Overabundant populations of large wild herbivores disrupt plant–pollinator networks in a Mediterranean ecosystem

Large herbivores are keystone species, so changes in their population abundance can have cascading effects on ecosystems. Over the last decades, many species of deer (Cervidae) have experienced unprecedented densities across many areas of the Northern Hemisphere, potentially leading to simplification of plant–pollinator communities, disruption of their interactions, and destabilization of ecological networks. Our study explores the impacts of increasing red deer (Cervus elaphus) densities on plant–pollinator networks.
We describe, to our knowledge, the first large wild herbivore manipulative experiment with increasing densities, consisting of comparable hectare-scale enclosures in a Mediterranean ecosystem. We simulated two current scenarios of deer overabundance: high densities (>30 individuals km−2) and hyper densities (>90 individuals km−2). We compared these scenarios to an adjacent control exclosure (no deer).
Deer herbivory reduced flower abundance of shrubs, as well as flowering plant and pollinator richness. Remaining plants and pollinators lost interactions, and some plants lost pollinator visitors. Network specialization and modularity decreased because modules (groups of strongly connected species) formed by herbs and specialist pollinators were gradually extirpated as deer density increased. This simplification increased network connectance and nestedness. Network robustness (a measure of stability) remained unaltered because the dominant plant, which attracted most pollinators, was unpalatable to deer.
We conclude that, in overabundant deer scenarios: (1) impacts on plant–pollinator networks will increase with increasing deer density; (2) plant–pollinator networks will be eroded, especially if composed of palatable, rare plants, visited by specialist pollinators; but (3) plant–pollinator network stability will not be affected if dominant plants are generalists and unpalatable.