The multidimensional (and contrasting) effects of environmental warming on a group of montane tropical lizards

Abstract

Abstract Mountains are cradles for biodiversity and crucibles for climate‐driven species loss, particularly for tropical ectotherms. Constriction on activity and amplified heat stress are two key sources of warming‐driven vulnerability in tropical montane ectotherms. These threats, however, might be counterbalanced if rising temperatures also release organisms from limits on activity induced by cold stress. We used biophysical modelling to estimate activity patterns and thermal stress under warming in a group of summit‐dwelling Anolis lizards (A. armouri and A. shrevei) endemic to the Caribbean island of Hispaniola. Activity is currently constrained by the prevalence of temperatures too cold for activity. Under warming, our models predict expanded hours of potential activity and reduced cold stress, especially under a high emission scenario. Crucially, release from cold stress does not come at the expense of heightened exposure to heat stress. This result arises from a steep mismatch between these species’ warm‐adapted ecology and the surprisingly cold environments they occupy. Yet, resilience in some dimensions belies vulnerability along others, particularly with regard to critical macrohabitat. We capitalized on a long‐term monitoring dataset to predict forest distributions under warming. Our models predict upslope shifts in montane cloud forests that may constrict the high‐elevation pine forests to which these lizards are inexorably linked. Warming‐driven macrohabitat loss can ‘pin’ the montane endemics into progressively shrinking ranges, especially since a rising cloud forest also facilitates upslope transport of a close relative, A. cybotes (a species associated with broadleaf forests). Many tropical ectotherms (including these anoles) are adapted to forest edges, a feature often associated with a relatively warm‐adapted ecophysiology. When such species are also found in cool environments, such as those found on mountaintops, warming‐amplified thermal stress is surprisingly limited. Therefore, the direct effects of warming on tropical ectotherms are quite broad, and can even include potential benefits to fitness‐based activities. Rising temperatures may often present a dual‐edged sword: warming simultaneously releases these organisms from constraints on activity while exposing them to other threats. Whether due to the direct or indirect effects of climate warming, exceptional vulnerability may indeed reside where biodiversity is highest.