Latitudinal gradients in species interactions are widely cited as potential causes or consequences of global patterns of biodiversity. However, mechanistic studies documenting changes in interactions across broad geographic ranges are limited. We surveyed predation intensity on common prey (live amphipods and gastropods) in communities of eelgrass (Zostera marina) at 48 sites across its Northern Hemisphere range, encompassing over 37° of latitude and four continental coastlines. Predation on amphipods declined with latitude on all coasts but declined more strongly along western ocean margins where temperature gradients are steeper. Whereas in situ water temperature at the time of the experiments was uncorrelated with predation, mean annual temperature strongly positively predicted predation, suggesting a more complex mechanism than simply increased metabolic activity at the time of predation. This large‐scale biogeographic pattern was modified by local habitat characteristics; predation declined with higher shoot density both among and within sites. Predation rates on gastropods, by contrast, were uniformly low and varied little among sites. The high replication and geographic extent of our study not only provides additional evidence to support biogeographic variation in predation intensity, but also insight into the mechanisms that relate temperature and biogeographic gradients in species interactions.

The rapid evolution of non‐native species can facilitate invasion success, but recent reviews indicate that such microevolution rarely yields expansion of the climatic niche in the introduced habitats. However, because some invasions originate from a geographically restricted portion of the native species range and its climatic niche, it is possible that the frequency, direction, and magnitude of phenotypic evolution during invasion have been underestimated. We explored the utility of niche shift analyses in the red seaweed Gracilaria vermiculophylla, which expanded its range from the northeastern coastline of Japan to North America, Europe, and northwestern Africa within the last 100 years. A genetically informed climatic niche shift analysis indicates that native source populations occur in colder and highly seasonal habitats, while most non‐native populations typically occur in warmer, less seasonal habitats. This climatic niche expansion predicts that non‐native populations evolved greater tolerance for elevated heat conditions relative to native source populations. We assayed 935 field‐collected and 325 common‐garden thalli from 40 locations, and as predicted, non‐native populations had greater tolerance for ecologically relevant extreme heat (40°C) than did Japanese source populations. Non‐native populations also had greater tolerance for cold and low‐salinity stresses relative to source populations. The importance of local adaptation to warm temperatures during invasion was reinforced by evolution of parallel clines: Populations from warmer, lower‐latitude estuaries had greater heat tolerance than did populations from colder, higher‐latitude estuaries in both Japan and eastern North America. We conclude that rapid evolution plays an important role in facilitating the invasion success of this and perhaps other non‐native marine species. Genetically informed ecological niche analyses readily generate clear predictions of phenotypic shifts during invasions and may help to resolve debate over the frequency of niche conservatism versus rapid adaptation during invasion.

Within our lakes, streams, estuaries, and oceans, there is an astounding chemodiversity of secondary metabolites produced by microbes, algae, and invertebrates. Nearly 30 years of study have yielded hundreds of examples in which secondary metabolites alter the foraging behavior or fitness of aquatic consumers, or both. However, our understanding of the mechanisms that mediate the fate and consequences of these metabolites in aquatic consumers remains in its infancy. Interactions between metabolites and consumers at the molecular and biochemical level are the purview of modern pharmacology, which is rooted in the long history of human–drug interactions and can be adopted for ecological studies. Here, we argue that a pharmacological approach to consumer–prey interactions will be as productive within aquatic systems as it has been for understanding terrestrial systems. We review the diversity of secondary metabolites in aquatic organisms, their known effects on the feeding behaviors and performance of aquatic consumers, and the few studies that have attempted to describe their biochemical manipulation within consumer tissues, i.e., their absorption, distribution, metabolism (including detoxification), and excretion. We then highlight vexing issues in the ecology and evolution of aquatic consumer–prey interactions that would benefit from a pharmacological approach, including specialist-versus-generalist feeding strategies, dietary mixing, nutrient–toxin interactions, and taste. Finally, we argue that a pharmacological approach could help to predict how consumer–prey interactions are altered by global changes in pH, water temperature and ultraviolet radiation, or by pollution. Arguably, the state of knowledge of aquatic consumer–prey interactions is equivalent to that faced by ecologists studying terrestrial herbivores in the 1970s; the literature documents profound variation among consumers in their feeding tolerances for secondary metabolites without a thorough understanding of the mechanisms that underlie that variation. The subsequent advancement in our understanding of terrestrial herbivores in the intervening decades provides confidence that applying a pharmacological approach to aquatic consumers will prove equally productive.

Populations evolve generalist, specialist, and plastic strategies in response to environmental heterogeneity. Describing such within-species variation in phenotype and how it arises is central to understanding a variety of ecological and evolutionary topics. The literature on phenotypic differences among populations is highly biased; for every one article published on a marine species, at least 10 articles are published on a terrestrial species and eight focus on terrestrial plants. Here, I outline what we know from the marine literature about geographic variation in phenotype in the sea, with a principal focus on local adaptation. The theory of environmental “grain” predicts that the most likely evolutionary response (e.g., local adaptation, phenotypic plasticity, generalism, and balanced polymorphism) depends on the spatial scale of environmental variation relative to the distance that an organism disperses. Consistent with these predictions, phenotypic plasticity is stronger among invertebrates with geographically broad dispersal versus restricted dispersal (i.e., planktonic-dispersers versus direct-developers). However, contrary to predictions, the relative frequency, and spatial scale of local adaptation is not consistently greater among direct-developers relative to planktonic disperers. This indicates that the likelihood of local adaptation depends on other organismal or environmental traits. Two of the most vexing issues that remain include (1) predicting the extent to which barriers to dispersal are a cause versus consequence of phenotypic differentiation and (2) delineating the relative importance of evolutionary forces that favor or impede local adaptation. Understanding the mechanistic basis of the geography of phenotypic differences, or phenogeography, has gained recent momentum because of a need to predict impacts of global climatic change, anthropogenic disturbances, and dispersal of organisms to non-native habitats.