Indicators of impacts: using an integrated approach to quantify the impact of invasive species and trigger management intervention

Biological invasions are a key driver of global change and a significant threatening process for many species already heavily impacted by habitat loss and altered disturbance regimes. High abundance of an introduced species can have a number of direct and indirect effects on the native community, impacting native species and potentially restructuring ecosystems. Furthermore, some native species also have the capacity to respond positively to anthropogenic change, reaching artificially high numbers, and having the kind of influence on an ecosystem not dissimilar to invasive exotic species. However, the ecological impact of many overabundant species remains poorly understood, and in many cases, costly management is undertaken on the assumption that the species must be having negative effects. This new project aims to use the conceptual ideas of ‘invasion biology’ and ‘surrogate ecology’ to identify robust indicators of impact for more efficient conservation management. We propose that by using an integrated approach of determining the ecological impact of overabundant species, the transition point where a species shifts from a benign introduction to a damaging invader can be identified. This project aims to increase our appreciation of how overabundant species impact ecological communities and to determine easily measured indicators of when and where management will be most effective. New scientific insights will have implications for practical conservation management while increasing our appreciation of the role of surrogates in monitoring the impacts of species overabundance.

The experimental release of a biological control agent to manage yellow crazy ant invasion on Christmas Island

Supercolonies of the invasive yellow crazy ant Anoplolepis gracilipes are a major and ongoing threat to biodiversity values on Christmas Island. To date, the management of supercolonies has depended on surveillance, monitoring and control using toxic bait (mostly fipronil), particularly through aerial baiting programs in 2002, 2009 and 2012. While this program has been very effective in suppressing YCA supercolonies and there are encouraging signs of recovery in many treated areas, new supercolonies continue to form. There is widespread concern for the sustainability of this program in terms of its expense, non-target impacts, and the resources it diverts from other conservation programs. Recent research conducted by La Trobe University, funded by the Director of National Parks and endorsed by the Christmas Island Crazy Ant Scientific Advisory Panel, indicates that long-term, sustainable suppression of supercolonies could be achieved through the introduction of a host-specific biological control agent that would indirectly affect yellow crazy ants by reducing carbohydrate supply provided by scale insects, a key resource implicated in supercolony dynamics. The yellow lac scale Tachardina aurantiaca is likely to be the single biggest contributor to the honeydew economy of supercolonies across the island. This species, like all other scale insect species on the island, is not native to Christmas Island.

A key natural enemy of Tachardina, the parasitoid microhymenopteran wasp Tachardiaephagus somervillei has now successfully been imported, reared and released on Christmas Island. Monitoring of supercolonies as part of this initial experimental release are ongoing. We will know the effectiveness of this management project in due course. In the meantime, there are some Conversation articles about the project you may find interesting (read them HERE and HERE)

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 previously inhibited exotic species (secondary invaders) by altering these key attributes of the recipient community. This research demonstrates the importance of properties of the recipient community in determining invasion

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. I have looked at this phenomenon from the perspective of how exotic land snails have responded to these changes (see PUBLICATIONS). The logical next questions are to understand how other invasive species on Christmas Island – namely wolf snakes, giant centipedes, black rats and feral cats – respond to these changes to test whether their success is also facilitated by the ant-scale mutualism. Please get in contact if you are interested in collaborating on Christmas Island invasive species projects or if you are interested in testing ideas of secondary invasion in other ecosystems.


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