Global declines in structurally complex habitats are reshaping both land and seascapes in directions that may change how biological communities respond to warming. Here, we test whether the widespread loss of kelp habitats through overgrazing by sea urchins changes fish community structure in directions that systematically alter warming sensitivity. We use simulations and comparisons of communities from 5996 sites across 19 ecoregions to test for thermal diversity shifts related to habitat. We find that the realized thermal affinities and ranges of fishes from kelp and urchin barrens differ, but only in regions with high initial response diversity. Fish communities in warm-temperate barrens host relatively more warm-affinity species than neighbouring kelp beds, highlighting that urchin herbivory can exacerbate tropicalization processes. By contrast, relatively cool-affinity species colonize cool-temperate barrens and explain apparent lags with ocean warming in these locations. Evidently, urchins are agents of ecological change with implications for climate resilience.

Aquatic ectotherms often attain smaller body sizes at higher temperatures. By analysing ~15,000 coastal-reef fish surveys across a 15oC spatial sea surface temperature (SST) gradient, we found that the mean length of fish length in communities decreases by ~5% for each 1oC temperature increase across space. This equated to a 50% decrease in mean length from 14 to 29oC mean annual SST. We found that trophic guild composition shifts from domination by herbivores and planktivores in the tropics, to invertivores and piscivores in cooler waters. By investigating the contribution of trophic composition to community-level mean length, we show ~25% of temperature-related changes could be attributed to trophic composition at the warmest sites, but <1% at colder temperatures. Our findings suggest that small changes in temperature will lead to large changes in fish community body sizes, driven both by community trophic composition in warm waters and mean sizes within trophic guilds.

Aquatic ectotherms often reach smaller body sizes at higher temperatures. By analysing ~15,000 coastal-reef fish surveys across a 15oC spatial sea surface temperature gradient, we show mean community-level fish length decreased by ~5% for each 1oC warming, equating to a decrease in mean community-level body-length by 50%, or mean weight by 90% from 14-29oC. We further show dominant trophic guild composition shifts from invertivores and piscivores, to herbivores and planktivores, as water temperature increases. By investigating the contribution of trophic-composition to overall community-mean-length, we show ~25% of temperature-related changes could be attributed to trophic-composition at the warmest sites, but close to zero at colder temperatures. Our findings suggest that small changes in temperature will lead to large changes in fish community body sizes, driven both by changes in community trophic composition in warm waters, and by changes to the average body sizes of fish within trophic guilds across all temperatures.

Climate change and fisheries exploitation are dramatically changing the species composition, abundances, and size spectra of fish communities. We explore whether variation in abundance-size spectra, a widely studied ecosystem feature, is influenced by a critical parameter thought to govern the shape of size-structured ecosystems—the relationship between the sizes of predators and their prey (predator-prey mass ratios, or PPMRs). PPMR estimates are lacking for vast numbers of fish species, including at the broader trophic guild scale. Using measurements of 8,128 prey items in gut contents of 97 reef fish species, we established PPMRs for four major trophic guilds (piscivores, invertivores, planktivores and herbivores) using linear mixed effects models. To assess theoretical predictions that higher mean community-level PPMR leads to shallower size spectrum slopes, we compared observations of mean community-level PPMR with size spectrum slopes for coastal reef sites distributed around Australia. PPMRs of individual fishes were remarkably high (median ~71,000), with significant variation between different trophic guilds (~890 for piscivores; ~83,000 for planktivores), and ~8,700 for whole communities. Community-level PPMRs were positively related to size spectrum slopes, broadly consistent with theory, however, this pattern was also influenced by the latitudinal temperature gradient. Tropical reefs showed a stronger relationship between community-level PPMRs and community size spectrum slopes than temperate reefs. The extent that these patterns apply outside Australia, and consequences for community structure and dynamics, are key areas for future investigation.

Global declines in structurally complex habitats are reshaping both land and seascapes in directions that affect biological communities’ responses to warming. Here, we test whether widespread loss of kelp habitats through sea urchin overgrazing systematically changes warming sensitivity of fish communities. Community thermal affinity shifts related to habitat were assessed by simulating and comparing fish communities from 2,271 surveys across 15 ecoregions. We find that fishes in kelp and urchin barrens differ in realized thermal affinities and range sizes, but only in regions where species pools have high variability in species’ thermal affinities. Barrens on warm-temperate reefs host relatively more warm-affinity fish species than neighbouring kelp beds, highlighting acceleration of tropicalization processes facilitated by urchin herbivory. By contrast, proportionally more cool-affinity fishes colonize barrens at high temperate latitudes, contributing to community lags with ocean warming in these regions. Our findings implicate urchins as drivers of ecological change, in part by affecting biological resilience to warming.

Amongst the more widely accepted general hypotheses in ecology is that community relationships between abundance and body size follow a log-linear size spectrum, from the smallest consumers to the largest predators (i.e., “bacteria to whales”). Nevertheless, most studies only investigate small subsets of this spectrum, due to extreme size classes that deviate from these linear expectations. In this study, we fit size spectra to field data from 45 rocky and coral reef sites along a 28° latitudinal gradient, and spanning 11 orders of magnitude in body size, from 3 μg to 150 kg. We found that 97% of the variation in abundance along this ‘extended’ size gradient was described by a single linear function across all sites. Moreover, consistent ‘wobbles’ were also observed, with subtle peaks and troughs in abundance along the spectrum, that related strongly to sea temperature and local site conditions.

The frequency distribution of individual body sizes in animal communities (i.e. the size spectrum) provides powerful insights for understanding the energy flux through food webs. However, studies of size spectra in rocky and coral reef communities typically focus only on fishes or invertebrates due to taxonomic and data constraints, and consequently ignore energy pathways involving the full range of macroscopic consumer taxa. We analyse size spectra with co-located fish and mobile macroinvertebrate data from 3,391 reef sites worldwide, specifically focusing on how the addition of invertebrate data alters patterns. The inclusion of invertebrates steepens the size spectrum, more so in temperate regions, resulting in a consistent size spectrum slope across latitudes, and bringing slopes close to theoretical expectations based on energy flow through the system. These results highlight the importance of understanding contributions of both invertebrates and fishes to reef food webs worldwide.