J. Timothy Wootton

Knowledge of how pathogens, in the context of a changing environment, shape the structure of ecological communities remains limited, and little information exists on the ability of small-scale field experiments to predict the consequences of large-scale environmental impacts. We addressed these issues using the emergence of a widespread epidemic (sea star wasting disease, SSWD) on rocky intertidal shorelines. Prior experiments demonstrated strong reductions in sessile species diversity in response to the local experimental removal of the sea star Pisaster ochraceus [R. T. Paine, Am. Nat. 100 , 65–75 (1966); R. T. Paine, Oceologia 15 , 93–120 (1974)] at our site. In surveys of sea star populations over 8 y spanning the onset of the SSWD epidemic, P. ochraceus populations declined by 90%, then subsequently recovered to about one third of pre-epidemic size under an endemic disease phase. Using four different types of temporal data, we documented large increases in populations of the competitively dominant mussel Mytilus californianus following P. ochraceus decline, which were of similar magnitude to those observed in prior field experiments. As in the field experiments, sessile species richness and diversity declined strongly following the onset of the SSWD epidemic as mussels were released from predator control. Our results reveal a strong effect of a pathogen in regulating ecological community structure by disrupting the role of a keystone species, a result anticipated by prior small-scale field experiments.

Strong ecological gradients along heterogeneous environments play an important role in shaping population differentiation across species ranges. Thus, the selective pressure of environmental variation on phenotypic variation strongly affects an organism's ability to persist across diverse or new environments. We investigated the spatial variability of biological responses in the intertidal bivalve Mytilus californianus to highlight the costs and trade‐offs of local adaptation and phenotypic plasticity across various functional traits in a dynamic environment, the marine intertidal. To test this, we performed a reciprocal transplant experiment with M. californianus individuals originating from the upper and lower intertidal measuring relevant phenotypic traits, followed by whole genome sequencing (WGS). We determined that morphological traits in individuals demonstrated phenotypic plasticity when moved to new environments, whereas physiological traits such as metabolism exhibited constraints in plasticity. Additionally, mussels from high intertidal zones, which experience greater heat and aerial exposure stress, maintained lower metabolic rates and showed increased frequencies of non‐synonymous mutations in functionally relevant heat shock proteins when compared to low intertidal mussels. These results suggest that morphological and physiological traits responded differently to spatially varying selection within the marine intertidal.

Populations in nature are comprised of individual life histories, whose variation underpins ecological and evolutionary processes. Yet the forces of environmental selection that shape intrapopulation life-history variation are still not well-understood, and efforts have largely focused on random (stochastic) fluctuations of the environment. However, a ubiquitous mode of environmental fluctuation in nature is cyclical, whose periodicities can change independently of stochasticity. Here, we test theoretically based hypotheses for whether shortened (‘Fast’) or lengthened (‘Slow’) environmental cycles should generate higher intrapopulation variation of life history phenotypes. We show, through a combination of agent-based modelling and a multi-generational laboratory selection experiment using the tidepool copepod Tigriopus californicus, that slower environmental cycles maintain higher levels of intrapopulation variation. Surprisingly, the effect of environmental periodicity on variation was much stronger than that of stochasticity. Thus, our results show that periodicity is an important facet of fluctuating environments for life-history variation.

Coexistence in ecological communities is governed largely by the nature and intensity of species interactions. Countless studies have proposed methods to infer these interactions from empirical data, yet models parameterised using such data often fail to recover observed coexistence patterns. Here, we propose a method to reconcile empirical parameterisations of community dynamics with species‐abundance data, ensuring that the predicted equilibrium is consistent with the observed abundance distribution. To illustrate the approach, we explore two case studies: an experimental freshwater algal community and a long‐term time series of displacement in an intertidal community. We demonstrate how our method helps recover observed coexistence patterns, capture the core dynamics of the system, and, in the latter case, predict the impacts of experimental extinctions. Collectively, these results demonstrate an intuitive approach for reconciling observed and empirical data, improving our ability to explore the links between species interactions and coexistence in natural systems.

Herbivores induce plants to undergo diverse processes that minimize costs to the plant, such as producing defences to deter herbivory or reallocating limited resources to inaccessible portions of the plant. Yet most plant tissue is consumed by decomposers, not herbivores, and these defensive processes aimed to deter herbivores may alter plant tissue even after detachment from the plant. All consumers value nutrients, but plants also require these nutrients for primary functions and defensive processes. We experimentally simulated herbivory with and without nutrient additions on red alder ( Alnus rubra ), which supplies the majority of leaf litter for many rivers in western North America. Simulated herbivory induced a defence response with cascading effects: terrestrial herbivores and aquatic decomposers fed less on leaves from stressed trees. This effect was context dependent: leaves from fertilized-only trees decomposed most rapidly while leaves from fertilized trees receiving the herbivory treatment decomposed least, suggesting plants funnelled a nutritionally valuable resource into enhanced defence. One component of the defence response was a decrease in leaf nitrogen leading to elevated carbon : nitrogen. Aquatic decomposers prefer leaves naturally low in C : N and this altered nutrient profile largely explains the lower rate of aquatic decomposition. Furthermore, terrestrial soil decomposers were unaffected by either treatment but did show a preference for local and nitrogen-rich leaves. Our study illustrates the ecological implications of terrestrial herbivory and these findings demonstrate that the effects of selection caused by terrestrial herbivory in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.

Cross‐ecosystem fluxes can intertwine otherwise disparate food webs, but the effects of biodiversity at the genotypic level on fluxes across ecosystems boundaries is not known. Fresh leaves, which vary in traits such as defensive compounds against terrestrial herbivores, drop off trees and enter streams, providing a vital resource for riverine organisms. We demonstrate substantial variation in decomposition rates among individual trees in four different rivers in the Olympic Peninsula of Washington State, USA. We show that locally derived red alder leaf litter decomposes on average 24% faster than red alder leaf litter introduced from other riparian zones. Within rivers, leaves downstream of their parent trees decompose nearly as quickly as leaves from local trees. Leaves upstream of the parent tree decomposed as slowly as leaves from trees growing alongside different rivers. Over time, aquatic decomposer communities have locally adapted to the specific trees supplying the riparian subsidies. In energy‐limited environments, such as small shaded streams, consumers must be efficient foragers. Our results indicate that this pressure for efficiency has led to adaptation at a particularly fine scale. More broadly, these results illustrate how genetic diversity and the effects of selection in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.