Most species on earth are rare, sometimes being restricted to very small areas of the world, and often occurring in very low numbers. This is well known. What is not so well known is why most species are rare. Work conducted by a team led by Dr David Bilton at Plymouth University, is helping to provide some answers to this question, and also helping us to understand how rare and common species may respond to future climate change.
There are three main ideas as to why related species show dramatically different range sizes; these relate to evolutionary age (i.e. how long a species has had to spread), dispersal limitation (i.e. how good a species is at moving to new places) and fundamental niche breadth (i.e. the range of environmental conditions a species can tolerate). Work at Plymouth University has evaluated the relative importance of these three factors amongst closely related rare and common species, and demonstrated that there is often a fundamental link between niche breadth, in the form of temperature tolerance, and range size. Put simply, how common or rare a species is can often be accurately predicted from its ability to cope with high and low temperatures. Species with narrower temperature tolerances are far more restricted, and it is these species which seem likely to be the most vulnerable to on-going climatic changes.
Dr Piero Calosi, working with Dr Bilton and Professor John Spicer, investigated tolerance to heat and cold in closely related species of European diving beetles which live in mountain streams, and vary dramatically in their geographical range size, from species restricted to single mountain systems in southern Europe to those which are found from the Mediterranean to northern Sweden. The ability to tolerate temperature extremes was a very good predictor of the geographical range size of species, far more so than how good these species are at flying from place to place, or how long ago they evolved. As with much of the biota of Europe, these beetles survived ice ages in southern Europe, moving north as the climate warmed 12,000 years ago. Present-day widespread species are those which have been able to retain their southern range boundaries, and expand north, something which requires relatively high tolerance to both heat and cold. Species with narrower temperature tolerances are far more restricted, and it is these species which seem likely to be the most vulnerable to ongoing climatic changes. As well as having limited temperature tolerance, these rare species also have limited ability to adjust their tolerance windows as temperatures change. Given that climate warming will also lead to a reduction in the availability of suitable aquatic habitats in Mediterranean areas, such restricted species would appear to be doubly threatened in the future.