Invasive Species


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•  Introduced alien species can escape into local ecosystems and become invasive

The species within any given ecosystem can be broadly described as either endemic or alien

  • Endemic species are those which are native to a defined geographic region (e.g. koalas in Australia)
  • Alien species are those that have been transferred from their natural habitat to a new environment

If an introduced alien species should have a detrimental effect upon the pre-existing food chains, it is classified as invasive

  • Invasive species typically threaten the biodiversity of the ecosystem they are introduced into by displacing native species

Invasive Species in Australia

alien species

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•  Competitive exclusion and the absence of predators can lead to reduction in the number of endemic species

    when alien species become invasive

The competitive exclusion principle states that two species cannot occupy identical niches within a community

  • According to competitive exclusion, one species will have a competitive advantage and survive at the expense of the other
  • Evolution via natural selection results in adaptations that function to minimise direct competition between species in nature

Invasive species possess several advantages that allow them to displace endemic species via competitive exclusion:

  • They typically possess a large fundamental niche (can occupy a wider area than endemic species)
  • They will often have faster reproduction rates (are capable of forming a larger population than endemic species)
  • They commonly lack a predator capable of limiting their survival (due to being introduced into a foreign environment)
  • They may possess certain features that are suited to the new environment (e.g. more aggression, better foraging, etc.)

Impact of Invasive Species on an Ecosystem

invasive species

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•  Study of the introduction of cane toads in Australia and another local example of an introduced alien species

Case Study One:  Cane Toads

The cane toad was introduced in Australia in 1935 to control the spread of the sugar cane beetle that was damaging crops

  • It failed to limit the beetle population (as the beetles live at the tops of sugar canes where the toads couldn’t reach them)

The cane toad has reproduced rapidly and is now classified as an invasive species that endangers the native wildlife

  • It has a wide diet and has depleted the prey population for native insectivores
  • It secretes a toxic chemical from its skin that has poisoned native predators
  • It has carried diseases that have been transmitted to native species of frogs and fish

Since its introduction, the cane toad has spread across the northern fringes of Australia and has proven difficult to eradicate

  • The cane toad has no natural predators in Australia to reduce its population size and now number in the millions

Cane Toad Distribution in Australia

cane toad

Case Study Two:  Wild Rabbits

European rabbits were introduced to Australia with the arrival of the First Fleet (ostensibly for recreational hunting)

  • Australian conditions promoted a population explosion (mild winters allowed rabbits to breed the entire year)
  • It was the fastest spread ever recorded for any mammal anywhere in the world

Since their introduction, rabbits have had a devastating effect on native Australian ecology:

  • They have killed native plant species by ringbarking (removing a strip of bark that prevents future growth beyond that point)
  • The loss of native plants has left the topsoil exposed and vulnerable to extensive erosion from wind and rain

Various strategies have been employed to control rabbit populations with varying effect:

  • 'Rabbit-proof’ fences were built to try and contain rabbit populations 
  • Methods of hunting and poisoning have been utilised, along with efforts to destroy rabbit warrens
  • Rabbit-borne diseases have been released with moderate success (e.g. myxomatosis virus)