The effects of neighbouring tree islands on pollinator density and diversity, and on pollination of a wet prairie species, Asclepias lanceolata (Apocynaceae)

The Everglades (Florida, USA) is a mosaic of different habitats. Tropical and temperate trees grow on patches of high ground (tree islands) surrounded by lower elevation wetland communities (marl prairie). Tree islands of various sizes provide nesting substrate, larval host plants and floral resources for insect pollinators. Herbaceous plants in the open surrounding wetlands may also depend on these pollinators. We investigated pollinator diversity and abundances in both tree island and marl prairie habitats using transect sampling methods and estimated pollination success of the milkweed Asclepias lanceolata, an insect-pollinated marl prairie species, in relation to distance from and size of the closest tree island. On a total of 11 bayhead tree islands, we found that insect diversity and abundance were greater on the edge of larger tree islands (20-30 m(2)) than on smaller tree islands (5-10 m(2)). Pollinator diversity and abundance in the marl prairie decreased with increasing distance from tree islands. Pairs of potted A. lanceolata plants were placed in the marl prairie at distances up to 1000 m from small and large tree islands. Fruit and seed production were highest for plants placed less than 25 m from tree islands and decreased with increasing distance. Our results suggest that tree islands are an important source of pollinators for the plants in the tree island and surrounding wetland habitats. This landscape-based study illustrates how overall landscape structure affects important biotic interactions, particularly plant-pollinator relationships. Our findings have far-reaching ecological implications for the reproductive success of plants in small, isolated populations that may depend on insect vectors for pollination.