Elevational and local climate variability predicts thermal breadth of mountain tropical tadpoles

The climate variability hypothesis posits that increased environmental
thermal variation should promote species with broader thermal tolerance
breadths, while stable environments should promote thermal specialists.
This hypothesis has been tested on large spatial scales, such as latitude
and elevation, but less so on smaller scales which reflect the experienced
microclimate. Here, we estimated thermal tolerance limits of 75 species of
amphibian tadpoles from an aseasonal tropical mountain range of the
Ecuadorian Andes, distributed along a 3500 m elevational range, to test
the climatic variability hypothesis at a large (elevation) and a small
(microhabitat) scales. We show how species from less variable thermal
habitats, such as lowlands and those restricted to streams, exhibit
narrower thermal tolerance breadths than highland and pond-dwelling
species respectively. Interestingly, while broader thermal tolerance
breadths at large scales are driven by higher cold tolerance variation
(heat-invariant hypothesis), at local scales they are driven by higher
heat tolerance variation. This contrasting pattern may result from
divergent selection on both thermal limits to face environmental thermal
extremes at different scales. Specifically, within the same elevational
window, exposure to extreme maximum temperatures could be avoided through
habitat shifts from temporary ponds to permanent ponds or streams, while
minimum peak temperatures remained invariable between habitats but
steadily decreased with elevation. Therefore addressing the effects of
habitat conversion is crucial for future research on resilience to climate
change.