The following post features the slides and (roughly) what I said when presenting part of my research at the Association for Tropical Biology and Conservation annual conference held in Honolulu, Hawai’i the other week (12 – 16 July 2015). I was presenting in the ‘invasive species’ session on the Thursday afternoon (last session of the last day!) which included many interesting talks. I present this here for anyone who happened to miss the talk but was otherwise interested in some of the content. Throughout the post the relevant text comes after the slide:
Invasive Giant African Land Snails (Achatina fulica) have little to impact in rainforest
Aloha, today I’m going to tell you about some research I’ve conducted as part of my PhD research, where we looked for any impacts of invasive Giant African Land Snails, when in high-abundance, in a natural – rainforest – setting. And I have obviously given away the answer in the title of this talk – we determined that this potentially damaging invader was nothing more than an aesthetic atrocity – in the context of this study. So how did we come to this conclusion…
I want to begin briefly by talking about what it means to be ‘invasive’ – the term we reserve for only the most established or problematic introduced species. So a species gets this tag if they are in a stable, self-replacing population; There is an abundance of that species; And they are spreading or able to spread. The scientific-ecological definition (according to most) purposefully excludes environmental impact from this definition, however – where the definition is more applied in a conservation management sense – the definition of ‘invasive’ very much includes environmental impact – but not demonstrated, quantified impacts – rather the assumption of impact. Because there’s lots of them that are continuing to spread, they must be having an affect- RIGHT? Now this is done because species are spreading around the place very quickly and more often than not the biological and ecological data doesn’t exist to properly determine potential impact. A precautionary approach dominates management as it wouldn’t be the end of the world if you assumed a species was having an impact when they weren’t, but the consequences could be very costly if you assumed NO-IMPACT and you were wrong. So for many introduced species, it remains untested as to whether this assumption of impact is valid, or whether some highly invasive species really have little to no impact
So let me introduce to you all 100 of the worst invasive species as identified by a global network of scientific and policy experts, of which this guy, the Giant African Land Snail, native to east Africa, is a proud member. So these guys make the list as they’re prolific invaders of the tropics worldwide, very common on many Indo-Pacific islands and on essentially all tropical mainlands. Their spread has been attributed to the classic combination of accidental introduction through global commerce and trade, as well as deliberate introductions in places as a food resource.
These guys are generally very successful in a new location because they have all those traits that makes a good invader. They grow large, they grow quickly, they can eat almost anything, anywhere, and with every individual able to produce around 1000 eggs in their lifetime, they can reach high densities very quickly.
An so what are the assumed impacts of these self replacing populations in high densities that are spreading in natural ecosystems. Well, they could be directly altering plant community composition through herbivory, They could be indirectly altering nutrient cycling through consumption of excessive plant material, both live and dead. They could be altering native land snail assemblages through competition or by fouling the habitat. And I say COULD for all these points because they are all unquantified assumed impacts based on some observations and anecdotes, and this ‘they must be having an affect’ assumption that a highly abundant species will be causing change that I introduced at the start. And considering how wide-spread this species is, it’s very surprising that there is this conspicuous lack of empirical research quantifying this species impact in NATURAL ECOSYSTEMS.
Which brings me to my study site. We do not have Giant African Land Snails on the Australian mainland. Good quarantine procedures have kept them out thus far. However, they are present out on the external Australian Territory of Christmas Island in the Indian ocean – where we a fairly confident they were deliberately introduced during Japanese occupation of the island in WWII.
Christmas Island itself is an isolated limestone island that looks a little like a Scottish terrier. They grey areas here are cleared vegetation – mostly for phosphate mining – but there’s no more clearing of rainforest for that anymore and almost all of the 70% of the island that remains covered in tropical rainforest is designated national park.
Now the rainforest of the island is quite unique and in someways backward to what most of you would be familiar with. This is what natural, intact, ‘good’ rainforest looks like on Christmas Island. Very open and structurally simple.
Rainforest looks this way due to the actions of the highly abundant native Red Land Crab. These guys eat through the leaf litter and most seedlings to create this kind of open rainforest as well as keeping Giant African Land Snails out through active predation. However, over the last 15 odd years we’ve seen the formation of invasive yellow crazy ant supercolonies – when in mutualism with exotic scale insects – which causes locale extinctions of the crab as the ants predate them.
As a result, you get massive structural changes in the forest, as well as the creation of enemy-free space, which is a big tick for Giant African Land Snails. And now we have patches of rainforest with high density populations of these secondary invaders.
AND so we ask the questions – what are ecological impacts of now having highly-abundant Giant African Land Snails in these rainforests in terms of leaf litter and seedling recruitment dynamics. Remember this species is described as a generalist feeder so these seem like the important ecological processes Giant African Land Snails might be impacting.
To address these questions we conducted an exclusion experiment at a site of high Giant African Land Snail abundance on the island, in which we monitored the leaf litter dynamics- biomass and percent cover – and seedling recruitment and survivorship inside an out of paired exclusion plots. Here is an example of an exclusion plot, which was simply some avian mesh with a copper-band around the top, which is affective as snails dislike crossing copper. Its reacts with their mucous and gives them a kind of electric shock. Plots were 4 m2, 2 x 2 and the experiment was conducted over 6 months – one wet season of GALS activity.
So Giant African Land Snails at the site were somewhat attracted to the coating in the mesh so sometimes you would see these sort of congregations, however the copper-banded fences did their job. Giant African Land Snail density was significantly reduced by our exclusion treatment relative to our open controls where Giant African Land Snail density was as high as 6 snails per square metre. So that’s good. We have an experiment. Now lets see if we found some treatment effects.
First up leaf litter. At the top we have biomass and at the bottom we have % cover and you can see that over the coarse of the experiment leaf litter declined. This was simply natural decomposition occurring over a wet season, so that TIME was identified as a significant driver for leaf litter dynamics. For leaf litter cover there was also an interaction effect of Time and Treatment where cover decreased less rapidly on exclusion plots. However, there was no significant treatment effect on either of these variables. Giant African Land Snail exclusion did not affect these leaf litter variables.
If we look at seedling recruitment dynamics we see the same story – if not a little less interesting. The number of new recruits each month declines over the experiment and that is the only significant effect. Recruitment and mortality of seedlings was the same on exclusion or control plots from month to month over the course of the experiment. Giant African Land Snail exclusion did not affect these seedling dynamics variables. Why?
Does it have something to do with activity? So these data are the lumping of 635 observations of nocturnal Giant African Land Snail activity of which we only ever saw half of the individuals doing anything. They were more-or-less all on the ground, but half were closed off inside their shell. Of the 334 observations of activity, only half of those were of Giant African Land Snails feeding. Many were moving and almost as many were simply doing nothing. Of the feeding observations, essentially all were made of Giant African Land Snails eating dead leaves.
So it appears that although Giant African Land Snails can be found up on vegetation and sometimes (anecdotally) consuming fresh leaf material of some species,
The majority of the time they’re foraging around on the ground chewing through leaf litter, of which you see this kind of damage – but their active density is not such that this could be experimentally separated from the natural decomposition rate.
So this is really what the title of this talk should of been in terms of leaving you all on the edge of your seats…… Invasive Giant African Land Snails, damaging invader? Aesthetic atrocity?, or both?
Well in the case of high-density populations of these invaders in rainforest on Christmas Island, these guys are pretty much just an aesthetic atrocity in terms of the two important ecological processes that we looked at in this study.
And so in the end you’d have to describe here, an example of one of the 100 worst invasive species in the world, having very little to no impact in a native rainforest. I’d like to thank the funding bodies that contributed to the project, a few mates that helped me out in the field, and of course you for listening, Here are the various ways you can get in contact with me so feel free to write them down or come find me for a chat if your interested in invasion ecology on Christmas Island. Cheers.
As always the talk went well and the conference was enjoyable. Prolific ‘twitterer’ @D0CT0R_Dave was in the audience so the key points of the talk were prominently featured for the Twitter audience. Check out #ATBC15 for a whole bunch of other going-ons from the conference.