Loss of spatial structure after temporary herbivore absence in a high-productivity reed marsh

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.