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33 Amphibian Ecotoxicology
- University of California Press
- Chapter
- Additional Information
The imperiled status of numerous amphibian species worldwide suggests that current research efforts need to take new, biologically relevant directions in order to understand the influence of chemical contamination. A recent summary of the current state of understanding concerning declining amphibians indicates that airborne contaminants are important but that “existing test protocols might be inappropriate” to evaluate their influence (Wake, 1998). We advocate adopting several approaches, experimental designs, and analyses that will promote a better understanding of the effects that chemicals can have on individuals, populations, and communities. We also provide examples of these approaches and the types of hypotheses that could be tested. Further, we attempt to show how the results of these approaches pertain to population and community regulation, and thus, how these results may be linked to conservation efforts. Incorporating Realism into Experiments Future research should incorporate greater diversity into studies (e.g., more species, genetic variation within and among geographic populations, and ontogenetic effects) and should consider the effects on multiple life history stages. One of the primary weaknesses in past studies has been the use of non-native amphibian species (e.g., African clawed frogs [Xenopus laevis ]) or broadly distributed species (e.g., northern leopard frogs [Rana pipiens]) with little consideration to local chemical contamination or specific environmental problems. Although we understand why such studies were initially useful and efficient, targeting ecologically relevant species is now more valuable for solving conservation problems. For example, direct comparisons of larvae of species with different feeding modes (e.g., planktivores versus detritivores) could increase our understanding of how morphology and behavior influence susceptibility . Chemicals that quickly adhere to sediments, for instance, may affect only detritivores whereas chemicals taken up by algae would primarily affect planktivores. Future research should also focus on the diversity of life modes and the rates of development, such as comparing species with direct versus indirect development or short versus long larval periods. A short period of larval development in a contaminated environment would reduce exposure. Species with a high degree of plasticity in length of the larval period may be better able to adapt to contaminated environments by minimizing their exposure as larvae (but see Larson, 1998). Further , we know little about the chemical susceptibility of species with narrow versus wide distributions, which may reflect evolutionary history or other ecological adaptations. Such a comparison might yield insight concerning declining species or endemic species that have small or restricted ranges that may make them particularly vulnerable to environmental stressors. A study comparing the susceptibility of multiple species tested against a well-known phylogeny (e.g., Ranidae) would aid our understanding of the evolution of chemical tolerance. We could evaluate important evolutionary considerations such as if declining species share common ancestors or whether unaffected species share common features enabling them to persist in the presence of higher levels of contamination. Thus, by incorporating the diverse features that taxa possess into toxicology studies, we can achieve a better understanding of how chemical exposure affects or is affected by morphology, physiology , behavior, ecology, and evolutionary history. One of the most interesting features of many amphibian species is their complex life cycle. Unfortunately, this added complexity necessitates understanding the chemical effects at several stages (e.g., egg, larva, and metamorphic individuals) and usually in completely different environments (e.g., aquatic versus terrestrial). It is likely that for some contaminants only one environment, and hence only one stage, is affected , thereby making single-stage studies of less practical use. Studies that directly compare the susceptibility of aquatic larvae to post-metamorphic terrestrial juveniles or adults have the benefit of elucidating the relative importance of aquatic versus terrestrial pollution. Further, such studies may uncover correlations in responses (phenotypic or genetic) between stages that can enhance our understanding of the differential sensitivity of species or if tolerance to chemicals is constrained or promoted. In addition, studying the ontogenetic changes in susceptibility may increase our understanding of the effects that the timing of chemical application has on a species. For example, pulsed applications of herbicides and insecticides in an agricultural landscape might coincide with particular amphibian developmental stages. Finally, examining carryover 241 TH I RTY-TH R E E Amphibian Ecotoxicology RAYMOND D. SEMLITSCH AND CHRISTINE M. BRIDGES effects of chemical exposure from one life stage to another (e.g., tadpole to adult) across seasons can help reveal detrimental effects that are not apparent initially but are expressed in a later stage or at a later time. For instance, there is little known...