Large Herbivore Impacts on Demographic Characteristics and Population Dynamics of an Endangered Orchid (Spiranthes parksii Correll)
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Spiranthes Parksii Correll is an endangered orchid of Texas. Populations of S. parksii are threatened by habitat loss and degradation resulting from human population growth and attendant development and resource extraction. Conservation easements have been established for the protection of the species, but little is known about the population dynamics of S. parksii and the biotic and abiotic factors which drive them. A baseline of ecological information is necessary to the creation of effective management plans for the conservation of this species. The work presented here is an experimental investigation of the affects of large herbivores on S. parksii population dynamics. We explored direct impacts of herbivore browsing as well as indirect impacts resulting from removal of neighboring vegetation on S. parksii population fluctuations and the demographic parameters underlying them. To determine direct herbivore affects on demographic characteristics and their implications for S. parksii population dynamics, we established 32, 1.5mx1.5m plots in areas of high S. parksii abundance. Each of the plots was randomly assigned one of two treatments: herbivore exclusion and control. Demographic parameters were recorded for each individual located within the plots. The data suggest that large and small herbivore browsing affects different lifecycle transitions through removal of photosynthetic and reproductive tissue. These transition alterations have the potential to affect S. parksii population growth. To investigate the indirect impacts of herbivory on S. parksii population dynamics, we established 32 5mx5m plots. Each plot was randomly assigned one of four fertilizer treatments: control, nitrogen addition, phosphorus addition, and nitrogen and phosphorus additions. One half of each plot was randomly assigned one of two biomass removal treatments: unaltered control and biomass removal during orchid dormancy. Demographic data was collected for individuals within the plots. Plots with phosphorus and nitrogen phosphorus additions generally differed regarding orchid demographic variables from nitrogen addition plots and unfertilized controls. Also, available light tended to be greater during orchid flowering in plots with biomass removed than in unclipped controls. A complete understanding of both direct and indirect herbivore impacts on S. parksii population dynamics is necessary for effective conservation of this endangered orchid.
Wonkka, Carissa Lyn (2010). Large Herbivore Impacts on Demographic Characteristics and Population Dynamics of an Endangered Orchid (Spiranthes parksii Correll). Master's thesis, Texas A&M University. Available electronically from