Rewilding means many things to many people. At its simplest, it involves the removal of stock-grazing. Where the vegetation has been over-grazed for a long-period, we hypothesise that the surviving species will have similar traits and when grazing is removed we expect that new colonists would be more nutritious, more palatable, more decomposable and have fewer anti-herbivore defences. We tested these hypotheses using a suite of long-term experiments, each with a paired sheep-grazed plot and an exclosure with no sheep-grazing, at Moor House NNR in northern England. In 2015, we took replicate samples of common species, found in both the grazed plot and exclosure, and samples of seven “focal” species, that had either proliferated from a very small base, or had invaded, in the absence of sheep-grazing. Samples were analysed for macro-nutrients, micro-nutrients and proxies for plant digestibility, decomposability and herbivore defence. Finally, we charted the abundance of the focal species over the last 70 years to assess the speed of increase in the absence of grazing.
Results/Conclusions
The focal species all had at least one macro-nutrient in much greater concentration than the common species. Six of the seven species were also much more palatable, i.e. they had lower ADF/NDF concentrations, had greater useable energy and/or crude protein; the exception was Dryopteris dilatata, a fern. For micro-nutrients the pattern was less clear with one or two species showing increased values for some elements. Compared to the common species, the focal species also tended to have low C:N ratios (faster decomposition) and lower Si concentrations (lower herbivore defence). Taken together, these results indicate that after sheep-grazing has ceased, the species that are increasing/colonizing have been suppressed/extirpated because of their overall good nutritional status. This is evidence supports Frank Fraser Darling’s “wet desert hypothesis” which stated that the low diversity of the British uplands was caused by sheep over-grazing. Charting the abundance of the focal species through time shows almost no increase, or very little detectable increase, in the sheep-grazed plots, but a slow increase where grazing is removed; taking between 20-30 years (depending on experiment) for the increase to begin. This suggests that it will take some time for species diversity to increase in simple rewilding schemes.