A couple of years ago I published an observational study on invasive ants facilitating exotic snails by killing native crabs. That’s the specifics that may be of limited interest, but the interactions (invader-facilitated invasion success) are broadly relevant to anyone studying the dynamics of multi-invaded complex systems. The overall conclusion is that we need to better recognise influential invasive species as intrinsic properties of recipient communities. You can quickly skim the paper by watching the video below, then I highly recommend reading the rest of this short blog that summarises the whole thing:
Humans have been really good at moving species all over the world, and in turn, exotic species have been really good at becoming abundant and influential in their new environments. However, sometimes the conditions in a new environment wont allow an exotic species to get their foot in the door and become invasive. For that to happen, conditions need to change – and who better placed to drive that change than other, previously successful, exotic species.
It is well established that the impacts of invasive species can seriously alter ecosystem properties. For example, the invasion of a highly-flammable grass will increase fire intensity and frequency in that environment. What is less established is how these altered ecosystems can now be invaded by other, previously unsuccessful, exotic species – in short, exactly how one exotic species will indirectly help another.
The concept being described here is that of ‘secondary invasion’. Most people will have an implicit understanding of the idea based on the more everyday notion of ‘secondary inflections’. A body contracts some pathogen (successful primary invader) making the body ill (altering the conditions of the environment) which leads to the body contracting other infections it otherwise wouldn’t have caught (previously unsuccessful secondary invader).
In ecological research, secondary invasion is a key part of the ‘invasional meltdown hypothesis’ (two invaders working together that amplifies impacts and accelerates more invasions) but was not explicitly defined as such by the authors. Actually, the concept itself remains undefined in the scientific literature and in desperate need of some dedicated investigation and empirical evidence.
A heavily invaded rainforest ecosystem on Christmas Island has long provided some of the strongest evidence in support of the ‘invasional meltdown hypothesis’, and is now also lending key support to this concept of secondary invasion.
The highly abundant red land crab dominates Christmas Island (natural densities of around 1 crab every square meter) and is responsible for the unique open structure and sparse ground layer typical of the rainforest ecosystem on the island (see below). Enter the invasive yellow crazy ant, that when fueled by the sugar of exotic honeydew-producing scale insects, cause local extinctions of the red land crab. The deletion of this ecosystem-engineer causes a pulse recruitment of seedlings and the build-up and persistence of leaf litter. The influence of the crab as a predator is also removed. The end result is an environment with a completely different set of conditions that other invasive species will respond to.
These changes were recognised early on as a great benefit to the exotic giant African land snail, which had been hanging-out on the island since WWII but had never been able to enter the rainforest without being quickly eaten by abundant red land crabs. The yellow crazy ant (primary invader) had indirectly caused the invasion success of the giant African land snail (secondary invader) by deleting the red land crab and creating enemy-free space (altering the conditions of the environment).
However, the giant African land snail is just one of many land snail species on Christmas Island – most of which are exotic and all of which are very small (20 – 1 mm in length). In a study published earlier this year in Biological Invasions, Pete Green and myself investigated whether the invasion success of these snails also depended on the yellow crazy ants altering the environment.
We found that in rainforest where the yellow crazy ant had removed the red land crab these smaller snails were an order of magnitude more abundant – the difference of only a few individuals compared to hundreds for every sample of leaf litter. When comparing intact to impacted rainforest there was no difference in total number of species (~20) or species composition as calculated from presence absence data, meaning all species were essentially at all sites.
This pattern of success was markedly different from what had been initially observed for the giant African land snail. These smaller species were able to enter the rainforest where crabs were present without issue. Yet they still responded positively (increased abundance) when the invasive ants deleted the native crabs. So how does this help inform our concept of secondary invasion?
When considering what determines invasion success, we need to establish when a species goes from simply being ‘exotic’ to ‘invasive’. It is generally considered that ‘invasive’ be reserved for those species that are well established in highly abundant populations, with the capacity to quickly spread and potential to impact the environment. That means the conditions limiting a species could be at any of the earliest points on the invasion pathway – either at the transport, entry or establishment stage.
What we have here on Christmas Island – with our investigation of the entire land snail fauna – is evidence for two distinct pathways of secondary invasion. A true-entry model (the giant African land snail that could not get into the rainforest at all) and a population-release model (the other smaller exotic species that could enter the rainforest but only persist at very low abundances). Both were indirectly facilitated by primary invaders to establish high abundant populations – by definition, ‘invasive’ – but from different starting points.
Often the focus of research is at a global scale, with big data, attempting to make the broadest generalization on the nature of something (which is of course important). The results and implications of this publication demonstrates that the intricate details of specific phenomena will continue to be understood through dedicated small-scale investigations, on literally some of the littlest species.