Cascading extinctions due to loss of indirect ecological interactions

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The current high rates of species extinctions are well publicised and it is clear that much of this is due to human activities such as overexploitation, habitat destruction and global environmental change due to greenhouse gas emissions. The loss of species is not just a moral and aesthetic issue. It has been demonstrated repeatedly that the ecosystem services (food production, soaking up of carbon emissions, pest control, flood control etc.) that human societies rely on are positively related to species diversity in ecosystems. The protection of biodiversity is therefore a major priority for governments. The effectiveness of this protection depends on how well we understand the processes that lead to species declines. Sometimes this is obvious: The collapse of cod stocks is clearly due to overfishing. Often however, such direct impacts are followed by secondary extinctions of other species, for not always obvious reasons, with the danger that this leads to a cascade of further extinctions and ecosystem collapse. Predicting these cascades is challenging and requires a detailed understanding of how the interconnectedness of species in ecosystems affects the transmission of human impacts on one species to other species that are not directly linked to it. This is particularly important for species at higher trophic levels (carnivores) which are most vulnerable to extinction. The idea has long existed that species of carnivore that specialise on different prey have positive effects on each other by limiting their prey populations and thereby preventing one prey species from outcompeting the other. A consequence of this is that if a carnivore is lost from the community, its prey is released from control and may subsequently out-compete the other prey species leaving the other carnivore without food and facing extinction. This is potentially an important mechanism by which extinction cascades occur, however, it is difficult to obtain experimental evidence for such effects. We have done preliminary experiments with simple insect communities in the laboratory which have demonstrated that the removal of one carnivore species does indeed lead to the extinction of others in just a few generations. The challenge now is to scale this model system up to a more realistic scale. We propose to carry out experiments in field-based mesocosms - roughly 2 meter cubed enclosures in which we can control the exact composition of the ecological community. We will assemble communities of insects in these and impose specific harvesting regimes on target species and follow the indirect impact this has on the other species and, in particular, whether the predicted extinction cascades occur. Within this setting we can manipulate important variables, such as the strength of competition among prey species, and test their impact on the outcome. We can also test the prediction that more species-rich and complex communities will be more resistant to extinction cascades, which would mean that as biodiversity is lost, the chance that further losses trigger extinction cascades increases.