Larval dispersal drives trophic structure across Pacific coral reefs

Interspecific dispersal constraints suppress pattern formation in metacommunities

Decisions to disperse from a habitat stand out among organismal behaviours as pivotal drivers of ecosystem dynamics across scales. Encounters with other species are an important component of adaptive decision-making in dispersal, resulting in widespread behaviours like tracking resources or avoiding consumers in space. Despite this, metacommunity models often treat dispersal as a function of intraspecific density alone. We show, focusing initially on three-species network motifs, that interspecific dispersal rules generally drive a transition in metacommunities from homogeneous steady states to self-organized heterogeneous spatial patterns. However, when ecologically realistic constraints reflecting adaptive behaviours are imposed-prey tracking and predator avoidance-a pronounced homogenizing effect emerges where spatial pattern formation is suppressed. We demonstrate this effect for each motif by computing master stability functions that separate the contributions of local and spatial interactions to pattern formation. We extend this result to species-rich food webs using a random matrix approach, where we find that eventually, webs become large enough to override the homogenizing effect of adaptive dispersal behaviours, leading once again to predominately pattern-forming dynamics. Our results emphasize the critical role of interspecific dispersal rules in shaping spatial patterns across landscapes, highlighting the need to incorporate adaptive behavioural constraints in efforts to link local species interactions and metacommunity structure. This article is part of the theme issue 'Diversity-dependence of dispersal: interspecific interactions determine spatial dynamics'.

Coral reef fishes reveal strong divergence in the prevalence of traits along the global diversity gradient

Coral reefs are experiencing declines due to climate change and local human impacts. While at a local scale these impacts induce biodiversity loss and shifts in community structure, previous biogeographical analyses recorded consistent taxonomic structure of fish communities across global coral reefs. This suggests that regional communities represent a random subset of the global species and traits pool, whatever their species richness. Using distributional data on 3586 fish species and latest advances in species distribution models, we show marked gradients in the prevalence of size classes and diet categories across the biodiversity gradient. This divergence in trait structure is best explained by reef isolation during past unfavourable climatic conditions, with large and piscivore fishes better represented in isolated areas. These results suggest the risk of a global community re-organization if the ongoing climate-induced reef fragmentation is not halted.

Congruent trophic pathways underpin global coral reef food webs

Ecological interactions uphold ecosystem structure and functioning. However, as species richness increases, the number of possible interactions rises exponentially. More than 6,000 species of coral reef fishes exist across the world's tropical oceans, resulting in an almost innumerable array of possible trophic interactions. Distilling general patterns in these interactions across different bioregions stands to improve our understanding of the processes that govern coral reef functioning. Here, we show that across bioregions, tropical coral reef food webs exhibit a remarkable congruence in their trophic interactions. Specifically, by compiling and investigating the structure of six coral reef food webs across distinct bioregions, we show that when accounting for consumer size and resource availability, these food webs share more trophic interactions than expected by chance. In addition, coral reef food webs are dominated by dietary specialists, which makes trophic pathways vulnerable to biodiversity loss. Prey partitioning among these specialists is geographically consistent, and this pattern intensifies when weak interactions are disregarded. Our results suggest that energy flows through coral reef communities along broadly comparable trophic pathways. Yet, these critical pathways are maintained by species with narrow, specialized diets, which threatens the existence of coral reef functioning in the face of biodiversity loss.

Sweepstake reproductive success and collective dispersal produce chaotic genetic patchiness in a broadcast spawner

A long-standing paradox of marine populations is chaotic genetic patchiness (CGP), temporally unstable patterns of genetic differentiation that occur below the geographic scale of effective dispersal. Several mechanisms are hypothesized to explain CGP including natural selection, spatiotemporal fluctuations in larval source populations, self-recruitment, and sweepstake reproduction. Discriminating among them is extremely difficult but is fundamental to understanding how marine organisms reproduce and disperse. Here, we report a notable example of CGP in the Antarctic limpet, an unusually tractable system where multiple confounding explanations can be discounted. Using population genomics, temporally replicated sampling, surface drifters, and forward genetic simulations, we show that CGP likely arises from an extreme sweepstake event together with collective larval dispersal, while selection appears to be unimportant. Our results illustrate the importance of neutral demographic forces in natural populations and have important implications for understanding the recruitment dynamics, population connectivity, local adaptation, and resilience of marine populations.

Effect of the plastic pollutant bisphenol A on the biology of aquatic organisms: A meta-analysis

Plastic pollution is a global environmental concern. In particular, the endocrine-disrupting chemical bisphenol A (BPA) is nearly ubiquitous in aquatic environments globally, and it continues to be produced and released into the environment in large quantities. BPA disrupts hormone signalling and can thereby have far-reaching physiological and ecological consequences. However, it is not clear whether BPA has consistent effects across biological traits and phylogenetic groups. Hence, the aim of this study was to establish the current state of knowledge of the effect of BPA in aquatic organisms. We show that overall BPA exposure affected aquatic organisms negatively. It increased abnormalities, altered behaviour and had negative effects on the cardiovascular system, development, growth and survival. Early life stages were the most sensitive to BPA exposure in invertebrates and vertebrates, and invertebrates and amphibians seem to be particularly affected. These data provide a context for management efforts in the face of increasing plastic pollution. However, data availability is highly biased with respect to taxonomic groups and traits studies, and in the geographical distribution of sample collection. The latter is the case for both measurements of the biological responses and assessing pollution levels in water ways. Future research effort should be directed towards biological systems, such as studying endocrine disruption directly, and geographical areas (particularly in Africa and Asia) which we identify to be currently undersampled.

Towards a multi-trophic extension of metacommunity ecology

Metacommunity theory provides an understanding of how spatial processes determine the structure and function of communities at local and regional scales. Although metacommunity theory has considered trophic dynamics in the past, it has been performed idiosyncratically with a wide selection of possible dynamics. Trophic metacommunity theory needs a synthesis of a few influential axis to simplify future predictions and tests. We propose an extension of metacommunity ecology that addresses these shortcomings by incorporating variability among trophic levels in 'spatial use properties'. We define 'spatial use properties' as a set of traits (dispersal, migration, foraging and spatial information processing) that set the spatial and temporal scales of organismal movement, and thus scales of interspecific interactions. Progress towards a synthetic predictive framework can be made by (1) documenting patterns of spatial use properties in natural food webs and (2) using theory and experiments to test how trophic structure in spatial use properties affects metacommunity dynamics.

Responses of coral reef fishes to past climate changes are related to life-history traits

Coral reefs and their associated fauna are largely impacted by ongoing climate change. Unravelling species responses to past climatic variations might provide clues on the consequence of ongoing changes. Here, we tested the relationship between changes in sea surface temperature and sea levels during the Quaternary and present-day distributions of coral reef fish species. We investigated whether species-specific responses are associated with life-history traits. We collected a database of coral reef fish distribution together with life-history traits for the Indo-Pacific Ocean. We ran species distribution models (SDMs) on 3,725 tropical reef fish species using contemporary environmental factors together with a variable describing isolation from stable coral reef areas during the Quaternary. We quantified the variance explained independently by isolation from stable areas in the SDMs and related it to a set of species traits including body size and mobility. The variance purely explained by isolation from stable coral reef areas on the distribution of extant coral reef fish species largely varied across species. We observed a triangular relationship between the contribution of isolation from stable areas in the SDMs and body size. Species, whose distribution is more associated with historical changes, occurred predominantly in the Indo-Australian archipelago, where the mean size of fish assemblages is the lowest. Our results suggest that the legacy of habitat changes of the Quaternary is still detectable in the extant distribution of many fish species, especially those with small body size and the most sedentary. Because they were the least able to colonize distant habitats in the past, fish species with smaller body size might have the most pronounced lags in tracking ongoing climate change.

Extensions of Island Biogeography Theory predict the scaling of functional trait composition with habitat area and isolation

The Theory of Island Biogeography (TIB) predicts how area and isolation influence species richness equilibrium on insular habitats. However, the TIB remains silent about functional trait composition and provides no information on the scaling of functional diversity with area, an observation that is now documented in many systems. To fill this gap, we develop a probabilistic approach to predict the distribution of a trait as a function of habitat area and isolation, extending the TIB beyond the traditional species-area relationship. We compare model predictions to the body-size distribution of piscivorous and herbivorous fishes found on tropical reefs worldwide. We find that small and isolated reefs have a higher proportion of large-sized species than large and connected reefs. We also find that knowledge of species body-size and trophic position improves the predictions of fish occupancy on tropical reefs, supporting both the allometric and trophic theory of island biogeography. The integration of functional ecology to island biogeography is broadly applicable to any functional traits and provides a general probabilistic approach to study the scaling of trait distribution with habitat area and isolation.

Colonisation rate and adaptive foraging control the emergence of trophic cascades

Ecological communities are assembled and sustained by colonisation. At the same time, predators make foraging decisions based on the local availabilities of potential resources, which reflects colonisation. We combined field and laboratory experiments with mathematical models to demonstrate that a feedback between these two processes determines emergent patterns in community structure. Namely, our results show that prey colonisation rate determines the strength of trophic cascades - a feature of virtually all ecosystems - by prompting behavioural shifts in adaptively foraging omnivorous fish predators. Communities experiencing higher colonisation rates were characterised by higher invertebrate prey and lower producer biomasses. Consequently, fish functioned as predators when colonisation rate was high, but as herbivores when colonisation rate was low. Human land use is changing habitat connectivity worldwide. A deeper quantitative understanding of how spatial processes modify individual behaviour, and how this scales to the community level, will be required to predict ecosystem responses to these changes.