ReviewThe amphibian decline crisis: A watershed for conservation biology?
Introduction
The discipline of conservation biology emerged specifically to understand, quantify and ameliorate the current biodiversity crisis (Soulé, 1986). Amphibians are playing an increasingly prominent role in this issue. The current era of research into amphibian declines followed the First World Congress of Herpetology in 1989 (Wake, 1991, Wake, 1998), though reports of declines go back at least 25 years before then (Pechmann and Wilbur, 1994). However, the Congress certainly precipitated an upsurge of interest as within a few years declines were reported for more than 500 amphibian species out of an estimated global total (at that time) of over 4000. A recent report from the IUCN’s Global Amphibian Assessment indicates that as many as a third of amphibian species, now estimated at over 5700, have undergone severe declines or extinction (Stuart et al., 2004) and that neotropical montane, stream-associated species are at particular risk. The declines have various likely causes, but there has been widespread controversy about their significance (Pechmann et al., 1991, Pechmann and Wilbur, 1994, Alford and Richards, 1999, Blaustein and Keisecker, 2002). In this review we: (1) briefly summarize research on causative factors (more comprehensive reviews on this aspect have been published elsewhere); (2) discuss the difficulties of extrapolation from experiments revealing possible causes of decline to demonstrating effects at the levels of populations and geographic ranges; and (3) consider how the study of amphibian declines relates to wider biodiversity issues, and the prospects for preventing continued amphibian declines. It is, however, important to be clear about how amphibian declines are defined (Green, 1997). In particular, we can identify declines in numbers of populations at the relatively large scale of biogeographic range, and fine-scale declines of specific populations in terms of overall size. Both are important but, as we shall see, the methods appropriate for investigating them can differ substantially.
Section snippets
Habitat destruction, agrochemicals and chemical pollution
Some reasons why amphibians decline have been understood for decades (Collins and Storfer, 2003). Temporal patterns of amphibian decreases reflect the major impact of habitat destruction in Europe during the mid-twentieth century, in contrast with more recent declines elsewhere (Houlahan et al., 2000). This conclusion has been contested, with the suggestion that global declines only began in the 1990s (Alford et al., 2001), but intensive arable farming starting in the mid-20th century has
Measuring effects at the distributional level
The overall significance of amphibian declines is manifest at the geographical range scale. Determining trends across a range requires an estimation of numbers of occupied sites at the landscape level (Green, 1997), usually with substantial resource implications due to the amount of effort involved. It is nevertheless essential because instability – or even declines – of local populations may not be reflected in overall changes at larger geographic scales. Green frog R. clamitans populations,
Amphibians and the wider biodiversity crisis
Within the context of the wider biodiversity crisis, two fundamental questions about amphibian declines arise: (1) Are amphibians particularly good indicators of a wider biodiversity malaise? (2) Are amphibians declining faster than other taxa? Since 1989 amphibians have been widely advocated as excellent ‘biological indicators’ or sensitive indicators of environmental health (e.g., Blaustein and Wake, 1990, Vitt et al., 1990, Wyman, 1990, Wake, 1991). These claims have been reiterated in more
Conclusions
What, then, should be the priorities for research into the amphibian decline phenomenon? In a thought-provoking essay, Caughley (1994) opined that conservation biologists would be unable to realise their goals of saving species from extinction unless empirical case studies of declining populations were underpinned by more theory. He argued that conservation biology is therefore progressing in two directions – enshrined within the ‘declining population paradigm’ and the ‘small population
Acknowledgements
We thank the many research students and fellows who have worked in our laboratories over past years, and English Nature, The Leverhulme Trust, The Darwin Initiative and the Natural Environment Research Council for financial support. We are also grateful to B. Schmidt, J. Gerlach and an anonymous review for constructive criticism of an earlier manuscript.
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