In biology, overabundant species refers to an excessive number of individuals and occurs when the normal population density has been exceeded. Increase in animal populations is influenced by a variety of factors, some of which include habitat destruction or augmentation by human activity, the introduction of invasive species and the reintroduction of threatened species to protected reserves. Population overabundance can have a negative impact on the environment, and in some cases on the public as well. There are various methods through which populations can be controlled such as hunting, contraception, chemical controls, disease and genetic modification. Overabundant species is an important area of research as it can potentially impact the biodiversity of ecosystems. Most research studies have examined negative impacts of overabundant species, whereas very few have documented or performed an in-depth examination on positive impacts. As a result, this article focuses on the negative impact of overabundant species.
Definitions When referring to animals as "overabundant", various definitions apply. The following classes explore the different associations with overabundance:
The inconvenience of animals in a certain region or area that threatens human livelihood, for example the tropics are considered to contain an overabundant population of the Anopheles mosquito which carries the malaria parasite. The population density of a preferred species has been reduced by another species population which is then considered as overabundant, for example predator populations of lions and hyenas reducing zebra and wildebeest numbers. A species population within a specific habitat exceeds the carrying capacity, for example national parks reducing herbivore populations to maintain and manage habitat equilibrium. The entire equilibrium consisting of animal and plant organisations is already out of balance, for example existing populations colonising new habitat. Out of all these classifications, class 4 is considered the most significant due to consequent ecological impacts.
Causes Overabundance may occur naturally, for example after weather events such as a period of high rainfall in which habitat conditions become optimal. However, other contributing factors include:
Anthropogenic disturbances Natural habitats are altered by human activity resulting in habitat fragmentation, decrease in forest densities and wild fires. Other human disturbances include restrictions on hunting, agricultural land modification and predator removal or control within a region or area. The consequent change in land use and the presence or withdrawal of human influence can trigger a rapid increase in both native and non-native species populations.
Invasive species can be better adapted to specific environments Invasive species are often overabundant as they outcompete native species for resources such as food and shelter which allows their population to thrive. Other factors influencing population growth include the lack of native predators or the less common presence of the introduced species within native predator habitat.
Overabundance due to translocation of threatened species to protected areas Some methods in managing threatened species involve reintroducing species to enclosed reserves or island areas. Once these species are introduced, their populations can become overabundant as these areas serve to protect the targeted species against predators and competitors. This occurred for the Bettongia lesueur, the burrowing bettong, which was reintroduced to the Arid Recovery reserve in Australia: their population has increased from 30 to approximately 1532 individuals. Due to the damage within this reserve their population is considered overabundant.
Potential impacts Overabundant species can have an adverse impact on ecosystems. Within ecosystems food resources and availability, competitors, and species composition can be negatively impacted on.
Impacts of overabundant herbivores
A common impact from overabundant herbivores is vegetative damage by overgrazing, where overgrazing refers to the effect of grazing having reached a level where other biodiversity within the ecosystem becomes threatened. Overgrazing can occur in both terrestrial and marine environments and can alter vegetation as well as the composition of vegetation. Population densities and the composition of fauna can also be negatively impacted on. Additionally, permanent ecological damage can be caused by overgrazing before maximum carrying capacity has been reached. Trophic relationships (i.e. feeding relationships in the ecosystem) can be altered by overabundant species, potentially causing a trophic cascade. Trophic cascades impact vegetation as well as invertebrates (including microorganisms) and birds. Furthermore, predator behaviour and populations may be indirectly affected.
Impacts of overabundant predators Overabundant predators are considered harmful to local biodiversity as they prey on native species, compete for resources and can introduce disease. They can decrease native mammal populations and, in some cases, can cause species to become extinct which results in a cascading ecological impact. Examples of invasive species include: "cats (Felis catus), rats (Rattus rattus), mongoose (Urva auropunctata), stoats (Mustela erminea)" and red foxes (Vulpes vulpes). Such species have contributed to the extinction of approximately 58% of modern-day mammals, birds and reptiles. In Australia, red foxes and feral cats have contributed to many native mammals becoming threatened or extinct which has led to diminished vegetation as foraging mammals have an important ecological role in maintaining a healthy landscape. A particular example is where grassland vegetation diminished to shrub land as a result of seabirds being preyed on by Arctic foxes. Seabirds have an essential ecological role which consists of helping to maintain nutrient levels and soil fertility. "Invasive predators also threaten 596 species classed as "vulnerable" (217 species), "endangered" (223), or "critically endangered" (156), of which 23 are classed as "possibly extinct."
Impact on society It can be very costly to control or eradicate overabundant species. For example, fencing regions as a protective measure against red foxes can cost approximately $10, 000 per kilometre while baiting an area of 35,0002 kilometres can cost about $1.3 million.
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