Habitat attributes mediate top-down and bottom-up drivers of community development in temperate and tropical algae

Global threats to ecosystems have galvanized ecologists to better understand the anthropogenic impacts of biotic and abiotic homogenization on ecological trajectories. However, contemporary understanding of spatiotemporal trajectories in ecosystems composed of competitive and consumptive interactions is largely observational or, if more mechanistic, based on simplified models or select aspects of ecological function (e.g., stability of primary production). Here, we describe a pair of 9-month experiments that explore the influence of top-down versus bottom-up drivers of temporal variation within consumer-resource assemblages in tropical and temperate marine ecosystems. Specifically, we examine the trajectories of tropical and temperate algal communities and metacommunities in response to nutrient and herbivory gradients to assess the drivers of spatiotemporal heterogeneity in ecological dynamics. We find that temporal variation (i.e., spatiotemporal asynchrony) in both tropical and temperate algal communities and metacommunities is largely driven by nutrient gradients (i.e., bottom-up effects), whereas herbivory gradients (i.e., top-down effects) appear to have a weaker influence only on tropical algal assemblages. Importantly, we find that the interactive effects of habitat heterogeneity, herbivory levels, and nutrient levels are strongest early (i.e., within 150 days) in algal successional dynamics at the metacommunity scale and that algal communities appear to converge on greater similarity, regardless of environmental conditions, within 300 days. These findings support the notion that temporal dynamics within a trophic level are partially determined by adjacent trophic levels, highlighting the importance of considering biotic and abiotic drivers of spatiotemporal asynchrony both within and across trophic levels within communities.