Abstract
Abstract Grazing can significantly impact spatial heterogeneity and conservation value of ecosystems. Earlier work revealed that overgrazing may stimulate persistent vegetation collapse in low‐productivity environments where vegetation survives by concentrating scarce resources within its local environment. However, it remains unclear whether grazer fluctuations may cause persistent vegetation changes in high‐productivity systems where dense stands facilitate their own survival by hampering grazer access. Here, we experimentally tested how the release from grazing by greylag geese (Anser anser) affects spatial vegetation structure in a highly productive, brackish marsh in which dense reed (Phragmites australis) stands and bare roosting areas coexist. Next, we assessed the resilience of the change in vegetation patterning by reintroducing the geese after a 2‐year exclosure period. During herbivore exclusion, vegetation rapidly colonized the bare areas, while reintroduction of herbivores generated a clear species‐specific response. Specifically, the pioneer species, Bolboschoenus maritimus, was immediately eradicated, while the dense and high structure of P. australis facilitated its own persistence by limiting grazer access. Surface accretion (~1 cm/year) during herbivore exclusion further amplified this herbivore‐inhibiting feedback, because greylag geese primarily rely on waterlogged conditions for grubbing. Synthesis and applications. Our results indicate that temporary reductions in herbivore numbers may induce persistent unfavourable changes in the spatial structure of a high‐productivity system. It is therefore important to first assess whether vegetation changes are naturally reversible or persistent. If state shifts are indeed persistent, sufficiently high grazer densities must be maintained to warrant the favourable heterogeneous system. If changes in vegetation structure negatively impact grazer densities, active management such as sod cutting or mowing may be required to restore ecosystem structure and functions.
Highlights
High spatial heterogeneity is often desired by ecosystem managers as it typically stimulates ecosystem‐level productivity, biodiversity and resilience (Pringle, Doak, Brody, Jocqué, & Palmer, 2010; Stein, Gerstner, & Kreft, 2014; van Nes & Scheffer, 2005)
Our results indicate that temporary reductions in herbivore numbers may induce persistent unfavourable changes in the spatial structure of a high‐productivity system
We followed the following procedure: we first tested whether the response variable was best described by a linear or a nonlinear regression over the plot gradient based on Akaike's information criterion (AIC)
Summary
High spatial heterogeneity is often desired by ecosystem managers as it typically stimulates ecosystem‐level productivity, biodiversity and resilience (Pringle, Doak, Brody, Jocqué, & Palmer, 2010; Stein, Gerstner, & Kreft, 2014; van Nes & Scheffer, 2005). When these interactions are self‐promoting, for instance by stimulating vegetation growth in vegetated patches and inhibiting vegetation development in bare patches, they may theoretically lead to nonlinear ecosystem dynamics and even multiple stable states if such feedbacks are strong enough (Rietkerk & van de Koppel, 1997; Scheffer, Carpenter, Foley, Folke, & Walker, 2001; van de Koppel et al, 1997) In such cases, structural changes in vegetation patchiness as a result of herbivore fluctuations may persist and management strategies aimed at restoring original herbivore numbers may be insufficient (Abraham, Jefferies, & Alisauskas, 2005; Jefferies, Jano, & Abraham, 2006; Peterson, 2002). We demonstrate that a temporary reduction of herbivory may provoke long‐lasting changes, as it allows the vegetation to exert self‐reinforcing feedbacks that exclude herbivores
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
