Invasional Meltdown

During our class trip to the Glendon campus, we came across many different invasive species that were just taking over large areas of land and showed no signs of stopping. Just the number and quantities of these species present is surprising. Invasive species are fairly ubiquitous in our forested landscape.

How do they become so successful at being so invasive? Some hypothesized mechanisms include better reproductive ability, rapid vegetative growth, adaptation and allelopathy. Another interesting hypothesis is invasional meltdown.

What is invasional meltdown?

It is the interaction between multiple foreign species that help each other accelerate their dominance. The combination of two or more invasive species interacting with one another could hasten their spread in habitats foreign to them, and dominate local habitats, leading to further degradation and environmental change.

Studying on Islands

Islands can serve as model systems for understanding how biological invasions affect community structure and ecosystem function. A study carried out by O’ Dowd et al showed how the introduction of the alien crazy ant Anoplolepis gracilipes indirectly caused seedling recruitment, seedlings and reduced litter breakdown. In the forest canopy, the ant formed new associations with the honeydew-secreting scale insects that ended up accelerating and diversifying impacts. In the canopy trees, foraging ants caused an increase of host-generalist scale insects and growth of sooty moulds, leading to canopy dieback and even deaths of canopy trees. Thus, a native keystone species suffered as a result of introduced mutualists and promoting invasional meltdown. The results also showed how the removal of a keystone species by an invasive one through ‘bottom up’ and ‘top down’ effects. These effects end up completely transforming island rain forest ecosystems.

 

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Impacts of invasion of island rainforest by the yellow cray ant (a) uninvaded site with open understorey maintained largely by the foraging activities of the red land crab. (b) Invaded site 1-2 years after ant invasion with a dense and diverse seedling cover and think litter layer. 

 

 

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