Much has been made recently of how different butterfly species are likely to adapt to habitat restoration and to climate change that opens up potentially new ranges for colonization. A considerable about of research is being undertaken to develop mathematical models that will help conservationist figure out which ones may benefit and which will be adversely affected. Wilson et al from the University of Exeter, Cornwall Campus, have been working on one such model for the silver spotted skipper, Hesperia comma, a relatively rare species in southern England where the team did its research.
The team developed a model that they hoped would be consistent with observed changes in the skipper’s range over the course of the past 18 years. Hesperia comma in England is a butterfly with very specific ecological needs; it is restricted to calcareous grasslands where it lays its eggs on short tufts of sheep’s fescue grass. Intensive agriculture has replaced the grazing that once maintained the skipper’s habitat; less that 20% of the original habitat remains. Moreover, rabbits that formerly helped maintain the grasslands in the absence of grazing were decimated by a disease called myxomatosis. By 1982 the species had dwindled to only about 70 populations covering a total land area of just over 2 square kilometers. Since 1982, rabbit populations have bounced back, and active conservation efforts have been undertaken to re-establish livestock grazing and to reintroduce H. comma to these improved habitats. At the same time, climate change appears to have opened up a number of new habitats that now exhibit microclimates warm enough to support the species.
Despite these improvements, the authors note, the butterfly expanded only into areas where it had previously been established; not into newly improved habitats or in areas where climate change had opened up new areas that would seem to be appropriate. Using the 18 years of available population data, the team created a mathematical model that they used to try to predict where new populations would be most likely to succeed. The model correctly predicted most of the observed population expansion, including the extinction of two populations introduced as part of conservation programs. The development of models like this for species of concern can be very useful to conservation of threatened butterfly species, the researchers explain. “Approaches similar to those described here can be used to model rates of expansion in different landscapes, to predict those species that will be able to shift their distributions in response to climate change, and those that will require active conservation management to do so.”
Many species – like H. comma – “may only be able to change their distributions in response to climate change in landscapes where there is sufﬁcient density of habitat to allow expansion.” The reason the butterfly isn’t expanding into the new seemingly suitable habitats, Wilson et al point out, is that – like many butterfly species that are threatened or in decline – H. comma has very specialized ecological requirements that go beyond simple habitat requirements. Even when climate change or habitat improvement would seem to provide appropriate new habitat, habitat fragmentation probably represents a major barrier to recolonization, the article notes. The same is likely to be true of many other specialized butterfly species, which will fail to thrive despite habitat improvement or climate change that, to all outward appearances, would seem to be ideal. Generalist butterflies are much more likely to benefit from both habitat restoration and global warming. Read the full article from Proceedings of the Royal Society B ((2009) 276, 1421–1427, doi:10.1098/rspb.2008.0724; published online 25 February 2009): Wilson et al 2009 Proc. R. Soc. B habitat fragmentation and range expansion