Biological invasions and habitat degradation represent the two major threats to biodiversity. However, due to the frequent co-occurrence of these factors, the ultimate mechanism driving biodiversity loss is often unclear. Effective conservation actions depend on accurate identification of the relative roles of biological invasion and habitat degradation to biodiversity loss, including their interacting effects on the structure and functioning of food webs. Indeed, biotic interactions can modify stressor effects, and key ecological properties including energy flux, vulnerability of communities to biodiversity loss and resilience to perturbations are related to the architecture of food webs.
Thanks to the stable isotope-based description of food web structures, this project will provide robust information on the effect of habitat degradation, species invasion and their interaction on biodiversity and on the architecture of invaded food webs. The project will focus on aquatic Mediterranean habitats, and specifically on fish communities including species of high conservational and commercial value. The quantification of trophic interactions will allow to measure the impact of invaders on native species, as well as the limiting effect of natives against invaders. A network analysis will provide unprecedented insights on the effect of invasive species on the vulnerability to biodiversity loss and resilience to additional stressors of invaded food webs under varying habitat conditions.
Results and procedures arising from this study will represent a step toward building a better quantitative understanding of how interactions between habitat degradation and biological invasions drive their net effect on the structure and functioning of biotic communities, providing clear advices supporting conservation management. Although our case studies rely on aquatic ecosystems, the approach we present is founded on ecological theory and is thus transferable to most other ecosystems.
Field studies explicitly testing the interactive effects of habitat degradation and biological invasions at the whole food web level are lacking (Bruder et al. 2019). Similarly, quantification of interaction strengths and effects of invasive species on the demographic and dynamic properties of native ones are generally restricted to species pairs. The quantitative description of consumptive and competitive interactions based on stable isotope analysis and Bayesian mixing model proposed in this project will allow to apply classical ecological theory and models to complex, real food webs. Results and procedures arising from this study will represent a step toward building a better quantitative understanding of how interactions between habitat degradation and biological invasions drive their net effect on the structure and functioning of biotic communities. Notably, aquatic food webs often experience multiple biological invasions (Jackson 2015). In this context, the approach proposed here would also allow to quantify the direct and indirect effects of interspecific interactions among co-occurring invaders on invaded communities, a largely neglected research subject (Jackson 2015).
Reconstruction of complex food webs depicting trophic links actually realized between native and invasive species, coupled with information on species density and body mass, will improve our ability to quantify the impact of invasive species on the biomass density of native ones, including species of conservational value or exploited commercially. This would represent a major advancement in the possibility of pest categorization of invaders, whose classification is often lacking due to difficulties in the quantification of their economic impact in invaded ecosystems, particularly in aquatic habitats (Havel et al. 2015). In parallel, quantification of the limiting effect of native species on invaders will provide a direct measure of the biotic resistance by native communities. Quantification of biotic resistance at the species level would represent an useful support to management strategies aimed at preserving local biodiversity while reducing the ecological and economic impact of invaders. Comparison of biotic resistance among undisturbed and degraded habitats will also allow to quantify if and how much habitat degradation reduces the ability of invaded communities to mitigate invasion.
Notably, the quantitative description of food webs will allow to measure their sensitivity to perturbations based on long-standing, broadly accepted mathematical measures of community resilience (May 1972, Allesina et al. 2015), and more recent measures of community vulnerability to biodiversity loss (Dunne et al. 2002, Calizza et al. 2015). This will allow to quantify how much invasive species and habitat degradation, by modifying the food web structure, affect the possibility of native communities to cope with climate- and human-driven environmental changes, enabling a better prediction and management of native species response.
We argue that sustainable long-term management strategies must consider 'interactive effects' models of native species decline in degraded habitats to be successful and cost-effective (Bruder et al. 2019). Thus, the insights on network-mediated effects gained via the approach we propose can substantially increase the efficiency of invaded ecosystem management. Although our case studies rely on aquatic ecosystems, the approach we present is founded on ecological theory and is thus transferable to most other ecosystems.