Density-dependent resource partitioning of temperate large herbivore populations under rewilding

In tropical grazer assemblies with abundant large predators, smaller herbivores have been shown to be limited by predation and food quality, while the larger species are regulated by food abundance. Much less is known about herbivore resource partitioning in temperate grazing ecosystems, where humans typically regulate large animal abundances. The Oostvaardersplassen ecosystem in the Netherlands is a unique multispecies assemblage of cattle, horses, red deer, and geese developed after the initial introduction of a few individuals in 1983. During the first 35 years, this herbivore assemblage without predation or human regulation gradually changed into an increasing dominance of the smaller herbivore species. Carrying capacity was reached around 2008, after which numbers started fluctuating depending on winter conditions. From 2018, management changed and population numbers became regulated for biodiversity and animal welfare reasons; however, population numbers still remained close to carrying capacity for several years. We used eDNA metabarcoding of dung to quantify the diet composition of cattle, horses, red deer, and geese, annually in early winter from 2018 to 2021 and calculated their niche overlap. We found strong interspecific diet overlap. Horse and cattle diets remained mostly stable with fluctuating densities of the different species, while only red deer diet showed density dependence. Interspecific niche overlap decreased with increasing red deer population size, the most abundant species. When calculated as total energy expenditure, we found that niche overlap was more linked to red deer abundance than to total herbivore energy intake. We suggest that red deer changed their diet mainly in response to their own population size, reducing their niche overlap in relation to their population increase. In this case, resource competition reduced resource availability and forced herbivores to consume different plant taxa. We conclude that in this predator-free temperate ecosystem, inter- and intraspecific resource competition are key factors structuring this assemblage of different size herbivores. We find a general competitive advantage of the more diet-flexible red deer over horses and cattle, but with also clear signs of resource partitioning.