Research

My research focuses on three interconnected themes: (1) climate-driven range lags in alpine and arctic plants, (2) plant-pollinator interactions in a changing world, and (3) insect eco-evolutionary responses to climate change.

1. Climate-driven range lags in alpine and arctic plants

Mountains can harbor high levels of biodiversity and endemism; however, alpine and arctic regions are also warming faster than the global mean, forcing plants to disperse to higher elevations or latitudes. These range shifts, however, are often slower than the rate of climate change, creating lags between species’ expected and observed ranges (see figure of illustrative elevational range shift). I want to know: are range lags growing with accelerating climate change? Are extinctions accruing faster than colonizations? And can we predict which species will expand or contract their ranges and where extinctions and colonizations will likely occur?

I am studying climate-driven range lags in alpine and arctic plants across Europe over the last 60+ years of climate change with Sabine Rumpf (University of Basel). We assembled a database of ~9,000 vascular plant species, which spans over 120 years, and contains ~105 million survey data across Europe. I am using this database to build species distribution models to quantify the temporal dynamics of range lags. Quantifying these range dynamics reveals how plants have responded to climate change and provides the groundwork for forecasting future range dynamics.

2. Plant - pollinator interactions in a changing world

Climate-driven plant-pollinator interaction rewiring

Species are not shifting their ranges in perfect synchrony, and these asynchronous shifts can alter species interactions. Theory predicts that novel interactions will disproportionately impact community dynamics and recent empirical studies demonstrate that plant performance can be greatly reduced by novel plant competitors in novel climates. Not just plants are shifting their ranges of phenology, but also the pollinators upon which plants rely, which can "reshuffle" plant competition for pollinators. What are the effects of changing plant-pollinator communities on species coexistence under climate change?

At the ETH Zürich, we experimentally simulated climate change by transplanting an alpine community downslope, exposing plants to the climates and novel plant and pollinator communities that they may face with climate change. We found that experimentally-simulated climate change rewired plant-pollinator interactions and often reduced seed set (Richman et al. 2020, Global Change Biology). We expected plants to suffer because they had lost their focal pollinators. Instead, we found that these pollinators were favoring novel plant competitors, suggesting that reshuffled competition for pollinators may be an unexpected driver of climate change impacts on plant coexistence.

Effects of competition for pollinators on plant coexistence

Classical theories in ecology, such as R* theory or modern coexistence theory, predict coexistence in consumer-resource interactions. Yet, we lack comparable theories for mutualism. Some of my theory shows that the Competitive Exclusion Principle also applies to mutualism: plants cannot coexist if they are most limited by the same sets of pollinators (Johnson & Bronstein 2019, Ecology). Coexistence requires that species partition their mutualists and other limiting resources. I also derived a coexistence theory framework for investigating how mutualism and competition jointly affect coexistence (Johnson 2021, Ecology). I show that failing to account for mutualisms can lead to erroneous conclusions about coexistence.

Mounting concerns about the effects of climate change and pollinator decline have fueled recent calls to study the roll of pollinators in maintaining plant diversity. With Jonathan Levine (Princeton), I studied how competition for pollinators influences plant coexistence and I simulated pollinator decline (Johnson et al. 2022, Nature). In a large field study mimicking a patchy meadow, I compared the coexistence of annual plants when competing for ambient pollinators versus under a pollen addition treatment alleviating competition for pollinators.

We found that competition for pollinators destabilized plant competition and eroded plant coexistence, mainly because pollinators favored abundant over rare plant species, effectively alleviating their intraspecific competition. By experimentally simulating pollinator decline, we showed that reductions in plant population growth rates were well predicted by their abilities to attract remaining pollinators. This suggests that pollinator decline may erode plant diversity by reshuffling plant competitive hierarchies and driving competitive exclusion.

Eco-evolutionary dynamics of mutualism

Mutualisms are critical for the maintenance of biodiversity; yet, their evolutionary origins remain enigmatic. Intriguingly, evidence suggests that many mutualisms have evolved from antagonistic interactions. With Judith Bronstein and Régis Ferrière (University of Arizona), I proposed a new hypothesis for the evolution of mutualism: the Co-Opted Antagonist Hypothesis, in which species evolve to “co-opt” an antagonist to perform a beneficial function, which changes the ecology of the interaction and favors coevolution of traits that lead to net mutualism (Johnson et al., 2021: Nature Communications).

For example, some plants may have co-opted insect search behaviors for host plants to disperse their pollen. I evaluated our hypothesis by building an eco-coevolutionary model of a community in which a hawkmoth pollinates some of its larval host plants, which was fully-parameterized with ecological data. Our model predicts coevolution of mutualism despite increased plant interactions with a voracious herbivore, with model trait values that are statistically indistinguishable from our empirical estimates. Our theory reconciles how mutualisms can evolve even without very tight partner fidelity as well as how ecological context influences evolution, and vice-versa.

3. Insect eco-evolutionary responses to climate change

Insects are directly affected by climate change because - as ectotherms - their physiology is largely driven by temperature. Predicting insect responses to climate change requires models that more mechanistically capture temperature effects on insect biology. I quantified temperature effects on the life history traits of the plant bug, Largus californicus, to parameterize mechanistic population models with Priyanga Amarasekare (UCLA). Our model accurately predicted complex field dynamics bracketing 25 years of climate change (Johnson et al. 2016, Functional Ecology). Our study was highlighted as a “template for future studies of insect populations” (Bewick 2016).

I also developed a framework predicting climate change impacts on insect fitness and population dynamics with Lauren Buckley (University of Washington). We show that thermal performance curves, a common approach for quantifying species’ thermal responses, systematically underestimate climate change impacts on insect fitness by simplifying survival dynamics across the life cycle (Johnson et al . 2023, The American Naturalist). By integrating thermal responses into a mechanistic population model, we show that accurately predicting species' responses to climate change requires considering how multiple fitness components respond to temperature over the life cycle.