Bird-made fruit orchards in northern Europe:: nestedness and network properties

Frugivorous birds are involved in the distribution of fleshy-fruited plants. In a temperate forest in northern Europe, we investigated how fruit-eating birds and habitat variation affect the local distribution of these plants and what the consequences are to the species composition of the fruit-bearing plant community. We subdivided the forest into 25x25 quadrates and mapped the distribution of all fleshy-fruited plant species (18 species) expected to have their seeds dispersed by birds. Our specific aims were (i) to test if the distribution of the fleshy-fruited species in the forest was clumped, (ii) nested, and (iii) to describe the set of species as an interacting network. Our results show that bird-dispersed fleshy-fruited species in a temperate forest constitute a set of orchards with a strong spatial structure. The distributions of species were highly correlated and nested. We suggest that factors involved in dispersal and colonization are mainly responsible of a nested structure, where rare species are only found together with more widespread species and species-poor sites only contain widespread species. Besides, the community had small-world topology with high clustering and short path length between species. This makes the network robust against random perturbations.