Abstract
Many ecosystems are grazed by livestock or large, wild herbivores and exist as mosaics of different vegetation communities. Changing grazing could have an impact on heterogeneity as well as on composition. A long-term, large-scale grazing experiment that maintained existing low-intensity sheep grazing, tripled it, removed it and partially substituted sheep grazing by cattle grazing was set up on a mosaic of upland vegetation types. The impact of changing grazing regimes was assessed in terms of changes in temporal and spatial species and functional beta diversity. Removal of grazing had the highest impact on species replacement, whilst increased grazing was closest to maintaining the original species complement. Wet heath and Molina mire had the lowest turnover, but wet heath showed the highest changes in unidirectional abundance as it contained species capable of increasing in abundance in response to changing grazing intensity. Agrostis-Festuca and Nardus grasslands displayed the highest level of balanced species replacement reflecting their more dynamic vegetation. In functional terms, there was no clear separation of communities based on their grazing preference, all were relatively resistant to change but Nardus grassland was the most resistant to the removal of grazing. The increased offtake associated with increased grazing led to a degree of homogenisation as grazing tolerant species associated with preferred communities increased in the unpreferred ones. Decisions about grazing management of the uplands involve many trade-offs, and this study identified potential trade-offs between stability and homogenisation to add to existing ones on the biodiversity of different groups of species and on ecosystem services.
Highlights
Many ecosystems are grazed by domestic livestock or large, wild herbivores
The vegetation types were distributed relatively evenly across the plots within blocks, with some differences between blocks, e.g., Block F had a higher proportion wet heath whilst Blocks A and B had a high proportion of Molinia mire (Pakeman et al 2019)
Communities were assessed as preferred—AgrostisFestuca grassland, moderately preferred—Molinia mire, bracken, Carex mire, and unpreferred—Nardus grassland and wet heath—based on observational studies (Hunter 1962) and from the capacity of different communities to sustain grazing animals from stocking rates in experimental studies from the same region (Pakeman 2004, 2014)
Summary
Many ecosystems are grazed by domestic livestock or large, wild herbivores. Changing grazing management or herbivore population sizes can drive changes within plant communities leading to shifts in plant species abundances and diversity (Hester et al 2006) and often to shifts in ecosystem function driven through changes in the functional traits of the species present (Milchunas et al 1998). Many communities exist as mosaics driven by interactions between grazing and underlying differences in edaphic or topographic conditions and dominant species in one community may be subordinate species in other communities and vice versa (Rodwell 1991, 1992) Depending on how these dominant and subordinate species react to changes in grazing, a change can lead to increased heterogeneity or homogeneity in species composition and functional traits as abundances and distributions change in response. Within a mosaic of communities that share species, increased grazing could reduce heterogeneity as it would promote species more tolerant or resistant to grazing in communities of intermediate or low preference to the herbivores (Pakeman and Nolan 2009), increasing their similarity to communities of higher preference. Changes in the identity of grazer can influence vegetation dynamics; more selective grazers such as sheep reduce diversity compared to less selective grazers such as cattle (Tóth et al 2018)
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