Animal populations are always changing. In some years, a population may grow rapidly as more animals are born than die. In others, it may shrink because deaths, emigration, disease, or limited food outweigh reproduction and immigration.
The basic rule is simple: a population grows when individuals are added faster than they are lost, and it declines when losses exceed gains. The challenge is understanding why those rates change.
What determines the size of an animal population?
A population is a group of individuals of the same species living in the same area. Its size depends on four basic processes: births, deaths, immigration, and emigration.
Births add individuals to a population, while deaths remove them. Immigration occurs when animals enter the population from elsewhere, and emigration occurs when they leave.
For a population with little movement in or out of its range, births and deaths are especially important. If 100 animals are born during a year and 70 die, the population increases by 30. If 70 are born and 100 die, it decreases by 30.
This balance can change from one season or year to the next as environmental conditions change.
Why do animal populations grow?
A population can grow when animals have enough food, water, shelter, and suitable habitat to survive and reproduce. When many individuals reach reproductive age and produce offspring, the number of animals can increase quickly.
Population growth is often strongest when resources are abundant and competition is relatively low. Young animals may have higher survival rates, adults may produce more offspring, and fewer individuals may die from starvation or disease.
Some species can increase particularly quickly because they reproduce at a young age, produce many offspring, or reproduce several times during their lives. Others reproduce slowly and produce only a small number of young, so their populations generally change more gradually.
Population growth does not continue indefinitely, however. As the number of animals increases, the same environment must support more individuals.
What is carrying capacity?
Carrying capacity is the largest population size that an environment can support over a particular period under particular conditions.
It is not a permanent number. Carrying capacity can change when the environment changes.
For example, an area with abundant vegetation may support many herbivores during a period of favorable weather. A drought can reduce plant growth and water availability, lowering the number of animals the habitat can support.
When a population approaches the limits of its environment, competition for resources generally becomes stronger. Animals may have more difficulty finding food, territory, nesting sites, or mates. These pressures can reduce reproduction or increase mortality, slowing population growth.
If a population temporarily exceeds what its environment can support, food shortages and other pressures can cause its numbers to fall.
Why do populations decline?
Animal populations decline when individuals are lost faster than they are added. Several factors can produce this imbalance.
Limited food can cause starvation, reduce reproduction, or make animals more vulnerable to disease and predators. Loss of habitat can remove feeding areas, nesting sites, shelter, or breeding grounds. Disease can spread more easily when animals live close together, although diseases can affect populations at many densities.
Predation also influences population size. Predators remove prey, while changes in prey abundance can in turn affect predator populations. These interactions can produce repeated rises and falls rather than a population remaining at one fixed size.
Severe weather can have major short-term effects as well. Droughts, floods, unusually cold periods, heat, storms, and wildfires can reduce survival or destroy resources and habitat.
Human activities can also change population growth by altering habitat, introducing pollutants, harvesting animals, or changing the availability of food and water.
How do density-dependent factors control populations?
Some population pressures become stronger as a population becomes more crowded. These are called density-dependent factors.
Competition is one example. When many animals depend on the same limited food source, each individual has less access to that resource. Disease and parasites can also spread more readily when individuals are in close contact.
Predation can sometimes have density-dependent effects as well. When prey become abundant, predators may find them more easily and consume more of them. The resulting decline in prey can then reduce food available to predators.
These processes can create a form of natural population regulation. They help prevent populations from increasing indefinitely under stable environmental conditions.
What are density-independent factors?
Other factors can affect populations regardless of how crowded they are. These are called density-independent factors.
Extreme weather is a common example. A major flood can kill animals in a population whether there are many individuals in the area or relatively few. A severe wildfire, drought, or extreme temperature event can similarly affect survival without depending directly on population density.
The distinction is useful, but natural events do not always fit neatly into one category. The effect of an environmental event can depend on a population’s condition, habitat, and other circumstances.
Why don’t animal populations simply grow steadily?
Population growth is rarely a smooth upward line. Birth rates and death rates change as conditions change, so population size often rises and falls.
A population may increase when food is plentiful. More animals then consume more resources, increasing competition. Reproduction may slow, mortality may rise, and the population may decline. After the population becomes smaller, resources may become more available again, allowing reproduction and survival to improve.
This can produce population cycles, in which numbers repeatedly increase and decrease.
Predator-prey relationships can contribute to such cycles. When prey become abundant, predators may have more food and produce more surviving offspring. More predators can then reduce the prey population. As prey become scarce, predator numbers may eventually fall, allowing prey numbers to recover.
Not every population follows a regular cycle. Environmental variation, migration, disease, competition, and interactions with other species can make population changes irregular.
What is exponential population growth?
When resources are effectively unlimited and population growth is not strongly restricted, a population can grow at an increasingly rapid rate. This pattern is called exponential growth.
The key feature is that the number of new individuals depends partly on how many reproductive individuals are already present. A larger population can produce more offspring, potentially making the increase faster over time.
Exponential growth is most useful as a model of what can happen under favorable conditions. Real animal populations generally encounter limits such as food shortages, predators, disease, competition, and habitat constraints.
What is logistic population growth?
Logistic growth describes population growth that slows as a population approaches its carrying capacity.
A population may initially grow rapidly when resources are abundant. As its size increases, competition and other limiting factors become stronger. Growth then slows, and population size may fluctuate around the environment’s capacity to support it.
The logistic model is useful because it captures an important principle: environmental resources place limits on sustained population growth.
In nature, however, populations do not necessarily settle at one stable number. Carrying capacity can shift, and populations may overshoot available resources before declining.
Why can populations suddenly crash?
A population crash occurs when mortality rises sharply, reproduction falls, or both happen at once.
A sudden shortage of food can produce a crash, particularly if a population has become very large. Disease can spread through a dense population and cause substantial mortality. A severe environmental event can also remove large numbers of individuals or destroy critical habitat.
Population crashes do not always mean a species is permanently declining. If enough individuals survive and the habitat recovers, reproduction can eventually increase the population again.
But repeated crashes can become dangerous when a population remains small for long periods. Fewer individuals may mean fewer breeding opportunities, reduced genetic diversity, or greater vulnerability to further environmental changes.
How do births and deaths vary within a population?
Not all individuals contribute equally to population growth. Age and life stage matter.
A population with many young individuals may have strong potential for future growth as those animals mature and reproduce. A population dominated by older individuals may have less reproductive potential, depending on the species.
Survival also varies by age. Young animals may face high mortality before reaching adulthood, while adults may survive at higher rates. In other species, older individuals may experience increasing mortality.
Scientists study these differences using life tables, survival rates, age structures, and reproductive rates. Such information helps explain not only how large a population is but also where it may be heading.
How does migration change population size?
Births and deaths are not the only ways populations change. Animals can move between populations.
Immigration increases the number of individuals in an area, while emigration decreases it. Seasonal migration can therefore cause dramatic changes in the number of animals present without corresponding changes in reproduction or mortality.
Movement between populations can also influence long-term population stability. If animals can move into an area where local numbers have fallen, immigration may help the population recover. Conversely, if habitat becomes unsuitable, animals may leave and cause the local population to shrink.
Why do some populations recover after declining?
Recovery depends on whether the factors causing the decline are reduced and whether enough individuals remain to reproduce.
If food and habitat become available again, reproduction and survival may improve. Immigration from nearby populations can also increase numbers. In some cases, reducing hunting or other sources of mortality can allow a population to rebuild.
Recovery is not guaranteed. If habitat has been permanently lost, a population may have nowhere suitable to expand. Small populations can also face additional difficulties because finding mates may become harder and random events can have a larger effect on their survival.
The balance behind population change
Animal population growth is ultimately a balance between gains and losses. Births and immigration add individuals; deaths and emigration remove them.
What makes population ecology complicated is that these processes are connected. More animals can increase competition for food. Changes in food can alter reproduction and survival. Predators respond to prey abundance, while prey respond to predation. Weather and habitat conditions can shift the resources available to both.
As a result, animal populations rarely remain at a fixed size. They expand when conditions favor survival and reproduction, decline when losses become greater than gains, and fluctuate as organisms interact with one another and with their changing environment.
