Food, water and space are major factors that limit the size of a population because organisms need enough resources to survive, grow and reproduce. When a population becomes large enough that these resources become scarce, competition increases, reproduction may decline, and deaths may rise. The population may then stop growing, shrink, or fluctuate around a level that the environment can support.
Why resources limit population growth
A population can grow when more individuals are being added through reproduction than are being lost through death or other forms of removal. Early in population growth, resources may be abundant, allowing individuals to obtain enough food, water and living space to survive and reproduce successfully.
As the number of organisms increases, however, the same resources must be shared among more individuals. Eventually, shortages can slow population growth.
Food provides energy and nutrients needed for metabolism, growth and reproduction. Water is essential for the chemical processes that keep cells functioning. Space provides room for organisms to obtain resources, find shelter, establish territories, build nests or otherwise carry out activities necessary for survival.
The limiting effect does not necessarily occur because a resource disappears completely. A resource can become limiting when there is not enough available for every individual to obtain what it needs.
How food controls population size
Food availability can strongly influence how many organisms an environment can support. When food is plentiful, individuals generally have better access to the energy and nutrients needed for growth and reproduction. This can allow a population to increase.
As the population becomes larger, food may become harder to obtain. Individuals may spend more time searching for food, encounter greater competition, or receive less nourishment. Poor nutrition can reduce growth, survival and reproductive success.
Food shortages can also affect populations indirectly. When many individuals depend on the same food source, competition may become intense. Some individuals may obtain enough food while others obtain too little. The resulting differences in survival and reproduction can slow population growth.
The type of food matters as well. A habitat may contain large amounts of material that an organism cannot eat or cannot efficiently use. Population size therefore depends on the availability of suitable food, not simply the total amount of potential food in an environment.
How water limits populations
Water is another fundamental resource. Organisms require it for processes ranging from cellular chemistry to temperature regulation and transport of substances within the body.
In environments where water is scarce, individuals may compete for limited water supplies. A population may remain relatively small because the available water cannot support additional individuals.
Water availability can also change with environmental conditions. A population may grow when water is abundant and decline when drought reduces the amount available. In some environments, seasonal changes in rainfall or water supplies can therefore produce corresponding changes in population size.
The importance of water varies among species. Organisms adapted to dry environments generally have physiological or behavioral characteristics that allow them to survive with less water than species that require consistently moist conditions. Even so, every species has limits to how much water stress it can tolerate.
Why space becomes a limiting resource
Space is more than an empty area in which organisms live. It can provide access to food, water, shelter, nesting sites and other resources.
When population density increases, individuals may have less usable space available to them. Some species respond by defending territories, while others compete for nesting sites, hiding places or access to particular areas.
Crowding can also affect behavior and reproduction. In some animals, limited space increases encounters and competition among individuals. Plants can become crowded as well, causing them to compete for access to light, water and nutrients in the soil.
The amount of space an environment contains is therefore only part of the issue. What matters is the amount of usable habitat available to a population and the resources that habitat contains.
Resource limits create competition
When a resource is abundant, individuals may rarely interfere with one another’s access to it. As demand approaches supply, competition becomes more important.
Competition can occur within the same species, known as intraspecific competition, or between different species, known as interspecific competition.
For example, members of the same population may compete for the same food, water or nesting sites. Different species may also depend on overlapping resources and compete when those resources become scarce.
Competition does not always involve direct fighting. It can occur simply because one organism consumes a resource that another organism would otherwise have used.
What carrying capacity means
The resource limits of an environment help determine its carrying capacity: the population size that an environment can sustain over time under particular conditions.
Carrying capacity is not necessarily a fixed number. It can change when environmental conditions change.
If food becomes more abundant, additional water becomes available, or suitable habitat expands, an environment may support a larger population. If drought reduces water, food supplies decline, or habitat is lost, the carrying capacity may fall.
This means that a population does not have one permanent maximum size. Its sustainable size depends on the conditions affecting survival and reproduction.
Population growth slows as resources become scarce
When a population is small and resources are plentiful, it can often grow rapidly. As population density increases, however, individuals encounter stronger resource limitations.
This produces a pattern known as density-dependent population regulation. The effects of competition, limited food, limited water and restricted space become stronger as more individuals occupy the same environment.
Eventually, resource shortages can make the number of deaths and the number of individuals leaving the population comparable to the number being added through reproduction. Population growth then slows or stops.
If conditions become unfavorable enough, deaths can exceed births and the population can decline.
Food, water and space interact
These resources do not operate independently. A shortage of one can make the others more important.
For example, a habitat may contain plenty of food but too little water to support a large population. Another habitat may have abundant water but insufficient food. A population may also have enough food and water in an area that is too small to provide sufficient nesting sites or shelter.
The resource that most strongly restricts population growth can change over time. Rainfall might determine population size during a dry period, while food availability becomes more important after water becomes plentiful.
Population size is therefore shaped by the combined availability of resources and by how organisms use them.
Resource limits keep populations from growing indefinitely
No population can increase indefinitely in a real environment because resources and environmental conditions impose limits. As population density rises, the availability of food, water and usable space per individual generally falls, increasing competition and reducing the ability of additional individuals to survive and reproduce.
These limits help regulate population size. When resources become more abundant, populations may grow; when resources become scarce, growth slows or the population declines. The balance between resource availability, reproduction and survival is what keeps populations within the limits imposed by their environment.
