Blog by Dr Richard Bradbury, Head of Environmental and Research section, RSPB Centre for Conservation Science

How vulnerable are species to extremes in weather? This is an important question, given that increases in extreme weather are among the more obvious manifestations of climate change.

Previous research (see here and here) has shown that butterflies are sensitive to extreme weather, so we examined sensitivity of butterfly population dynamics to variation in weather conditions across their European geographic ranges. The resulting paper is the first from the PhD of our student Simon Mills, with his great team of supervisors at Sheffield and Reading Universities. Simon was able to make use of the fantastic long-term butterfly data-sets that have been collected by a veritable army of volunteers across Europe – his study spanned 35 years of data for 12 species, from over 900 sites, ranging from Finland to Spain.

By constructing population growth rate models that included all these data, Simon could examine how sensitivity to temperature and precipitation varied with life-stage and with location in a species’ range, controlling for strong density dependence effects.

We found that range edge populations were more sensitive to weather variation than those in the centre of species ranges, that precipitation events were as important as temperature events and that there were different sensitivities to extreme weather events at different times in the butterfly life cycle. This strongly suggests that we should expect to see geographic variation in the response to climate changes among different populations of the same species – perhaps not surprisingly, populations at the range edge will be most sensitive, so that is where we might expect to see the most rapid changes.

How do we use information like this to assess risk to species from climate change?

Getting our heads around climate risk is a focus for another of our PhD students, Chris Wheatley. His first PhD paper, again with his other supervisors – this time from York University and the BTO, tried to make sense of the slightly bewildering (at least to me!) array of methods that have been proposed to assess climate change risk to species. These different methods have arisen out of a range of contexts – some, for instance, in data-rich situations (eg for UK butterflies and birds) where lots is known about species trends, and others in more data-poor situations, where less is known about species trends. The big question is whether they are all equally valid approaches, in all situations. 

Photo: Brimstone – one of the species found to be more sensitive to extremes of weather at its range edges.

We compared 12 different methods, to see whether they consistently assign species to the same risk categories. Crucially, we also checked whether any of the methods actually perform well at identifying climate-threatened species, using historical data for British birds and butterflies to assign risks and using more recent data for validation. Again, the availability of such datasets is fantastic testament to all those volunteers who so diligently collect these data year-on-year. 

Chris found that the different vulnerability assessment methods are definitely not consistent with one another when considering the same species, so the different methods cannot be used interchangeably. The results of our validation provide some more support for trend-based rather than purely trait-based approaches, although further validation will be required as data become available.

So, with this knowledge, we were then given an interesting challenge from Natural England – to assess the climate risk to as many English species as we could. This took a massive effort, ably led by our friends at BTO, but involving many others – from University partners to those in the species societies who curate the data.

Following the results above, we used risk assessment methods that incorporated species trends and assessed over 3000 plants and animals.  Species were selected for their occurrence in England, the primary focus of the study, but climate change impacts were assessed across Great Britain, to widen their geographical relevance.

These species vary greatly how much data we have on them, so we employed two approaches – for all species, we used a basic risk assessment that compared projected future changes in potential range with recently observed changes. For a sub-set of 402 better studied species, we were able to conduct a more comprehensive assessment, including potentially confounding and exacerbating factors such as dispersal ability and habitat availability.

The basic assessment classified 21% of the 3000+ species at high risk and 6% at medium risk of range loss under a B1 climate change scenario. More species were classified as having a medium (16%) or high (38%) opportunity to potentially expand their distribution.  The more comprehensive assessment produced slightly more pessimistic results, classifying 35% of the 402 species at risk of range loss, while 42% may expand their range extent – but the overall balance is still for more species to benefit than lose out, in an English context.  Whilst the overall pattern was clear, confidence was generally low for individual species, with the exception of well-studied groups such as birds.

Photo: The balance of risk and opportunity varied between taxonomic groups and habitats, with upland habitats and bryophytes and vascular plants containing the greatest proportion of species at risk from climate change.

This is where it is important to link back to Simon’s study. England has a significant proportion of species at their poleward range limit; the balance of risks and opportunities from climate change may be different elsewhere. In addition, it’s important to point out that an assessment at the scale of England needs to be considered in the light of the wider population of these species. Some, like Dartford Warbler are projected to do well in England under climate change, and indeed are already doing so, but these increases at the poleward edge of their range could be offset by declines further south (see previous blogs here, here and here. This is why it makes it more important than ever to make sure we have a robust system of reserves and protected areas in which these species can settle and flourish as they move north.

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