Green, Peter T., Dennis J. O’Dowd, Kirsti L. Abbott, Mick Jeffery, Kent Retallick, and Ralph Mac Nally. Invasional meltdown: Invader-invader mutualism facilitates a secondary invasion. Ecology, vol. 92, no. 9, 2011, pp. 1758-1768.
Green et al. discuss the increasing prevalence of species invasion in all ecosystems due to human invasion and note the unexplored idea that interactions between invasive species in the same environment could have notable consequences, such as impacting invasion success and propagule pressure. The focus is brought to the invader-invader mutualism between the yellow crazy ant and honeydew-secreting scale insects, two bioinvaders of Christmas Island that together create yellow crazy ant supercolonies. Their interactions facilitate a secondary invader, the giant African land snail, by suppressing the native red land crab. The authors observe the relationship between the mutualistic invaders and the giant African land snail over the course of 7 years to gain insight on the poorly-understood effects of interactions between invasive species.
The authors used Bayesian hierarchical models to determine if the effect of the ant-scale insect invasion influences the spread of giant African land snails, which they tracked over the course of 7 years. Potential reversal effects of suppressing the ant-scale insect supercolonies were explored, as well as the effect of suppressing the red land crab on giant African land snail populations through site comparisons and experiments.
The probability of giant African land snails invading sites with abundant red land crabs is 14 times lower than that for sites with no crabs or supercolonies, and 253 times lower than that for sites with supercolonies and a suppressed crab population. Managing these supercolonies at certain sites reverses the probability of snail invasion and recovers the crab population for those sites. At sites with supercolonies, almost all snail mortality is due to starvation, and ants only feed on dead snails.
Significance for Palau:
Palau is an isolated archipelago with a plethora of unique endemic species inhabiting its many islands; as a similarly isolated land mass, Christmas Island has evolved into a unique environment, housing many endemic species of its own. This study demonstrates how just two small exotic organisms, introduced to the environment through human interference, can become a massive threat to biodiversity in a given area. Furthermore, it demonstrates the importance of studying the interactions between invaders, as well as the success of suppressing the invasive populations to recover native populations. As a republic with an economy heavily based on tourism, Palau continues to be subjected to more and more invasive species that disrupt the delicate native ecosystems. This study is especially relevant to conservation in such a place, as it lays some of the groundwork required for remediating the natural environments of isolated land masses such as Palau.