Overpopulation

Overpopulation Definition

Overpopulation refers to a population which exceeds its sustainable size within a particular environment or habitat. Overpopulation results from an increased birth rate, decreased death rate, the immigration to a new ecological niche with fewer predators, or the sudden decline in available resources. Therefore, overpopulation describes a situation in which a population in a given ecosystem limit the resources available for survival.

Overpopulation Effects

Overpopulation can have several effects on the environment, as well as other species within an ecological system. Indeed, human overpopulation has resulted in technological advances which have increased human lifespan and fertility, and consequently placed pressure on global resources. Such effects are such that the planet is currently in a novel geological epoch called the Anthropocene. In general, overpopulation results in an ecological disruption as resources are depleted. This disruption can lead to the decline of other populations which compete for the same resources. Typically, such effects result in the cycling between periods of population growth and periods of population decline until it can reach homeostasis within a particular ecological niche. Some examples of naturally regulated population growth are rodents, rabbits, and various insect populations (e.g., army worms and locusts).

In situations of overpopulation caused by the introduction of a foreign species for which they have no natural predators, they can become an invasive species. An example is the inadvertent introduction of zebra mussels to the North American water systems. Since zebra mussels are natively from the Black Sea and Caspian Sea, they have no natural predators in the foreign ecosystems of North America and parts of Europe. As such, zebra mussels quickly became an invasive species, clogging water treatment pipes, affecting power plants, and impacting the local freshwater fish populations. It is estimated that the overpopulation of zebra mussels has cost approximately $5 billion USD since their introduction. The image below illustrates an infestation of zebra mussels on a North American lock due to the overpopulation of zebra mussels in the North American waterways. Other economic effects of overpopulation include those caused by crop destruction, as seen with the overpopulation of rabbits in Australia. While the overpopulation of rabbits destroyed farmers crops, leading to poor yields, the continent also experienced a loss of native plant species, as well as the removal of precious topsoil due to erosion.

Another effect of the overpopulation of one species, is the increased population growth of the natural predators of such species. This effect is generally considered to be positive, as the predator population serves to control the overpopulated prey species. Such effects also serve to drive evolutionary changes as the prey species evolves to avoid increased predation.

Solutions for Overpopulation

Historically, there have been several situations for which overpopulated species could not be managed naturally. In these instances, issues with overpopulation have been overcome using a variety of methods. One of the most common causes of overpopulation is the introduction of foreign species to a new ecological niche for which they have no natural predators. A famous example is the introduction of rabbits to Australia in the 19th century, where they had no natural predators. In an attempt to control the overpopulation of rabbits in Australia, several different methods were employed. Poison, hunting, a rabbit-proof gate, and the introduction of predators (e.g., ferrets and cats) were some methods used in an attempt to control the rabbit population. However, after these methods failed, scientists released the myxoma virus into the rabbit population. Myxoma virus is a rabbit-specific virus that successfully reduced the rabbit population by approximately 500 million.

Causes of Overpopulation

The overpopulation of a species can result from a variety of factors. The most common include:

1. The introduction of a foreign species for which it has no natural predators. Often, such species become invasive, as seen in the above examples of zebra mussels and the introduction of rabbits in Australia.


2. An increased birth rate will result in population growth, which can lead to the overpopulation of a species if such growth exceeds the resources within a particular geographic area.


3. Decreased mortality rates can result in the overpopulation of a species if the increased lifespan of a species results in limiting the available resources within an ecological niche.


4. A reduction in available resources can result in overpopulation if the amount of available resources cannot sustain the population within that region. Some examples include desert environments or times of drought which make crops and other sources of food scarce.

Quiz

1. Which of the following is NOT a cause of overpopulation:
A. Increased mortality rate
B. Increased birth rate
C. Decreased mortality rate
D. Absence of predators

Answer to Question #1

A is correct. Increased population growth due to an increased both rate and decreased mortality rate and the absence of predators can all cause a species to become overpopulated. An increased mortality rate would have the opposite effect.

2. Zebra mussels in North America and rabbits in Australia are examples of what effect of overpopulation?
A. Decreased food availability
B. Increased population growth of natural predators
C. Invasive species
D. None of the above

Answer to Question #2

C is correct. Both the introduction of zebra mussels and rabbits to Australia resulted in invasive species that had catastrophic consequences on the local economy and ecosystem. In North America, zebra mussels clog water treatment and power plants, and have caused billions of dollars as a result. In Australia, rabbits destroyed crops and precious top soil, devastating the natural plant species and farming economy.

References

• Flowerdew J and Sumpton K. (1985). The ecological effects of the decline in Rabbits (Oryctolagus cuniculus L.) due to myxomatosis. Mammal Review. 15(4): 151-186.


• Petersen B. (1972). Reproductive efficiency and overpopulation. Population Review. 16(1): 60-63.


• Ratcliffe F. (1955). Review of Myxomatosis in Australia, 1950-1955. Journal of the Australian Institute of Agricultural Science. 21(3): 130-133.


• Strayer D, Caraco N, Cole J, Findlay S, and Pace M. (1999). Transformation of Freshwater Ecosystems by Bivalves: A case study of zebra mussels in the Hudson River. BioScience. 49(1): 19-28.

Overpopulation