Impacts of an Invasive Snail (Tarebia granifera) on Nutrient Cycling in Tropical Streams: The Role of Riparian Deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N.

外来物种与栖息地退化是驱动环境变化的两大核心因素，且二者常同步发生。在淡水生态系统中，邻近陆地植被的退化可通过改变资源可获得性，助力外来物种的入侵与扩散。本研究以特立尼达岛的水生生态系统为研究对象，探究完整河岸覆被(riparian cover)的存在如何调控入侵植食性螺类瘤拟黑螺(Tarebia granifera)对氮(N)循环的影响。我们在保留或移除河岸植被的样地中，定量测定螺类生物量、生长速率及氮排泄量，以明确螺类种群动态与氮排泄特征和河岸带(riparian zone)健康状况之间的关联。我们在3条新热带区溪流中，于冠层开阔与冠层闭合两类生境中测定螺类生物量与氮排泄量，进而估算单位生物量及单位面积的氮排泄速率。冠层开阔生境中的螺类生物量较冠层闭合生境高2至8倍，单位面积氮排泄速率则为后者的3至9倍。在冠层开阔生境中觅食的螺类可获取更为丰富的食物资源，同时表现出更高的生长速率与单位生物量氮排泄速率。生态系统总氮需求估算结果显示，冠层完全闭合、半闭合及开阔生境中，螺类氮排泄量分别可满足生态系统总氮需求的2%、11%与16%。本研究结论表明，人为干扰导致的河岸冠层流失可在热带溪流网络沿线形成螺类生物量、生长及氮排泄的热点区域，进而改变入侵螺类对氮生物地球化学循环的影响效应。