Species are responding to climate change by shifting their distributions polewards and/or uphill. However, the rates at which distributions are changing vary greatly among species. An understanding of how species’ distributions are changing, and what drives the rate of distribution change, is necessary in order to identify which conservation strategies have the potential to facilitate range shifts and to prevent population and species losses.
I studied the responses of southerly-distributed butterflies in Britain to four decades of climate change. I found that not only did the rates of species’ distribution area, northern range margin and abundance changes vary amongst species, but that rates of change also varied within species over time. Some of the variation in distribution change was explained by species’ abundance trends; species required stable or increasing abundances in order to expand their distribution areas. Once species were expanding their range, however, the rate of expansion was affected by the amount of suitable habitat available in the landscape. The application of a simulation model, SPEED, to project species’ distribution change supported these conclusions; all species modelled were capable of distribution expansion given sufficiently high population growth rate. Moreover, increased habitat availability allowed greater rates of distribution expansion.
The observed trend towards more negative abundance changes in recent years suggests that habitat quality has deteriorated. Results from this thesis showing that stable or increasing abundance trends are a prerequisite for distribution expansion imply that conservation strategies should focus on improving species’ abundance trends through increasing habitat quality. Results also support the notion that conservation efforts aiming to protect and restore as much natural habitat as possible, in order to facilitate species’ distribution expansion under climate change, are likely to be effective
