Effects of density on spacing patterns and habitat associations in the Neotropical Glassfrog Espadarana Prosoblepon

Background/Question/Methods Social behaviors are often context dependent, and behaviors observed at high population densities may be altered or disappear at low densities. Male Glassfrogs (Family: Centrolenidae) defend leafy territories along Neotropical streams where females deposit clutches of eggs on the vegetation. We quantified abundance and spacing of individual male glasfrogs on 4 permanent stream transects over 5 pre-decline years and 6 post-decline years in El Copé, Panama.  Anecdotally, we noticed that certain areas of the stream consistently had higher densities of males of the species Espadarana prosoblepon, suggesting either that males were selecting particular physical features of the environment, or perhaps that areas of high density offered some benefit in mate attraction or predator avoidance. In 2004, amphibians at this site experienced mass mortality and widespread population declines following the invasion of a pathogenic fungus.  E. prosoblepon abundance declined by 77%, but this species has persisted at this site, allowing us to compare spacing and habitat use at high and low densities. To document whether male frogs were associated with habitat features differently in high and low density years, we quantified habitat characteristics on 48 10-m stream reaches before and after the invasion of Bd.

Results/Conclusions  Pre-decline density across all transects was 0.034 ± 0.02 males/m and that declined to 0.019 ± 0.02 males/m post-decline. Nearest neighbor distance did not change with density, and averaged 0.692 ± 0.74 m across all transects and years. Post-decline the number of hotspots decreased from 30 to 20, but 18 of 20 (90%) post-decline hotspots were located in the same locations as pre-decline clusters, although comprised of all new individuals. To document whether male frogs were associated with habitat features differently in high and low density years, we quantified habitat characteristics on 48 10-m stream reaches before and after the invasion of Bd. Principle components analysis showed that males were strongly associated with the amount of leafy vegetation along the stream in both years. Post-decline stream substrate and hydrology increased our ability to predict male frog abundance. Because dispersion patterns are not affected by changes in population density, we conclude that male E. prosoblepon are selecting large scale habitat features that have not greatly changed in the 12 year study period. This is the first report of lek-like behavior or resource defense for the prolonged breeder E. prosoblepon. This study also reveals that adult frog spacing and locality stays consistent after Bd invaded.