Extreme warming restructures communities along gradients in stress and biodiversity

Background/Question/Methods

Predicting how climate warming will impact living systems requires understanding how ecological responses manifest across existing environmental gradients that structure populations and communities, and yet the increasing frequency and severity of extreme events challenges our notions about diversity, disturbance, and stability. Field observational studies in complex, changing environments provide opportunities to explore the range of possibilities in light of natural history and ecological theory. Here, we investigate population- and community-level impacts of a multi-year marine heatwave in a species-rich rocky shore ecosystem using observations from an eight-year time series of intertidal seaweed diversity. We explore dynamics within and across elevations and functional groups, using joint species distribution modeling and community analyses. We hypothesized that 1) community abundance would be most sensitive at higher, more physiologically stressful elevations, especially among larger, more emergent taxa, and 2) species and communities would shift lower elevations if stressful conditions limited upper distributions. These predicted changes would lead to parallel gradients in environment, diversity change, and stability of community biomass.

Results/Conclusions

While local and regional species richness remained stable, all seaweed functional groups experienced significant, largely synchronous declines in abundance (25% decline over eight years, on average). Gradients in recovery after the hottest years of the heatwave were contingent on species and environment: recovery at higher elevations was accelerated by recruitment of sessile invertebrates, while seaweed diversity and compensatory dynamics at lower elevations promoted community stability. Despite partial recovery of seaweeds, many species shifted towards lower elevations (33 cm, on average). Local species richness promoted population and community stability (R2 = 0.91) more strongly than elevation or variation in species elevational niches. These results suggest that species niche differences that mediate population responses and species interactions may underlie relationships between diversity and stability. We speculate that life history traits, which remains underexplored in seaweeds, may be key to understanding the tight relationships we observed between species diversity and community stability that emerged during an unprecedented and prolonged heatwave. Our observations and inference suggest non-linear relationships between biodiversity and stress responses across existing environmental gradients that may shift habitat distribution and ecosystem productivity.