Anurans show a distinct dichotomy among the sources of energy used during larval growth and development, endotrophy and exotrophy. This distinction allows more exact definitions of direct development, ovoviviparity, viviparity, and related terms. Endotrophic larvae, whether as a non-hatched embryo or a free-swimming, non-feeding tadpole, gain immediate developmental nutrition solely from parental sources, most frequently from the yolk allotted to each egg during vitellogenesis. Exotrophic larvae are always free-swimming, feeding tadpoles and orally consume numerous sources of energy not derived from a parent. The morphology of exotrophic tadpoles is surveyed with an emphasis on oral structures. Speculative scenarios and hypotheses concerning the functions of the various morphologies and the relationship of trophic structures to the ecology of the tadpoles are offered. Larval morphology and behavior permit the recognition of six developmental guilds of endotrophic larvae and 18 ecomorphological guilds of exotrophic larvae; these guilds are fully characterized.

Guilds of Anuran Larvae: Relationships among Developmental Modes, Morphologies, and Habitats

Greater than 70% of the world's amphibian species are in decline. We propose that there is probably not a single cause for global amphibian declines and present a three-tiered hierarchical approach that addresses interactions among and between ultimate and proximate factors that contribute to amphibian declines. There are two immediate (proximate) causes of amphibian declines: death and decreased recruitment (reproductive failure). Although much attention has focused on death, few studies have addressed factors that contribute to declines as a result of failed recruitment. Further, a great deal of attention has focused on the role of pathogens in inducing diseases that cause death, but we suggest that pathogen success is profoundly affected by four other ultimate factors: atmospheric change, environmental pollutants, habitat modification and invasive species. Environmental pollutants arise as likely important factors in amphibian declines because they have realized potential to affect recruitment. Further, many studies have documented immunosuppressive effects of pesticides, suggesting a role for environmental contaminants in increased pathogen virulence and disease rates. Increased attention to recruitment and ultimate factors that interact with pathogens is important in addressing this global crisis.

/pdf/the-cause-of-global-amphibian-declines-a-developmental-mlzabcxp9m.pdf

The cause of global amphibian declines: a developmental endocrinologist's perspective

Acidic pond water may influence the reproduction of amphibians by causing direct mortality of embryos and larvae, and/or by disrupting trophic relationships between amphibians and other aquatic organisms. The embryo is the most sensitive stage of development and may abort soon after exposure to very low pH or eventually fail to hatch at a more moderate pH. Larvae are killed by disruption of Na and CI balance. The toxicity of pond water is governed by complex interactions of pH, temperature, and the concentrations of Al, Ca, and organic acids. The reduction in recruitment associated with acidification of ponds affects both the local distribution and abundance of amphibians. The relative importance of different mechanisms of acidification of amphibian breeding sites is unknown. Amphibian breeding ponds are generally small, low in buffering capacity, and darkly stained by humic substances, making it difficult to discriminate between the contribution of acid from atmospheric input and natural sources (such as sphagnum and organic acids). It is however quite clear that temporary ponds are extremely vulnerable to acidification by rain and that they require continued monitoring in order to detect future changes in biological and/or chemical characteristics.

The Influence of Acidic Pond Water on Amphibians: A Review

When species differ in their ability to cope with varying abiotic conditions, a change in an abiotic factor can potentially alter the outcome of biotic interactions. The purpose of this study was to examine this phenomenon. We tested the effects of an abiotic factor (pH) and two biotic factors (intraspecific and interspecific competition) on survival and growth to metamorphosis by larvae of two anurans (Hyla gratiosa and Hylafemoralis) in outdoor tanks. Two levels of pH (4.5 and 6.0) and three levels of density (0, 30, or 60 embryos per 580 L tank) of each species were arranged in a fractional factorial design and replicated three times. Larvae of H. gratiosa exhibited typical patterns of intraspecific density-dependent re- sponses: as density increased, survival and size at metamorphosis decreased and larval period increased. The interactions of the abiotic factor and the biotic factors were complex in this experiment. The addition of H. femoralis at high pH caused decreased survival and increased larval period of H. gratiosa, but at low pH these negative effects were not detected. Variation in pH had no effect on the size at metamorphosis of H. gratiosa. Interspecific competition from H. femoralis caused decreased size at metamorphosis of H. gratiosa only at the high initial densities. Some of the responses of H. femoralis were different than those of H. gratiosa. In this species, intraspecific competition affected only the size at metamorphosis. Low pH in- creased the survival of H. femoralis, but had no effect by itself on any other trait. Increased conspecific density and pH interacted only in their effect on the size at metamorphosis of H. femoralis. Interspecific competition resulted in decreased size at metamorphosis but only increased larval period at the high initial densities. Interspecific competition at the high density interacted with pH to increase survival and size at metamorphosis at low pH in contrast to those results at high pH. Overall, the larval period was the response most strongly affected in H. gratiosa while for H. femoralis the size at metamorphosis was the most strongly affected. Hyla gratiosa had more negative impact on H. femoralis than vice versa. This may be due to the larger body size of H. gratiosa. Low pH enabled H. femoralis to compete more effectively, because the negative impact of H. gratiosa was lessened. Even in a relatively simple simulation, complex interactions occurred between biotic and abiotic factors. This finding demonstrates the importance of multi-factor analysis in furthering our understanding of the structure of assemblages.

Effect of pH Variation of Interspecific Competition Between Two Species of Hylid Tadpoles

We examined the interactions of an abiotic factor (pH) and a biotic factor (density) on the survival and growth of two species of anuran larvae (Hyla gratiosa and Hyla femoralis) in outdoor tanks. Three levels of pH (4.3, 4.6, or 6.0) and three levels of density (0, 30 or 60 embryos) were arranged in a blocked design and replicated three times for Hyla gratiosa. At the end of this experiment the effects of pH (4.3, 4.6, or 6.0), density of H. femoralis (30 or 60), and prior use by H. gratiosa (at 0, 30, or 60 larvae per tank) on the survival and growth of H. femoralis, were examined. Higher density increased larval period and decreased size at metamorphosis of H. gratiosa. Lower pH decreased survival rate and also decreased size at metamorphosis. Body sodium concentrations were lowest at the low pH values. Lower pH increased the susceptibility of H. gratiosa tadpoles to the adverse effects of higher densities. For H. femoralis higher density decreased survival, increased larval period and decreased size at metamorphosis. Hyla femoralis also had lower survivorship at low pH and exhibited decreased size at metamorphosis. However, unlike the results with Hyla gratiosa, there were no interactive effects between pH and density for any of the life-history traits studied. The effect of previous colonization by H. gratiosa on H. femoralis survival was facilitative. Body sodium concentrations of H. femoralis were lowest at the highest pH value. Metamorphs of the same size had much lower levels of sodium in H. femoralis than H. gratiosa. In general, H. femoralis was less affected by pH variation than H. gratiosa. These results demonstrate that abiotic factors can interact strongly with biotic effects such as density and they suggest that interspecific interactions can be strongly modulated by the background abiotic environment.

Interaction of pH, density, and priority effects on the survivorship and growth of two species of hylid tadpoles

ABSTRAcr.-Ambystoma maculatum and Rana sylvatica larvae from three ponds in Marquette County, Michigan were raised in the laboratory at pH's 3, 4, 5 and the pond pH. At pH 3, all larvae died within 12 h after exposure. A. maculatum larvae reared at pH 4 and 5 had significantly slower rates of development and growth than larvae reared at pH > 5. The development of R. sylvatica larvae was not significantly different in any of the pH treatments. A survey of 55 county amphibian breeding ponds revealed that 42% of the ponds had a pH < 5.5 and thus, may be inhibiting development and growth of A. maculatum but not R. sylvatica. Based on alkalinity data, 57% of 42 ponds in 1984 were classified as sensitive or highly sensitive to further acidification by acid precipitation. Between May-July 1984, 20% of the A. maculatum breeding ponds dried prior to metamorphosis, producing a possible synergistic mortality effect from desiccation and low pH.

Pond Acidity and its Relationship to Larval Development of Ambystoma maculatum and Rana sylvatica in Upper Michigan

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Pond Acidity and its Relationship to Larval Development of Ambystoma maculatum and Rana sylvatica in Upper Michigan