- Population density
- Top-down and bottom-up regulation
- Critical number
- Endangered Species Act


[SLIDE 1]

Population density is the number of individuals per unit area.

Some populations are density-dependent such as rabbits, which become prey for coyotes and foxes when they are plentiful in nature. Density dependent limits increase as population density increases.

Other populations like locusts are not affected by other prey and can grow to large numbers. They are density independent but can be wiped out by weather events.


[SLIDE 2]

There are two kinds of population control regulation - top-down and bottom-up. Top-down regulation involves control of a species by predators. An example would be coyotes killing rabbits.

Bottom-up regulation happens through the scarcity of a resource. An example would be rabbits dying off because of lack of food.


[SLIDE 3]

The survival and recovery of a population depends on a minimum population base, known as the critical number. When a population drops below the critical number, surviving members become vulnerable, breeding fails, and extinction can be expected.

The loss of biodiversity is disturbing. Human activities have caused this decline and the extinction of many animals and plants. Human impacts are not density-dependent and can even go up as populations decline.


[SLIDE 4]

Concern for decline in species led to the Endangered Species Act that defined two categories of species that need protection - threatened and endangered.

Threatened species are species whose populations are declining rapidly.

Endangered species are at a critical number.

These definitions are assigned by the US Fish and Wildlife Service.