The effect of urbanisation and local environmental heterogeneity on phenotypic variability of a tropical treefrog

Abstract

1. Urbanisation reduces species richness and change community composition. However, little is known on how the phenotype of organisms with low dispersal ability respond to environmental changes associated with urbanisation in fast urbanizing centres, such as those in the Global South.

2. Here, we tested how urbanisation rate, local environmental heterogeneity, land surface temperature, and spatial gradients affect phenotypic traits associated with dispersal, resource acquisition, and performance, namely: body size, head shape, and leg length of the Dwarf Treefrog (Dendropsophus nanus) using a space-for-time substitution approach.

3. We took linear measurements from 768 individuals in 21 ponds along an urban gradient in central Brazil. We also measured local environmental variables and summarized them using Hill-Smith Principal Component Analysis. The spatial arrangement of ponds at multiple scales was described using Moran Eigenvector Maps. Those variables were then entered into a Structural Equation Model to test their direct and indirect effects on the mean and coefficient of variation (CV) of phenotypic traits. Additionally, we calculated the Scaled Mass Index as a proxy for fitness and estimated the adaptive landscape for body size, size-free leg length, and head shape. We also tested for spatial autocorrelation in traits.

4. Body size decreased from the periphery to the urban centre, whereas CV of body size and head shape had the opposite pattern. Body size increased, whereas CV of body size and head shape decreased in man-made ponds. The CV of leg length decreased with increasing land surface temperature. The remaining traits were not affected by any predictor variable. None of the traits were spatially autocorrelated. Both body size and head shape were under weak directional selection, but in opposite directions.

5. Our results suggest that the lack of a clear spatial variation in phenotypic traits can be due to a weak selection, due to a recent, although intense, urbanisation process. In conclusion, eco-evolutionary dynamics in tropical cities seem to have a different pace compared to temperate ones. Our results can contribute to building urban ecological theory that explicitly includes city age, their development, growth rate, and history.