Abstract
We analysed data collected between 1954 and 2000 from nine long-term experiments designed to assess the effects of sheep-grazing versus no-sheep-grazing at Moor House NNR, an Environmental Change Network site. The experiments were set up between 1954 and 1972 across a range of vegetation types typical of much of upland Britain. Data from this type of experiment are often difficult to analyse and we describe the procedures undertaken to prepare the data for analysis. We fitted the resultant data to the British National Vegetation Classification and used ordination techniques to assess the relative positions of the experiments to each other. Finally, we used Generalized Linear Mixed-effects Modelling within a Bayesian framework to model change of species taxonomic/physiognomic groups through time in both sheep-grazed and ungrazed treatments across all nine experiments; variables included species diversity, Shannon–Weiner index and derived data on occurrence and abundance of species groups based on taxonomy and physiognomy. Hurdle analysis was used to model the species groups; this analysis separated the change through time in both probability of occurrence (binomial distribution) and abundance (Poisson distribution).In the sheep-grazed plots (the “business-as-usual” treatment hence here designated the “control”) there was a reduction in species diversity and a decrease in abundance of vascular plants, grasses, lichens, liverworts and mosses; whereas herbs, sedges and shrubs increased. When probability of occurrence was considered, there was a reduction in number of presences of both lichens and liverworts. Thus, the status quo management of continuous sheep-grazing, even though reduced since 1972, has resulted in a marked change in species composition of these plant communities, with some winners and some losers, but overall they support the concept of biotic homogenization. It is likely that some of these changes were driven by external factors such as elevated atmospheric nutrient deposition. Removal of sheep grazing had some positive benefits; with the herbs, mosses, sedges and shrubs increasing, but faster reductions in grasses and liverworts. Sedges+rushes were stable. It suggested that future monitoring schemes might use either the probability of occurrence of liverworts and lichens, or the abundance of lichens, liverworts, grasses and mosses as sensitive indicators of change in upland Britain.Moreover, during the period that Moor House has been protected as a nature reserve some key plant species groups have declined in spite of reductions in grazing pressure. To reverse this trend requires some form of interventionist management. In order to increase the diversity of vascular plants some form of disturbance will probably be needed, but for bryophytes and lichens this remains a research question. If the results from these small-scale experiments are replicated at the landscape scale a reduction of sheep grazing pressure, for example in rewilding schemes, will have little effect on species composition over a 28–44 year period.
Highlights
IntroductionThere are many examples of such experimental studies, but there are two main types: the first are experiments that measure the effects of applied treatments in a single location, well-known examples include the early Breckland grass-heath experiments of A.S. Watt (Watt, 1957, 1960a,b, 1962) and more recent ones such as the Buxton Climate Change Impacts Laboratory (Bates et al, 2005; Grime et al, 2008), Cedar Creek Ecosystem Science Reserve (Wilson and Tilman, 1993; Tilman, 1994; Tilman et al, 1994), and the Park Grass Experiment at Rothamsted Experimental Station (Tilman et al, 1994; Silvertown et al, 2006)
The nine experiments were located across the Moor House reserve to cover the range of variation in the vegetation across the area, i.e. from relatively productive Agrostis-Festuca grassland on brown-earth soils and a calcareous flush at the neutral end of the soil spectrum through grasslands dominated by Festuca ovina or Nardus stricta, to rush (Juncus squarrosus), sedge (Eriophorum spp.) and dwarf shrub Calluna vulgaris, Erica tetralix or Empetrum nigrum-dominated vegetation on blanket bog
There was a discrepancy (Table 2) between the original description of Festucetum for Hard Hill and Little Dun Fell (Eddy et al, 1969) which was classified as H19 (Vaccinium myrtillus-Cladonia arbuscula heath: Festuca ovina-Galium saxatile sub-community)
Summary
There are many examples of such experimental studies, but there are two main types: the first are experiments that measure the effects of applied treatments in a single location, well-known examples include the early Breckland grass-heath experiments of A.S. Watt (Watt, 1957, 1960a,b, 1962) and more recent ones such as the Buxton Climate Change Impacts Laboratory (Bates et al, 2005; Grime et al, 2008), Cedar Creek Ecosystem Science Reserve (Wilson and Tilman, 1993; Tilman, 1994; Tilman et al, 1994), and the Park Grass Experiment at Rothamsted Experimental Station (Tilman et al, 1994; Silvertown et al, 2006). These multisite studies are less common than those on single sites and are more complex to analyse (Alday et al, 2013; Alday and Marrs, 2014)
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