Competition promotes adaptation at warming range edges. We established replicate, miniaturized ranges experiencing warming at the range edge using Lemna duckweeds, with and without their Spirodela competitor. Adaptation to warming range edges, driven by genetically based shifts in thermal traits, occurred only for populations that evolved in the presence of competitors.
Abstract
INTRODUCTION: As climate change accelerates, whether populations can adapt fast enough to warming is a central question in ecology, evolution, and conservation. Many species are already facing severe heat stress at the warm edges of their geographic range, where without an evolutionary response, local extinction and range contraction are often predicted. Most predictions of whether populations will adapt at warm range edges, however, ignore species interactions. A particularly important and unresolved question therefore is how interspecific competitors influence adaptive responses at warm range edges, especially given that climate-driven range shifts are expected to intensify competitive pressure at warm range edges.
RATIONALE: To overcome the empirical challenge of testing for the synergistic effects of competition, evolution, and warming across space and over time, we devised replicate, miniaturized landscapes using duckweeds (Lemna spp. and Spirodela polyrhiza) as a model plant community. We established Lemna species ranges with or without their S. polyrhiza competitor and across a thermal gradient in space that ranged from cooler, benign temperatures at their range core to warmer temperatures at their range edge. We then experimentally increased temperatures to simulate climate warming at the range edge and allowed genetically diverse Lemna populations to evolve across the landscape for a total of 32 days (10 to 15 generations). By reciprocally transplanting 288 populations across 72 sites, we then quantified how competitors influenced adaptation at the cooler range core and warming range edge.
RESULTS: We found that interspecific competitors constrained local adaptation at the range core, consistent with conventional predictions that negative demographic effects from competition lower the potential for adaptive evolution. By contrast, we found that adaptation to warming at the range edge was possible only when populations had evolved with interspecific competitors. Moreover, competitors enabled the evolution of higher heat resistance and increased thermal breadth at the warm range edge, but not at the cooler range core. These changes in thermal traits were associated with smaller and more fragmented plant units, a trait that likely enhances both performance under heat stress and competitive ability. Taken together, our results at the warm range edge show that competition can promote adaptation to warming when there is a shared evolutionary response to both biotic and abiotic drivers of selection.
CONCLUSION: We provide empirical support for a general and overlooked mechanism through which even antagonistic interactions could contribute to range-edge adaptation. Our results highlight that competitors could therefore play a key role in buffering species against the threat of range contractions under warming, and may contribute to the lack of widespread range contractions and biodiversity loss observed thus far at the warm range edge. Overall, we highlight the urgency of including community context in predicting future range shifts. Fortunately, our results suggest that accounting for antagonistic interactions does not necessarily hinder evolutionary responses to climate change.