Top-down vs. bottom-up control on vegetation composition in a tidal marsh depends on scale.
The relative impact of top-down control by herbivores and bottom-up control by environmental conditions on vegetation is a subject of debate in ecology. In this study, we hypothesize that top-down control by goose foraging and bottom-up control by sediment accretion on vegetation composition within an ecosystem can co-occur but operate at different spatial and temporal scales. We used a highly dynamic marsh system with a large population of the Greylag goose (Anser anser) to investigate the potential importance of spatial and temporal scales on these processes. At the local scale, Greylag geese grub for below-ground storage organs of the vegetation, thereby creating bare patches of a few square metres within the marsh vegetation. In our study, such activities by Greylag geese allowed them to exert top-down control by setting back vegetation succession. However, we found that the patches reverted back to the initial vegetation type within 12 years. At large spatial (i.e. several square kilometres) and temporal scales (i.e. decades), high rates of sediment accretion surpassing the rate of local sea-level rise were found to drive long-term vegetation succession and increased cover of several climax vegetation types. In summary, we conclude that the vegetation composition within this tidal marsh was primarily controlled by the bottom-up factor of sediment accretion, which operates at large spatial as well as temporal scales. Top-down control exerted by herbivores was found to be a secondary process and operated at much smaller spatial and temporal scales.
Conservation Ecology Group, Groningen Institute of Evolutionary Life Sciences GELIFES, University of Groningen, CC Groningen, The Netherlands.
Kelly
K
Elschot
Ecosystem Management Research Group, University of Antwerp, Universiteitsplein, Wilrijk, Belgium.
Anke
A
Vermeulen
Flanders Hydraulics Research, Flemish Government, Berchemlei, Antwerpen, Belgium.
Wouter
W
Vandenbruwaene
Conservation Ecology Group, Groningen Institute of Evolutionary Life Sciences GELIFES, University of Groningen, CC Groningen, The Netherlands.
Jan P
JP
Bakker
NIOZ Royal Netherlands Institute for Sea Research, Department of Estuarine and Delta Systems and Utrecht University, AC Yerseke, the Netherlands.
Tjeerd J
TJ
Bouma
Sovon, Dutch Centre for Field Ornithology, GA Nijmegen, The Netherlands.
Julia
J
Stahl
Natuurbeschermingsvereniging De Steltkluut, AH Terneuzen, The Netherlands.
Henk
H
Castelijns
Ecosystem Management Research Group, University of Antwerp, Universiteitsplein, Wilrijk, Belgium.
Stijn
S
Temmerman
D000818:Animals; D017753:Ecosystem; D005777:Geese; D019015:Geologic Sediments; D010944:Plants; D053833:Wetlands