|Institution:||University of Auckland|
|Keywords:||Invasive species; Decomposition; Invertebrate; Ecology|
|Full text PDF:||http://hdl.handle.net/2292/3395|
Invasive plant species in natural ecosystems have been shown to have a wide range of potential impacts on community composition and ecosystem function. Alligator weed (Alternanthera philoxeroides) is invasive in New Zealand and elsewhere. An aggressive competitor and difficult to control, its impacts in productive ecosystems are widely documented. However, little information exists about alligator weed???s effects in natural ecosystems. This thesis therefore investigated potential effects of alligator weed on decomposition and nutrient cycling, as well as on invertebrate and plant communities in a ???native??? ecosystem, thus integrating effects on ecosystem processes and community composition. These aspects were compared between vegetation dominated by alligator weed or by one of two native sedges, Isolepis prolifer and Schoenoplectus tabernaemontani, in a northern New Zealand lake. In addition, the potential of shading to control alligator weed was assessed. Herbivory by the biocontrol agent Agasicles hygrophila resulted in substantial alligator weed defoliation, differing in timing and magnitude from biomass dynamics of the native sedges. Alligator weed also decomposed faster than either native sedge. Changes in decomposition dynamics led to a shift towards fungivore dominated beetle communities. This illustrates the potentially complex ecosystem effects of biocontrol agents as well as invasive weeds. Alligator weed decomposition rates and invertebrate communities were more similar to those of I. prolifer than S. tabernaemontani. This pattern was attributed to the greater chemical and architectural similarity of alligator weed to I. prolifer compared with S. tabernaemontani. Invasive plants??? impacts may thus be partially influenced by the degree of similarity between the invasive species and the dominant vegetation they invade. Alligator weed was associated with decreased cover of native plants, potentially placing them at greater risk of local extinction. Greenhouse and field experiments showed that shading significantly reduced alligator weed growth. However, clonal support from unshaded areas, and competitive release from less shade tolerant plants, may potentially impede the success of shade based control. This research demonstrates that alligator weed can have a range of complex and multi-trophic effects in a natural ecosystem. Further, characteristics of both invasive and competing vegetation can influence the success of cultural control methods such as shading.