Browsing by Author "Nick M. Haddad, Committee Chair"

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    The Effects of Habitat Fragmentation and Connectivity on Plant Disease
    (2009-12-22) Johnson, Brenda Lynn; Nick M. Haddad, Committee Chair; Kevin Gross, Committee Member; Charles Mitchell, Committee Member
    Within a large-scale habitat corridor experiment, I performed both experimental and observational studies to determine the effects of habitat fragmentation, habitat edge, and patch connectivity on the movement and incidence of fungal plant diseases. Increased spread of infectious disease is often cited as a potential negative effect of habitat corridors, and increases in the amount of habitat edge that are inevitable byproducts of corridor creation could also impact the incidence and development of plant disease. However, the impacts of corridors and habitat edges on plant disease dynamics remain empirically untested. Using sweet corn and southern corn leaf blight as a model plant-pathogen system, I experimentally tested the impacts of connectivity and habitat fragmentation on pathogen movement and disease development. I found that corridors do not facilitate the movement of wind dispersed plant pathogens, that connectivity of patches does not enhance levels of foliar fungal plant disease, and that edge effects are the key drivers of plant disease dynamics. Over time, less edgy patches had higher proportions of diseased plants, and distance of host plants to habitat edges was the greatest determinant of disease development. To test the effects of habitat connectivity and edge on the incidence of naturally occurring plant disease, I surveyed foliar lesions on three native Lespedeza species. I found that connectivity of habitat patches did not affect levels of disease and that incidence of wind dispersed foliar fungal diseases was significantly higher close to habitat edges, further demonstrating that edge effects play an important role in plant disease dynamics. I also found that density of host plants was significantly higher farther from habitat edges, contradicting previous studies that relate higher host densities to increased disease load. Environmental variables also showed strong edge effects, with significantly higher temperatures and light intensities at the interior of habitat patches, providing possible mechanisms for these disease patterns. Results from both studies show that worries over the potential harmful effects of connectivity on disease dynamics are misplaced, and that, in a conservation context, diseases can be better managed by mitigating edge effects.
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    The effects of patch shape and connectivity on nest site selection and reproductive success of the Indigo Bunting
    (2004-03-01) Weldon, Aimee Jean; Nick M. Haddad, Committee Chair; Christopher E. Moorman, Committee Member; Theodore R. Simons, Committee Member
    Habitat fragmentation and its associated effects have been blamed for the recent population declines of many Neotropical migratory bird species. Increased predation and parasitism resulting from edge-related effects have been implicated for poor nesting success in many studies, mostly of forest interior species. However, little attention has been devoted to disturbance-dependent birds. In this study, I examine how patch shape and connectivity in fragmented landscapes affects the reproductive success of disturbance-dependent bird species, specifically the Indigo Bunting (Passerina cyanea). I conducted my study in a landscape-scale experimental system of similar-area habitat patches that differed in connectivity and in shape. Shapes differed between edgy and rectangular forms, where edgy patches contained 50% more edge than rectangular patches. I tested whether edgy patches function as ecological traps for species with strong edge preferences, by leading them to select dangerous habitats. Indigo Buntings preferentially selected edgy patches over rectangular patches, but experienced significantly lower reproductive success in edgy patches early in the season. Although predation pressure intensified in rectangular patches late in the season, seasonal fecundity was still significantly lower in edgy patches, providing the first empirical evidence that edges can function as ecological traps for Indigo Buntings. A second objective of my study was to evaluate the efficacy of conservation corridors for disturbance-dependent bird species. Conservation corridors have become a popular strategy to preserve biodiversity and promote gene flow in fragmented landscapes, but corridors may also have negative consequences. I tested the hypothesis that corridors can increase nest predation risk in connected patches relative to unconnected patches. Nest predation rates increased significantly in connected patches compared to unconnected rectangular patches, but were similar between connected patches and unconnected edgy patches. This suggests that the increase in predator activity in connected patches is largely attributable to edge effects incurred through the addition of a corridor. This is the first landscape-scale study to experimentally demonstrate the potential negative effects of conservation corridors.