Abstract
Effective management of wetland quantity and quality is crucial for effective conservation of declining amphibian populations. In particular, frogs and toads that employ aggregative breeding strategies may suffer negative population impacts in response to changes in availability of aquatic breeding habitat, including overabundance of suitable habitat, if density of conspecifics attending aggregations is positively correlated with reproductive success. Here we document such a positive relationship, potentially the first example of a component Allee effect in an anuran, in the critically endangered Houston toad (Bufo houstonensis). We assessed the relationship between mean yearly chorus size and reproductive success of males at the pond level using an information theoretic model selection approach and a two-sample t-test. The chosen model contained the single variable of mean yearly chorus size to predict probability of reproduction, as selected using the Akaike Information Criterion corrected for small sample size and Akaike weight. Mean chorus sizes were significantly higher among ponds exhibiting evidence of reproduction than in those that showed no evidence of reproduction. Our results suggest that chorusing alone is a poor proxy for inference of population stability and highlight a need for reassessment of widely-used amphibian monitoring protocols. Further, amphibian conservation efforts should account for potential Allee effects in order to optimize benefits and avoid underestimating critical population thresholds, particularly in species exhibiting rapid population declines.
Highlights
Wetland quantity and quality are a primary concern in conservation of amphibians that have an aquatic larval stage [1]
We examined whether there is a relationship between the mean number of calling male B. houstonensis attending choruses within a breeding season and evidence of reproductive activity at the pond level, and whether there is a minimum threshold of mean yearly chorus size required to observe reproduction
A total of 101 mean yearly chorus size (MCS) values were included in our analyses, of which 45 MCS values were 0
Summary
Wetland quantity and quality are a primary concern in conservation of amphibians that have an aquatic larval stage [1]. Draining of existing wetlands or replacement of ephemeral wetlands by more permanent impoundments (e.g. livestock ponds) may have negative impacts on amphibian populations [1]. Management strategies may call for construction of artificial wetlands to mitigate these negative impacts. Frogs and toads, employ aggregative breeding strategies which serve to dramatically increase population density at a highly local scale [4,5,6]. Given well-documented amphibian declines [7,8], the effects of increased breeding habitat availability on aggregation size and overall reproductive output of individual breeding sites merit greater scrutiny than has far been empirically documented. Smaller or less dense breeding aggregations may attract fewer females, thereby reducing mating probability for males attending smaller choruses, and may have subsequent negative population impacts. A reduction in mate-finding ability at low population density that has negative effects on individual fitness components and/or per capita population growth rate is an example of the Allee effect [9]
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