One of the outstanding questions in urban evolutionary biology is whether there are universal or predictable responses to urbanization, and if so, what factors influence urban tolerance. As small-bodied ectotherms, reptiles are particularly sensitive to many of the challenges of urban life. For example, reptiles appear to be more sensitive to habitat fragmentation than birds, mammals, or amphibians. In their recent publication, Tim Doherty and colleagues combined these two ideas to ask if there are any global patterns in reptile responses to anthropogenic habitat modification. Although they did not explicitly address urbanization, perhaps the patterns they observe in response to other forms of anthropogenic habitat modification (e.g., mining, agriculture, logging, plantations) can inform our understanding of reptile responses to urban habitat modification.
Using a meta-analysis approach, they surveyed literature from 1981-2018, compiling 56 studies that reported anthropogenic habitat effects on abundance. They then calculated standardized mean differences in abundance and tested to see if responses were related to a suite of ecological and organismal characters: body size, clutch size, reproductive mode, habitat specialization, range size, disturbance type, vegetation, and climate. Within this framework, they predicted that species more sensitive to anthropogenic habitat modification would: (1) be larger-bodied, (2) lay smaller clutches, (3) be oviparous, (4) have smaller range sizes, and (5) be habitat specialists. This list of traits might sound familiar to you — these same traits are often investigated with respect to urban tolerance, for example in mammals and Anolis lizards.
Their meta-analysis resulted in a whopping 815 effect sizes calculated for 376 species from across the globe. In general, the effect of anthropogenic habitat modification on reptile abundance was predominantly negative, although there was substantial variation among species. Among the taxa responding less negatively to anthropogenic habitat in the figure below, you might notice some familiar urban friends: Dactyloidae (anole lizards) and Gekkonidae (e.g., your friendly house gecko).
And one group stood out as responding positively to anthropogenic disturbance: Phrynosomatidae, which includes the spiny lizards (Sceloporus) common to disturbed and urban environments of the West Coast of North America as well as side-blotched lizards (Uta) an horned lizards (Phrynosoma) that appear to thrive in anthropogenic conditions. One possible explanation for this result proposed by the authors is that Phrynosomatids tend to occupy arid habitats, which may resemble human-modified environments in both climate and structural habitat, making the transition to anthropogenic environments less drastic compared to other taxa.
When the authors examined what traits were associated with negative responses to anthropogenic habitat modification, they found that species with small range sizes and small clutch sizes responded more negatively, as predicted. Somewhat surprisingly, other attributes that are often considered important such as body size, reproductive mode, specialization, and climatic preferences did not significantly influence responses. Moreover, unlike in Anolis lizards, the authors found no phylogenetic signal in tolerance of anthropogenic disturbance across their global dataset of reptiles.Together, these findings suggest that characteristics of different anthropogenic disturbance types are more important than evolutionary relatedness or local habitat features (e.g., climate).
Overall, this analysis demonstrates that reptiles are extremely sensitive to anthropogenic habitat modification of all sorts, which in their words is “a key cause of reptile population declines”. Yet a unifying set of traits underlying this sensitivity did not emerge, highlighting the difficulty in developing a trait-based predictive framework for species responses to anthropogenic habitat change.
Featured header image: Sceloporus lizard, photo by Kristin Winchell (CC-BY-NC)
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