1. Increased global temperatures caused by climate change are causing species to shift their ranges and colonize new sites, creating novel assemblages that have historically not interacted. Species interactions play a central role in the response of ecosystems to climate change, but the role of trophic interactions in facilitating or preventing range expansions are largely unknown.
2. The goal of our study was to understand how predators influence the ability of range-shifting prey to successfully establish in newly available habitat following climate warming. We hypothesized that fish predation facilitates the establishment of colonizing zooplankton populations, because fish preferentially consume larger species that would otherwise competitively exclude smaller bodied colonists.
3. We conducted a mesocosm experiment with zooplankton communities and their fish predators from lakes of the Sierra Nevada Mountains in California, USA. We tested the effect of fish predation on the establishment and persistence of a zooplankton community when introduced in the presence of higher- and lower-elevation communities at two experimental temperatures in field mesocosms.
4. We found that predators reduce the abundance of larger bodied residents from the alpine and facilitate 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.
5. We found support for a shift to smaller species with climate change through two mechanisms: 1) the direct effects of warming on developmental rates and, 2) size-selective predation that altered the identity of species’ that could colonize new higher elevation habitat. Our results suggest that predators can amplify the rate of range shifts by consuming larger bodied residents and facilitating the establishment of new species. However, the effects of climate warming were dampened by reducing the average body size of community members, leading to no net change in ecosystem function, despite higher growth rates. This work suggests that trophic interactions play a role in the reorganization of regional communities under climate warming.
The methods for this experiment are described in the full text and supplement of the paper "Predators drive community reorganization during experimental range shifts". In short, we collected aquatic copepods and cladocerans from three lakes in the Sierra Nevada mountains of California in 2016. At that time we also characterized a subset of physical and chemical characteristics of those three focal lakes.
We used those zooplankton communities in an experiment where we added lower elevation communities to higher elevation communities in the presence and absence of fish predation. We collected zooplankton at the beginning and end of the experiment, preserved them in ethanol, then identified and measured individuals using a microscope. We also measured chlorophyll a, following the protocol described in the main text of the paper.
Details regarding how to interpret and use the raw data generated in this experiment are described in the "Usage Notes" ReadMe files.
The units and description of variables are in the first sheet of the excel file, "Metadata". There are 4 data sheets total.