Distribution, abundance and size distribution of the alien invasive pulmonate, Theba pisana, in the West Coast National Park, South Africa: Is there cause for concern?

The distribution, abundance and size distribution of the invasive land snail, Theba pisana, in the West Coast National Park (WCNP), South Africa was investigated. The Park was divided into 1 km2 grids within each of which five 1 m2 quadrat counts, taken at 50 m intervals, of both dead and alive snails were recorded and mapped. Of a total of 106 grids (535 quadrat samples), 19 % and 46 % of the study area contained live and dead snails respectively. However, densities were generally low and only 18% of grids contained more than zero but less than 10 live snails m-2. The average density of live snails within the park (4.04 m-2 ± 24.91) was significantly lower than both the density of dead snails (8.77 m-2 ± 21.25) and the density in more disturbed habitats outside the park (57 m-2 ± 96.25). As alien species do well in disturbed habitats, the near pristine environment of the park could have limited their distribution. Also, while most snails (dead or alive) were found along roads, densities decreased dramatically with distance from the road. Even though recent summer temperatures of the WCNP are consistent with the norm for the region, the average rainfall is considerably less than normal, which could also have contributed to the present restricted distribution of T. pisana within the Park. In early autumn, the field population consisted of two size classes, large snails of about 14 mm and small snails of 6 mm diameter. By the end of winter, most snails were much smaller (2-7 mm) with very few snails exceeding 10 mm. During spring, the snails grew rapidly and a larger cohort (11 mm) replaced the smaller one. Thus, T. pisana in the WCNP probably has an annual lifecycle. The Western Cape is characterised by a Mediterranean climate and thus T. pisana is well adapted for this region. Based on its present and past distribution, its invasion into the WCNP should not be taken lightly and more studies concerning the threat to the fynbos biome, is necessary to conserve the rich biodiversity of the region. (** See published paper at https://ir.sun.ac.za/cib/handle/123456789/718)本研究针对南非西海岸国家公园（West Coast National Park, WCNP）内入侵性陆生蜗牛（Theba pisana）的分布格局、种群丰度与体型分布特征展开调查。研究将该公园划分为1 km²的网格单元，每个网格内以50 m间隔布设5个1 m²样方，对样方内存活与死亡的该种蜗牛进行计数并记录，随后完成空间制图。本次研究共覆盖106个网格单元（合计535个样方样本），其中19%与46%的研究区域分别分布有存活与死亡的该种蜗牛。但种群整体密度普遍偏低，仅18%的网格单元内存活蜗牛密度处于0~10只·m⁻²区间内。公园内存活蜗牛的平均密度（4.04只·m⁻² ±24.91）显著低于死亡蜗牛密度（8.77只·m⁻² ±21.25），同时也显著低于公园外受干扰程度更高的生境中的种群密度（57只·m⁻² ±96.25）。鉴于外来入侵物种在受干扰生境中更易扩张繁衍，该公园近乎原生的生态环境可能限制了该种蜗牛的分布范围。此外，尽管绝大多数该种蜗牛（无论存活与否）均沿道路沿线分布，但其种群密度随距道路距离的增加呈急剧下降趋势。尽管WCNP近年夏季气温与该区域长期气候均值相符，但年均降雨量显著低于正常水平，这一因素可能同样加剧了该公园内T. pisana的分布受限情况。早秋时节，野外种群包含两个体型等级：直径约14 mm的大型个体与直径6 mm的小型个体。至冬末时，绝大多数个体体型缩小至2~7 mm，仅极少数个体直径超过10 mm。春季期间，蜗牛生长速率显著提升，一个直径约11 mm的更大龄组取代了原有的小型个体。由此推测，WCNP内的T. pisana种群可能具有一年生生活史。西开普省属于地中海气候区，因此该种蜗牛对该区域具备良好的适应性。基于该物种当前与历史分布情况，其入侵WCNP的风险不容忽视，未来需开展更多针对其对凡波斯（fynbos）生物群系威胁的相关研究，以保护该区域丰富的生物多样性。**详见已发表论文：https://ir.sun.ac.za/cib/handle/123456789/718