Little snails dig on leaves, indifferent to crabs

Secondary Invasion Part 3 – First, a brief recap… Yellow crazy ants invade rainforest on Christmas Island… They kill the native red land crab… And facilitate the high abundance of a whole community of exotic land snails (read the publication or the blog). So we know the mechanism of invasion success when it comes to those land snails…? Crab deletion right? Hmm, not quite the full story. 

That’s because when you delete this crab from this rainforest, you’re not just removing them, but also all of what they do. These highly-abundant omnivorous-detritivorous ecosystem-engineers are opportunistic predators, and leaf litter, seed, fruit and seedling consumers. This influence creates an open forest with limited habitat and resources for other kinds of species … like exotic snails. When the crabs are gone, there is no replacement of that predation and consumption, and so the rainforest community significantly changes….

The question is, is it the removal of a predator (creation of enemy-free space) or the build-up and persistence of leaf litter (habitat augmentation) or a little bit of both that the little exotic snails are responding to? So Pete and I wanted to answer this, but because litter and crab dynamics are so closely linked, any observational study would only provide a best estimate to infer the mechanism. And wanted to determine that mechanism…. Time to do some controlled field experiments.

So we tackled this in a number of different ways, the major experiments being full-factorial manipulations of crabs density (high and low) and leaf litter biomass (high and low). If you were a little snail, it didn’t matter what crab density was, so long as leaf litter was high, you’re abundance was high! Very strong evidence that it is all about that one mechanism – habitat augmentation.

A take home message here is – when we combine this study with an earlier experiment looking at the biggest exotic snail on the island – we find ourselves looking at a single organism (the crab), that is influencing a whole community of exotic species (the snails) simultaneously by top-down (predation on big snail) and bottom-up (habitat augmentation for little snails) processes!

For the actual science of this story, you should definitely read the paper and contact the authors if you have any questions.

O’Loughlin LS & Green PT (2016) Habitat augmentation drives secondary invasion: an experimental approach to determine the mechanism of invasion success. Ecology 97: 2458-2496.

K!E#25 by Luke S. O’Loughlin


This comic has two direct predecessors:

Secondary Invasion Part 1: which was published as part of #ESA15comic – the unofficial comic of the 2015 Ecological Society of Australia conference in Adelaide. You can read that 31 page comic HERE.


Secondary Invasion Part 2: Started the whole ‘Christmas Island Club’ analogy and was featured as part of KAPOW! ECOLOGY Issue 1 (K!E#15). You can read the associated blog HERE


Secondary Invasion Parts 4 & 5 are all currently in review; comics will follow!

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