The role of overgrazing and anthropogenic disturbance in shaping spatial patterns of distribution of an invasive seaweed.
Natural and human-generated disturbances are widely acknowledged as key drivers of biological invasions. Understanding the role of spatial patterns of compounded disturbances in determining invader establishment and spread can enhance the effectiveness of control strategies. Using the framework of 1/f noise models and by means of a field experiment, we evaluated how the simultaneous exposure of Posidonia oceanica meadows to disturbances (namely overgrazing and rhizome uprooting from vessel anchoring) differing in spatial extent and degree of autocorrelation influenced the establishment and spread of the invasive seaweed, Caulerpa racemosa. Effects on invader establishment and spread were evaluated by means of mixed-effect models relating spatial patterns of disturbance and presence or abundance of C. racemosa, quantified through spectral coefficients. The spatial distribution of C. racemosa mimicked that of overgrazing, both variables showing positive autocorrelation. Yet, C. racemosa was unable to disperse from disturbed patches into adjacent intact areas. The uprooting of rhizomes did not promote the establishment of C. racemosa, but magnified (∼18 times) the proliferation of the exotic seaweed when coupled with overgrazing. Synthesis and applications. Fish overgrazing of Posidonia oceanica lessens the resistance of seagrass beds to invasion by Caulerpa racemosa. Thus, conservation strategies (e.g. marine protected areas) aiming to restore over-exploited fish populations may indirectly enhance the susceptibility of P. oceanica meadows to invasion by C. racemosa. Uprooting of P. oceanica rhizomes, such as that caused by boat anchoring, exacerbates the effects of overgrazing. The anchoring of recreational vessels should be, therefore, strictly regulated in areas characterized by intense grazing of P. oceanica leaves. More generally, our study shows that native herbivores may indirectly facilitate invasion by reducing the resistance of resident plant communities and suggests that sounded strategies for controlling the establishment and spread of invasive species require taking into account the interactive effects of natural and anthropogenic disturbances.