258 Indicator Species Figure The migration of trees across North America and Europe after the end of the last glaciation, revealed by pollen remains in lake sediments and peat (A) The spread of oaks in North America, with radiocarbon ages in thousands of years (contours) and the present range of the genus (shaded) (B) Estimates of the overall rates of spread of trees on two continents, based on data of the type shown in part A Reproduced from Williamson M (1996) Biological Invasions London: Chapman & Hall storms), and even locally cooler conditions All these complex changes should show up in changes in the distributions and abundances of organisms They Species are proving to be extremely sensitive indicators of contemporary climate change, where historical records allow decent reconstruction of former and current distributions Populations of Edith’s checkerspot butterfly Euphydryas editha are disappearing from southern California and northern Mexico, at the current southern end of its distribution, and from more lowland sites; sites where previously recorded populations still exist are on average 21 further north than sites where populations went extinct (Figure 4) These are exactly the changes we would expect in a warming world Twenty years ago in northwest Europe, little egrets Egretta garzetta (small white herons) used to be rare visitors from the Mediterranean Now they are breeding in northern France and southern England in an astonishing expansion of range Populations of many other European birds, butterflies, and other organisms are spreading north at the present time, as the climate warms Of course, none of this tells us whether the climate change that is certainly happening is ‘‘natural’’Fit could have happened anyway and may have nothing to with anthropogenically produced greenhouse gassesFor whether it is indeed due to human activities Using species as indicators of climate change tells us unequivocally that the earth’s climate is changing, but so does the mercury in the thermometer What neither tells us is why, and no end of work on species as indicators will solve that dilemma As we have already seen, this situation is not unique to climate change It generally holds whenever we use species as indicators of the state of the environment Indicator species can tell us whether an environment is, or is not, changing They not tell us why the changes are taking place That almost always requires additional detective work, although knowledge of an organism’s biology will frequently provide valuable clues Three examples, using birds as indicators, illustrate the problem in more detail Birds as Indicators of Large-Scale Environmental Changes Birds are widely used indicators, because in Europe, North America, and other parts of the world where there are large armies of amateur bird watchers their populations and distributions have been recorded well enough, for long enough, to reveal major environmental trends Peregrine Falcons and DDT The catastrophic collapse of peregrine falcon Falco peregrinus populations throughout the northern hemisphere in the 1950s signaled widespread contamination of the environment by chlorinated hydrocarbon insecticides, first DDT, then other compounds such as aldrin and dieldrin The total, and rapid, disappearance of these dramatic birds signaled to