Exploring the mechanisms of change in species’ distribution: Garden warbler (Sylvia borin) as a case study

Background/Question/Methods The anticipated future increases in global surface temperatures are likely to have major impacts on the distribution of species. Predicting future species’ distributions is a key area of importance in ecological research, which is largely being addressed through the use of climate envelope models. While climate envelope models may indicate the broad direction of likely changes in distribution, they fail to incorporate non-climatic factors that are important determinants of species’ distributions within their current range, and incorporate neither population dynamics nor dispersal, therefore observed responses may differ greatly from these projections.  My recent results from simulation models demonstrate that where species are structured across an environmental gradient, a species’ range may lag behind its “climate envelope” and the rate of movement of a range may not remain constant throughout a period of climate change.  Garden warbler (Sylvia borin) is a long-distance afro-palearctic migrant passerine which has part of its breeding range in the UK.  The British breeding population has declined by more than 25% in the past 30 years, however this pattern is regionally variable, and in Scotland they have increased by 264% during the same time period, which represents a northwards range expansion. This study uses data from the Repeat Woodland Bird Survey (RWBS) collected from the same sites throughout the UK during two time periods, 1981-1988 and 2003-2004.  Garden warbler occupancy data from the two episodes have been converted to a single measure of colonisations and extinctions between the two time periods in order to investigate the roles of changing climate and habitat suitability in driving the recent range dynamics of this species. 

Results/Conclusions Results to date have shown that changes (relative to the starting point) in spring temperature, spring rainfall and low vegetation cover affect the probability of colonisation and extinction between the two time periods.  Increasing deer numbers can lead to reduced low vegetation cover, suggesting a potentially important role of grazing management in mitigating the impact of climate change on garden warbler populations.  The results demonstrate that the dynamics of this species’ range within the UK are being driven by both the shift in the climatic gradient and changes in woodland management.  I argue that there is an urgent need for more mechanistic based projections of species’ range dynamics that account for potential synergistic effects between different environmental drivers.