Global amphibian declines: sorting the hypotheses
TL;DR: More studies are needed to connect the suspected mechanisms underlying both classes of hypotheses with quantitative changes in amphibian population sizes and species numbers, and to identify the hypotheses and conditions under which the various causes operate alone or together.
Abstract: Reports of malformed amphibians and global amphibian declines have led to public concern, particularly because amphibians are thought to be indicator species of overall envi- ronmental health. The topic also draws scientific attention because there is no obvious, simple answer to the question of what is causing amphi- bian declines? Complex interactions of several anthropogenic factors are probably at work, and understanding amphibian declines may thus serve as a model for understanding species declines in general. While we have fewer answers than we would like, there are six leading hypotheses that we sort into two classes. For class I hypotheses, alien species, over-exploitation and land use change, we have a good understanding of the ecological mechanisms underlying declines; these causes have affected amphibian populations negatively for more than a century. However, the question remains as to whether the magnitude of these negative effects increased in the 1980s, as scientists began to notice a global decline of amphibians. Further, remedies for these problems are not simple. For class II hypotheses, global change (including UV radiation and global climate change), contaminants and emerging infectious diseases we have a poor, but improving under- standing of how each might cause declines. Class II factors involve complex and subtle mechanistic underpinnings, with probable interactions among multiple ecological and evolutionary variables. They may also interact with class I hypotheses. Suspected mechanisms associated with class II hypotheses are relatively recent, dating from at least the middle of the 20th century. Did these causes act independently or in concert with pre- existing negative forces of class I hypotheses to increase the rate of amphibian declines to a level that drew global attention? We need more studies that connect the suspected mechanisms underlying both classes of hypotheses with quantitative changes in amphibian population sizes and species numbers. An important step forward in this task is clari- fying the hypotheses and conditions under which the various causes operate alone or together.
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American Museum of Natural History1, Columbia University2, University of Hamburg3, Sao Paulo State University4, University of Richmond5, University of the Western Cape6, Natural History Museum7, University of Texas at Arlington8, Yahoo!9, Florida Fish and Wildlife Conservation Commission10, California Academy of Sciences11, University of Michigan12, National University of Colombia13, McGill University14
TL;DR: A new taxonomy of living amphibians is proposed to correct the deficiencies of the old one, based on the largest phylogenetic analysis of living Amphibia so far accomplished, and many subsidiary taxa are demonstrated to be nonmonophyletic.
Abstract: The evidentiary basis of the currently accepted classification of living amphibians is discussed and shown not to warrant the degree of authority conferred on it by use and tradition. A new taxonomy of living amphibians is proposed to correct the deficiencies of the old one. This new taxonomy is based on the largest phylogenetic analysis of living Amphibia so far accomplished. We combined the comparative anatomical character evidence of Haas (2003) with DNA sequences from the mitochondrial transcription unit H1 (12S and 16S ribosomal RNA and tRNAValine genes, ≈ 2,400 bp of mitochondrial sequences) and the nuclear genes histone H3, rhodopsin, tyrosinase, and seven in absentia, and the large ribosomal subunit 28S (≈ 2,300 bp of nuclear sequences; ca. 1.8 million base pairs; x = 3.7 kb/terminal). The dataset includes 532 terminals sampled from 522 species representative of the global diversity of amphibians as well as seven of the closest living relatives of amphibians for outgroup comparisons. The...
1,994 citations
Cites background from "Global amphibian declines: sorting ..."
...…including habitat loss and fragmentation (Green, 2005; Halliday, 2005) but also possibly due to global environmental changes (Donnelly and Crump, 1998; Blaustein and Kiesecker, 2002; Heyer, 2003; Licht, 2003) and such proximate causes as emerging infectious diseases (Collins and Storfer, 2003)....
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01 Jan 2004
896 citations
TL;DR: In this article, the authors explore the relationship between current European distributions of amphibian and reptile species and observed climate, and project species potential distributions into the future by quantifying the magnitude and direction of modelled distributional shifts for every species.
Abstract: Aim We explore the relationship between current European distributions of amphibian and reptile species and observed climate, and project species potential distributions into the future. Potential impacts of climate warming are assessed by quantifying the magnitude and direction of modelled distributional shifts for every species. In particular we ask, first, what proportion of amphibian and reptile species are projected to lose and gain suitable climate space in the future? Secondly, do species projections vary according to taxonomic, spatial or environmental properties? And thirdly, what climate factors might be driving projections of loss or gain in suitable environments for species?
895 citations
TL;DR: There is little evidence to suggest that they are more effective than other taxa as surrogate measures of biodiversity or habitat quality, and major challenges remain in extrapolating from experimental data to population level effects and in developing methodologies that will provide unbiased descriptions of amphibian population dynamics.
716 citations
Cites background or result from "Global amphibian declines: sorting ..."
...These claims have been reiterated in more recent literature (e.g., Collins and Storfer, 2003; Storfer, 2003; Blaustein et al., 2003)....
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...Some reasons why amphibians decline have been understood for decades (Collins and Storfer, 2003)....
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...Unlike the effects of habitat change that have been known for at least 100 years, our understanding of the effects of chemicals on amphibians is comparatively recent (Collins and Storfer, 2003)....
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TL;DR: In this article, the authors show that climate variability is a dominant factor affecting large wildfires in the western United States, an observation supported by palaeoecological data on charcoal in lake sediments and reconstructions from fire-scarred trees.
Abstract: Climatic variability is a dominant factor affecting large wildfires in the western United States, an observation supported by palaeoecological data on charcoal in lake sediments and reconstructions from fire-scarred trees. Although current fire management focuses on fuel reductions to bring fuel loadings back to their historical ranges, at the regional scale extreme fire weather is still the dominant influence on area burned and fire severity. Current forecasting tools are limited to short-term predictions of fire weather, but increased understanding of large-scale oceanic and atmospheric patterns in the Pacific Ocean (e.g., El NiSouthern Oscillation, Pacific Decadal Oscillation) may improve our ability to predict climatic variability at seasonal to annual leads. Associations between these quasi-periodic patterns and fire occurrence, though evident in some regions, have been difficult to establish in others. Increased temperature in the future will likely extend fire seasons throughout the western United States, with more fires occurring earlier and later than is currently typical, and will increase the total area burned in some regions. If climatic change increases the amplitude and duration of extreme fire weather, we can expect significant changes in the distribution and abundance of dominant plant species in some ecosystems, which would thus affect habitat of some sensitive plant and animal species. Some species that are sensitive to fire may decline, whereas the distribution and abundance of species favored by fire may be enhanced. The effects of climatic change will partially depend on the extent to which resource management modifies vegetation structure and fuels.
674 citations
Cites background from "Global amphibian declines: sorting ..."
...For example, amphibian declines are of particular concern to the conservation community, though a direct relationship with climatic change has been difficult to identify (Carey & Alexander 2003; Collins & Storfer 2003)....
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References
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TL;DR: A review of the ecological impacts of recent climate change exposes a coherent pattern of ecological change across systems, from polar terrestrial to tropical marine environments.
Abstract: There is now ample evidence of the ecological impacts of recent climate change, from polar terrestrial to tropical marine environments. The responses of both flora and fauna span an array of ecosystems and organizational hierarchies, from the species to the community levels. Despite continued uncertainty as to community and ecosystem trajectories under global change, our review exposes a coherent pattern of ecological change across systems. Although we are only at an early stage in the projected trends of global warming, ecological responses to recent climate change are already clearly visible.
9,369 citations
"Global amphibian declines: sorting ..." refers background in this paper
...Global change hypotheses are inherently complex, impacting individuals and populations either directly or indirectly, and perhaps taking years or decades to be manifest (Walther et al ., 2002)....
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TL;DR: These phenomena have two major biological implications: many wildlife species are reservoirs of pathogens that threaten domestic animal and human health; second, wildlife EIDs pose a substantial threat to the conservation of global biodiversity.
Abstract: Emerging infectious diseases (EIDs) of free-living wild animals can be classified into three major groups on the basis of key epizootiological criteria: (i) EIDs associated with “spill-over” from domestic animals to wildlife populations living in proximity; (ii) EIDs related directly to human intervention, via host or parasite translocations; and (iii) EIDs with no overt human or domestic animal involvement. These phenomena have two major biological implications: first, many wildlife species are reservoirs of pathogens that threaten domestic animal and human health; second, wildlife EIDs pose a substantial threat to the conservation of global biodiversity.
3,757 citations
"Global amphibian declines: sorting ..." refers background in this paper
...Emerging infectious diseases are diseases that are newly recognized, newly appeared in a population or rapidly increasing in incidence, virulence or geographical range (Daszak et al ., 2000, 2003)....
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...…typing showed only five variable nucleotides of 5918 total bases at 10 loci among 32 globally distributed chytrid strains and therefore has likely spread recently, supporting the argument that amphibian chytridiomycosis is an emerging disease (Daszak et al ., 1999, 2000; Morehouse et al ., 2003)....
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TL;DR: The recently completed research program (TREES) employing the global imaging capabilities of Earth-observing satellites provides updated information on the status of the world's humid tropical forest cover, indicating that the global net rate of change in forest cover for the humid tropics is 23% lower than the generally accepted rate.
Abstract: A recently completed research program (TREES) employing the global imaging capabilities of Earth-observing satellites provides updated information on the status of the world's humid tropical forest cover. Between 1990 and 1997, 5.8 ± 1.4 million hectares of humid tropical forest were lost each year, with a further 2.3 ± 0.7 million hectares of forest visibly degraded. These figures indicate that the global net rate of change in forest cover for the humid tropics is 23% lower than the generally accepted rate. This result affects the calculation of carbon fluxes in the global budget and means that the terrestrial sink is smaller than previously inferred.
1,961 citations
"Global amphibian declines: sorting ..." refers background in this paper
...In this issue, Kats & Ferrer show that the R. muscosa case is one example of many where a non-native species has led to the decline of native amphibians....
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TL;DR: Experimental data support the conclusion that cutaneous chytridiomycosis is a fatal disease of anurans, and it is hypothesize that it is the proximate cause of these recent amphibian declines.
Abstract: Epidermal changes caused by a chytridiomycete fungus (Chytridiomycota; Chytridiales) were found in sick and dead adult anurans collected from montane rain forests in Queensland (Australia) and Panama during mass mortality events associated with significant population declines. We also have found this new disease associated with morbidity and mortality in wild and captive anurans from additional locations in Australia and Central America. This is the first report of parasitism of a vertebrate by a member of the phylum Chytridiomycota. Experimental data support the conclusion that cutaneous chytridiomycosis is a fatal disease of anurans, and we hypothesize that it is the proximate cause of these recent amphibian declines.
1,872 citations
"Global amphibian declines: sorting ..." refers background in this paper
...The distinctive biology of the chytrid fungus, Batrachochytrium dendrobatidis , is highlighted by evidence that, unlike most chytrids, B. dendrobatidis is diploid and the first described vertebrate pathogen in the phylum Chytridiomycota (Berger et al ., 1998)....
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TL;DR: There are three major causes of global environmental change: increasing carbon dioxide in the atmosphere, alterations in the biogeochemistry of the global nitrogen cycle, and ongoing land use/land cover change as mentioned in this paper.
Abstract: While ecologists involved in management or policy often are advised to learn to deal with uncertainty, there are a number of components of global environmental change of which we are certain–certain that they are going on, and certain that they are human—caused. Some of these are largely ecological changes, and all have important ecological consequences. Three of the well—documented global changes are: increasing concentrations of carbon dioxide in the atmosphere; alterations in the biogeochemistry of the global nitrogen cycle; and ongoing land use/land cover change. Human activity–now primarily fossil fuel combustion– has increased carbon dioxide concentrations from °280 to 355 mL/L since 1800; the increase is unique, at least in the past 160 000 yr, and several lines of evidence demonstrate unequivocally that it is human—caused. This increase is likely to have climatic consequences–and certainly it has direct effects on biota in all Earth's terrestrial ecosystems. The global nitrogen cycle has been altered by human activity to such an extent that more nitrogen is fixed annually by humanity (primarily for nitrogen fertilizer, also by legume crops and as a by product of fossil fuel combustion) than by all natural pathways combined. This added nitrogen alters the chemistry of the atmosphere and of aquatic ecosystems, contributes to eutrophiction of the biosphere, and has substantial regional effects on biological diversity in the most affected areas. Finally, human land use/land cover change has transformed one—their to one—half of Earth's ice—free surface. This in and of itself probably represents the most important component of global change now and will for some decades to come; it has profound effects on biological diversity on land and on ecosystems downwind and downstream of affected areas. Overall, any clear dichotomy between pristine ecosystems and human—altered areas that may have existed in the past has vanished, and ecological research should account for this reality. These three and other equally certain components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused in turn by the extraordinary growth in size and resource use of the human population. On a broad scale, there is little uncertainty about any of these components of change or their causes. However, much of the public believes the causes–even the existence–of global change to be uncertain and contentious topics. By speaking out effectively, we can help to shift the focus of public discussion towards what can and should be done about global environmental change.
1,781 citations
"Global amphibian declines: sorting ..." refers background in this paper
...They occur together or separately, and like Vitousek (1994) , we will label both ‘land use change’....
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...…atmospheric concentrations of gaseous nitrous oxide, widespread distribution of synthetic organic compounds, altered biogeochemistry of global element cycles, harvesting of natural populations by humans, land use/ land cover change and biological invasions by non-native species (Vitousek, 1994)....
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...They occur together or separately, and like Vitousek (1994), we will label both ‘land use change’....
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...... UV and chemicals) Examples of global change include increasing concentrations of carbon dioxide in the atmosphere, increased nitrogen fixation, increased atmospheric concentrations of gaseous nitrous oxide, widespread distribution of synthetic organic compounds, altered biogeochemistry of global element cycles, harvesting of natural populations by humans, land use/l and cover change and biological invasions by non-native species ( Vitousek, ......
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