222 ❯ STEP 4. Review the Knowledge You Need to Score High
Population Ecology and Growth
Like many fields of biology, ecology contains hierarchies of classification. A populationis
a collection of individuals of the same species living in the same geographic area. A collec-
tion of populations of species in a geographic area is known as a community.Anecosystem
consists of the individuals of the community and the environment in which it exists.
Ecosystems can be subdivided into abiotic and biotic components: biotic componentsare
the living organisms of the ecosystem, while abiotic componentsare the nonlivingplayers
in an ecosystem, such as weather and nutrients. Finally, the biosphereis the entire life-
containing area of a planet—all communities and ecosystems.
Three more terms for you: (1) the nicheof an organism, which consists of all the biotic
and abiotic resources used by the organism; (2) population density,which describes how
many individuals are in a certain area; and (3) distribution,which describes how popula-
tions are dispersed over that area. There are three main types of dispersion patterns that you
should know (see also Figure 18.1):- Clumped:The individuals live in packs that are spaced out from each other, as in
schools of fish or herds of cattle. - Uniform:The individuals are evenly spaced out across a geographic area, such as birds
on a wire sitting above the highway—notice how evenly spaced out they are. - Random:The species are randomly distributed across a geographic area, such as a tree
distribution in a forest.
(a)(b)(c)Figure 18.1 Distribution patterns: (a) clumped; (b) uniform; (c) random.Population ecologyis the study of the size, distribution, and density of populations and
how these populations change with time. It takes into account all the variables we have
mentioned already and many more. The size of the population, symbolized N, indicates
how many individuals of that species are in a given area. Demographersstudy the theory
and statistics behind population growth and decline. The following is a list of demographic
statistics you should be familiar with for the AP Biology exam:
Birth rate Offspring produced per time period. Highest among those in the
middle of the age spectrum.
Death rate Number of deaths per time period. Highest among those at two
extremes of the age spectrum.