Bio Review Notes #56
ECOLOGY: POPULATIONS
Performance Objectives:
Ecology is the study of species in relation to their environments.
Populations include all those members of a species that can interbreed.
The role of a population in its ecosystem is called its niche.
The place where a species lives is called its habitat.

 Population: All members of a species that can interbreed or exchange genes with one another and leave fertile offspring.

Habitat: The place where a species lives.

Niche: The "role" or "occupation" or "way of life" of a species; its position in the workings of the ecosystem.

Exponential growth: Assume that a population increases by a proportion r in each generation. Then the rate of increase
dN/dT   =   r N
where   N = current population size and
r = "intrinsic rate of natural increase" (Malthusian constant)
This equation can be rewritten as
N   =   N0  er T
where N0 is the initial population size and T is the time in generations.
Under this model, the population size keeps increasing without limit.

Logistic growth: A more realiztic model assumes that the population size will reach some environmentally determined limit, the "carrying capacity," after which mortality limits further growth. Under this model,
dN/dT   =   r N  (K-N / K)
The resulting S-shaped (sigmoid) growth curve levels off as the population size (N) approaches the carrying capacity (K).

Density-dependent controls: Causes of mortality that increase with population size. Examples: limitations on food and space; increased stress and aggression; more predation and infectious diseases.

Density-independent controls act independently of population size. Examples: floods, earthquakes, weather extremes, habitat collapse (landslides, etc.).

r-selection: Populations suffering frequent, devastating, density-independent mortality are r-selected. Most r-selected species have widely fluctuating population sizes well below carrying capacity, selection favoring very rapid growth (large r), prolific reproduction, rapid dispersion, little or no parental care, heavy mortality to newly released eggs or other initial stages, short live spans, and early reproduction at small sizes.

K-selection: Populations suffering very little density-independent mortality (like humans and other large mammals) are K-selected. Most K-selected species have populations at or near carrying capacity (K), selection favoring economy of resources and efficiency ("making do" with less food or less space), long individual life span, and reproduction only at larger sizes. Fewer eggs or offspring are produced, but more stored food or parental care is devoted to each. Most mortality is density-dependent (from crowding, etc.) and tends to occur later in life.

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