Population Biology Basics

How Animal Populations Interact and Change Over Time

Grasshoppers are an example of r-selected animals.
Grasshoppers are an example of r-selected animals. Photo © Niels Busch / Getty Images.

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

Populations change over time due to births, deaths, and the dispersal of individuals between separate populations. When resources are plentiful and environmental conditions appropriate, populations can increase rapidly.

A population's ability to increase at its maximum rate under optimal conditions is called its biotic potential. Biotic potential is represented the letter r when used in mathematical equations.

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 exponentially, 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

Characteristics of r-selected species include:

  • early maturation
  • numerous, smaller young
  • shorter life spans
  • less parental care
  • a little competition for resources

Some environmental and biological factors can influence a population differently depending on its density. If population density is high, such factors become increasingly limiting on the success of the population. For example, if individuals are cramped in a small area, the disease may spread faster than it would if population density were low. Factors that are affected by population density are referred to as density-dependent factors.

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 intra-specific competition which occurs when individuals within a population compete with one another to obtain the same resources. Sometimes intra-specific 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.

Populations of animals interact with each other and their environment in a variety of ways.

One of the primary interactions a population has with its environment and other populations is due to feeding behavior.

The consumption of plants as a food source is referred to as herbivory and the animals that do this consuming are called herbivores. There are different types of herbivores. Those that feed on grasses are referred to as grazers. Animals that eat leaves and other portions of woody plants are called browsers, while those that consume fruits, seeds, sap, and pollen are called frugivores.

Populations of animals that feed on other organisms are called predators. The populations on which predators feed are called prey. Often, predator and prey populations cycle in a complex interaction. When prey resources are abundant, predator numbers increase until the prey resources wane. When prey numbers drop, predator numbers dwindle as well.

If the environment provides adequate refuge and resources for prey, their numbers may again increase and the cycle begins again.

The concept of competitive exclusion suggests that two species that require identical resources cannot coexist in the same location. The reasoning behind this concept is that one of those two species will be better adapted to that environment and be more successful, to the point of excluding the lesser species from the environment. Yet we find that many species with similar requirements do coexist. Because the environment is varied, competing species can use resources in different ways when competition is intense, thus allowing space for one another.

When two interacting species, for example, predator and prey, evolve together, they can influence the evolution of the other. This is referred to as coevolution. Sometimes coevolution results in two species that influence (both positively or negatively) from each other, in a relationship referred to as symbiosis. The various types of symbiosis include:

  • parasitism - one species (parasite) benefits more than the other species (host)
  • commensalism - one species benefits while a second species is neither helped nor injured
  • mutualism - both species benefit from the interaction