Extinction Rates and Butterflies RATES OF POPULATION EXTINCTIONS IN BRITISH invertebrates have now been measured in several different ways [(1), “Comparative losses of British butterflies, birds, and plants and the global extinction crisis,” J.
A. Thomas et al., Reports, 19 March, p. 1879] and may have relevance to estimating global extinction rates (2). We (1) have used the rate of extirpation of species from the whole of the British Isles, measured over the 20th century from the British Red Data Book (RDB), whereas Thomas et al. Use distribution changes in the last 20 to 40 years measured in about 3000 map grid cells by 20,000 volunteers. There are notable consistencies and differences in the conclusions of these two approaches.
The extinction of species from the whole of the British Isles is likely to be relatively accurately recorded: Rare species are actively sought, and only one of the 43 species recorded as likely extinct in the RDB has since been rediscovered (with a low and local population). The national extinction rate per century ranges from
0. 4% overall for the 14,000 insect species covered in the RDB to over 5% for the 60 species of butterflies and 7% for the 40 species of Odonata (the two bestrecorded taxa).
Both studies find the rate of loss of selected invertebrate taxa to be roughly the same order of magnitude as the rate of loss of plants and birds. In both studies, butterflies have a notably higher rate of loss than plants or birds, which is not a recording artifact, because these three taxa are well studied.
Given the relatively high local extinction rates of butterflies recorded by these and other studies, we disagree with the conclusion of Thomas et al. That butterflies represent good indicators for losses of other taxa. Rather, Thomas et al.’s study supports our suggestion (2, 3) that butterflies (being mostly warmth-loving and herbivorous) are atypical invertebrates that are relatively sensitive to climatic fluctuations and thus give a potentially misleading guide to extinction rates and human impacts.
CLIVE HAMBLER AND MARTIN R. SPEIGHT Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK.
1. C. Hambler,M. R. Speight, Conserv. Biol. 10, 892 (1996).
2. C. Hambler, Conservation (Cambridge Univ. Press, Cambridge, 2004).
3. C. Hambler, M. R. Speight, Br.Wildlife 6, 137 (1995).
Response HAMBLER AND SPEIGHT SUGGEST THAT butterflies have experienced amplified extinction rates in Britain, and thus their widespread use as indicators of change in insects (1, 2) is inappropriate. We consider this argument to be flawed, because of an artifact of recording.
It is widely accepted that comparisons of the proportion of species believed to have become extinct in different taxonomic groups will be biased if the groups being compared experienced different levels of past recording (1, 3). This occurs because the early species lists for undersampled groups contain a disproportionately high representation of common widespread species (4), and it is the rare and local species in a taxon, which tended not to have been recorded in the first place, that are especially prone to extinction (1, 3). McKinney (5) quantified this artifact in six groups (mammals, birds, molluscs, crustaceans, insects, and marine invertebrates) and obtained a strong correlation between the proportion of species recorded as being globally extinct against the proportion of species that was estimated to have been discovered (r2 = 0.82). We can extend this analysis to different groups of British insects using, like Hambler and Speight, the British RDBs as the main data source (see figure).
The figure, which represents change in
9. 2% of all known British insect species plus spiders, shows a similar relationship to McKinney’s, indicating that for groups in which “only” 90% of species had been listed a century ago, recorded national extinction rates were less than half those of groups in which 100% of species had been known.
Given the rigor of early butterfly recording, their documented declines were not unusual.
Nor are British butterflies atypically thermophilous, as Hambler and Speight claim.
The immature, not adult, stages define climatic constraints on insects (6), and distribution maps show that higher proportions of aculeate Hymenoptera and Orthoptera species than butterflies are restricted to the warmest regions of Britain; moths and dragonflies are similar to butterflies, while staphilinid beetles and woodlice are less confined to warm spots (7). Furthermore, because of climate warming, those butterfly species that are thermophilous experienced population increases in Britain that frequently mitigated the effect of habitat degradation (8).
Only four of the ten most rapidly declining butterfly species could be classed as thermophilous: The majority include alpine species.
We are also surprised that Hambler and Speight consider phytophagous insects to be unduly sensitive to environmental change. This contradicts their earlier statements
(9) , with which we agree (6), that specialists, such as taxa inhabiting rotting trees, are more threatened; moreover, the (well-recorded) taxa with the highest reported extinction rates in Britain have different lifestyles: carnivorous aquatic (dragonflies) and social terrestrial (bumblebees). In theory, parasitic species are the most vulnerable of all to change
(10) . Parasitoids are too poorly described to assess critically, but social parasites of ants have a disproportionately high representation in RDBs (6).
In conclusion, we do not claim that butterflies are ideal indicators of other insect changes, but they appear to be suffi
Ciently representative to be employed usefully, due to their comprehensive recording levels, as the only invertebrate taxon for which it is possible to estimate rates of decline in many parts of the world (1, 2).
JEREMY A. THOMAS* AND RALPH T. CLARKE