Exploitation of intertidal grazers as a driver of community divergence

Abstract

Summary 1.The possibility that different assemblages of species may represent alternative stable states has been the subject of much theoretical and empirical work. Alternative stable states may in theory arise from a perturbation of sufficient magnitude that pushes an assemblage from one stable equilibrium point to another. Overfishing is one such disturbance that can lead to cascading communitylevel effects. Yet, whether these different assemblages represent alternative stable states or are the consequence of chronic disturbance from fishing is still a matter of debate. Understanding the mechanisms that drive community stability is fundamental if we are to assess the consequences of anthropogenic impacts on the structure and function of ecosystems to better inform management of disturbed habitats. 2. To investigate the extent to which present-day community state is stable versus being maintained by chronic exploitation, we manipulated the time and intensity of physical disturbance and grazing by limpets in a system where over-harvesting of limpets has led to a regional-scale shift in community structure to one in which algal turfs have replaced barnacles as the primary space occupier in the mid-intertidal. 3. After a 1-year period since disturbance was applied, assemblages in disturbed areas were significantly different from undisturbed areas, but the timing of disturbance and its intensity had little effect on the outcome of succession. Undisturbed areas were highly resistant to new colonization and persisted unchanged throughout the study period. 4. Manipulation of limpet abundance in disturbed patches showed that, where present, limpets successfully prevented the recolonization of space by algal turfs. Moreover, there was evidence that grazing by limpets at the turf ⁄ open-rock boundary effectively pushed the turfs back, extending the area of open-rock. 5. Synthesis and applications. Our findings provide evidence that in this system the dominance by algal turfs does not represent an alternative stable state but that chronic exploitation of limpets leads to the persistence of this community. Conservation strategies aimed at protecting or enhancing limpet abundances (e.g. no-take marine reserves) should allow the gradual restoration of this community to its pre-disturbed state.

Density and distribution of Patella ferruginea in a Marine Protected Area (western Sardinia, Italy): Constraint analysis for population conservation

“…Ramos, 1998;Guerra-García et al, 2004;Espinosa, 2009), the protection of the hot spots of this species should be considered a priority. In fact, MPAs should play a fundamental role as a refuge area for the remaining populations (Espinosa et al, 2009a,b;Martins et al, 2010;Van Hoey et al, 2010). In order to prevent the extinction of P. ferruginea in Sardinia it will be important to combine the knowledge and experience of all stakeholders, as the Spanish experience has shown (MMAMRM, 2008).…”

Shells of Patella aspera as ‘islands’ for epibionts

“…crustose algae) are able to grow (Hawkins & Hartnoll, 1983;Jenkins et al, 2008;Martins et al, 2010). In contrast, the shells of limpets can support a dense cover of algae (including those that are grazed off of the rock) and animals.…”

Congruent Molecular and Morphological Diversity of Macaronesian Limpets: Insights into eco-evolutionary Forces and Tools for Conservation

“…The high levels of exploitation have contributed to severe declines in limpet abundance with many populations considered to be overexploited (Martins et al, 2008). The reductions in limpet abundance is also reported to be altering ecosystem services of the coastal areas, as it has been linked to the increase in turf-forming algae (Martins et al, 2010).…”