Can tropical forest lizards survive climate change?
Preliminary results from the Bay Islands of Honduras show that, contrary to expectations, some tropical forest lizards may actually cope well with increasing temperatures due to climate change. Previous studies have warned that, in tropical regions, open-habitat lizards might invade cooler forest habitat and drive forest-dwelling species to extinction.
A changing planet
Global temperatures are predicted to rise by as much as 7°C in the next century, and this has resulted in dire forecasts for the planet’s fauna and flora. ‘Cold blooded’ animals (or ectotherms), are thought to be particularly vulnerable to changes in climate because their body temperatures tend to match their immediate surroundings.
Historically, temperate-zone ectotherms were thought to be more vulnerable to warming because climate models have shown that temperatures will increase disproportionately at higher latitudes. However, several recent studies have reversed these predictions by showing that tropical ectotherms are thermal ‘specialists’ (ie they are adapted to stable thermal conditions) and are therefore more vulnerable to perturbations in ambient temperature.
As these studies have refocused the attention of many climate change biologists on the susceptibility of tropical species, follow-up questions have emerged: will all (or most) species within the tropics respond to climate change in the same way? Or, alternatively, are some more vulnerable than others?
The invasion hypothesis
A 2009 paper by Huey et al, ‘Why tropical forest lizards are vulnerable to climate warming’, suggested that, as a result of increasing temperatures in the Caribbean, open-habitat lizard species might invade cooler forest habitats as their native habitat becomes too hot.
As they enter the forest, however, they will encounter forest-dwelling species and may be forced into competition with them. The forest species are adapted to the cooler forest habitat and cannot escape increasing temperatures because the forest is already the coolest part of the tropical landscape.
As this invasion occurs, then, it is possible that the forest lizards may already be experiencing physiological stress, and the outcome of competition will likely favour the open-habitat species. Put simply, open-habitat species may drive forest species to extinction.
Anole research in the Bay Islands
Anoles (lizard species in the genus Anolis) make up a large component of neotropical biodiversity (the genus contains over 400 species). Many Caribbean islands are home to multiple anole species, and in many cases these species avoid competition by occupying different thermal environments (eg open-habitat and forest). The outcome suggested by the ‘invasion hypothesis’ described above, then, has the potential to dramatically reduce lizard biodiversity in the tropics.
In the Bay Islands of Honduras, we are testing the invasion hypothesis by examining the thermal ecology of anoles on four islands in the archipelago. Each island contains both open-habitat and forest-dwelling species. That the forest species are endemic on two of the islands, Utila and Roatan, means the conservation implications of invasion are especially serious.
Mapping the thermal environment
Initial analyses suggest that the open-habitat species on Cayo Menor may actually enjoy performance benefits from moderate increases in temperatureDuring the summer of 2010, we placed temperature-logging models in open and forest habitats on Cayo Menor to develop a detailed map (in space and time) of the thermal environment as seen from the perspective of a lizard. Additionally, in order to assess the physiological impact of temperature, we measured lizard running speeds at several temperatures that span their tolerance range. By combining these datasets with climate change projections, we will determine the likelihood that open-habitat species will enter the forest and compete with forest species.
Surprisingly, our initial analyses suggest that the open-habitat species on Cayo Menor, Anolis allisoni, may actually enjoy performance benefits from moderate increases in temperature, and will therefore have no reason to move into the forest.
Moreover, our temperature models are showing that the forest thermal environment is much more spatially heterogeneous than anticipated, meaning that there may be plenty of thermal refugia within the forest for the forest species, A. lemurinus, to escape increasing temperatures (at least initially).
These studies, and those that will build on them, demonstrate that projections about the impact of climate change on natural systems are difficult to make: the natural world – and every species within it – is idiosyncratic, often intractable to generalisation.
See also a piece written by Harvard biologist Jonathan Losos on his Anole Annals blog on this topic.