Lesson 16: Limits on Population
In this lesson, we will learn about population density and how populations are
controlled. We will also look at the concept of the critical number and how it
relates to the loss of biodiversity. Finally, we will look at the Endangered
Species Act which seeks to curb human impact on biodiversity.
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.
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.
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.
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.