BIO 190:The Class: Chapter 40: lesson 40

Animal Ecology

Before going into the lesson, open the link below to look at some very interesting patterns in the distribution of animals in relation to ecological variables.

Distribution Patterns

Ecology is the part of the field of biology which studies the relationship between organisms and their environment.  Ecologists look at living things in relation to several different levels of organization.  Below the biosphere, which we looked at in Chapter 39, is the next level of organization, the ecosystem.  An ecosystem is a large, self-sustaining system made up of living organisms and non-living components.  Below the ecosystem is the  community, which is a group of living organisms which share the same environment.  The next level is the population which is a group of organisms of the same species which occupy a particular space.  A deme is a local population of interbreeding members which shares a common gene pool.   The final level of the hierarchy is the individual organism which responds to the environment.  To the ecologist, environment includes both other living organisms and the non-living surroundings.

These living and non-living components of the environment or ecosystem are referred to as biotic and abiotic factors, respectively.  The biotic factors affecting an organism include other living plants, animals and microorganisms in the environment.  Abiotic factors consist of  non-living factors such as temperature, moisture, elevation, soil, nutrients, wind and slope.

Please open the links below to further explore the principles of ecology.
The Ecosystem
Habitat

Biotic Components of the Ecosystem

Biologists identify three biotic categories of the ecosystem: producers, consumers and decomposers.

Producers, which include algae, green plants and cyanobacteria, use the energy of the sun to synthesize the sugars from carbon dioxide through the process of photosynthesis.  Producers are autotrophs.  The energy stored by plants through photosynthesis is called production, of which their are two types, primary and gross productivity.  Primary productivity refers to the initial step in the process of production, the input of energy by plants.  It is recorded in terms of rate, that is, the energy rate per unit of time.  Gross productivity is the total rate of storage of energy.  Net primary productivity is the amount of energy available for growth after the energy consumption required for maintenance and reproduction is subtracted.  Another important term is that of biomass.    Unlike productivity, which is related to the rate at which energy is formed, biomass refers to the weight of dry organic material per unit of area, or the energy storied per unit of area at any given time.

Consumers and decomposers, known as heterotrophs, use energy from the self-sustaining producers.  Examples of consumers include herbivores, carnivores and parasites.  Decomposers are a kind of consumer which breaks down dead organisms or their fecal material into inorganic constituents.

The Food Chain

The manner in which the net primary productivity in a community supports the rest of the community may described through the concept of the food chain.  Food chains are descriptions of the energy flow in a system.  Plants, the primary producers, are eaten by consumers which are then eaten by other consumers.  In the grazing food chain, for example, herbivores eat green plants or algae, the base units of the food chain.  Small carnivores eat herbivores, and these small carnivores are consumed by larger carnivores.  There may be several levels of carnivores in the chain.  At the end of the chain are the top carnivores, which do not have predators, but which replenish the system through decomposition.  Food chains are most commonly woven together to form food webs, such as that illustrated in figure 40-3 of your textbook.

Decomposers have their own form of food chain, know as the detritus food chain.  Dead organic matter is decomposed by detritus feeders, such as worms, which is then used by microorganisms.

Trophic Levels

Each of the steps in a food chain, such as those described above, may be referred to as a trophic level.  The amount of energy transferred to the next trophic level is small, about 10% of the total available energy.  The usual number of steps in a chain is about four or five.  The biomass at each trophic level may be presented graphically as pyramids.  Pyramids of number (Eltonian pyramids) show the number of individual organisms that are transferred between trophic levels (figure 40-4A).  Pyramids of biomass show the total “bulk” of organisms at each trophic level.   These pyramids usually have the classic pyramidal form, although some may be inverted pyramids (figure 40-4B).  A third type of pyramid describes the rate of energy between trophic levels. (figure 40-4C).

Nutrient Cycles (biogeochemical cycles)

Nutrient cycles involve exchanges between living organisms and the essential elements from the environment which sustain them, such as air, water and rocks of the earth's crust.  Two of the most important nutrient cycles are the carbon and nitrogen cycles.  All living things depend on the availability of carbon.  Nitrogen, too, is essential to living organisms.   Please look at figures 40-6 and 40-7 for graphic representations of carbon and nitrogen cycles, respectively.

Ecological Dominance

Biological communities are characterized by a single species or group of species.  The dominant species may be the most numerous, most productive, or the largest.  In some way, the dominant species exerts the greatest influence on the community.  However, a dominant species is not necessarily essential to structure of the community.  Your textbook gives the example of the American chestnut as a species which was dominant, but not essential, to the community.  A species which is essential to the fundamental framework of a community is called a  keystone species.

Ecological Niche

An animal's niche is the animal's place within the total environment, and includes what it does and its relation to food and enemies.  The fundamental niche includes the entire set of conditions under which an animal can live and reproduce.   The realized niche is the actual set of conditions under which an animal population lives.  An important rule which relates to species and their niche is the principle of competitive exclusion.  This principle states that two species cannot occupy the same niche at the same time and the same place.  Such a circumstance would cause direct competition between the species for food.  Under these conditions, one species must change its niche, or move, or face extinction.

Interactions in Populations

Populations may interact in a variety of ways: competition, predator-prey, parasite-host, commensalism, mutualism and protocooperation.  In some cases, however, there is no interaction between populations of a community, i.e., neutral interaction.

Population Growth

Populations have the ability to reproduce in greater numbers than required to simply replace individuals in the population.  Where there are rich resources and little or no competition, populations can grow at exponential rates (see figure 40-13).  This kind of geometric growth is call the intrinsic growth rate, represented by the letter “r”.  Eventually populations tend to reach a level of density at which point growth levels out and fluctuates around that level.

To understand how population growth is checked by environmental factors, it is crucial to understand the concept of carrying capacity, which is the maximum density the environment can support,  represented by the letter K.  As populations grow to carrying capacity density, the growth curve changes from a geometric to logistic curve, the latter having a sigmoid-shaped curve (figure 40-13).  The mathematical expression of this growth curve is found in the logistic equation.

Factors that limit population growth fall into two broad categories: density-dependent and density-independent.  Density dependent mechanisms are those limiting factors which are the direct result of a high population density with concomitant competition for food and resources.  Examples of density dependent forces are infectious disease, predation, shelter, stress and emigation.   Density-independent factors are forces which limit population, but are not the result of population density.  Examples include harsh weather conditions and natural disasters-fires, drought and floods.

Once you have completed this assignment, you should:

Go on to Assignment 40
or
Go back to Chapter 40

E-mail the professor W. Sylvester Allred at Syl.Allred@NAU.EDU, or call (520)523-7214

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