• The distribution patterns of individuals in populations vary between species and may depend on biotic as well as abiotic factors.

• Clumped distribution may offer prey such as elk protection against predators.

• Random distribution often characterize r-selected species such as flowers that produce numerous seeds.

• Uniform distribution may occur for species such as creosote bushes that produce toxins to keep competitors at a distance.

What kind of distribution pattern do human populations exhibit?

• A population's growth rate (r) is its birth rate minus its death rate.

Under ideal conditions, a population can grow at an exponential rate and reach its biotic potential with high birth rates and low death rates.

If a pair of deer mice produce a net of 10 pups per breeding cycle, the population increases by a factor of 5, and will reach 6,250 mice in 5 cycles.

• Carrying capacity (K) is the maximum population size that a habitat can sustain a species indefinitely.

Real populations cannot sustain exponential growth (J curve) forever.

Eventually birth and death rates stabilize, and most populations exhibit logistic growth (S curve) when they reach the carrying capacity of the environment.

• Species that pursue r-selected reproductive strategy exhibit

• high biotic potential

• short lifespan

• early maturity

They are very adaptable and can often exploit new resources with exponential growth.

• Species that pursue k-selected reproductive strategy exhibit

• low biotic potential

• long lifespan

• late maturity

Their populations tend to be more stable, especially in undisturbed, mature areas.

• Some populations, especially r-selected species, may exhibit boom-and-bust cycles.

When resources are plentiful and/or predators are few, the high biotic potential may allow the population to briefly exceed (overshoot) the carrying capacity (K) of the habitat.

The resulting high death rate causes a population crash.

When resources recover, death rates decline, and the cycle repeats.

• Community dynamics may play a role in population cycles.

This example from Isle Royale in Michigan illustrates the inter-dependent nature of a predator-prey relationship.

The boom and bust cycles of the predator (wolves) often follow those of the prey (moose) population; other factors such as virus outbreaks and severe weather also play a role: the Parvo epidemic of the early 1980's caused the wolf population to crash.

• A top predator such as the gray wolf may play the role of a keystone species.

Wolf absence or presence in Yellowstone is a key factor on the diversity of the whole ecosystem.

• When wolves were extirpated from Yellowstone, elk populations increased, and overgrazed their willow food source.

Fewer willow thickets in turn caused decline of beavers.

Absence of beaver dams led to fewer ponds and decreased birds that depend on wetlands.

• When wolves were reintroduced into Yellowstone, elk populations decreased.

The willows regrew.

Beavers return and build dams.

Beaver dams create wetlands - habitat for birds and other wildlife.