Invader–invader mutualism facilitates secondary invasions in rainforest on Christmas Island
Mutualistic interactions between successful ‘primary’ invaders can produce reciprocal, positive population-level effects that amplify their impacts. These impacts may alter key properties of the recipient community to facilitate ‘secondary’ invaders. On Christmas Island, mutualism between the exotic yellow crazy ant Anoplolepis gracilipes and a variety of introduced honeydew-producing scale insects results in the formation of high-density crazy ant supercolonies that cause local extinctions of the dominant omnivore-detritivore, the red land crab Gecarcoidea natalis. Occurring at naturally high densities, these crabs could provide the rainforest community with considerable biotic resistance against invaders through their activities both as predators, and by limiting key resources such as leaf litter and seedlings. In this project I asked whether the ant-scale mutualism indirectly facilitated the success of secondary invaders – exotic land snails – by altering these key attributes of the recipient community. Land snails are common invaders on islands, and on Christmas Island 22 of the 38 species are introduced, including the notorious giant African land snail Achatina fulica. Where red crabs were absent, exotic land snails were an order of magnitude more abundant and the community differed compositionally from the uninvaded state. Field experiments that manipulated red crab density and leaf litter biomass, either singly or in combination, showed that increased land snail abundance was due to the augmentation of habitat and resources, rather than the creation of enemy-free space. This represents a ‘population-release’ model of secondary invasion, defined as the facilitation of invasive species from very low to very high densities. However, high density does not necessarily mean high impact. An exclusion experiment demonstrated that high densities of A. fulica had no detectable impact on seedling recruitment or leaf litter decomposition. This research demonstrates the importance of properties of the recipient community in determining invasion success, and highlights the role of invasive species themselves in altering those properties.