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University of Western Australia (2016)

Interactive effects of land-use change and rainfall decline on insect species networks

Murphy Mark Vincent

Titre : Interactive effects of land-use change and rainfall decline on insect species networks

Auteur : Murphy Mark Vincent

Université de soutenance : University of Western Australia

Grade : Doctor of Philosophy (PhD) 2016

Résumé
Terrestrial ecosystems are under threat from multiple global environmental change (GEC) drivers, with increasing evidence of non-additive interaction effects between them. Two of the most important drivers of biodiversity loss are land-use change (primarily habitat loss) and climate change. Historically these have been researched and managed as if they were largely independent processes, however recent research has indicated that synergistic interactions between them may be widespread. How these major drivers interact to affect biodiversity, ecological networks, and the functional performance of ecosystems is a vital area of research, contributions to which will aid the future conservation of fragmented ecosystems threatened by increasing climate change.
In this thesis, I test the interactive effects of land-use change and rainfall decline on native bee and wasp communities in fragmented woodlands in southwest Australia, a global biodiversity hotspot. I sampled insect communities in 48 woodlands along gradients of habitat loss, historic rainfall variation, and recent rainfall decline, and tested the interactive effects of these drivers using a factorial combination of treatment categories. In chapter two, I show that land-use change (habitat loss and edge effects) has driven substantial changes in invertebrate, bee and wasp communities, and these effects sometimes varied in magnitude across rainfall gradients. I show that recent rainfall decline has had a greater magnitude of effect on bee communities compared with wasp communities, and may be driving regional homogenisation in bee species composition. I also highlight the limited utility of spatial comparisons across standing gradients in historic rainfall for inferring future climate trends (i.e. space-for-time approach), due to contrasting effects compared with those observed for temporal trends in recent rainfall decline on the diversity of wasp communities. In chapter three I extend the analysis of global change effects to the response of host-parasitoid species interaction networks. I show once again that rainfall decline has had a greater impact on bee hosts of parasitoids rather than wasp hosts of parasitoids, and that it is driving homogenisation of species interaction networks across the study region. Moreover, I show that rainfall decline has negatively specialist parasitoids more than generalists, and that these are likely linked, via bottom-up changes to host availability, to declines in bee abundance. Most importantly, I found evidence for synergistic interactions between habitat loss and both historic rainfall and recent rainfall decline on the structure of species networks, with reductions in the richness of network modules being exacerbated at the dry end of both gradients. These reductions were linked to reduced occurrence of specialist parasitoids in networks. Finally, in chapter four I assessed the effects of interacting global change drivers on ecological function in bee and wasp communities. I assessed both the direct and indirect effects of drivers on parasitism rates and their stability through time, with potential mediating variables of habitat characteristics (tree mortality) and species network structure (module richness). I found strong evidence that global change is affecting ecosystem function via changes to network structure. Drier conditions measured by both rainfall gradients exacerbated the negative effects of habitat loss on parasitism rates and stability, with these effects being strongly mediated through the declines in network module richness that were found in chapter three.
In this work I have uncovered strong and wide-ranging effects of global change on insect communities, with interactions between land-use change and climate change being relatively common. Often the negative effects of one driver were exacerbated by the level of effect of another, highlighting important synergies among GEC drivers that are likely widespread in nature. These interactive effects altered species abundances, diversity and composition, and the structure and functioning of ecological networks – emphasising that anthropogenic influence is pervasive at multiple levels of community organisation. These findings are highly relevant for future conservation efforts, particularly in areas of the world where historic land-use impact has been high and where increasing drought is predicted under future climate change.

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