Plant Community Response to Landscape Connectivity and Patch Shape

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Title: Plant Community Response to Landscape Connectivity and Patch Shape
Author: Damschen, Ellen Ingman
Advisors: Nicholas M. Haddad, Committee Chair
James F. Gilliam, Committee Co-Chair
Thomas R. Wentworth, Committee Member
George R. Hess, Committee Member
Abstract: Land transformation is the single most important factor promoting the global loss of terrestrial biological diversity. Remaining habitat fragments contain more edges, less interior habitat, and are more isolated from other habitat fragments, all of which decrease rates of colonization following local extinctions, reduce reproductive rates and gene flow between populations, and ultimately lead to species extinctions. The best approach to prevent species loss, therefore, is to preserve greater areas of habitat. In many cases, however, habitat has already been fragmented and strategies are needed to configure and manage the remaining land. Land managers often create reserve networks that incorporate the use of landscape corridors, linear strips of habitat connecting isolated patches, to reduce species loss by increasing colonizations and decreasing extinctions. Most empirical tests of corridors have been limited to individuals and populations, leaving corridor effects on diversity largely unknown, especially at large spatial scales. Additionally, only a handful of studies have examined corridor effects on plants, which may be especially sensitive to the abiotic changes resulting from alterations in patch shape due to dispersal limitation. Using one of the best-replicated, large-scale habitat fragmentation experiments, I tested explicitly for corridor effects on plant community diversity and composition by examining the established plant community and the soil seedbank. My experimental design distinguished among the three possible ways corridors can affect between-patch processes: by acting as a movement conduit between connected patches ('connectivity effects'), by increasing area alone ('area effects'), and by intercepting organisms moving across the landscape and filtering them into connected patches ('drift-fence effects'). Additionally, I tested for the importance of within-patch edge effects because corridors increase the amount of edge relative to core habitat in a given patch. I provide evidence that corridors increase plant diversity through a combination of connectivity, drift-fence, and edge effects that can be largely predicted from plant dispersal modes. Biotically dispersed plant species (e.g., by birds and mammals) were most affected by connectivity effects, while passively-dispersed species (e.g., by wind or gravity) were most affected by drift-fence effects. Resource managers should consider these differential responses relative to their conservation goals for particular species or communities. In a related experiment, I tested for effects of habitat edges on plant performance, which are known to have impacts on abiotic conditions and biotic interactions. Edge effects have been especially well documented for forest-dwelling species along edges created by clearing or disturbing the surrounding habitat, but edge effects for historically open-habitat species along edges of forests have been virtually ignored. This is the case for many native herbaceous species in the southeastern United States that once existed in historically open longleaf pine forests but are now restricted to openings in modern densely planted pine forests. I tested empirically for edge effects of open habitat species by planting nine species of native longleaf pine forest herbs (three grasses, two asters, and four legumes) in equal densities at six distances (0, 6, 12, 25, 50, and 100 m) from an edge of a dense forest into an adjacent opening. I measured plant growth and flowering as well as available light and soil conditions. Using multivariate analysis of variance (MANOVA), I determined that plant growth and flowering were both significantly affected by the distance to the open habitat edge. Responses for individual species differed, in that some species performed best near the edge while others performed worst. Incident photosynthetically active radiation, soil moisture, and competitive interactions with other study species are likely responsible for the observed tradeoffs in performance. Resource managers should consider thinning the overstory of longleaf pine forests, establishing uneven age distributions within pine stands, and initiating regular low-intensity understory burns for the conservation of these understory species.
Date: 2005-09-19
Degree: PhD
Discipline: Zoology

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