Cascading extinctions due to loss of indirect ecological interactions

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.

Our project is focussed on aphids and their associated parasitoids but we expect that the insights gained from it to be widely applicable to the management of exploited populations (e.g. fisheries) and the conservation of biodiversity. Nevertheless, because the aphids included in our model system are pest species of commercial crops and the parasitoids are used as biological control agents, we expect that our results will be of benefit to this industry as well.
Beneficiaries:
Industry and food security: Aphids are sap feeding insects that can cause major crop damage as a direct result of their feeding but also, and possibly more importantly, as vectors of plant viruses. Of the approximately 4000 species of aphid, 250 are considered economically important as pests in agriculture and forestry and their economic cost to Britain has been estimated to run into the many 100s of millions of pounds. Parasitoids are commonly employed for the biological control of pest aphids but with mixed success. The key to successful pest management using biological control is a clear understanding of host-parasitoid population dynamics, which are inherently unstable, especially in simplified agricultural ecosystems. We will maintain our existing links with the industry and the associated academic researchers to disseminate the relevant findings from our work.
General public: We believe that for the general public to engage with an agenda of public spending on biodiversity conservation, it is vital that they are well informed about the value of biodiversity to society in terms of ecosystem services. We also feel it is important to divert the spotlight away from focussing on aesthetically pleasing species because, in our view, this carries the danger of giving the impression that biodiversity conservation is a luxury rather than a necessity. Through a series of school-based and public outreach activities we will engage all ages into thinking about biodiversity conservation and the interconnectedness of species within ecosystems.
Managers of exploited populations and conservationists: An understanding of how populations of species within communities are dynamically linked, even if they may be many trophic links removed from each other, is especially important to this user group. While many users in this group will be aware of these issues, often also from their own observations, there is currently still a lack of a general framework for including this ecological theory into practical biodiversity management. We will disseminate the management and conservation message from our work to these end-users through our existing network of contacts (mostly fisheries managers) and by presenting our work at the International Congress for Conservation Biology.