Managing Animal Overpopulation

Some animals have the ability to proliferate to such an extent that they disrupt the ecological balance of wildlife. A plague or infestation of an animal can have a devastating affect upon other animal populations and can upset the population dynamics of an ecosystem.

While the populations of many wild animal species are threatened; the populations of some others can become threatening; not only to man, but also to other animals which share their habitat. Here are some examples:


Rodents such as mice and rats are a problem worldwide. They cause physical damage to the fabric of buildings and can potentially spread disease. There are many different species of rodents, with those that are indigenous species not posing a significant problem (eg Australian native rodents that are protected).

It is always better to control rodents before a significant infestation develops. Chemicals (rodenticides) can control small numbers; but once a population reaches large enough numbers, further measures are needed. It is important that the use of rodenticides is accompanied by an improvement in environmental management at the site (eg. improved hygiene, removal of food sources and proofing, maintenance and repair of buildings.


Red squirrels are native to the UK, whereas grey squirrels are an introduced species. Issues have arisen as the grey squirrel is able to perform better than the red squirrel for a number of reasons including:

  • Competition between the two species for food. Both squirrel species feed on acorns from Oak trees. Red squirrels are only able to consume acorns when they have fully ripened whereas grey squirrels are able to digest the acorns before they become fully ripe, giving the grey squirrel an advantage, and restricting food supplies for the red squirrel population.
  • The grey squirrel carries a disease, the squirrel parapoxvirus, which does not appear to effect their health, but will kill red squirrels.
  • Red squirrels will not breed when under pressure.

As a result of the above issues, grey squirrel populations in the UK have increased while red squirrel populations have decreased, to the point where they are considered to be near threatened in the UK. Grey squirrels have also become a pest in places, removing bark from trees and damaging crops. In the roof of houses they can destroy insulation, damage electrical wiring and affect plumbing, potentially causing fires and floods.

The management of this situation is focused on the conservation of the red squirrel by reducing grey squirrel numbers. The UK government announced a mass culling program in 2006, encouraged by an earlier grey squirrel culling program in 1998 at the North Wales island of Anglesey which was successful in assisting the natural recovery of the remaining red squirrel population in the area.


Rabbits have become a source of both problems to the environment and native animal populations in areas of the world where they have been introduced by humans. Rabbits have large appetites and a high reproductive rate, having massive impacts upon agriculture as well as animal populations including livestock and wildlife as they are forced to compete with rabbits for food. Rabbits are responsible for serious erosion problems as they eat native plants which leave the topsoil exposed and vulnerable to sheet, gully and wind erosion. The effect of rabbits upon the ecology of Australia has been devastating.

It is believed that rabbits have been a significant factor in the loss of one eighth of all mammalian species, as well as un unknown number of native plant species.  

 A range of techniques have been used to attempt to control rabbit populations, including gassing, barriers, shooting, snaring and ferreting, though the most effective measures have been the introduction of diseases to the rabbit population such as myxomatosis and calcivirus, particularly in Australia. In Europe, the introduction of a genetically modified virus to commercially farmed rabbits has been beneficial to rabbit farmers, though the introduction of this virus to places such as Australia would see a rabbit boom and in increase in negative environmental effects.  


Mosquitoes are a threat to both human and animal populations through the transmission of viral diseases carried by Mosquito populations. One of the most significant of these diseases is Malaria, which is a leading cause worldwide of premature mortality and causes 5.3 million deaths annually. Mosquitoes are responsible for transmitting a range of other diseases including elephantiasis, yellow fever, dengue fever, epidemic polyarthritis, Rift Valley fever, Ross River fever, St Louis encephalitis, West Nile Virus, Japanese encephalitis, LaCross encephalitis Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE). EEE and WEE are found in the US, and causes disease not only in humans but in horses and some bird species. In the US, mosquitoes have also been found responsible for transmitting the West Nile Virus to over 140 different bird species.

Mosquito control is implemented across the world largely to protect humans from disease but is also utilised by wildlife managers in order to protect wildlife. A balance often needs to be sought in the reduction of mosquito populations, as the protection of humans and animals from mosquito transmitted disease is often gained through the use of chemical control, which can result in negative impacts upon human and animal populations.

Mosquito control can be used on a large scale, such as organised programs to reduce populations over a large area and on a small scale through individual actions to control themselves from mosquitos on their own property. Organised mosquito management programs today draw on the principles of integrated pest management, including the following measures:

  •     Source reduction - removal of mosquito breeding habitats
  •     Habitat modification – manipulating habitats to reduce breeding
  •     Biocontrol – introducing natural predators of mosquitoes
  •     Larvicide – using pesticides to reduce larval populations
  •     Adulticide – using pesticides to reduce adult populations

Some solutions for malaria control efforts in the third world are: mosquito nets, mosquito nets treated with insecticide (often permethrin), and DDT. The use of DDT as a mosquito control method has been the subject of a great deal of controversy. It is argued that DDT is the most effective weapon in combating mosquitoes and their diseases, yet the effects of DDT upon biodiversity are significant.