Range Changes in British Butterflies: the Roles of Climate, Habitat and Dispersal in Patterns of Spread

Abstract

Habitat associations and demographic parameters of four generalist butterfly species resident to Great Britain, namely Pararge aegeria (speckled wood), Aphantopus hyperantus (ringlet), Pyronia tithonus (gatekeeper) and Melanargia galathea (marbled white), were investigated. These species were chosen because they have variable habitat associations and have all expanded their range in recent years. UKBMS transect data was used to generate species specific values for both intrinsic rates of increase, r, and mean density in occupied habitats, , for the four study species. Results indicate that three of the four species studied occur at significantly different densities across two or more of their preferred habitats. High variation in intrinsic rates of increase across all species studies was documented. Results were used to inform the accurate parameterisation of a dynamic model framework used to simulate present-day ranges of the four study species.

Recent spread of species were simulated using spatial dispersal models across a gridded landscape of Great Britain, where cell suitability is modified between 0-1 according to (1) habitat suitability, (2) climate suitability or (3) all cells are given an equal suitability of unity. Spread was simulated with almost equal degrees of success in models run on grids (1) and (2). Model simulations run on grid (3) resulted in poor model outcomes and over-simulation of species current range extent. This suggests that both habitat and climate play a role in observed present day distributions of the four study species. For species whose recent expansion could be simulated well using these models, the best-fit model for each species was run into the future to simulate potential future spread. Future simulations suggest that Melanargia galathea and Pyronia tithonus will expand their range by 15.3% and 7.8% respectively under present day habitat suitability between 2009 and 2060.

Field data was used to investigate local and regional patterns of temperature at three study sites along a north-south transect in England and relate this to phenology of a chalk grassland specialist butterfly, M. galathea, and its preferred nectar source, Centaurea scabiosa. Results indicate that mean maximum daily temperature was significantly different the local (variable aspect/topography) and regional scale. Locally, the highest temperatures were observed on south and south west facing slopes and coldest on north facing slopes. Regionally, the highest temperatures observed in the south and coldest in the north. This means insect and plant experience different environmental conditions depending on local or regional situation. There is evidence that heterogeneity in the local environment at each of the three study sites results in an extended flowering period of C. scabiosa, increasing the amount of time nectar is available to pollinating insects. Topographically variability could thus act as a buffer to phenological mismatch induced by future climate change and could be used as a reserve selection criterion for conservation organisations. There is also evidence that both timing and duration of the flowering period of C. scabiosa varies at both the local and regional scales. The timing of the flight period of M. galathea varied among years and sites likely in relation to variable macro and microclimate. This has implications for future translocation studies whereby individuals are moved from one area of the country to another and must be considered if translocations are to be successful.

This thesis has highlighted ecological processes occurring at both fine and broad spatial scales that must be considered if model predictions are to be robust. Future research must continue to recognise the importance of an individualistic approach to forecasting responses of species to environmental change.