Top-down vs. bottom-up control on vegetation composition in a tidal marsh depends on scale

Abstract

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.

Human activities accelerated the degradation of saline seepweed red beaches by amplifying top‐down and bottom‐up forces

“…, He and Silliman , Elschot et al. ). Human activities have constituted different types of disturbances to natural coastal wetland ecosystems, which can alter plant composition and distribution by influencing top‐down and/or bottom‐up mechanisms (Crain et al.…”

“…, Elschot et al. ). Understanding the responses of salt marshes to these biotic and abiotic factors is fundamental for assessing structural shifts in these coastal ecosystems (Prach and Walker ).…”

“…These intertidal ecosystems can sequester carbon (blue carbon), maintain healthy fisheries, and protect shorelines. The dynamics of salt marsh communities are impacted by both top-down (e.g., consumers) and bottom-up (e.g., physical factors) forces (Kaminsky et al 2015, Rupprecht et al 2015, Elschot et al 2017. Understanding the responses of salt marshes to these biotic and abiotic factors is fundamental for assessing structural shifts in these coastal ecosystems (Prach and Walker 2011).…”

“…Understanding the responses of salt marshes to these biotic and abiotic factors is fundamental for assessing structural shifts in these coastal ecosystems (Prach and Walker 2011). A large body of literature has examined the dynamics of coastal salt marshes as a function of grazing pressure, drainage conditions, sedimentation accretion, and sea level rise (Leendertse et al 1997, Spivak et al 2007, He et al 2015, Elschot et al 2017). Human activities have constituted different types of disturbances to natural coastal wetland ecosystems, which can alter plant composition and distribution by influencing topdown and/or bottom-up mechanisms (Crain et al 2008, Farina et al 2016.…”

Disentangling grazing and light controls on algal communities in grassland and afforested streams

“…The relative importance of top-down control by herbivores and bottom-up control by environmental conditions on primary production within ecosystems has been the subject of a long-standing debate in ecology (Elschot et al, 2017;Gruner et al, 2008;Hunter & Price, 1992). Environmental conditions may determine the direction and magnitude of top-down and bottom-up forces that control freshwater communities and influence species diversity (Thompson & Townsend, 2005;Werner & Matthiessen, 2013).…”