Not enough is known about how rainfall affects tropical animals

Despite the fact that tropical forests host some of the most diverse species on the planet, very little is known about how animals in the rainforest are directly affected by precipitation. To address this issue, a team of researchers has developed an analytical tool that can be used by scientists to factor in the environmental conditions that are most optimal for any given species, including rainfall.

Study lead author Alice Boyle is an associate professor in the Division of Biology at Kansas State University.

“We understand exactly how most animals respond to temperature, but the same is not true for rain,” said Professor Boyle. “When animal biologists see rainfall effects in their studies, they assume it must be about how plants are responding to rainfall and how that affects the food supply for the organisms they’re studying. But there can be direct physiological consequences of rain related to feeding behavior, predation, pathogens, and more. There’s a lot more going on than food supply.”

The researchers propose that animals have a hygric niche, which they define as the collection of physiological, behavioral, and ecological processes that predict how warm-blooded organisms will perform under various levels of precipitation.

“Prior to this, there was no unifying conceptual framework to understand why responses to precipitation might differ between species or even within the same species, depending on the location of the study,” said Professor Boyle. “We’ve heard from scientists who have said, ‘Wow, how come I’ve never thought about this before?’ I think this new framework is probably going to change the way many people study the distributions, physiology, and demographic responses of endotherms.”

Study co-author Jeff Brawn is a professor in the Department of Natural Resources and Environmental Sciences at the University of Illinois. 

“This concept has implications for conservation of sensitive organisms, long term. In terms of planning where to invest conservation dollars or where to prioritize habitat, we should be looking at rainfall refugia where precipitation regimes are likely to stay intact over time,” said Professor Brawn.

The team developed the concept of the hygric niche using decades of bird and mammal research from the tropics to help distinguish between the effects of rainfall and moisture. However, weather patterns are shifting due to climate change.

“Human-caused changes to climate are resulting in some areas getting wetter, and other areas getting drier,” wrote the study authors. “Also, it is not just the amount of precipitation that is changing; the timing and magnitude of storms are also changing, and we have very little idea of how any of this will affect animals.”

The experts outlined the ways in which too much or too little precipitation can affect foraging behavior, reproductive and population growth rates, and competitive interactions among animals. 

“Even if you can see intact forest out to the horizon, if the precipitation regimes change, the integrity of that ecosystem may be compromised. And that’s concerning,” said Professor Brawn.

While this concept was conceived with tropical systems in mind, the researchers said it can be applied to ecosystems and organisms outside the tropics.

“The next steps involve the research community testing key assumptions and predictions of our model,” wrote the study authors. “One of the hardest but most important tasks is to understand whether rainfall affects different animal species for the same or different reasons. Is it really mostly about food, or are these less-obvious physiological costs more important than we thought? Answering these questions will be crucial to doing effective conservation and climate change mitigation in the tropics.”

The research is published in the journal Trends in Ecology and Evolution.

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By Chrissy Sexton, Earth.com Staff Writer