Potential component Allee effects and their impact on wetland management in the conservation of endangered anurans

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.

Figure 1. Frequency of observation and reproductive success of four chorus size classes.

Mean number of Houston toad (Bufo houstonensis) males attending choruses calculated from nights on which at least one male was heard calling (CHOR, grey bars) and number of ponds exhibiting evidence of reproduction (black bars) for four chorus size classes. Proportion of ponds with evidence of reproduction goes up as CHOR increases, with all ponds in the largest size class having evidence for reproduction.

Figure 2. Probability of Houston toad (Bufo houstonensis) reproduction as a function of mean chorus size.

X = observed reproduction. Line represents probability of reproduction as predicted using the logistic regression model [P(REP) = e ^{(−2.43+3.29*MCS)}/1+e ^{(−2.43+3.29*MCS)}]. Probability of reproduction approaches 100% at mean yearly chorus size of 2.0 males.