From habitat geometry to ecosystem functions in marine mussel beds

Background/Question/Methods

 In terrestrial habitats, the Equilibrium Theory of Island Biogeography and Metapopulation Theory have incorporated the effect of area and isolation to explain patch-occupancy dynamics. More recent theories predict effects of fragmentation on communities from the more complex interplay between size of habitat area and edge. As patch area decreases, the relative proportion of edge increases, with corresponding increase in the perimeter-area ratio. However, few studies have explicitly addressed the independent and interacting effects of perimeter and area on community and ecosystem dynamics. This study aimed to disentangle the relative effects of perimeter, area and their interaction on the structure of the macro-invertebrates community associated with mussel beds and on ecosystem functions measured by nutrient fluxes. We conducted our experiment along the coast of the St Lawrence estuary (Sainte-Flavie, Quebec). Live blue mussels were used to create artificial mussel transplants corresponding to nine combinations of area and perimeter, in a factorial design. Area and perimeter effects on biodiversity and on fluxes of oxygen and ammonium were assessed.

 

Results/Conclusions

Oxygen uptakes were significantly lower for mussel transplants with a small perimeter (160 cm) than a large perimeter (200 cm). Following the same trend, a higher released of ammonium was observed for transplant with a large perimeter. Area had no effect on oxygen fluxes and mortality rate, but there was a significant interaction between area and perimeter on mortality rate. The mortality rate of mussels within the transplants was higher for a large area (800 vs 1200 cm²) with a small perimeter. Preliminary results suggest a negative correlation between perimeter-area ratio and the diversity of associated macro-invertebrates. This study underlines the need to integrate the interactive effects of various metrics of landscape geometry to the study of the relationship between community dynamics and ecosystem functions in fragmented landscapes.