Disentangling assembly processes: environmental and biotic filtering jointly drive lagoon fish co-occurrence patterns

In coastal lagoons, fish community assembly is shaped by both environmental filters and biotic interactions, yet their relative importance remains unclear. In this study, we aimed to disentangle the primary drivers of fish community assembly in a tropical coastal lagoon. Additionally, we sought to identify shared or opposite environmental responses and potential biotic interactions that might help elucidate observed co-occurrence patterns. Results revealed that co-occurrence patterns in a tropical lagoon fish community are driven by a near-equal combination of these two processes, challenging the long-standing assumption that environmental filtering is the dominant driver in these systems. Using a joint species distribution model (JSDM) combined with functional guild classification and a literature survey, we found that 53% of co-occurrence patterns were driven by shared environmental responses, while 47% reflected residual correlations, potentially indicative of biotic interactions. Positive and negative associations were balanced, with correlations suggesting predator avoidance, schooling behavior, and possible competition. Our findings highlight that a pluralistic approach, accounting for both abiotic and biotic factors, is essential for a comprehensive understanding of community assembly. While many residual associations could not be confidently attributed to specific ecological processes due to limited trophic information, our results underscore the value of integrating species traits and ecological knowledge to interpret model outputs. We suggest that future research incorporate additional factors, such as indirect biotic interactions, seasonal effects, additional functional traits and environmental harshness, into the modelling of species distributions, particularly in disturbance-prone environments like coastal lagoons where the effects of environmental and biotic filtering can vary depending on environmental harshness.