Disturbance often results in small changes in community structure, but the probability of transitioning to contrasting states increases when multiple disturbances combine. Nevertheless, we have limited insights into the mechanisms that stabilise communities, particularly how perturbations can be absorbed without restructuring (i.e. resistance). Here, we expand the concept of compensatory dynamics to include countervailing mechanisms that absorb disturbances through trophic interactions. By definition, 'compensation' occurs if a specific disturbance stimulates a proportional countervailing response that eliminates its otherwise unchecked effect. We show that the compounding effects of disturbances from local to global scales (i.e. local canopy-loss, eutrophication, ocean acidification) increasingly promote the expansion of weedy species, but that this response is countered by a proportional increase in grazing. Finally, we explore the relatively unrecognised role of compensatory effects, which are likely to maintain the resistance of communities to disturbance more deeply than current thinking allows.
The combination of ocean warming and acidification brings an uncertain future to kelp forests that occupy the warmest parts of their range. These forests are not only subject to the direct negative effects of ocean climate change, but also to a combination of unknown indirect effects associated with changing ecological landscapes. Here, we used mesocosm experiments to test the direct effects of ocean warming and acidification on kelp biomass and photosynthetic health, as well as climate-driven disparities in indirect effects involving key consumers (urchins and rock lobsters) and competitors (algal turf). Elevated water temperature directly reduced kelp biomass, while their turf-forming competitors expanded in response to ocean acidification and declining kelp canopy. Elevated temperatures also increased growth of urchins and, concurrently, the rate at which they thinned kelp canopy. Rock lobsters, which are renowned for keeping urchin populations in check, indirectly intensified negative pressures on kelp by reducing their consumption of urchins in response to elevated temperature. Overall, these results suggest that kelp forests situated towards the low-latitude margins of their distribution will need to adapt to ocean warming in order to persist in the future. What is less certain is how such adaptation in kelps can occur in the face of intensifying consumptive (via ocean warming) and competitive (via ocean acidification) pressures that affect key ecological interactions associated with their persistence. If such indirect effects counter adaptation to changing climate, they may erode the stability of kelp forests and increase the probability of regime shifts from complex habitat-forming species to more simple habitats dominated by algal turfs.
Aim Biological invasions are among the main threats to biodiversity. To promote a mechanistic understanding of the ecological impacts of non-native seaweeds, we assessed how effects on resident organisms vary according to their trophic level.Location Global.Methods We performed meta-analytical comparisons of the effects of nonnative seaweeds on both individual species and communities. We compared the results of analyses performed on the whole dataset with those obtained from experimental data only and, when possible, between rocky and soft bottoms.Results Meta-analyses of data from 100 papers revealed consistent negative effects of non-native seaweeds across variables describing resident primary producer communities. In contrast, negative effects of seaweeds on consumers emerged only on their biomass and, limited to rocky bottoms, diversity. At the species level, negative effects were consistent across primary producers' response variables, while only the survival of consumers other than herbivores or predators (e.g. deposit/suspension feeders or detritivores) decreased due to invasion. Excluding mensurative data, negative effects of seaweeds persisted only on resident macroalgal communities and consumer species survival, while switched to positive on the diversity of rocky-bottom consumers. However, negative effects emerged for biomass and, in rocky habitats, density of consumers other than herbivores or predators.Main conclusions Our results support the hypothesis that seaweeds' effects on resident biodiversity are generally more negative within the same trophic level than on higher trophic guilds. Finer trophic grouping of resident organisms revealed more complex impacts than previously detected. High heterogeneity in the responses of some consumer guilds suggests that impacts of non-native seaweeds at higher trophic levels may be more invader-and species-specific than competitive effects at the same trophic level. Features of invaded habitats may further increase variability in seaweeds' impacts. More experimental data on consumers' response to invasion are needed to disentangle the effects of nonnative seaweeds from those of other environmental stressors.
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