SE CASC Researchers Show How Climate Change and Invasive Species Threaten Florida Treefrogs

FOR IMMEDIATE RELEASE

J. Alex Baecher | jbaecher@gmail.com
Brittany Salmons | basalmo2@ncsu.edu | 919-515-3766

Invasive species may negatively impact native species’ ability to adapt to climate change through direct competition or predation, leading to population and ecosystem decline. A new study quantifies how climate change and the spread of invasive species combine to create a dual threat to biodiversity.

Previously, researchers thought that environmental factors – like temperature and rainfall – primarily determined the broad limits to where species can live, while biological factors – such as competition and predation – primarily influence which species can coexist in individual habitats. These ideas are referred to as the ‘Environmental Filtering’ hypothesis. 

“For example, if one thinks of a species distribution as a rectangle, environmental factors define the overall size of the rectangle, whereas interactions with other species might determine whether or not the rectangle has holes in it; kind of like swiss cheese – areas where species are not locally present – or solid like cheddar cheese – species are present everywhere,” says senior author of the study, Brett Scheffers, an Associate Professor at the University of Florida.

However, with support from the Southeast Climate Adaptation Science Center, researchers found that these factors can interact to influence the relationship of native species with their physical environment.

“Climate change is rapidly expanding the range of suitable conditions for invasive species, encouraging invasions of novel species,” says Alex Baecher, the lead author and a PhD student at the University of Florida. “Invasive species also thrive under altered, or unstable, conditions—something climate change provides in spades – and invasive species tend to degrade the ecosystems they inhabit, resulting in a self-reinforcing collapse of biodiversity.”

To understand the interaction between climate change and invasive species, researchers used the Cuban treefrog as their model invasive species, which are potential competitors and predators of  two native treefrogs – green treefrogs (Hyla cinerea) and squirrel treefrogs (H. squirella) – in Gainesville, Florida.

Aptly named, treefrogs tend to climb and live above-ground in trees and here, different species of treefrog have different preferences in height. In order to meet their needs, some species live just above the ground whereas others live much higher in the canopy. However, they will move up or down when resources or conditions change. For example, if there is less rainfall than normal, a treefrog may respond by moving lower in the tree to find water closer to the ground. Likewise, treefrogs will move outside of their preferred height when a predator is nearby. But, which factor is more important if both rainfall and predation risks happen at the same time? How these kinds of factors interact and impact species movements is less understood. 

Researchers recreated a forest ecosystem in climate controlled containers to observe native treefrog movements while altering water availability and Cuban treefrog presence. 

“We essentially built a large in-door gymnasium for frogs to move in with water either distributed evenly from ground to ceiling or skewed towards the ceiling or towards the floor,” says Scheffers. “We wanted to know if frogs were willing to shift up and down to track experimental changes in water and then whether or not the presence of an invasive species disrupts this ability to track water.”

“We needed to understand if their movements were a response to water availability, or an attempt to evade interactions with other species, in this case the invasive predator, Cuban treefrogs,” adds Baecher. 

They found that Cuban treefrogs negatively impacted native species’ attempts to track water resources vertically (closer to the canopy or on the ground), likely due to fear of predation. So when climatic conditions change and water resources move, Cuban treefrogs may restrict native treefrogs from accessing resources, which could have a negative effect on the health of native species. 

“This finding challenges the long-standing ‘environmental filtering’ hypothesis, which would assume that climate and invasive species act independently, so their effects on native species would be no greater when they occur simultaneously,” says Baecher. “ Other studies show that hundreds of new invasive species may become established in the future due to climate change. Applying our experimental design would allow researchers to understand how other invasive species will impact native species under unstable conditions.”

The article, “Experimental evaluation of how biological invasions and climate change interact to alter the vertical assembly of an amphibian community,” was published in the Journal of Animal Ecology. Co-authors include Drs. Steve A. Johnson (University of Florida), Elizabeth A. Roznik (North Carolina Zoo), and Brett R. Scheffers (University of Florida). 

JAB was supported through a Graduate Preeminence Doctoral Fellowship from the University of Florida (UF), awarded through the School of Natural Resources and Environment (SNRE), as well as additional financial support provided by the UF SNRE Robin Nadeau Ecology Graduate Research Award and a grant from the U.S. Geological Survey Southeast Climate Adaptation Science Center (GRANT #G20AC00467). Funding was also provided to BRS by a UF Institute of Food and Agricultural Sciences (IFAS) Early Career Research Award and Alford P Sloan Foundation Early Career Fellowship Award.

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Note to Editors: The study abstract follows.

“Experimental evaluation of how biological invasions and climate change interact to alter the vertical assembly of an amphibian community”

Authors: J. Alex Baecher, Steve A. Johnson, Elizabeth A. Roznik, Brett R. Scheffers

Published: March 5, Journal of Animal Ecology

DOI: https://doi.org/10.1111/1365-2656.13899

Abstract: 

While biotic-abiotic interactions are increasingly documented in nature, a process-based understanding of how such interactions influence community assembly is lacking in the ecological literature. Perhaps the most emblematic and pervasive example of such interactions is the synergistic threat to biodiversity posed by climate change and invasive species. Invasive species often out-compete or prey on native species. Despite this long-standing and widespread issue, little is known about how abiotic conditions, such as climate change, will influence the frequency and severity of negative biotic interactions that threaten the persistence of native fauna.

Treefrogs are a globally diverse group of amphibians that climb to complete life-cycle processes, such as foraging and reproduction, as well as to evade predators and competitors, resulting in frog communities that are vertically partitioned. Furthermore, treefrogs adjust their vertical position to maintain optimal body temperature and hydration in response to environmental change. Here, utilizing this model group, we designed a novel experiment to determine how extrinsic abiotic and biotic factors (changes to water availability and an introduced predator, respectively) interact with intrinsic biological traits, such as individual physiology and behavior, to influence treefrogs’ vertical niche.  

Our study found that treefrogs adjusted their vertical niche through displacement behaviors in accordance with abiotic resources. However, biotic interactions resulted in native treefrogs distancing themselves from abiotic resources to avoid the non-native species. Importantly, under altered abiotic conditions, both native species avoided the non-native species 33 – 70% more than they avoided their native counterpart. Additionally, exposure to the non-native species resulted in native species altering their tree climbing behaviors by 56 – 78% and becoming more vertically dynamic to avoid the non-native antagonist.

Our experiment determined that vertical niche selection and community interactions were most accurately represented by a biotic-abiotic interaction model, rather than a model that considers these factors to operate in an isolated (singular) or even additive manner. Our study provides evidence that native species may be resilient to interacting disturbances via physiological adaptations to local climate and plasticity in space-use behaviors that mediate the impact of the introduced predator.