Susceptibility of indigenous aquatic plants to alien invasives : competitive interactions as influenced by nutrient levels and density

[摘要] This research investigated the susceptibility of South African indigenous aquaticplants to competition from invasive species, using the competitive interactions oftwo sets of aquatic plants as a potential indicator. These interactions werestudied in two separate experiments: the submersed weeds, Hydrilla verticillata(L.F.) Royle (Hydrocharitaceae) and an indigenous species, Lagarosiphon major(Ridley) Moss (Hydrocharitaceae) and; the floating weeds, Azolla filiculoidesLamarck (Azollaceae) and the indigenous Spirodela polyrhiza (L.) Schleid(Lemnaceae). Plants were grown under differing nutrient levels, and in anaddition series of eight different densities, using the reciprocal yield model toestimate competitive ability. The invasive Hydrilla outcompeted Lagarosiphon interms of mean length, dry mass, and survival. Major algal infestation in the highnutrient level of the Hydrilla/Lagarosiphon experiment altered light and nutrientconditions, which may have played a significant role in the lack of establishmentof Lagarosiphon and the poor growth performance of Hydrilla. The invasiveAzolla and indigenous Spirodela both performed well in terms of plant mass andincrease in number. While Azolla was affected by intraspecific competition, itshowed a steady increase in growth and multiplication with an increase innutrients. The individual mass of Spirodela plants was highest in the low nutrientlevel, and multiplication rates were greatest in the high nutrient level. Resultsindicate that the susceptibility of indigenous plants may be increased in highnutrientsystems, and that a continuous monitoring programme of aquatic alienspecies is vital in protecting our indigenous plants from extinction. This researchrecommends that the method of investigating competitive interactions betweenalien and indigenous plants be repeated with a variety of aquatic plants, as ameans of anticipating susceptibility to invasions.

[发布机构] University of the Witwatersrand