rate of root growth.    

            Species distribution depends on moisture. Some organisms are at home in rain forests where it rains every day. Others are adapted to life in deserts where water is in short supply. Well-aerated soil is filled with air passages allowing the circulation of gases such as oxygen, carbon dioxide, and nitrogen. Moisture clings to the surfaces of soil particles creating conditions that support bacteria, fungi, and protists. These soil microbes make chemical nutrients available to plants. Some microbes use up the nutrients, thus retarding plant growth.

            In an ecosystem, the biotic communities interact with the nonliving environment. Abiotic environmental factors control the distribution, size, reproduction, nutrition, and overall metabolism of the living communities.  

Chemical Cycling in Ecosystems

            Organisms require various chemical elements for growth and maintenance. Elements required in large amounts are known as macronutrients. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are macronutrients that makeup about 95 percent of the mass of all living things. Other elements such as copper, iodine, and manganese are required in very small amounts. Elements required in small amounts are known as micronutrients or trace elements. Earth contains only a fixed amount of these chemical elements. It is important that they be recycled quickly and efficiently.

            Elements follow a circular path from the abiotic environment to organisms and back again to the environment. This two-way exchange between the living and nonliving components within the ecosystem is called a cycle. Chemicals are continuously removed from the atmosphere, water, and land. They are used by living organisms and then returned in some form to the nonliving environment. A biogeochemical cycle is the cyclic movement of chemicals between living and nonliving components of the environment. Biogeochemical cycling is also called nutrient cycling.

            Elements such as carbon, oxygen, and nitrogen are found in large quantities in the oceans and atmosphere. These elements are often found combined with each other in these areas. Biogeochemical cycles are often classified by the storage site or reservoir of the element. Carbon, oxygen, and nitrogen participate in gaseous cycles because of their atmospheric reservoir and the fact that these elements are often found in gaseous form.

            The elements phosphorus, sulfur, calcium, magnesium, and copper are found bound into the solid matter of earth's crust. These elements are involved in sedimentary cycles for they are usually found in solid form in rock. The hydrologic cycle is the movement of water from sea to the land and back again to the sea. Movement of elements within and between the air, land, and water reservoirs is slower than movement of these elements between organisms.     

            Many elements are found combined in nature. Thus, biogeohemical cycles are often interconnected. Oxygen makes up about 20 percent of the atmospheric gases. Carbon, in the form of carbon dioxide, makes up about 0.03 percent of those gases. Oxygen is part of the water molecule. Both carbon dioxide and oxygen dissolve in water. In photosynthesis, carbon dioxide and water combine to form organic compounds. During the process, oxygen is released. Organisms use oxygen in aerobic respiration, releasing carbon dioxide and water.

                                                      The Carbon Cycle 

            Carbon dioxide moves from the atmosphere to producers who use it in photosynthesis. Consumers and decomposers eat the producers and each other. Carbon is passed through the food chain. During respiration, these organisms release carbon dioxide back into the atmosphere or waters. Carbon dioxide also enters the atmosphere when fossil fuels and wood are burned. Volcanic activity and the weathering of carbon-bearing rocks also add carbon dioxide. 

            Large amounts of carbon are found in ocean waters. It is dissolved as carbon dioxide or stored as calcium carbonate in rocks and animal shells. Carbon dioxide diffuses from the water to the atmosphere. It returns to the waters by precipitation. The remains of plants and animals may become compacted into carbonate rock. Limestone is a typical example.

                                                   The Nitrogen Cycle

            Nitrogen, like carbon, is an essential element for living organisms. Nitrogen gas (N2) makes up nearly 78% of the atmosphere. Unfortunately, plants, animals, fungi, and protists cannot use nitrogen in this form. Most nitrogen gas is converted into usable form by bacteria living in the root cells of certain plants. A small amount of nitrogen is converted to usable form by lightning.

            Bacteria and lightning convert nitrogen gas into nitrate or nitrite ions, ammonia gas, or ammonium ions. Nitrates dissolve in the soil water. They are taken up by the roots of plants and used to produce proteins and other organic nitrogen molecules. These nitrogen-containing molecules pass through the food chain. Animal wastes are converted to ammonia or ammonium ions by decomposers. Ammonium ions are converted to nitrites or nitrates and used by bacteria for energy. Other bacteria may convert the ammonia, nitrates, or nitrites back to nitrogen gas.

                                                The Phosphorus Cycle

            The phosphorus cycle consists of two interconnecting cycles. Phosphates in the rock and soil are taken up by plants. The plants are eaten by herbivores and phosphorus passes through the food chain. The phosphates reenter the soil in the form of animal wastes. This portion of the cycle is relatively rapid and localized.

            Some phosphates enter water systems and eventually find their way to the sea. Phosphates are used by algae and the algae are eaten by fish. In turn, the fish are eaten by birds. Bird waste, rich in phosphorus, is deposited on islands. Some of the phosphorus is washed into the oceans. The ocean sediments attract and bind phosphorus very tightly. Over long periods of time, phosphorus is returned to the land as mountains or islands rise from the sea bed. As the phosphorus is weathered or eroded, it returns to the oceans or is passed through the food chain. This portion of the cycle may take as long as a million years.

                                                       The Water Cycle 

            Water is the most abundant substance on earth's surface. Water moves from the atmosphere to the earth's surface in the form of rain, snow, or dew. Several routes are available to water that falls on land surfaces. It may runoff into rivers, lakes, streams, or the oceans. It may soak into the ground. There it may be taken up by the roots of plants. Water often seeps into the ground and becomes part of the groundwater supply. Groundwater may be stored in underground streams or lake-like areas called aquifers. This underground water supply moves by gravity and may come to the surface as a spring.

            Water returns to the atmosphere by evaporation or transpiration. Evaporation occurs when liquid water is converted to its gaseous form and moves from a surface. This movement may be from the surface of a lake, the soil, or an animal. Evaporation helps moderate the temperature in an area and concentrates minerals in the soil.

            Transpiration is a special case of evaporation. Transpiration refers to water lost from the surface of a plant, usually its leaves. This water had carried nutrients up from the roots and through the plant. The connection between the water cycle and other biogeochemical cycles again becomes evident.

            Not all the materials that cycle through an ecosystem are helpful to living things. Lead, mercury, cadmium, and radioactive strontium-90 are just a few examples. The proper functioning of the biosphere depends on the maintenance of its ecosystems. Ecosystems depend on the maintenance of dynamic balance and interconnections between their biotic and abiotic components. As you can see, everything is connected to everything else.