Functional diversity shapes the stability of reef fish biomass under global change

Understanding how environmental and human pressures impact the temporal stability of fish community biomass on shallow reefs is essential for effective conservation and management. These pressures influence community stability directly, by affecting species stability and asynchrony in species’ fluctuations. However, their effects may also indirectly depend on the functional traits of the species composing the community, which remains poorly understood. Here, we examine both direct and indirect, trait-mediated effects of environmental variability and human impacts on species biomass stability and asynchrony in 215 Australian shallow reefs. These communities span a 10-degree sea surface temperature (SST) gradient and have been monitored over 14 years. Our results indicate higher asynchrony in tropical reefs due to higher trait diversity and trait redundancy and higher species stability in temperate communities due to higher mean trophic level. Human impacts, through its negative effects on species stability and trait diversity, was the main destabilising factor of fish community biomass. Temporal change in SST destabilised species biomass while increasing mean trophic level in fish communities. Overall, our findings show that a comprehensive analysis of the multiple facets of functional diversity is crucial to better understand the long-term stability of marine ecosystems under global change.
Methods
We quantitatively evaluate the direct and indirect effects of environmental and human pressures on the temporal stability of fish community biomass. We use an extensive shallow reef monitoring dataset that spans the entire Australian continent during a sea warming period punctuated by multiple extreme marine heatwaves (2008-2021) with observable impacts on fish populations. We focus on 215 fish communities on shallow rocky and coral reefs distributed across Australia, along a spatial gradient of mean sea surface temperature (hereafter mean SST) that ranges from 13.9 to 25.4 °C (mean = 19.3 ± 3.1 °C; Supplementary Figure 1). For each community, we characterise two aspects of climate change that capture (i) the linear trend in sea surface temperature through time (hereafter SST change) [32] and (ii) the temporal variability in SST caused by increasingly frequent marine heatwaves (hereafter CVSST). We also consider the temporal variability in marine primary productivity, measured by chlorophyll a content (hereafter CVChlorophyll), as it can directly influence the stability of fish communities. We assess the intensity of human pressures using human gravity that integrates both reef accessibility and human population density, providing a relevant proxy of the intensity of multiple human activities on shallow reef ecosystems. We estimate four facets of functional diversity (i.e. CWMs, trait diversity, trait redundancy and trait distinctiveness) using four fish functional traits; species maximum length, growth coefficient, trophic level and maximum depth, which relate to both fish species responses to abiotic conditions (e.g. deepening) and fish species contributions to ecosystem functioning (e.g. food web).