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Dryad

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

Abstract

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 on herbivore resource partitioning in temperate grazing ecosystems, where humans are typically the regulators. In the Oostvaardersplassen ecosystem in The Netherlands, a unique multispecies assemblage of cattle, horses, red deer and geese developed after initial introduction of a few individuals in 1983. During the first 35 years, this herbivore assemblage without predation or human regulation gradually changed into increasing dominance of the smaller herbivore species. Carrying capacity was reached around 2008, after which numbers started fluctuating depending on winter conditions. A population crash, especially of red deer, in winter 2018 led to heavy societal debate around animal welfare, after which active population regulation was introduced. This suggests strong niche overlap and competition between these very different-sized herbivores, possibly due to their homogenising effect on vegetation composition and structure at high densities. We used eDNA metabarcoding of dung to quantify the diet composition of cattle, horse, red deer and geese, annually in early winter from 2018-2021 and calculated their niche overlap. Overall, we found strong diet overlap between species. The diet of horse and cattle remained mostly unaltered and it was the one of red deer that changed the most across the years. Niche overlap decreased with increasing red deer population size, the most abundant species. When calculated as total energy expenditure, we found niche overlap was more linked to the shifts in red deer than to the total herbivore energy fluctuation. We suggest red deer changed their diet mainly in response to their own population size, reducing their niche overlap with increasing red deer population. In this case, resource competition translated into shorter vegetation height, reducing resource availability and forcing herbivores to consume different plant taxa. We conclude that in this temperate ecosystem, inter- and intraspecific resource competition are key factors structuring community composition and dynamics from small to large herbivores, with a competitive advantage of the smaller species, but with also opportunities for resource partitioning.