Adam M. Siepielski

Heatwaves are intensifying under climate change, often triggering the catastrophic loss of predators. Still, it remains unclear whether their ecological impacts on ecosystems arise from direct thermal stress or from altered food web dynamics following predator loss. Using experimental tri-trophic fresh water food webs, we independently manipulated heatwave intensity (none, moderate or severe) and predator persistence (fish removed during the onset of heatwaves versus fish persisted during heatwaves) to disentangle these pathways. We show that predator loss, not heat alone, is the primary driver of community restructuring following a heatwave. Predator loss produced strong trophic cascades, and communities became dominated by large Daphniidae consumers that were less diverse, less temporally stable and more synchronized. These changes coincided with declines in primary producers despite increased nutrient availability from decomposing fish. By contrast, direct thermal effects were modest and depended strongly on trophic structure. When predators persisted, heatwaves intensified top-down pressure, producing declines in predator-sensitive consumers. When predators were lost, heatwaves instead magnified trophic disruptions by releasing vulnerable consumers from top-down control, generating substantial and persistent shifts in community structure. Ecosystems that retain predators may therefore be more resilient to heatwaves, underscoring predator conservation as a key strategy for maintaining ecosystem stability under accelerating climate extremes.

Chronically male-biased sex ratios are predicted to intensify sexual conflict and reduce female fitness. Yet empirical tests of female responses in natural settings are rare. We tested how female reproductive performance depends on social environment in the field using two closely related sympatric damselflies that differ strikingly in adult sex ratios: one experiences consistently male-biased sex ratios (Enallagma exsulans), whereas the other does not (E. traviatum). In field experiments, females from the chronically male-biased species were less sensitive to elevated male density, maintaining fertility despite increased harassment. By contrast, fertility declined under high male density in the species that naturally encounters more balanced sex ratios. Fecundity and survival remained largely unchanged, suggesting that fertility is the primary fitness component sensitive to variation in male density. Mixed-species trials showed that frequent heterospecific mating attempts imposed only modest additional costs to females. These results provide field-experimental evidence that closely related species can differ markedly in how female fertility responds to social environment under natural conditions. The correspondence between these differences and natural sex ratios is also consistent with a putative role for adaptation to persistent male bias.

Microbial respiration is a key biotic driver of climate change. Warming boosts microbial population growth, which increases biomass and respiration, potentially leading to more warming. This feedback might be disrupted by adaptation in thermal performance curves (TPCs) -whose shape describes how temperature drives growth. In this study, we uncover substantial genetic variation (G) in the intrinsic population growth rates (r) of the protist Tetrahymena thermophila, demonstrate a causal link between heritable variation in r and heritable variation in TPC shape, and show how this variation constrains predicted r-TPC shape evolution along specific evolutionary paths across temperatures. We also uncover Gene-by-Environment (G × E) variation in r, which results in specific signatures in TPC shape and predictable temperature-dependent TPC evolution that can erode heritable variation, thus reducing future evolutionary potential. Overall, we show how temperature-dependent evolution in microbial TPC shape-a linchpin of global ecosystem function-is determined by a combination of heritable and non-heritable variation in intrinsic growth rates.

Host populations often vary in the magnitude of coinfection they experience across environmental gradients. Furthermore, coinfection often occurs sequentially, with a second parasite infecting the host after the first has established a primary infection. Because the local environment and interactions between coinfecting parasites can both drive patterns of coinfection, it is important to disentangle the relative contributions of environmental factors and within‐host interactions to patterns of coinfection. Here, we develop a conceptual framework and present an empirical case study to disentangle these facets of coinfection. Across multiple lakes, we surveyed populations of five damselfly (host) species and quantified primary parasitism by aquatic, ectoparasitic water mites and secondary parasitism by terrestrial, endoparasitic gregarines. We first asked if coinfection is predicted by abiotic and biotic factors within the local environment, finding that the probability of coinfection decreased for all host species as pH increased. We then asked if primary infection by aquatic water mites mediated the relationship between pH and secondary infection by terrestrial gregarines. Contrary to our expectations, we found no evidence for a water mite‐mediated relationship between pH and gregarines. Instead, the intensity of gregarine infection correlated solely with the local environment, with the magnitude and direction of these relationships varying among environmental predictors. Our findings emphasize the role of the local environment in shaping infection dynamics that set the stage for coinfection. Although we did not detect within‐host interactions, the approach herein can be applied to other systems to elucidate the nature of interactions between hosts and coinfecting parasites within complex ecological communities.

A principal goal in ecology is to identify the determinants of species abundances in nature. Body size has emerged as a fundamental and repeatable predictor of abundance, with smaller organisms occurring in greater numbers than larger ones. A biogeographic component, known as Bergmann’s rule, describes the preponderance, across taxonomic groups, of larger-bodied organisms in colder areas. Although undeniably important, the extent to which body size is the key trait underlying these patterns is unclear. We explored these questions in diatoms, unicellular algae of global importance for their roles in carbon fixation and energy flow through marine food webs. Using a phylogenomic dataset from a single lineage with worldwide distribution, we found that body size (cell volume) was strongly correlated with genome size, which varied by 50-fold across species and was driven by differences in the amount of repetitive DNA. However, directional models identified temperature and genome size, not cell size, as having the greatest influence on maximum population growth rate. A global metabarcoding dataset further identified genome size as a strong predictor of species abundance in the ocean, but only in colder regions at high and low latitudes where diatoms with large genomes dominated, a pattern consistent with Bergmann’s rule. Although species abundances are shaped by myriad interacting abiotic and biotic factors, genome size alone was a remarkably strong predictor of abundance. Taken together, these results highlight the cascading cellular and ecological consequences of macroevolutionary changes in an emergent trait, genome size, one of the most fundamental and irreducible properties of an organism.

Explaining the maintenance of genetic variation in fitness‐related traits within populations is a fundamental challenge in ecology and evolutionary biology. Frequency‐dependent selection (FDS) is one mechanism that can maintain such variation, especially when selection favours rare variants (negative FDS). However, our general knowledge about the occurrence of FDS, its strength and direction remain fragmented, limiting general inferences about this important evolutionary process. We systematically reviewed the published literature on FDS and assembled a database of 747 effect sizes from 101 studies to analyse the occurrence, strength, and direction of FDS, and the factors that could explain heterogeneity in FDS. Using a meta‐analysis, we found that overall, FDS is more commonly negative, although not significantly when accounting for phylogeny. An analysis of absolute values of effect sizes, however, revealed the widespread occurrence of modest FDS. However, negative FDS was only significant in laboratory experiments and non‐significant in mesocosms and field‐based studies. Moreover, negative FDS was stronger in studies measuring fecundity and involving resource competition over studies using other fitness components or focused on other ecological interactions. Our study unveils key general patterns of FDS and points in future promising research directions that can help us understand a long‐standing fundamental problem in evolutionary biology and its consequences for demography and ecological dynamics.

Microbial respiration is a key biotic driver of climate change. Warming boosts microbial population growth, which increases biomass and respiration. This feedback might be disrupted by adaptation in thermal performance curves (TPCs) –whose shape describes how temperature drives growth. In this study, we uncover substantial genetic variation (G) in microbial intrinsic population growth rates ( r ), demonstrate a causal link between G variation in r and G variation in TPC shape, and show how this variation constrains r-TPC shape evolution along specific evolutionary paths across temperatures. We also uncover Gene-by-Environment (G × E) variation in r , which results in specific signatures in TPC shape and predictable temperature-dependent rapid TPC evolution but also lower G, which could reduce future evolutionary potential. Overall, we show how temperature-dependent evolution in a linchpin of global ecosystem function—microbial TPC shape—is determined by a combination of heritable and non-heritable variation in intrinsic growth rates.

Predators have a key role in structuring ecosystems^1,2,3,4. However, predator loss is accelerating globally^4,5,6, and predator mass-mortality events⁷ (MMEs)—rapid large-scale die-offs—are now emblematic of the Anthropocene epoch⁶. Owing to their rare and unpredictable nature⁷, we lack an understanding of how MMEs immediately impact ecosystems. Past predator-removal studies^2,3 may be insufficient to understand the ecological consequences of MMEs because, in nature, dead predators decompose in situ and generate a resource pulse⁸, which could alter ensuing ecosystem dynamics by temporarily enhancing productivity. Here we experimentally induce MMEs in tritrophic, freshwater lake food webs and report ecological dynamics that are distinct from predator losses^2,3 or resource pulses⁹ alone, but that can be predicted from theory⁸. MMEs led to the proliferation of diverse consumer and producer communities resulting from weakened top-down predator control^1,2,3 and stronger bottom-up effects through predator decomposition⁸. In contrast to predator removals alone, enhanced primary production after MMEs dampened the consumer community response. As a consequence, MMEs generated biomass dynamics that were most similar to those of undisturbed systems, indicating that they may be cryptic disturbances in nature. These biomass dynamics led to trophic decoupling, whereby the indirect beneficial effects of predators on primary producers are lost and later materialize as direct bottom-up effects that stimulate primary production amid intensified herbivory. These results reveal ecological signatures of MMEs and demonstrate the feasibility of forecasting novel ecological dynamics arising with intensifying global change.