DEVELOPING A BETTER UNDERSTANDING OF INTRA AND INTER-SPECIFIC VARIATION IN RESPONSES TO CLIMATE CHANGE

The existence of intra- and inter-specific variation in species’ distribution change in response to climate change indicates that the drivers of species’ distribution change vary both among species and within species over time. The results presented in this thesis can be drawn together to give an overall picture of how species’ distribution changes vary, and the environmental and species’ traits that drive this variation.

The most significant conclusion reached from analyses in this thesis is that species’ abundance trends were of over-riding importance in determining distribution change. In my analyses of empirical data (Chapter 3), range expansion was confined to the subset of species with stable or increasing abundance trends. The application of the SPEED model to project distribution change amongst southerly-distributed species (Chapter 5) provided a different approach to assessing the determinants of distribution change, and results emphasised the importance of population growth rate for distribution expansion. SPEED model projections demonstrated that increased population growth rates allowed species to overcome the limitations of low habitat availability and/or low dispersal ability. The importance of population growth for distribution expansion has been shown in other studies (e.g. Willis et al., 2009b), and the work presented here suggests that population growth is likely to be the most important determinant of whether or not a species expands its distribution area in response to climate change. Species’ abundance trends vary over time (Chapter 2), therefore understanding the drivers of population change will be necessary to identify which conservation strategies are required to improve abundance trends and facilitate species’ distribution expansion.

The second conclusion is that habitat availability is an important determinant of the rate of distribution expansion. Amongst those species which did show positive abundance trends, the rate of distribution expansion depended upon the species-specific habitat availability (Chapter 3). This substantiates the suggestion that the greater habitat availability of generalist compared to specialist species contributed to the faster rates of distribution expansion observed amongst generalist species (in the analysis of temporal variation in species’ distribution change; Chapter 2). Other studies have also made the inference that differences in habitat availability between generalist and specialist species contribute to observed differences in rates of distribution expansion (e.g. Warren et al., 2001, Davey et al., 2012) and my results provide evidence for their conclusions. Results of the SPEED model also identified habitat availability as an important determinant of distribution change, particularly at low population growth rates (Chapter 5). Thus

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increased habitat availability not only facilitated distribution spread but also helped to prevent distribution decline, because larger areas of habitat supported more individuals, which reduced the risk of population extinction.

It is clear that inter-specific variation in habitat availability contributes to the variation among species in rates of distribution expansion, and species’ habitat availability is also likely to vary over time as habitat is lost or restored. Furthermore it has been shown that species habitat

associations are likely to vary in response to environmental change. Butterfly species in Britain were shown to make use of a wider range of habitat types during warmer years, yet, despite an increase in average temperatures over time, the overall trend was towards narrower habitat breadths (Oliver et al., 2012). It was suggested that this narrowing of habitat breadths was due to habitat deterioration, and therefore there are likely to be multiple drivers of species’ habitat associations, which will in turn affect species’ habitat availability.

The third conclusion is that dispersal ability may not always be an important driver of species’ distribution change. Empirical analyses of the drivers of distribution change (Chapter 3), suggested that dispersal ability may not always be an important driver of species’ distribution change, despite some previous research suggesting that it may be (Warren et al., 2001, Anderson

et al., 2012) but supporting other studies showing little role for dispersal (Angert et al., 2011). It

may be that when studying dispersal ability alongside other potential explanatory variables, dispersal was relatively unimportant. This suggestion is supported by my SPEED modelling results, which showed that greater dispersal ability was only advantageous to range expansion when species’ population growth rate, habitat availability and/or climatic suitability were sufficiently high (Chapter 5). Thus if abundance trends and habitat availability are the primary limitations on distribution change, then dispersal ability may only be important when species have both sufficient habitat availability and positive abundance trends; these conditions were certainly not met for the majority of species in the empirical analyses in Chapter 3. Both the empirical analyses and the modelling results emphasise the importance of considering multiple aspects of species’ life history simultaneously when trying to understand or to project species’ distribution change in response to climate change (Huntley et al., 2010), and imply that the importance of species’ traits are likely to vary depending on the environmental conditions experienced.

The research in this thesis has improved our understanding of species distributional responses to climate change by highlighting that species responses are temporally inconsistent and that this inconsistency is related primarily to temporal variation in species’ abundance trends. Simulation modelling provided further insight and suggested that the importance of traits, such as habitat

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availability and dispersal ability, for distribution change are likely to vary depending upon population growth rate, which may help to explain why other studies have found that species’ traits had low explanatory power (e.g. Angert et al., 2011).