Modelling Cystoseira s.l.-turf competition via a space-implicit model with border dynamics

This paper presents a model of competition between the arborescent canopy-forming algae Cystoseira s.l., and the smaller size algae forming turf on rocky bottoms. The two algal assemblages are both grazed by sea urchins. The algae and invertebrates involved in the model play an important role in the health of coastal ecosystems and particularly in the conservation of biodiversity-rich habitats of temperate nearshore rocky reefs: the model presented is intended to help describing real ecological processes of canopy degradation and recovery. The model is a 3-species space implicit food web. Its distinctive feature is the inclusion of community border effects in interactions, as only the individuals laying along the canopy border of Cystoseira s.l. forests take part in recruitment and competition with algal turfs, and undergo grazing by sea urchins. Also endogenous recruitment of turf involves only border individuals. These border effects are taken into account in the same way of herd behavior in the mathematical ecology of animal species. Some important features of the model phase space are investigated, focussing on the capacity of the system to mimic the transitions among ecologically different states of the rocky bottom (canopy, turf, and barren).