The role of an ecosystem engineer’s density in modulating the response to a heat wave

Background/Question/Methods It is widely accepted that biodiversity promotes ecosystem functioning through different mechanisms. While we know that the strength of biodiversity effect on functioning depends on the role that species play in the system (e.g. keystone species/ecosystem engineer) rather than the number of species, whether and how such a role may change under extreme climatic events and whether responses are driven by the density of the keystones remains uncertain and more studies are needed. The bivalve Mytilaster minimus is an ecosystem engineer that form dense mussel beds, increasing spatial complexity and hosting biodiversity hotspots across the intertidal Mediterranean rocky shores. Here we assessed the effects of a heat wave on the metabolic functioning of the whole mussel bed (mussels + associated species), M. minimus individuals alone at different density and species associated to the bivalve without mussels. To do so, in autumn 2018, in Southern Italy, along 3 levels of density (low, medium, high), we scraped randomly 6 quadrats 10 x 10 cm (N = 54). Replicates were divided in two groups, one exposed to increasing temperature (T = 38 °C) and the other maintained at environmental temperature. We adopted the heterotrophic oxygen consumption (mg/l O₂ l^-1h^-1g-DW^-1) as a proxy of ecosystem functioning

Results/Conclusions

The metabolic rates of the ecosystem engineer M. minimus alone, under both treated and controlled condition, did not differ from the metabolic rates of the mussel beds; this would indicate that M. minimus play a key role in driving the functioning. Metabolic rates of M. minimus alone and of the whole mussel bed community decreased with the increasing of the density of the mussels. Conversely, when the metabolic rate of the macrofaunal community was considered no pattern was depicted. In addition, the thermal stress didn’t affect neither the mussel bed community, nor the ecosystem engineer M. minimus nor the species assemblages associated to the bivalve (macrofaunal community). However, at medium density, both the metabolic rates of the mussel alone and those of the mussel bed under stressful conditions differed from those under controlled conditions, indicating the presence of mechanisms responsible for this response. We hypothesize that a balance between facilitation and intraspecific competition that varies according to density and environmental conditions could explain that pattern.