Space-use by feral cattle and horses shapes vegetation structure in a trophic rewilding area

Feral cattle (Bos taurus) and horses (Equus ferus caballus) are commonly introduced to European rewilding areas to halt vegetation succession and to conserve light-demanding species. Yet, we still do not understand how the habitat preference of animals shapes vegetation structure at the landscape scale. Here, we used spatial preference modeling to understand drivers of space-use based on GPS-collared horses and cattle in a 120-ha rewilding area in Denmark. Using a time series of a satellite-based vegetation productivity index, we tested the ability of animal space-use to explain changes in vegetation, as well as the trend of its spatial variability at the reserve scale, as a measure of landscape-scale vegetation heterogeneity. We expected that animal space-use would be driven mainly by topography and vegetation characteristics and that highly used areas with open vegetation would remain open. We, indeed, found that vegetation density and landscape connectivity were good predictors of space-use preference for both cattle and horses. Additionally, both cattle and horses were strongly attracted to an artificial shelter located inside the reserve, warranting consideration of the use and placement of artificial infrastructure. Space-use diverged during periods of resource scarcity emphasizing the value of introducing a variety of herbivore functional types for optimizing structural ecosystem heterogeneity. As expected, we found that cattle and horses slow down vegetation succession in highly used areas, as shown by the negative correlation between changes in growing season productivity and intensively used areas dominated by short herbaceous and shrubby vegetation. We could also show that the highly used areas showed the largest reductions and the fastest recovery in vegetation greenness following the pan-European drought in 2018. A ~2/3 reduction in herbivore population size subsequent to the drought was followed by a general greening of the landscape, but with no clear relationship with space-use intensity. Our study supports that trophic rewilding with year-round grazing can limit vegetation densification at the landscape scale under near-natural conditions. This is pertinent in the face of accelerating succession toward increasingly dark and tree-dominated vegetation in temperate Europe's natural areas, and the associated biodiversity loss.