Limited niche partitioning and minor effects of interspecific competition promote the near-neutrality of large mammal herbivores

Dietary niche partitioning and emergent neutrality are proposed as two alternative mechanisms for the coexistence of diverse large herbivores. However, there is still a lack of solid evidence in the field to distinguish these two bodies of theory. To examine the role of alternative theories in the coexistence of sika deer and roe deer, we compared the population and individual dietary niche dynamics in sympatric and allopatric areas in Northeast China Tiger and Leopard National Park and applied DNA metabarcoding. Although cryptic niche partitioning occurs between the two deer species, its extent is largely limited and much smaller than the intraspecific niche variation among seasons. No significant effects of competition on niche dynamics have been detected for either ungulate. Niche partitioning is not always greater in sympatric areas or during the nongrowing season. The intraspecific niche shifts synchronously between ungulates. For both deer species, greater individual specialization occurs during the growing season, with certain individuals specializing in grass species, most likely driven by a trade-off between nutritional requirements and tolerance to chemical defenses. Collectively, these results strongly support a minor effect of competition on dietary niche dynamics and a comparable competitive ability in terms of feeding, which promotes the near-neutrality of the two deer species in a temperate forest. However, determining the generality of the coexistence mechanism still urges more studies in the future to test more herbivorous species in more ecosystems.