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Novel interactions and species’ responses to climate change

Periodic Reporting for period 3 - NICH (Novel interactions and species’ responses to climate change)

Reporting period: 2019-02-01 to 2020-07-31

A great challenge facing ecologists today is predicting the responses of species, communities and ecosystems to global climate change. Progress will hinge on our ability to predict how responses are shaped by evolution and by changes to species interactions. As well as altering interactions among species that already co-occur in communities today, climate change is leading to the large-scale reshuffling of species distributions, giving rise to novel interactions among species that did not previously co-occur. However, the ecological and evolutionary consequences of novel interactions remain poorly understood. The NICH project combines cutting-edge experiments and process-based modeling to address three questions about the impact of novel competitors on responses to climate change in alpine plant communities:

(1) How will novel interactions impact species’ responses to climate change? We are testing the ecological consequences of novel competitors for population persistence, and the potential for longer-term evolutionary responses, by transplanting whole plant communities across elevation gradients to simulate future competitive scenarios faced by focal alpine plants.

(2) Do species traits predict the outcome of novel interactions? A mechanistic understanding of competitive effects is essential to predict impacts of novel interactions. We are testing how climate affects the outcome of competition among pairs of species planted along an elevation climate gradient, and whether these effects can be predicted using species’ functional traits.

(3) What are the implications of novel competitive interactions for species’ ranges dynamics under climate change? We will use process-based species distribution models, parameterized with our experimental data, to explore the consequences of changing competitive interactions for range dynamics under climate change.

This project will advance our understanding of species’ responses to climate change, and develop approaches that can be applied to a diversity of other ecological settings. It also tackles fundamental questions in ecology, shedding light on the mechanisms shaping species distributions. By linking experimental community ecology and biogeography, the project pushes the limits of our ability to predict the dynamics of complex ecological systems.
Until now we have established the field experiments to address Questions 1 and 2, stated in the previous section. To tackle Question 1, we established a whole-community transplant experiment at four sites along an elevation gradient in the western Swiss Alps. In this experiment, focal alpine plants interact with either their current alpine plant community or with plant communities from lower elevations, and we are characterizing their ecological and evolutionary responses to changing competitor identities as climate warms. Preliminary results from the first three seasons of data collection indicate that novel plant communities accelerate rates of local extinction of our focal alpine plants under climate warming. These effects are most pronounced under moderate levels of warming, implying that novel species establishment could be a key driver of biodiversity loss in the coming decades (Nomoto & Alexander, in review). Currently, we are using phenotypic and fitness data to investigate how natural selection from changing climate and species interactions acts on our focal species, and combining these data with molecular tools for one focal species to investigate its potential for longer-term evolutionary responses to climate change.

In a controlled glasshouse experiment we investigated the potential effects of changing plant-soil biota interactions, which arise as species migrate to higher elevation with climate change, on the coexistence of novel plant competitors. Our results revealed positive effects of low elevation soil biota on plant performance, but with only one exception the soil biota effects did not alter model predictions for the qualitative outcome of competition between high and low elevation plant species (Cardinaux et al., 2018). We used the field experiment to investigate consequences of novel interactions for ecosystem processes. Our preliminary results suggest that low elevation plants colonizing alpine communities can modify effects of climate warming on components of the soil carbon cycle, at least in the short term (Walker et al., in review). In a review article, we place these effects of novel species within the wider context of how above- and belowground linkages are affecting vegetation responses to climate change in mountains (Hagedorn et al., 2019).

We set-up a second large-scale field experiment to address Question 2. In this experiment we compete 14 plant species from high and low elevation in pairwise combinations in three sites at low, intermediate and high elevation, and collect data on their performance and functional traits. We are testing how the outcome of these interactions varies with climate, and the extent to which the outcome of competition can be predicted by species’ functional traits. So far we have collected and are analysing data from the first growing season. We also plan to use these data to develop models that address Question 3.

The important role that species interactions play in mediating effects of climate within ecological communities poses great challenges for forecasting responses of biodiversity, and in particular species’ distributions, to climate change. We published an article that lays-out the need for experiments that quantify outcomes of novel interactions in ways that can inform species distribution models, and outlines a research agenda to achieve this goal (Alexander et al. 2016). Furthermore, to provide a baseline for our empirical research we synthesised current knowledge about the mechanisms affecting disequilibrium range dynamics in mountain ecosystems. We published a review paper on this topic (Alexander et al. 2018), where we also developed a mechanistic community model of disequilibrium range dynamics across elevation gradients, providing a baseline for the models that will be used to address Question 3.
The two large-scale field experiments that we have established are innovative approaches that will advance our current understanding about the responses of species to novel interactions following climate change. They will also contribute substantially to our understanding of the ecological and evolutionary factors controlling species’ distributions by providing so far missing experimental tests of hypotheses underlying range-limit theory, such as the existence of trade-offs between climate tolerance and competitive ability. The data arising from these experiments will help us to assess the ability of species’ traits to predict competitive outcomes among plants, and therefore how far we can generalize from these experiments to other plant communities. Using this information we plan to develop and parameterize process-based species distribution models for these plant species that account for novel competitive interactions, to provide improved predictions of climate change impacts on natural communities and ecosystems.