APES 3.7-3.8: Population Growth Impacts and Migration

Overpopulation or overabundance

A condition in which a species' population exceeds the carrying capacity of its environment, meaning available resources and ecological conditions cannot sustainably support all individuals.

Carrying capacity

The largest population of a species that an environment can support over time with its available food, space, shelter, and other resources.

What assumption must be made when interpreting a population increase as overpopulation?

An increase in numbers alone is not sufficient; the population must also be causing resource depletion, ecological damage, or other effects indicating that it is near or beyond environmental limits.

What factors can cause a population to exceed its carrying capacity?

Increased birth rates, decreased mortality, reduced predation, and large-scale migration can all produce overabundance. Human activities can also increase food supplies or shelter for a species.

How can large-scale migration contribute to local overpopulation?

When many individuals move into an area, the local population can exceed the area's carrying capacity even if the species was not previously overabundant there. Increased competition for food, space, and shelter may result.

How can overpopulation lead to migration?

When food, space, or shelter become insufficient, individuals may move into areas that the species does not typically inhabit in search of resources.

What happens to individuals when a population exceeds carrying capacity?

Competition for food, space, and shelter increases. Individuals may migrate, experience starvation or disease, or die because essential resources are insufficient.

How can overpopulation weaken an entire species?

Limited resources can reduce average body mass and reproductive success while increasing disease and parasite loads. These effects may occur even when the population is numerically large.

Fecundity

The reproductive capacity of a population or individual. In overpopulated populations, fecundity may decline because inadequate nutrition and other stresses reduce successful reproduction.

Why is the judgment that a population is 'overpopulated' partly value-based?

The biological effects can be measured, but deciding whether those effects are unacceptable depends on human purposes and values. A population may be labeled overpopulated when it is considered harmful, costly, dangerous, or damaging to other species.

Competitive exclusion principle

Two species competing for the same limiting resource generally cannot coexist indefinitely at constant population levels. One species may exclude the other, or the species may shift resource use or habitat.

How can supplemental feeding create wildlife overpopulation?

Providing food increases survival and reproduction and can concentrate animals in one area. The resulting population may exceed the natural carrying capacity once feeding stops or when other resources become depleted.

How can reduced hunting or trapping contribute to wildlife overabundance?

Lower harvest rates increase survival, allowing populations to grow when predation or other mortality is insufficient to offset reproduction.

What ecological effects can overabundant herbivores produce?

Intense browsing and overgrazing can remove vegetation, alter habitat structure, reduce plant diversity, and change conditions for insects, birds, and other organisms.

Why can deer overpopulation cause biodiversity loss even in heavily forested regions?

Deer may overbrowse understory plants and alter forest structure, eliminating certain grassland and woodland species. The resulting habitat changes can also reduce populations of birds and small mammals that depend on particular vegetation.

What economic and safety problems can result from deer overpopulation?

High deer numbers can damage crops and forest regeneration, increase the cost of protective fencing, and raise the frequency of vehicle collisions. Protecting forests may cost more than establishing them.

How can agricultural expansion and protected urban or park areas increase goose populations?

Agricultural fields provide abundant food, while parks and cities provide refuge where hunting is restricted or prohibited. Together, these factors can raise survival and support larger flocks.

What are the ecological consequences of severe goose overgrazing in Arctic habitats?

Repeated grubbing and grazing can remove tundra and marsh vegetation, expose bare peat, increase erosion, and create hypersaline, oxygen-poor mud. Plant and animal biodiversity may decline sharply.

How can cormorant overabundance affect fisheries and aquaculture?

Cormorants may consume large quantities of farmed or wild fish, reducing aquaculture harvests and affecting recreational or conservation fisheries.

What is immunocontraception, and why is it useful in wildlife management?

Immunocontraception uses vaccines or related treatments to reduce fertility without killing animals. It can regulate population growth, but injectable methods require close access to animals and may take time to reduce population size.

What is a major limitation of injectable versus oral immunocontraceptives?

Injectable treatments require animals to be captured or approached individually. Oral treatments could reach animals more easily, but they are less developed.

What population-control strategies can be used for overabundant or invasive species?

Approaches include regulated hunting or trapping, habitat modification, relocation, fertility control, egg or nest treatment, predator restoration, and direct removal. Each method has ecological, economic, practical, and ethical trade-offs.

Why might direct culling be controversial even when a population is ecologically harmful?

Culling can reduce ecological or economic damage, but people may object on ethical, cultural, or animal-welfare grounds. Public acceptance can determine whether a management strategy is feasible.

How can sterilization help reduce domestic pet overpopulation?

Spaying or neutering prevents unwanted litters and lowers the number of offspring entering shelters. Over time, fewer births can reduce shelter crowding and euthanasia.

Why is domestic pet overpopulation difficult to quantify?

Animals continually move among households, shelters, rescue groups, and other locations, while some are euthanized or adopted. Incomplete records make nationwide population totals difficult to determine.

How do predator-prey interactions commonly produce population cycles?

A prey increase can support predator growth, but predator numbers usually rise after prey numbers do. Increased predation then reduces the prey population, followed by predator decline when food becomes scarce.

Why does predator population growth typically lag behind prey population growth?

Predators require time to reproduce in response to abundant prey. Consequently, predator abundance often reaches its maximum after the prey population has already begun to decline.

Lotka-Volterra equations

A mathematical model used to describe idealized predator-prey population cycles. It helps represent interactions in which prey growth supports predator growth and predation reduces prey abundance.

What can happen to a predator population after a prey population crashes?

Predators may experience starvation, reduced reproduction, and high mortality because food is scarce. Their decline can eventually allow the prey population to recover.

How can predation drive evolution in prey populations?

Predators preferentially remove more vulnerable individuals, favoring inherited traits that improve escape, concealment, or defense. Predators may also evolve in response, producing coevolution.

Why does removing predators not necessarily prevent prey overpopulation?

Without predators, prey may grow rapidly while resources remain abundant. The population can then overshoot carrying capacity and undergo a severe crash when food or habitat becomes limiting.

Maximum sustainable yield

The greatest long-term harvest that can be taken from a population without causing its sustained decline. Population models are used to estimate this value in fisheries and other managed populations.

What are direct and indirect methods for estimating wildlife population size?

Direct methods count animals, either across an entire area or in a sample that is extrapolated. Indirect methods infer abundance from signs such as feces, dens, nests, or other evidence of presence.

How does a sample count estimate the size of a larger population?

Researchers count individuals in a representative subsection and extrapolate using the sampled area, density, habitat coverage, and animal behavior. The estimate becomes unreliable if the sample is not representative.

Mark-recapture method

A population-estimation technique in which individuals are captured, marked, released, and later recaptured. The proportions of marked and unmarked individuals provide an estimate of total population size.

What quantitative relationship is commonly used in a simple mark-recapture estimate?

If $M$ individuals are initially marked, $C$ are captured later, and $R$ of the second sample are marked, then the estimated population is $N \approx \frac{MC}{R}$. This assumes marking does not change survival or recapture probability and that the population is reasonably closed.

Why can estimating fish populations require different assumptions from estimating terrestrial animal populations?

Fish populations are strongly affected by movement through connected waterways, while some terrestrial populations occupy more defined areas. Migration may therefore be negligible for isolated waters but essential for migratory fish such as salmon.

What is an introduced species?

A species transported by human activity to a region where it did not previously occur. Introduction can disturb ecological relationships and alter resource competition.

Why can introduced species become invasive in a new environment?

They may encounter abundant resources, few effective predators or pathogens, and little competition from native species. Under these conditions, their population can increase rapidly.

Invasive species

An introduced species whose population expands substantially and causes harmful effects to agriculture, fisheries, native ecosystems, or other human interests.

What ecological problems can an invasive mute swan population cause?

Mute swans can damage aquatic vegetation and displace or harass native waterfowl. Rapid annual population growth can intensify these effects.

How does human success in supporting larger populations affect other species?

Expansion of human populations and resource use can reduce the populations and habitats of other species. This represents competition for limiting resources on a global scale.

How can human overpopulation affect ecosystem services and human well-being?

Large human populations may degrade services such as food production, water purification, and climate regulation. They can also reduce health and well-being and displace other species.

What evidence is used to argue that human activity may be exceeding Earth's ecological limits?

Evidence includes declining ecosystem services, ecological-footprint assessments, climate disruption, biodiversity loss, and the crossing of planetary boundaries for safe use of the biosphere.

Ecological footprint

An accounting measure of the biologically productive land and water area needed to provide the resources a population consumes and absorb the wastes it produces. It can be used to assess whether human resource use exceeds Earth's capacity.

What ethical issue is central to debates about human overpopulation?

One issue is whether humans have a moral responsibility to preserve enough habitat and resources for viable populations of other species. This involves balancing human needs against biodiversity and long-term biosphere stability.