The effects of water velocity and sediment composition on competitive interactions between native and invasive species in a spring fed river
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Hydrilla (Hydrilla verticillata) is an invasive species that is problematic globally and also in the San Marcos River where it competes with native species. Hydrilla has been described as the “perfect aquatic weed” because it is able to propagate under a wide range of environmental conditions including low nutrient and variable light conditions (Langeland 1996). Treatment methods for control of non-native aquatic plants can be restricted due to the co-occurrence of native endangered species, requiring an integrated approach of several methods for restoration, including removal by hand, and manipulating environmental factors to encourage growth of native species. I conducted a competition study to determine if a native species can out-compete non-native species under a set of environmental conditions. The experiment was conducted within Spring Lake at the headwaters of the San Marcos River, Hays Co, Texas between 03/28/2014 and 05/21/2014. I used a three-factor replacement design; (water velocity, substrate type, and competitive pressure) to assess competitive interaction between a native species (Illinois pondweed) and non-native species (hydrilla). Illinois pondweed (Potamogeton illinoensis) and hydrilla (Hydrilla verticillata) were potted in monoculture (intraspecific competition) and mixtures (interspecific competition) using sand or silt sediment, and high or low velocity for a period of seven weeks. Above- and belowground dry biomass, total stem length, and number of stems were measured. Across all treatments, pondweed demonstrated significantly (P<0.05) higher growth rates than hydrilla. Substrate type and monocultures were not statistically significant factors in plant growth. However, growth indices indicated that total dry biomass of both plants was slightly higher in sand substrate and high velocity. I also found intraspecific competition was greater than interspecific competition for both species. Both species produced more biomass when in monoculture and less biomass in mixtures. Therefore, data from this study comparatively better environmental conditions for Illinois pondweed to successfully out-compete hydrilla are in sand substrate and high velocity. These strategies could be used to enhance EAHCP efforts in the San Marcos River where invasive plant management options are limited. Part of the EAHCP plan includes planting native species after removal of non- natives. Illinois pondweed may not be a suitable plant for remedial gardening, not because it is an inferior competitor with hydrilla, but because it may not be competing with it at all. In fact, Illinois pondweed may be aiding hydrilla propagation by slowing water velocity, thus accumulating fine sediments and creating suitable habitat for hydrilla.