According to a new study, the range of canopy temperatures in tropical forests at the bottom of mountains overlaps considerably with those at the top of the mountains, which suggests that canopy animals likely have the physiology that might allow them to move across a mountain gradient freely unhindered by the climate.
Animals that live in trees in the tropics are likely to be better at crossing mountains and dealing with climate change compared to ground-dwelling animals, a new study published in Frontiers in Ecology and the Environment suggests.
In the tropics, mountains are typically considered to be barriers for lowland animals. According to a popular hypothesis put forward by ecologist Daniel H. Janzen in 1967, this is because the tropics have a relatively steady climate, with temperatures never getting too hot or too cold. Tropical organisms there tend to be adapted to very narrow ranges of temperatures. So an animal adapted to warm temperatures living at the bottom of a mountain might not be able to tolerate the colder temperatures it encounters as it moves higher up.
“Think about a layered cake,” Brett Scheffers, an assistant professor in the department of wildlife, ecology and conservation at the University of Florida, Gainesville, U.S., told Mongabay. “One band represents a range of temperatures, the next band represents another range of temperatures and so on. There’s not a lot of overlap between those bands. What this means is that animals living on the bottom of a mountain will likely never have experienced temperatures at the top of the mountain and vice versa.”
This suggests that tropical animals could have a harder time “escaping” from a changing climate because of the temperature constraints that mountains impose.
However, temperatures within a tropical forest are not uniform. Trees, for instance, create large vertical temperature gradients — that is, it is cooler at the base of a tree, under its shade, compared to the top of the tree that’s that exposed directly to the sun.
To see what this variability means for tropical animals’ ability to adapt to climate change, Scheffers and his colleagues monitored temperatures of the soil, ground and canopy of tropical forests across mountains in Madagascar, the Philippines and Australia.
Overall, the team found that tree canopies experienced a wide range of temperatures compared to the soil or the ground. This implies that animals living in trees are exposed to a wider variety of temperatures than those living near the ground: During the day, canopy animals are under the hot sun, while at night, the lack of vegetation above exposes the animals to colder temperatures compared to ground-dwelling creatures.
Previous studies have found that this is often the case. In the rainforests of Panama, for example, researchers have found that ants living in the canopy can tolerate heat that is 3.5 to 5 degrees Celsius (6.3 to 9 degrees Fahrenheit) higher than ant species living in the constant shade near the ground.
Scheffer’s study also found that the range of canopy temperatures in forests at the bottom of mountains overlapped considerably with those at the top of the mountains. By contrast, there was very little overlap in soil or ground temperatures between lowland and upland forests.
“Since the climate [of the canopy] is shared across mountains, it suggests that canopy animals likely have the physiology that allows them to move across a mountain gradient freely unabated by climate,” Scheffers said. “And this movement has implications for where the animals can go, which could ultimately impact the speciation or extinction of the species.”
In short, tree-dwelling tropical animals are more likely to be able to move across mountain passes and “climb” away from climate change than ground-dwelling animals, Scheffers said.
“The hypothesis and data are very reasonable,” Janzen, currently at the University of Pennsylvania, told Mongabay. “My concept applies to any instance that compares things in more fluctuating environments with those in more constant environments. My old example was the case that I could spot back then. Were I to write the same paper today, I would have simply listed more of these contrasts, and understory versus canopy is certainly one of them.”
However, not all tree-dwelling animals might be successful at dealing with environmental stress, Scheffers warns.
“There may be exceptions to the rule,” he said. “We are putting out a perspective about what might be happening out there. And we hope that lots of people test it, challenge it.”
Citation: Scheffers, B.R. and Williams, S.E. (2018). Tropical mountain passes are out of reach – but not for arboreal species. Front Ecol Environ 2018; doi: 10.1002/fee.1764.