Invasional Meltdown on an Oceanic Island

             Invasive species can be one of the most disruptive disturbances possible for an ecosystem to encounter.  Once such a species has established itself within a community, the likelihood of effective management against it becomes minimal.  For my community ecology class, we were required to read a paper about the invasion of alien crazy ants (Anoplolepis gracilipes) on Christmas Island.  The ants have caused a catastrophic shift in the island’s ecosystem, and displaced the native keystone species.

            Before alien crazy ants invaded Christmas Island, the red land crab was a keystone species of sorts. It fed on the ground vegetation and seedlings in the understory of the local tropical rainforest, and did so extremely effectively.  This changed drastically with the introduction of the crazy ant.

            The crazy ant has been so impactful on the ecological community for many reasons. First, the ecosystem in place was a very simple one, making its susceptibility to potential invasions much higher and more devastating. This was compounded by the fact that the crazy ants routinely swarm and kill land crabs they come in contact with.  With the thinning of the crab population came the regrowth of the forest’s understory; species diversity has risen on Christmas Island since the introduction of the alien crazy ants.  Additionally, litter breakdown on the forest floor has decreased drastically.

            The crazy ants have also had an extremely negative impact on the canopy trees within the forest.  The introduction of crazy ants coincided with the introduction of a species that they have commonly had a mutualistic relationship with: the scale insect.  Scale insects congregate in canopy trees in high densities and generally lead to the introduction of sooty moulds. The mould leads to canopy dieback, and in some cases, the death of the tree itself.

            Since the introduction of the alien crazy ant on Christmas Island, the entire ecosystem that was in place has completely collapsed. The red land crabs that were once the keystone species keeping the forest floor clear of saplings are much less common than they were in the past, and the forest canopy that was once full has become fragmented due to the infestations of scale insects.  While it is extremely difficult to stop such invasions from happening, more effort needs to be placed into the development of effective management strategies for stopping the spread of invasive species.

paper:

http://www.uvm.edu/~ngotelli/Bio%20264/ODowd.pdf