A population increases when more individuals are added than removed. It decreases when more individuals are removed than added. In biology, the main processes behind these changes are births, deaths, immigration, and emigration.
These four processes determine whether the number of individuals in a population grows, shrinks, or stays roughly the same. Their relative importance can change over time as food, water, habitat, predators, disease, climate, and other environmental conditions change.
The four processes that change population size
For most populations, the basic relationship is:
Population change = births + immigration − deaths − emigration
Births add individuals, while deaths remove them. Immigration occurs when individuals move into a population, and emigration occurs when they leave it.
For example, a population of deer can grow rapidly if many fawns are born and few deer die or leave the area. But even a population with many births can decline if deaths and emigration are greater than the number of new individuals entering it.
In some populations, especially plants or organisms with limited movement, births and deaths are the dominant factors. In mobile animals, immigration and emigration can also have a major effect.
Births can drive population growth
The number of new individuals entering a population through reproduction is one of the most direct causes of population increase.
When conditions are favorable, organisms may have more offspring that survive to reproductive age. Food availability, suitable habitat, access to mates, temperature, and parental care can all affect reproductive success.
Population growth is not determined simply by how many offspring are produced. What matters is how many survive and eventually contribute to the next generation. A species can produce large numbers of offspring while its population remains stable or declines if most offspring die before reproducing.
The birth rate describes how frequently new individuals are added through reproduction relative to the population. A high birth rate can contribute to rapid growth when deaths and movement out of the population remain low.
Deaths can cause a population to shrink
Deaths reduce population size. A population may experience increased mortality when individuals face shortages of food or water, disease, extreme weather, predation, competition, habitat loss, or other environmental pressures.
Age also matters. Very young and very old individuals may have different risks of death from those faced by individuals in the prime of life.
A population does not necessarily decline simply because its death rate is high. If births are high enough to offset deaths, the population can still grow. Population size depends on the balance between individuals being added and individuals being removed.
Immigration and emigration can change local populations
Births and deaths are not the only ways population size changes. Individuals can move between populations.
Immigration increases the size of a population when individuals enter an area. Emigration decreases it when individuals leave.
Movement can be especially important for animals that travel between habitats. For example, a population in one area may grow because individuals regularly arrive from nearby areas, even if local reproduction is relatively low.
This movement also connects populations. A population that loses many individuals through emigration may decline, while another population receiving those individuals may increase.
Environmental conditions limit population growth
A population cannot grow indefinitely in a real environment. As its size increases, individuals generally place greater demands on the resources they share.
Food, water, shelter, nesting sites, territory, and other necessities can become harder to obtain. Crowding can also increase competition and make diseases easier to spread.
These pressures are examples of factors that can become stronger as population density increases. Density-dependent factors are environmental influences whose effects are related to how crowded a population is.
When resources become scarce, individuals may grow more slowly, reproduce less successfully, or die at higher rates. These effects can reduce population growth.
Carrying capacity sets a moving limit
The environment’s ability to support a population is often described by its carrying capacity. This is the approximate population size that a particular environment can sustain over time under particular conditions.
Carrying capacity is not a permanent number. It can change when environmental conditions change.
A habitat with abundant food and water may support more individuals than the same habitat during a severe drought. Changes in vegetation, disease, predators, temperature, or available shelter can also alter how many individuals the environment can support.
As a population approaches the limits imposed by its environment, competition and other pressures can become stronger. Growth may slow, stop, or reverse.
Some factors affect populations regardless of their density
Not every population change is caused by crowding or competition. Density-independent factors can affect populations regardless of how many individuals are present.
Severe storms, floods, wildfires, droughts, freezes, and other sudden environmental events can kill individuals or destroy habitat. Their effects may be especially dramatic when they occur suddenly or across a large area.
These events can sometimes cause a population to fall sharply even when food and other resources had previously been sufficient.
Predators, competitors, and disease can change population size
Interactions with other organisms can strongly influence population growth.
Predators remove individuals from prey populations. Competition can reduce access to food, water, space, or mates. Parasites and diseases can reduce survival or reproduction.
These relationships can also interact. For example, a decline in available food may weaken individuals, making them more vulnerable to disease. Changes in predator numbers can alter prey survival, while changes in prey abundance can affect predator populations.
As a result, population size is often shaped by several interacting factors rather than by one cause.
Population growth can be rapid when conditions are favorable
When resources are abundant and environmental resistance is relatively weak, a population can increase quickly. If each generation produces enough surviving offspring to replace the parents and add more individuals, the number of individuals can rise at an accelerating rate.
This pattern is called exponential growth. It is possible when a population has abundant resources and relatively few constraints on reproduction and survival.
In nature, however, exponential growth usually cannot continue indefinitely. As population density rises, resources become more limited and other pressures become stronger.
A more typical long-term pattern is logistic growth, in which population growth begins rapidly but slows as the population approaches the environment’s carrying capacity.
Population size can fluctuate rather than simply rise or fall
Populations do not always move steadily in one direction. Their sizes can rise and fall repeatedly.
A population might increase when food is plentiful, then decline when food becomes scarce. Predator numbers may respond to changes in prey abundance, creating further changes in the prey population. Weather and seasonal conditions can add another layer of variation.
Some fluctuations are relatively predictable, while others are irregular. The important point is that population size reflects changing conditions and interactions rather than a fixed tendency to grow or shrink.
Population growth also depends on age structure
The number of individuals in a population does not tell the whole story. Their ages matter because individuals at different life stages may have different survival and reproductive rates.
A population containing many individuals that have not yet reached reproductive age may have relatively low reproduction in the short term but substantial potential for future growth. A population dominated by older individuals may have a different growth pattern.
This is why ecologists examine age structure when trying to understand how a population may change over time.
Why populations sometimes remain stable
A population is stable when individuals are being added at roughly the same rate that they are being removed.
That can happen when births approximately balance deaths and immigration approximately balances emigration. It can also occur through changing combinations of these processes.
Stability does not mean that nothing is happening. Individuals are continually being born, dying, arriving, and leaving. The overall number simply remains relatively similar because gains and losses balance each other.
Ultimately, population size is the result of this balance. More births and immigration than deaths and emigration produce population growth; more deaths and emigration than births and immigration produce population decline. Environmental conditions determine how strongly those four processes operate, which is why population size can change as ecosystems and conditions change.
