A population cannot grow indefinitely because its environment has a limited supply of food, water, space, shelter, and other resources. As a population becomes larger, competition for these necessities usually increases. Predators, disease, parasites, environmental changes, and other pressures can also reduce survival or reproduction. Together, these factors limit how large a population can become.
The specific factors that restrict growth depend on the species and its environment. Some populations are mainly limited by food availability, while others may be constrained by disease, nesting sites, water, predation, or competition with other species.
Population growth depends on births, deaths, immigration, and emigration
A population changes when individuals are added or removed. Births and immigration increase population size, while deaths and emigration decrease it.
When births and immigration consistently exceed deaths and emigration, the population grows. When the opposite occurs, it declines. Population growth therefore depends not only on how many individuals are present but also on how successfully they survive and reproduce.
Early in population growth, resources may be plentiful relative to the number of individuals. Under favorable conditions, a population can increase rapidly. But as more individuals occupy the same environment, resources become harder to obtain and other limiting pressures become stronger.
Resources become limiting as populations get larger
Every environment has finite resources. Organisms need some combination of food, water, nutrients, light, space, shelter, and suitable places to reproduce.
When a population is small, individuals may have relatively easy access to these resources. As the population grows, more individuals compete for the same supplies. Food may become scarce, water may be harder to find, and suitable nesting or breeding sites may become occupied.
This competition can reduce growth in several ways. Individuals may grow more slowly, produce fewer offspring, have difficulty finding mates, or become more vulnerable to death.
Carrying capacity sets a limit
The carrying capacity of an environment is the largest population size that the environment can sustain over time under particular conditions.
Carrying capacity is not a fixed number that applies permanently to a species. It can change when environmental conditions change. A drought, loss of habitat, increase in food supply, severe winter, or other environmental change can alter the number of individuals an area can support.
When a population approaches or exceeds its carrying capacity, competition and other limiting pressures generally become stronger. Population growth slows, and the population may fluctuate around the level that the environment can support.
Density-dependent factors become stronger as populations become crowded
Some limiting factors have effects that depend strongly on population density. These are called density-dependent factors.
Competition is one example. When many individuals depend on the same limited resource, each individual has less access to it. Disease can also become more important in dense populations because individuals are more likely to encounter one another and transmit infections.
Predation may also become more important when prey are abundant. A large prey population can provide predators with more opportunities to find food, potentially increasing predation pressure.
Other density-dependent pressures include parasitism, territorial competition, and competition for mates or breeding sites.
Disease and parasites can slow population growth
Infectious diseases can limit population growth by reducing survival or reproduction. Their effects may become particularly strong when individuals live close together.
Parasites can have similar effects. They may consume resources from their hosts, damage tissues, weaken the host, or reduce its ability to reproduce. In some populations, disease and parasitism can contribute substantially to fluctuations in population size.
The effect is not necessarily constant. Environmental conditions, population density, immunity, and interactions among species can all influence how strongly disease affects a population.
Predators can limit prey populations
Predation removes individuals from a population and can therefore restrict its growth. The strength of this effect depends on both predator and prey populations.
When prey become abundant, predators may have more food and may survive or reproduce more successfully. An increase in predators can then increase the number of prey being killed. If prey become scarce, predators may have more difficulty finding food, which can reduce predator numbers or reproduction.
These interactions can produce population fluctuations rather than a simple fixed limit.
Competition occurs within and between species
Individuals of the same species often compete for resources such as food, territory, mates, and nesting sites. This is intraspecific competition.
Competition can also occur between different species. When two species depend on the same limited resource, an increase in one species can reduce the resources available to the other.
Competition becomes especially important when resources are scarce. It can lower growth, survival, or reproductive success and thereby prevent a population from continuing to increase.
Some limiting factors do not depend on population density
Not every factor becomes stronger as a population becomes larger. Density-independent factors can affect populations regardless of their density.
Extreme weather, droughts, floods, wildfires, severe temperature changes, and other environmental disturbances can cause deaths or reduce reproduction in populations of many different sizes.
For example, a major drought can reduce available water and food whether a population is small or large. Such events can temporarily lower a population below the level that its environment could otherwise support.
Population growth is usually controlled by several factors at once
There is rarely one universal factor that limits a population. Several pressures can operate simultaneously.
A population might have sufficient food but too little suitable habitat. Another might have abundant habitat but experience strong predation or disease. Conditions can also change over time, causing the factor that most strongly limits growth to change as well.
The result is that population size reflects the combined effects of resource availability, environmental conditions, interactions with other organisms, and the population’s own ability to survive and reproduce.
Ultimately, population growth is limited because individuals cannot reproduce and survive without sufficient resources and suitable environmental conditions. As population size increases, limiting pressures often become stronger, preventing unlimited growth.