A test of recruitment limitation in some stream insects

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The general aim of this research is to examine the contribution of different life history stages to the local population dynamics of some stream insects. Understanding the factors that limit population size is a challenging problem of widespread interest among ecologists, and also of vital importance to sound practice in conservation and environmental management. Traditionally, freshwater ecologists have focused on the aquatic, juvenile stages of insects, with considerably less attention devoted to the transition stages between the aquatic juveniles and terrestrial adults. We will concentrate on the recruitment of eggs from the terrestrial into the aquatic medium and the conversion of those eggs into neonates and help fill a substantial knowledge gap about the ecology of these life stages. It seems likely that either or both of these stages may act as populations 'bottlenecks'. Simply finding a suitable location to lay eggs may limit population size. Many stream insects lay their eggs on the underside of stream rocks that are emergent from the water's surface and we will test whether the supply of emergent rocks might limit egg recruitment. Further, egg-laying females may be influenced by the spatial arrangement of emergent rocks (egg-laying sites) and we will test also how this influences recruitment. Once the eggs hatch, the mobility of neonates will dictate whether or how long any localised effects of emergent rock limitation persist and, by measuring neonate movement, we will identify the spatial scale relevant to studies about population dynamics. Alternatively, effects of emergent rock limitation may be overwhelmed by high mortality of eggs or neonates, suggesting a different underlying mechanism for recruitment limitation. A lack of any pattern between emergent rocks, eggs and neonates, would suggest that populations size is set by different life stages and subsequent research efforts can be directed at those stages. Thus, our research will identify whether bottlenecks occur for egg and neonate life history stages, how strong those bottlenecks might be, and the spatial scales at which the restrictions occur. These results are of fundamental importance to ecological models of population dynamics. Information on how insects use emergent rocks, will be of use to river managers and rehabilitation projects.