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Top-down vs. bottom-up control on vegetation composition in a tidal marsh depends on scale
Kelly Elschot1, Anke Vermeulen2, Wouter Vandenbruwaene3
1Conservation Ecology Group, Groningen Institute of Evolutionary Life Sciences GELIFES, University of Groningen, CC Groningen, The Netherlands.
Plos One
|February 4, 2017
Summary
In tidal marshes, sediment accretion (bottom-up) drives long-term vegetation change over decades, while goose foraging (top-down) has minor, short-term impacts at local scales.
Area of Science:
- Ecology
- Environmental Science
- Plant Ecology
Background:
- The interplay between herbivore (top-down) and environmental (bottom-up) controls on vegetation dynamics is a key ecological question.
- Tidal marsh ecosystems are influenced by both grazing pressure and abiotic factors like sediment accretion.
- Understanding the scale-dependent effects of these controls is crucial for predicting marsh resilience.
Purpose of the Study:
- To investigate the relative importance of top-down (goose foraging) and bottom-up (sediment accretion) controls on vegetation composition in a dynamic tidal marsh.
- To determine how spatial and temporal scales influence the impact of herbivory and environmental factors on marsh vegetation.
- To test the hypothesis that these controls can co-occur but operate at different scales.
Main Methods:
- Utilized a dynamic tidal marsh system with a significant Greylag goose population (Anser anser).
- Observed local-scale herbivore foraging impacts (bare patches) and their recovery rates.
- Assessed large-scale, long-term vegetation changes in relation to sediment accretion rates and sea-level rise.
Main Results:
- Greylag geese exerted top-down control locally by creating small, bare patches through grubbing.
- These disturbed patches reverted to their initial vegetation type within approximately 12 years.
- At larger spatial (km²) and temporal (decades) scales, high sediment accretion rates (> sea-level rise) drove vegetation succession towards climax types.
Conclusions:
- Vegetation composition in this tidal marsh is primarily driven by bottom-up sediment accretion operating at large spatial and temporal scales.
- Top-down control by goose herbivory is a secondary process with significant effects only at smaller spatial and temporal scales.
- Scale-dependency is critical in understanding the balance between biotic and abiotic forces shaping ecosystem structure.
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