Large herbivores change the direction of interactions within plant communities along a salt marsh stress gradient

Abstract

Question: How multiple abiotic stress factors combined with herbivory affect interactions within plant communities is poorly understood. We ask how large herbivore grazing affects the direction of plant-plant interactions along an environmental gradient in a salt marsh.Location: Grazed (cattle) and ungrazed salt marshes of the Dutch Wadden Sea island Schiermonnikoog. Here, patches of tall plant communities, dominated by the tough, unpalatable species Juncus maritimus Lam., are found alternating with low-statured, intensively grazed plant communities.Methods: Along the inundation gradient, we measured plant species composition and plant species traits (specific leaf area, specific root length, maximum height and abundance) inside and outside J. maritimus patches in grazed and ungrazed areas. In addition, we measured soil structure parameters (bulk density, soil porosity, clay depth), multiple limiting conditions for plant growth (soil salinity, soil redox, plant canopy light interception), plant biomass, presence of herbivores and abundance of soil macro-detritivores.Results: Under grazing, the palatable grasses Elytrigia atherica (Link) Kergu elen and Festuca rubra L. were positively associated with J. maritimus, while shadeintolerant Puccinellia maritima (Huds.) Parl. and Juncus gerardii Loisel. were negatively associated with this species. Furthermore, macro-detritivore presence was higher inside J. maritimus patches. In ungrazed areas E. atherica and F. rubra were negatively associated with J. maritimus, while P. maritima and J. gerardii were rare. In both grazed and ungrazed conditions the directions of species associations were independent of the inundation gradient. Analysis of species traits and abiotic conditions suggested that associational resistance (a facilitation type) was important in grazed areas. In ungrazed areas, light competition was the likely dominant process.Conclusions: The direction of species associations within these salt marsh communities was strongly affected by grazing, not by the underlying stress gradient. Measurement of species traits indicated that plant-plant interactions shifted from competitive to facilitative under grazing. Besides grazing, cross-trophic facilitation of soil disturbing macro-detritivores may play an importantthus far ignoredrole in structuring plant communities.

Biotically driven vegetation mosaics in grazing ecosystems: the battle between bioturbation and biocompaction

“…Grazing herbivores spend an increasing proportion amount of time in preferred grazing patches (Howison et al. ), therefore increasing urine and dung inputs (Ruess and McNaughton ). In addition, repeated visits by herds to palatable plant‐dominated patches exacerbate these effects, as urine deposition in these preferred areas promotes nutrient‐rich plant growth (McNaughton ).…”

“…, Howison et al. ), thereby reducing their bioturbating effects. This speeds up soil compaction and decreases water infiltration (Howison et al.…”

“…This speeds up soil compaction and decreases water infiltration (Howison et al. ), hence inducing (biotic‐driven) local drought conditions. In nutrient‐based grazing lawns, the eventual return of taller plant species is often simply attributed to patch choice of grazing herbivores (moving to a more preferred patch locally or seasonal migration) and resulting in increased light competition by taller growing species (Frank et al.…”

“…However, bioturbation provides a novel mechanistic explanation for the active reversal of the compacting effects of large herbivores through burrowing, nesting, and foraging activities that occur even in the permanent presence of grazers (Howison et al. ). As illustrated using a model in Fig.…”

Facultative grazing and bioturbation by macrodetritivores alter saltmarsh plant–plant interactions under stress

“…Such cross‐trophic facilitation may profit plants by extending their niche breadth (Bruno, Stachowicz & Bertness ) and influence plant community composition (Howison et al . ; Schrama et al . ).…”