Testing how global warming changes freshwater communities

Elton Prize shortlisted Article

This blog post is provided by Natalie T. Jones and tells the #StoryBehindThePaper for the article “Predators drive community reorganization during experimental range shifts”. Natalie has been shortlisted for the 2020 Elton Prize for this article.

Natalie completed this research as a postdoctoral fellow working with Dr. Jonathan Shurin at the University of California, San Diego. She is currently a postdoctoral fellow at the University of Queensland in Australia and loves empirically testing predictions from ecological theory using different plant and animal systems.

Climate change is altering species’ distributions, creating novel communities of interacting competitors, predators, parasites and mutualists. I wondered if including trophic interactions, which are important in determining species competitive hierarchies, could shape how new communities assemble.

Even as a warming atmosphere favours colonization by species from lower latitudes or altitudes, predators may prevent prey from expanding their ranges to newly suitable sites.  For example, competitively inferior species would typically be unable to establish in new sites if residents competitively exclude would-be colonists. However, if predators forage in a density-dependant manner, more abundant residents will be a superior resource for predators, especially if they are a more substantial meal. This preferential predation on residents could provide a colonization opportunity for range shifting species. We end up with an apparent competition scenario, where weaker competitors would be excluded but for the dominant predator’s preference for their enemy.

We set out to test the role of apparent competition for the establishment of range shifting species using aquatic zooplankton communities. This is a great study system because decades of work have documented that competitive ability is correlated with body size in zooplankton and that fish preferentially predate on larger plankton.

Fortunately, my postdoctoral supervisor for this work, Dr. Jonathan Shurin, has established an alpine lake monitoring program in the Eastern Sierra mountain range including Yosemite National Park. We used that data, including Dr. Celia Symons data from her 2016 Proc B paper to select the lakes to collect zooplankton communities from.

The biggest challenge was locating our highest elevation lake, aptly named Secret lake. After three failed attempts, we successfully found the lake, under snow, at 3,320 m.

Secret lake under ice. Picture credit Natalie Jones
Co-authors Hamanda and Adriana with Natalie taking a lunch break after collecting zooplankton for the experiment. Picture credit Scott Forster

By the seventh trip to Secret lake, the ice had melted and the lake was full of beautiful jet-black daphnia, Daphnia melanica, and the large bright red copepod, Hesperodiaptomus shoshone. These were the largest freshwater zooplankton I had ever seen. They are completely visible to the naked eye. D. melanica, reminded me of a watermelon seed. The black color is a response to the intense ultraviolet light at high elevations.

Zooplankton from Secret lake. D. melanica and some H. Shoshone are visible. Video credit Natalie Jones

Once we had collected zooplankton communities from lakes at three elevations, we set up the entire experiment at one of the University of California’s research stations, The Sierra Nevada Aquatic Research Laboratory (SNARL) This station not only has the best acronym, it is expertly managed by the amazing Dr. Carol Blanchette, has great lab space and is only ten minutes away from the town of Mammoth Lakes.

We set up a mesocosm experiment using cattle tanks near the experimental streams. After Fedex delivered our predators, juvenile trout, we were ready to go. We simulated a range shift by adding lower elevation communities to higher elevation communities, in either the presence or absence of fish. At the beginning and end of the experiment, we identified and counted the number of individuals of each species, then measured their body length. The change in the identity, abundance and body size of the species were the responses we focused on.

The University of California field station where we did the experiment. Picture credit Natalie Jones
Dr. Celia Symons filling up tanks before we added the zooplankton communities. Picture credit Natalie Jones
An aerial view of the experiment (green tanks) next to the experimental streams at SNARL. Video credit Carol Blanchette, SNARL.

We found that fish predation reduces the abundance of larger-bodied residents from the alpine lake, facilitating the establishment of new lower-elevation species. In addition, fish predation and warming independently reduced the average body size of zooplankton by up to 30%. This reduction in body size offset the direct effect of warming-induced increases in population growth rates, leading to no net change in zooplankton biomass or trophic cascade strength.

What do these results mean? Well, our results suggest that predators can amplify the rate of range shifts by consuming larger-bodied residents and facilitating the establishment of new species.  This work suggests that trophic interactions play a role in the reorganization of regional communities under climate warming.

The scientific community needs figure out how ecological communities will shift in response to climate change. It’s crucial for understanding where, who, when and how species will go extinct, or not. I think that continuing to leverage our collective understanding about all types of species interactions, e.g., mutualisms, parasitism and facilitation, then applying that understanding to generating predictions about climate change driven range shifts is a good direction to pursue. We hope that the results of our work add a new piece to the puzzle.

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