The wider implications of amphibian population declines

Since the herpetological community first became aware of them in 1989, the sudden and catastrophic declines that have occurred among amphibian populations in many parts of the world have become the focus of intense research (DeWeerdt, 2001). What messages are there in the results of this research for the wider conservation community? Many amphibians have declined, along with species belonging to other taxa, as the direct result of habitat destruction, degradation and change. While it is important to monitor such events, the primary focus of the Declining Amphibian Populations Task Force (DAPTF) is to investigate the numerous and wholly unexpected declines that have occurred within national parks, nature reserves and other protected areas, where habitat loss should not be a factor. The most obvious implication of these declines is that they raise serious doubts about the effectiveness of protected areas as a means for conserving biodiversity. As David Wake, one of the founders of the DAPTF, put it, 'putting a fence around biodiversity just isn't working'. There have been two recurring features of amphibian declines in protected areas. Firstly, many have been very sudden, with populations disappearing over a period of 1 or 2 years. Secondly, while some amphibian species have been affected, others have not. These two features have helped to foster the implicit assumption that there is a single, globally acting cause for amphibian declines that affects only certain kinds of amphibian species. This concept of a 'smoking gun' has stimulated a great deal of innovative and exciting research into potential causes, notably increased UV-B radiation, climate change, pollution and disease. However, it is becoming increasingly clear that, while all these factors have played a part, to varying degrees in specific declines, no single factor can be held responsible for all amphibian declines. Indeed, there is increasing evidence that amphibian declines are caused by complex synergistic interactions between a number of causal factors. In a very recent study, Kiesecker et al. (2001) present evidence that, in the western USA, climate change and increased UV-B radiation have acted together to create circumstances in which amphibians are more likely to succumb to a pathogenic fungus. Another implication of amphibian declines, therefore, is that the causes of extinction are often not simple or obvious, and that we should not assume that similar phenomena have a common cause. By focusing their attention on amphibians, there is a danger that researchers, like those working within the DAPTF, may come to assume that they are looking at phenomena that are unique to amphibians. This is clearly not the case. All the factors so far identified as being harmful to amphibians, such as UV-B, climate change and pollution, are harmful to other taxa. While the diseases that are harmful to amphibians, such as the fungal disease chytridiomycosis which is affecting amphibians throughout the world, is specific to amphibians, it is becoming increasingly clear that disease is a major threat to many wildlife populations. To give but two very recent examples, populations of vultures in India have been all but wiped out in the last 2 years, and a disease is currently sweeping through California's oak trees. Why wildlife populations seem to be becoming more susceptible to infectious disease in one of the major questions currently facing conservation biologists (Carey, 2000).