Species sensitivity and community structure: an integrated approach to understanding how one affects the other

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There is, quite rightly, much concern expressed over the future of animal and plant species; for example the decline in farmland birds or the increasing rarity of many plants and insects. However, each species does not exist in isolation. Instead, one species interacts with other species, and those species, in turn, interact with yet others and so they form highly complex ecological networks, such as food webs. Usually studies are focussed on either species or whole networks, but I will take an integrated approach by considering both, for example, by examining the biology and sensitivity of species in the context of their place in ecological networks (how well connected they are into the network). I will address this in asking the following questions: (1) are the species that are least well connected into networks those that are most sensitive to changes to the environment, such as habitat fragmentation?, and (2) are the least well connected species the most likely to be rare and declining? However, changes in the abundance of species and how well connected they are in networks in turn affect the structure of whole networks. Therefore knowing the identity of individual species in the networks will help us to understand ecological networks better, even though in many previous studies species' identity has not been considered. In this study I will investigate ecological networks in woodland patches. Woodland used to cover most of Britain, but the patches I will study are the fragments that remain, and they vary both in their size and how close they are to other patches. I will simultaneously study two different ecological networks in these woodland patches: one is the network of flowers and the insects that visit and pollinate them, and the other is the network of plants, aphids and aphid predators. I will measure how well connected each species is in a network and relate this to (1) its sensitivity to woodland fragmentation, (2) its national rarity and decline and (3) biological characteristics of the species, such as how long it lives, how quickly it reproduces, or how mobile it is. I will also ask how habitat fragmentation affects whole networks and whether it makes them less resilient to the extinction of species. Hoverflies feature in both these networks since adults are flower visitors and the larvae of many species feed on aphids. Therefore I will ask whether a larva that is well connected into its food web gives rise to an adult that is well connected into its plant-pollinator network. The two networks will be linked through their shared species (hoverflies and plants) raising the possibility of extinction 'vortices' in the networks, where the loss of one species causes the loss of others that are dependent on it, and the loss of yet more species dependent on those, and so on cycling through the two linked networks. Often studies examining the effects of environmental change on biodiversity consider only individual species or whole networks. By taking an integrated approach I will provide a novel understanding of how species and ecological networks influence each other to affect biodiversity as a whole.