Department Aquatic Ecology

Warming-related community turnover in terrestrial and freshwater

This project contributes to the Blue Green Biodiversity Research Initiative – an Eawag-WSL collaboration focusing on Biodiversity at the interface of aquatic and terrestrial ecosystems.

A critical challenge in ecology is to understand how changes in biodiversity and community composition across the Earth’s biomes will keep pace with the rapid rate of anthropogenic climate change. It has been demonstrated that community composition responds to gradients in temperature in both terrestrial and aquatic systems across the globe: arctic and alpine terrestrial plants, phytoplankton, birds and butterflies, and numerous other ecto- and endothermic taxa. Range shifts towards higher latitudes and elevations are now commonly observed for many species and systems, as organisms shift their geographical distributions in order to track their thermal requirements. With rising temperatures at a given location, the presence or abundance of species adapted to higher temperatures is expected to increase, whereas species adapted to lower temperatures may decline and eventually become excluded. Such directional shifts in community composition in favor of warm-affinity species are referred to as “thermophilization”, a phenomenon that is increasingly documented in both aquatic and terrestrial plants and animals. Yet, patterns of thermophilization are idiosyncratic, varying across taxonomic groups and ecosystems, highlighting the need for systematic, hypothesis-driven comparisons.

We are conducting a comparative analysis between terrestrial and freshwater ecosystems and across trophic guilds, in order to investigate potential drivers of variation in rates of thermophilization. We aim to generate a more mechanistic understanding of the variation underlying community-level responses to climate warming and will enable the identification of taxonomic groups and communities that are particularly vulnerable to local extirpations under future scenarios of climate warming. We hypothesize that differences in the rates of thermophilization will depend on the magnitude, rate, variance and timing of environmental warming in each ecosystem, as well as variation in the characteristics of the organisms across trophic guilds including: generation time, mobility, thermal strategy (ecto- vs. endothermy), and the breadth of thermal tolerance. Specifically, we expect thermophilization to be more pronounced at sites with stronger, less variable and more recent warming, and in organisms with shorter generation times (e.g. phytoplankton compared to plants, Fig. 1a), higher mobility (e.g. birds compared to fish, Fig. 1a), that are ectotherms, and that have narrower thermal tolerances.

We are applying estimates of species’ thermal traits to existing aquatic and terrestrial community data, measured over time and/or space, in order to create estimates of thermophilization (e.g. Fig 1a). We are also testing the degree to which individual species’ thermal traits predict their relative growth rates in a community in response to warming (e.g. Fig. 1b). We are using existing datasets on terrestrial and freshwater communities in Switzerland and around the world. The Swiss datasets cover numerous trophic and taxonomic groups spanning vascular plants, butterflies and birds in the terrestrial realm, and phytoplankton, zooplankton and fish in the freshwater realm. We focus on datasets acquired in Switzerland, due to the climatic and geographic comparability of ecosystems (e.g. alpine or peri-alpine), harmonization of sampling schemes for numerous taxonomic groups, and comparability of the temporal and spatial range across which the data were taken. These datasets include nationwide re-survey data of plants, butterflies and birds from terrestrial systems as part of Swiss Biodiversity Monitoring Program (BDM), as well as lake phytoplankton and zooplankton data generated by the cantons, and spatially extensive lake and river fish surveys performed by collaborators. Furthermore, we include analyses of globally sampled communities for the purposes of broader scientific comparison. These include surveys of huge spatial extent (e.g. the USA EPA National Lakes Assessment Survey and Swedish Lake and River Fish surveys), long-time series (e.g. LTER data in the US and re-sampled forest surveys across Europe), RivFishTime data for global freshwater fish and BioTime data for terrestrial and aquatic taxa.