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Many species of amphibians have declined substantially in distribution or number of populations in the United States and globally, and a variety of anthropogenic and natural factors have been suggested as causal agents in these declines (e.g., Green, 1997a,b; Lannoo, 1998, 2003; Alford and Richards, 1999; Houlahan et al., 2000, 2001; Semlitsch, 2000; Alford et al., 2001; Lannoo et al., Introduction, Part One this volume). Evidence for the causal action of these agents derives from many types of sources, all of which are important in seeking to understand amphibian population declines. Clear causeand -effect relationships have been demonstrated in some cases, but the number of such cases is remarkably small given the number of species thought to be declining. More frequently , researchers have reported associations between putative causal agents and population declines. Although the weight of such correlative or circumstantial evidence is strong in many cases, it remains weak in others, usually due to the paucity of research. Nevertheless, such observations can be helpful by providing insight into the relative importance of causal factors and by suggesting which factors warrant the highest priority for further research or management action. The purpose of this analysis is to evaluate the relative frequency of factors implicated as adversely affecting amphibian populations in the U.S., and to identify taxonomic and geographic patterns for these effects. In this analysis I rely on informed investigators for information about causal factors and thus encompass the gamut of knowledge from well-documented studies to speculations. Such expert opinion may provide the best insight available about the factors responsible for declines . I have used a consistent method of evaluation for each species by reviewing earlier drafts of the accounts written for the U.S. species represented in this book. These species accounts are provided in a standardized format and are written by individuals who have substantial experience with the subject species. To the extent that sufficient information is provided in the accounts, I compile the factors implicated as adversely affecting populations and then assess the status of each of these species with regard to the stability of its distribution and number of populations. I also compare the frequency of adverse factors among the species relative to species status and region of the United States. A secondary purpose of this analysis is to identify taxonomic and geographic patterns in species status. In particular, I evaluate differences between the western United States and elsewhere, as amphibian declines are often thought to be greatest in the western United States (Hayes and Jennings, 1986; Corn, 2000; Lannoo et al., Introduction, Part One, this volume). Methods I reviewed earlier drafts of the species accounts for each amphibian native to the United States (this volume) for information to classify the species by status and to identify factors thought to be affecting its populations. The total number of species accounts available at the time was 267 (91 of 103 anurans and 176 of 185 caudates). The species accounts were written to address a standardized list of topics by authors with extensive experience with the subject species. The status of each species was determined relative to any change in its geographic distribution in the United States, or change in number of sites within this range. Status categories were Increasing, No Change, Net Extirpations, Major Decline, and Not Determined, as defined in Table EP-1. Information for this classification was provided primarily by the sections “Historical versus Current Distribution,” “Historical versus Current Abundance” and “Conservation.” Two categories (Net Extirpations and Major Decline) were combined for many analyses and are referred to as “adversely affected species.” Although not a specific topic required by the standardized format for the species accounts, most authors provided information on causal factors. A factor was included in the present analysis only if it was implicated by the authors (based on either known or suspected relationships) as affecting the persistence of populations, that is, it was associated with extirpations of whole populations or portions inhabiting a given area, or it was deemed a substantial threat to the persistence of populations. The factor was not included if it was indicated as affecting only individual health or mortality, such as moderate levels of disease or predation. More than one factor was implicated for many species. Factors were categorized according to descriptions in Table EP-2. For the three most commonly implicated factors (i.e., land use change, exotic species, and chemical contamination), the specific land use factors (Table EP-2), exotic taxa, and chemical...

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