Populations are groups of individuals belonging to the same species that live in the same region at the same time. Populations, like individual organisms, have unique attributes such as:
- growth rate
- age structure
- sex ratio
- mortality rate
In most instances, resources are not unlimited and environmental conditions are not optimal. Climate, food, habitat, water availability, and other factors keep population growth in check due to environmental resistance. The environment can only support a limited number of individuals in a population before some resource runs out or limits the survival of those individuals. The number of individuals that a particular habitat or environment can support is referred to as the carrying capacity. Carrying capacity is represented by the letter K when used in mathematical equations.
Populations can sometimes be categorized by their growth characteristics. Species whose populations increase until they reach the carrying capacity of their environment and then level off are referred to as K-selected species. Species whose populations increase rapidly, often exponetially, quickly filling available environments, are referred to as r-selected species.
Characteristics of K-selected species include:
- late maturation
- fewer, larger young
- longer life spans
- more parental care
- intense competition for resources
- early maturation
- numerous, smaller young
- shorter life spans
- less parental care
- little competition for resources
There are also density-independent factors which affect populations regardless of their density. Examples of density-independent factors might include a change in temperature such as an extraordinarily cold or dry winter.
Another limiting factor on populations is intraspecific competition which occurs when individuals within a population compete with one another to obtain the same resources. Sometimes intraspecific competition is direct, for example when two individuals vie for the same food, or indirect, for example when one individual's action alters and possibly harms the environment of another individual.