It took a great deal of clever biological detective work see Ratcliffe, 1980 to link the decline of peregrine populations to the accumulation of these persistent pesticides up the food c
Trang 1ornithologists that something was seriously wrong with the
environment, but what? It took a great deal of clever biological
detective work (see Ratcliffe, 1980) to link the decline of
peregrine populations to the accumulation of these persistent
pesticides up the food chain, resulting in eggshell thinning, reproductive failure, and (in extreme cases) direct poisoning
of adult birds Although some populations have now recovered, signaling a recovery in environmental quality, the species is still missing from many parts of its former rangeFsome coastal populations in England, for instance Nobody knows why
Migratory Songbird Declines in North America
In North America, considerable concern is currently being expressed over widespread declines in summer migrant birds, particularly warblers Unlike the so-called amphibian decline, nobody questions the phenomenon; just like the amphibian decline, nobody really knows why it is happening There is no doubting the data; many species are indeed declining very quickly, in as clear an indication as one wants that something
is wrong with the environment, but what? Several possibilities exist, and they are unlikely to be mutually exclusive One ex-planation focuses on the destruction of tropical forests in the birds’ wintering areas Another suggestion is that there are other unknown problems there or on the migration routes
A third possibility is extensive habitat fragmentation and urbanization in the breeding forests of the eastern seaboard This human modification of the northeast forests markedly increases nest losses of migrant songbirds to jays, crows, cowbirds, and racoons, all species that thrive in the slipstream
of urban humans
Declines in Formerly Common Farmland Birds in Northwest Europe
In the intensively agricultural areas of northwest EuropeF over the whole of lowland England, for exampleFa whole raft
of formerly ‘‘common farmland birds’’ are also in steep de-cline (Tucker and Heath, 1994) They include skylarks (Alauda arvensis ), European tree sparrows (Passer montanus), corn buntings (Milaria calaudra), gray partridges (Perdix perdix), and song thrushes (Turdus philomelos) Here the problem is now
reasonably well understood, though many details remain unresolved Modern farming is so efficient and clean that there
is little for the birds to eat Weeds are killed with herbicides, which remove both seeds and rich sources of insects that feed
on the weeds The crop itself is sprayed to remove insects and
is harvested so efficiently that few seeds are spilled on the way Modern farms are biodiversity deserts, an indication of the power of people to squeeze nature to the margins while ap-parently maintaining a green and pleasant land If present trends continue, skylarks will be rare birds in Britain in
20 years
Species as Indicators of Biodiversity The Nature of the Problem
Common sense suggests that the known losses of plants and birds from European farmland will go hand-in-hand with much more poorly documented declines in many other, less familiar and cryptic taxa, from land snails to glowworms, and hoverflies to harvest spiders In other words, changes in the distribution and abundance of well-known groups should
Figure 4 The fate of 151 previously recorded populations of Edith’s
checkerspot butterfly, Euphydryas editha, in western North America.
The populations ranged from northern Mexico to southern Canada
and were visited by Camille Parmisan and other biologists between
1992 and 1996 Populations that had disappeared because of habitat
degradation (e.g., loss of usable host plants) were omitted from the
analysis Dividing the populations into five, evenly spaced latitudinal
bands between 301 N and 531 N (A) reveals that significantly more
southern populations have gone extinct than northern populations;
sites where previously recorded populations still exist were, on
average, 21 further north than sites where populations were extinct
Extinctions were also higher at lower altitudes (B) (n is the number
of populations in each latitudinal or altitudinal band) Both results are
consistent with the effects of global climate warming on the butterfly,
leading to a northward and upward shift in its geographical range
Reproduced with the permission of McMillan Journals Ltd, from
Nature 282 (1996), page 766.
Indicator Species 259