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Abstract Parasites can profoundly alter host communities. However, the impact of parasites can vary from one community to another. Understanding why the impact of parasites varies across communities is challenging because it requires (i) separating the direct effects of the parasite on the host species from the indirect effects it exerts through the ecological interactions among the hosts and (ii) identifying how the presence of one host alters rates of infection in another.Freshwater fish communities in Trinidad have recently been invaded by a generalist parasitic nematode. This event, combined with our long‐term mark‐recapture studies of these communities, which began long before the invasion, presented a replicated natural experiment. In this experiment, we measured how host demographic rates responded to two ‘treatments’: stage of parasite establishment (before, and 1–2 years, 3–5 years and 6–8 years after invasion) and type of community (killifish‐guppy communities and killifish‐only communities). This design allowed us to infer the direct and indirect impacts of the parasite invasion on killifish communities and determine how the presence of guppies altered infection rates in killifish.The parasitic invasion drastically altered killifish‐guppy communities: the ratio of killifish to guppies changed from 1:2 before the invasion to 1:16 after the invasion. Living with guppies amplified the effects of the parasite on killifish, such that parasite‐related mortality rates of large adult killifish were twice as high in communities with guppies compared to those without.This effect was driven by a doubling of infection rates in large killifish that live with guppies. In a parallel study, we identified the same patterns of size‐ and community‐dependent infection rates of killifish in five separate river systems, implying that this pattern is general in this system.Our study provides mechanistic insight into how parasites alter community structure under natural conditions, via their direct and indirect impacts on host demographic rates. Our work highlights the value of long‐term field studies for our understanding of the impact of parasites on community structure and of ecological interactions in general. 

Abstract 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. 

Density-dependent selection, which promotes contrasting patterns of trait means at different population densities, has a long history in population genetics and ecology. The unifying principle from theory is that density-dependent selection operates on phenotypic traits whose values counter the effects of whatever ecological agent is limiting population growth, be it resource competition, predators, or pathogens. However, the complexity inherent in density dependence means that the same selective process can generate multiple outcomes, depending upon the details of how population density affects vital rates and the age or size structure of a population. Failure to appreciate the potential for multiple outcomes confounded many early studies of the process. Nonetheless, careful empirical work in laboratory studies, long-term field studies, and studies of sexual selection demonstrates the wide reach of density-dependent selection. The inconsistent outcomes observed in these studies call for renewed research into how the details of density dependence channel adaptive responses. 

Female rainbow fish that mate with males exhibiting rare color patterns gain fitness through the reproductive success of their sons. 

In structured populations, persistence under environmental change may be particularly threatened when abiotic factors simultaneously negatively affect survival and reproduction of several life cycle stages, as opposed to a single stage. Such effects can then be exacerbated when species interactions generate reciprocal feedbacks between the demographic rates of the different species. Despite the importance of such demographic feedbacks, forecasts that account for them are limited as individual-based data on interacting species are perceived to be essential for such mechanistic forecasting—but are rarely available. Here, we first review the current shortcomings in assessing demographic feedbacks in population and community dynamics. We then present an overview of advances in statistical tools that provide an opportunity to leverage population-level data on abundances of multiple species to infer stage-specific demography. Lastly, we showcase a state-of-the-art Bayesian method to infer and project stage-specific survival and reproduction for several interacting species in a Mediterranean shrub community. This case study shows that climate change threatens populations most strongly by changing the interaction effects of conspecific and heterospecific neighbours on both juvenile and adult survival. Thus, the repurposing of multi-species abundance data for mechanistic forecasting can substantially improve our understanding of emerging threats on biodiversity. 

Abstract Dispersal is a central life history trait that affects the ecological and evolutionary dynamics of populations and communities. The recent use of experimental evolution for the study of dispersal is a promising avenue for demonstrating valuable proofs of concept, bringing insight into alternative dispersal strategies and trade‐offs, and testing the repeatability of evolutionary outcomes.Practical constraints restrict experimental evolution studies of dispersal to a set of typically small, short‐lived organisms reared in artificial laboratory conditions. Here, we argue that despite these restrictions, inferences from these studies can reinforce links between theoretical predictions and empirical observations and advance our understanding of the eco‐evolutionary consequences of dispersal.We illustrate how applying an integrative framework of theory, experimental evolution and natural systems can improve our understanding of dispersal evolution under more complex and realistic biological scenarios, such as the role of biotic interactions and complex dispersal syndromes.