Herbivory mediates the expansion of an algal habitat under nutrient and CO2 enrichment

Certain environmental conditions facilitate the control of primary producers by herbivores. Environmental change can, therefore, mediate the strength of consumption relative to production such that the abundance of primary producers is altered, potentially driving phase-shifts from one habitat type to another (e.g. the displacement kelp forests by mats of turfs along temperate coasts). Here, we assessed the extent to which herbivores may counter the increased cover of kelp-competitors (i.e. turfs) anticipated to be caused by enriched nutrients and CO2. Specifically, we experimentally enriched nutrients and CO2 and quantified the change in cover of algal turfs in the presence and absence of gastropod grazers. In the absence of grazers, turf algae responded positively to the combination of enriched nutrients and CO2 such that they occupied >95% of available space. In contrast, where grazers were present, sparse covers of algal turfs were maintained even under CO2 and nutrient enrichment (i.e. turfs occupied <5% of available space). This result indicates that consumption by grazers increased where enrichment of nutrients and CO2 facilitated greater algal cover such that cover continued to be largely restricted. By recognising the environmental conditions which will maintain or disrupt the balance between the processes of production and consumption, we may improve forecasts regarding the probability of habitat stasis or shifts.