Benthic megafauna and the functioning of macroalgal forests and urchin barrens

Two temperate rocky reef food-web models, representing the trophic diversity of the Mediterranean rocky reef communities, were built for the two stable states: macroalgal forests and barren grounds, which are characterized by opposite amounts of erect macroalgal biomass. The food webs described here include 46 functional nodes or groups (comprising auto- and heterotroph species) and elucidate how the loss of erect macroalgal biomass from rocky reefs modifies the trophic roles of the different components of the community and how these changes modify its functioning. Results reveal that both alternative states exhibit a dominance of consumers of low trophic levels (TLs), with most of the energy flow occurring within the first three TLs. Detritus plays a significant role in energy recycling, with a substantial proportion of energy flowing into and from detritus in both alternative states. Transfer efficiency from detritus is higher than from primary producers, emphasizing the importance of recycling in supporting ecosystem stability. Functional group analysis indicates different biomass and production distributions between macroalgal forest and barren states, with corresponding rearrangements in the intensity of biomass flow as well as the roles of the distinct functional groups. Benthic megafauna covers different functional roles in barrens and supports a stable food web, counterbalancing the significant reduction in biomass and biodiversity of meio- and macro-fauna observed in this system. Network flow analysis reveals similar complexity and meta-stability between the two systems, despite differences in primary production and energy utilization.