Associate Editor, Meredith Root-Bernstein discusses the short-term effects of rewilding projects and the recently published paper, Experimental rewilding enhances grassland functional composition and pollinator habitat use by Garrido et al.

Rewilding has attracted attention as an emerging approach to nature conservation in areas where large animals and their ecosystem functions are missing. In Europe, ecological processes carried out by large herbivores may have been significantly altered or lost twice: first with the extinctions and range reductions of animals like aurochs, tarpans, wisent or moose, and secondly with the more recent loss of traditional grazing regimes of livestock, in the form of land abandonment (or indeed, industrial farming). Large grazers and browsers may play a number of important roles in maintaining particular habitat types or successional patterns, for example by creating “intermediate disturbances” to the soil and herbs or to shrubs and trees; by stimulating or repressing growth or plant-plant competition in certain plant species; or through nutrient recycling in the form of their urine, dung and biomass. Despite an increasing number of restoration and habitat management projects that are inspired by the idea of rewilding, there is not always data to assess the scale and timing of the ecological impacts of reintroducing large herbivores to habitats now lacking them.

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‘insect species, in particular pollinators, are sensitive to these small-scale habitat structural and compositional changes, adjusting their behaviour to benefit from them’

This study by Garrido et al. is thus a welcome addition to our understanding of the effects of reintroducing a large grazer to abandoned grasslands in wood-pasture mosaics. The study took place in Sweden, near Uppsala.  In this area there are currently roe deer, which browse and sometimes graze, and moose, which primarily browse. Wild horses were present in southern Sweden at the beginning of the Holocene, and feral domestic horses have been present there from perhaps 4700 years ago to 1932. Thus, the loss of wild or feral horses likely left an ecological gap, with no animals carrying out grazing functions. The experiment reintroduced Gotland Russ horses at a density of about 4 horses per 10 ha, and set up grazing exclusions matched with horse-grazed sites. The researchers predicted that reintroduction of grazing would increase the number of plant species due to an increase in plant species with ruderal traits—that is traits adapted to early successional sites, after disturbances—and that it would attract a greater richness of pollinators (bees and butterflies), compared to the matching grazing exclusion sites.

Over three years, the researchers found an increase in community ruderal traits (larger leaf size and lower plant height) compared to a reduction in ruderal traits in the ungrazed sites.  Species richness was also higher in grazed than ungrazed plots. In addition, the mean number of bee and butterfly species, and the time they spent feeding and resting in grazed plots, was higher than in ungrazed plots. The number of butterfly and bee species observed on grazed plots increased with observed plant richness.

This study demonstrates that the reintroduction of a large grazer with an ecological history in the region can lead to rapid responses in the number of plant species, functional type and form. The particular role of ruderal species and ruderal functional traits has often been surprisingly overlooked in studies of disturbance ecology. It is also useful and interesting to see that insect species, in particular pollinators, are sensitive to these small-scale habitat structural and compositional changes, adjusting their behaviour to benefit from them. This study thus gives an idea of the scale and kind of changes that one might expect to see in rewilding projects over short time scales. How ecological changes due to reintroductions will accumulate over medium and long terms has been documented even less, but hopefully further research will provide us with an increasing number of case studies of ecological transformation.

Read the full paper, Experimental rewilding enhances grassland functional composition and pollinator habitat use in Journal of Applied Ecology.