READING AND QUESTIONS: BIOTIC RELATIONSHIPS IN ECOSYSTEMS
POPULATIONS, COMMUNITIES AND ECOSYSTEMS
In studying organisms in nature, ecologists often look at a particular group of organisms in a particular type of
natural setting. The simplest grouping of organisms in nature is a population. A population includes all
individuals of a particular species within a certain area. All the pecan trees in a forest make up a population. All
the bullfrogs in a pond make up a population. Populations can also be considered as parts of larger groups. All
the populations of different organisms within a given area make up a community. For example, all the frogs, fish,
algae, plants and other living things in and around a pond make up a pond community.
An ecosystem includes a community and its physical environment. In an ecosystem, both the biotic
(living) and abiotic (nonliving) factors are included. There is an ongoing exchange of materials between the
abiotic and biotic parts of an ecosystem. All the ecosystems of the earth are linked to one another. Organisms
move from one ecosystem to another. Water and other inorganic substances pass from one ecosystem to
another. Also, organic compounds, with their stored energy, are transferred between ecosystems.
AUTOTROPHIC AND HETEROTROPHIC NUTRITION
An ecosystem includes all kinds of organisms — microorganisms, plants, fungi and animals. These organisms
interact in many ways, but their nutritional and energy relationships are among the most important.
Autotrophs are organisms that can make their own food using carbon dioxide. Most autotrophs carry on
photosynthesis, which requires sunlight and water. A few, however, carry on chemosynthesis; bacteria found in
deep vents in the ocean floor are an example. Directly or indirectly, autotrophs provide the food for heterotrophs
— those organisms that cannot synthesize their own food.
Heterotrophs are divided into several groups according to what they eat and how they obtain their food.
Heterotrophs include herbivores, carnivores, omnivores and saprotrophs. Herbivores are animals that feed only
on plants. Rabbits, cattle, horses, sheep and deer are herbivores. Carnivores are animals that feed on other
animals. Some carnivores are predators and some are scavengers. Predators, such as lions, hawks and
wolves, attack and kill their prey and feed on their bodies. Scavengers are carnivores that feed on dead animals
they find. Vultures and hyenas are scavengers. Omnivores are animals that feed on both plants and animals.
Humans and bears are omnivores. Saprotrophs are organisms that obtain nutrients by breaking down, or
decomposing, the remains of dead plants and animals. Many bacteria and fungi function as saprotrophs.
SYMBIOTIC RELATIONSHIPS
Symbiosis is a term used to describe a long-term relationship between two different species that live in close
association with each other and in which at least one of the organisms benefits. There are three types of
symbiotic relationships: mutualism, commensalism and parasitism.
In mutualism, both organisms benefit from their association. For example, termites have cellulosedigesting bacteria living in their digestive tracts. Without these microbes, termites could not get nutrients from
the wood they eat. In turn, the termites provide the bacteria with food and a place to live. Cows have a similar
association with the microbes that live in their digestive tracts.
Lichens consist of algal and fungal cells. Both types of cells benefit from this association. It allows them
to live in environments in which neither could survive alone. Through photosynthesis, the algae produce food for
themselves and for the fungi. The fungi provide moisture and the structural framework and attachment sites in
which the algae grow.
Peas, clover and alfalfa are legumes. Legumes have nodules on their roots in which nitrogen-fixing
bacteria grow. The bacteria convert nitrogen gas from the air in the soil into forms usable by the plants. In this
relationship, the plants are supplied with the nitrogen compounds they need, while the bacteria are given an
environment in which they can grow and reproduce.
In commensalism, one organism benefits from the symbiotic relationship and the other is not affected.
For example, pilotfish are small fish that live with sharks. They eat the scraps left over from the shark's feeding.
Thus, the shark provides the pilotfish with food. As far as is known, the pilotfish neither helps nor hurts the
shark. Barnacles may attach themselves to the large body surface of a whale. Barnacles are sessile (unable to
move on their own) and rely on water currents to bring them food. The movements of the whale provide them
with a constantly changing environment and food supply. The whale is not affected by the presence of the
barnacles.
In parasitism, one organism benefits from the symbiosis and the other is harmed. The organism that
benefits is called the parasite, while the organism that is harmed is called the host. Some parasites cause only
slight damage to their hosts, while others kill the host. Tapeworms, for example, are parasites that live in the
digestive tracts of various animals. There, they are provided with nutrients and an environment in which to grow
and reproduce. However, the host is harmed by the presence of the tapeworms. The loss of nutrients and
tissue damage caused by the worm can cause serious illness. There are also parasitic plants that grow on other
plants. Two examples of plant parasites are mistletoe and Indian pipe.
COMPETITION IN ECOSYSTEMS
Each type of organism within an ecosystem has a particular part of the environment in which it lives. This is its
habitat. For example, the habitat of a slime mold is the damp floor of a forest. Because of the complex
interactions that occur within an ecosystem, each species also plays a particular role in an ecosystem. The role
of a species in an ecosystem is its niche. An organism's habitat is part of its niche, but only part. Also included
are how, when and where it obtains nutrients, its reproductive behavior, and its direct and indirect effects on the
environment and on other species within the ecosystem.
In a balanced ecosystem, each species occupies its own niche. It occupies a particular territory (its
habitat) and obtains nutrients in a particular way. Competition arises when the niches of two species overlap.
The greater the overlap — in other words, the more requirements the two species have in common — the more
intense the competition. Competition between two different species is called interspecific competition. As the
resources being competed for become more scarce, the competition becomes more intense. Eventually, one of
the species is eliminated from the ecosystem, leaving the more successful species to occupy the niche.
Competition also occurs between members of the same species. This is called intraspecific competition.
The intensity of the competition between members of the same species is affected by population density and the
availability of resources. If conditions become harsh, those individuals with the most helpful adaptations will
survive. The less-well-adapted individuals will not.
PRODUCERS, CONSUMERS AND DECOMPOSERS
In all but a few small ecosystems, the autotrophs are plants and other photosynthetic organisms. They trap
energy from sunlight and use it for the synthesis of sugars and starch. These substances can be changed to
other organic compounds that are needed by the plant, or they can be broken down for energy. Heterotrophs
can only use the chemical energy stored in organic compounds for their life processes. These organic nutrients
must be obtained from the bodies of other organisms — either plants or animals. Because autotrophs are the
only organisms in an ecosystem that can produce organic compounds (food) from inorganic compounds, they
are called producers. Since heterotrophs must obtain nutrients from other organisms, they are called
consumers.
Saprotrophs play an important role in an ecosystem. They function as organisms of decay, or
decomposers. They break down the remains of dead plants and animals, releasing substances that can be
reused by other members of the ecosystem. In this way, many important substances are recycled in an
ecosystem.
FOOD CHAINS AND FOOD WEBS
Within an ecosystem, there is a pathway of energy flow that always begins with the producers. Energy stored in
organic nutrients synthesized by the producers is transferred to consumers when the plants are eaten.
Herbivores are the primary consumers, or first-level consumers. The carnivores that feed on the herbivores are
secondary consumers. For example mice feed on plants and are primary consumers. The snake that eats the
mouse is a secondary consumer, while the hawk that eats the snake is a tertiary consumer, or third-level
consumer. Since many consumers have a varied diet, they may be second-, third-, or higher-level consumers,
depending on their prey. Each of these feeding relationships forms a food chain, a series of organisms through
which food energy is passed. A simple food chain, with arrows showing the direction of energy flow, can be
shown as:
grass
field mouse
great horned owl
where the grass is a producer, the field mouse is a primary consumer, and the owl is a secondary consumer.
Feeding relationships in an ecosystem are never just simple food chains, however. There are many
types of organisms at each feeding level, and there are always many food chains in an ecosystem. Usually,
each organism is part of several different food chains. These food chains are interconnected to form a food
web. A simple food web could be shown as:
mountain lion
deer
snake
rabbit
Harris' hawk
small owl
field mouse
frog
cricket
trees
shrubs
grasses
At every level in an ecosystem, there are organisms that act as decomposers. The decomposers make use of
the wastes and remains of all organisms in the system. They use the energy they find in these materials for their
own metabolism (life processes). At the same time, they break down organic compounds into inorganic
compounds and make substances available for reuse. The decomposers are the final consumers in every food
chain and food web.
PYRAMIDS OF ENERGY AND BIOMASS
The amount of energy available in a food web decreases with each higher feeding level. This happens because
only a small fraction of the energy taken in as food becomes stored as new tissue. Much of the food eaten is not
digested and absorbed. Furthermore, a large part of the energy in food is used for respiration and maintenance.
This energy is lost as heat. As a result, only about 10 percent of the energy taken in at any feeding level is
passed to the next feeding level. Therefore, higher trophic (or feeding) levels can support fewer organisms.
The amount of energy available in an ecosystem is commonly shown in the form of a trophic pyramid, or
energy pyramid. The greatest amount of energy is present in the producers, which form the base of the pyramid,
and the least energy is present at the top of the pyramid, which are the highest-level consumers. Because the
amount of available energy decreases so steeply, there are usually no more than four or five trophic (feeding)
levels in an ecosystem.
The following diagram is an example of a trophic pyramid. In a field, green plants are the producers,
trapping the sun's energy for photosynthesis. For every 1000 calories of energy absorbed by the plants, only 100
are stored. Thus, 100 calories are available to a primary consumer, such as a cricket that feeds on the plants.
As a secondary consumer, a toad that eats the cricket receives only 10 of the original 100 calories consumed by
the cricket. At the top of the energy pyramid, a snake that eats the toad receives only 1 calorie of the 10 calories
received by the toad. This amount is only 0.001 percent of the energy originally absorbed by the plants at the
base of the pyramid.
energy of tertiary consumer (e.g., snakes)
energy of secondary consumer (e.g., toads)
energy of primary consumer (e.g., crickets)
energy of producers (e.g., green plants)
Since the total amount of energy available decreases with each higher trophic level, the total mass of
living organisms that can be supported at each level decreases too. This relationship can also be represented
by a pyramid. The relationship, known as the biomass pyramid, shows the relative mass of the organisms — the
biomass — at each trophic level. The greatest amount of biomass is in the lowest level, the producers. The
least is found in the highest level of consumers.
BIBLIOGRAPHY
Maton, Anthea, and Jean Hopkins, Susan Johnson, David LaHart, Charles W. McLaughlin, Maryanna Q.
Warner, and Jill D. Wright. 1994. Ecology, Earth's Living Resources. Englewood Cliffs, NJ: Prentice
Hall.
Miller, Kenneth R., and Joseph Levine. 1991. Biology. Englewood Cliffs, NJ: Prentice Hall.
QUESTIONS: Answer the following questions on a separate paper using complete sentences.
1.
Explain the difference between a population and a community.
2.
List the three types of symbiosis.
3.
Explain why each higher level of a tropic pyramid must be smaller than the one below it.
4.
Can two different species occupy the same habitat over a long period of time? Can they occupy the same
niche over a long period of time? Explain.
Using the food web diagram, assume that a disease caused most of the rabbits in the ecosystem to die. Use
that assumption to answer the following questions:
5.
Predict what effects this would have on the (a) hawk population, (b) grasses and (c) field mouse
population. Explain the basis for your prediction.
6.
Which, if any, of these effects is certain to occur. Explain.
7.
Explain why predictions about changes in ecosystems contain a high degree of uncertainty.