Ecology and Conservation of Plants in Fragmented Landscapes: Studies of How Habitat Corridors and Edges Alter Insect Herbivory and Seed Dispersal by Birds.

摘要

Habitat fragmentation is a leading cause of population decline and species extinction. Isolating organisms in remnant habitat patches can reduce gene flow and increase organisms' vulnerability to local threats, e.g., disease or predation. Fragmentation also creates habitat edges---altering abiotic conditions in edgy patches---and this can profoundly change the availability of resources and the type and frequency of organisms' interactions. Habitat corridors are popular management tools to mitigate the negative effects of fragmentation, but few studies have assessed the impact of corridors on plants. Corridors can benefit plants by increasing dispersal and gene flow, or corridors can be detrimental to plants by increasing movement and abundance of plants' antagonists. Because corridors are linear landscape features, they also introduce long habitat edges to a landscape. My dissertation research focused on the impact of habitat corridors and edges on plants and plant-animal interactions, specifically insect herbivory and seed dispersal by birds.;I conducted my dissertation research in the world's best-replicated, landscape-scale corridor experiment, at the Savannah River Site (SRS) in South Carolina, USA. I used a single species of native, annual, fruiting plant, Solanum americanum, for all three studies presented here. Solanum americanum is commonly eaten by grasshoppers, which are the primary herbivores at SRS; Solanum americanum is a myrmecophytic plant, frequently defended by ants against herbivores; and S. americanum fruits are consumed by a variety of seed-eating birds. I found that corridors did not increase grasshopper abundance or herbivory in connected patches, and plants in connected patches did not produce fewer fruits. Instead, habitat patch shape, which determines the edge-to-area ratio of a patch, was a more important driver of grasshopper abundance, herbivory and plant fruit production. I also found that grasshopper abundance in the experimental landscapes is strongly determined by edge effects, and an edge gradient in herbivory pressure determined the amount of protection ants provided to plants against herbivory. Finally, I found that corridors did increase rates of seed dispersal by birds, but this corridor effect was only evident during winter; it was not evident during summer, when most seed-eating birds at SRS are territorial.