Spider community responses to Chromolaena odorata invasion, grassland type and grazing intensities

Biological indicators measure components of the biota and are used to give general information about complex ecosystems in which they occur, playing key roles in conservation planning and management. This study illustrates the impact of habitat change by factors that are extrinsic to the habitats in question and the importance of spider responses in aiding management decisions. The spider responses illustrated existence of environmental change and represent responses of other biota. The conclusions drawn from this study have important management implications for protected areas with grazing herbivores and occurrence of alien invasive plants. Grazed sites showed the highest abundance, diversity and species richness, while ungrazed had the lowest. The implications from this study are that no grazing has negative implications on lower trophic levels, whereas grazing seems to result in favourable conditions for optimal abundance, diversity and species richness. The higher abundance, diversity and species richness associated with grazed sites could result from increased ground cover, greater variation in habitat structure, increased plant diversity and enhanced soil/plant nutrient concentrations. But, ungrazed sites in turn become more monotonous and provide less habitat diversity. However, the characteristic species for each grazing intensity level demonstrates the difficulty in making generalizations for management even for closely related species. The results further opposed the assumption that grazing lawns are a result of overgrazing and thus highly undesirable. This grassland type in comparison to tall bunch grassland displayed the highest spider diversity and species richness. This evidence further supports the conclusion that grazing lawns are steady state communities of their own and not a sub-set of any other grassland type. Therefore, veld management decisions that eradicate grazing lawns are negative for the park as the fauna and flora associated with this grassland type will be lost, leading to cascading effects. Additionally, this study illustrated that habitat modification by invasion of invasive alien plant species has detrimental consequences for the endemic fauna. C. odorata invasion results in a monotonous habitat structure. Consequently, structural heterogeneity is a primary determinant for spider diversity as opposed to abundance of prey, because plant height and architecture drive spider colonization. Therefore, removal of alien invasive weeds results in returning a system to close approximation of its condition prior to disturbance with both structure and function recreated. Assemblage patterns can be selected as endpoints to measure the ecological rehabilitation; thus, the non-significant differences in assemblage patterns of the control versus cleared sites imply that the system is rehabilitating with clearing without further management intervention. This study adds to the limited information on the implications of grazing intensities, grassland types, short and long-term invasion and clearing of an alien invasive plant on spider communities.生物指示物（biological indicators）可对生物群落组分进行测定，用于提供其所在复杂生态系统的整体信息，在保护规划与管理中发挥关键作用。本研究阐明了所关注栖息地的外在驱动因素对栖息地变化造成的影响，以及蜘蛛类群响应在辅助管理决策制定中的重要价值。蜘蛛类群的响应可反映环境变化的存在，同时也可代表其他生物群落的响应模式。本研究得出的结论，对存在放牧植食动物以及外来入侵植物（alien invasive plants）出现的保护区具有重要的管理启示意义。放牧样地的物种多度、多样性及物种丰富度均为最高，而未放牧样地则最低。本研究的启示在于：禁牧会对低营养级（trophic levels）生物产生负面影响，而放牧则可为实现最优的物种多度、多样性及物种丰富度营造有利条件。放牧样地所具备的更高物种多度、多样性与物种丰富度，可能源于更高的地表覆盖度、更丰富的栖息地结构异质性、更高的植物多样性，以及土壤/植物养分浓度的提升。反之，未放牧样地则会逐渐趋于单一化，栖息地多样性也随之降低。不过，不同放牧强度梯度下的特征物种表明，即便对于亲缘关系较近的物种，也难以形成普适性的管理方案。本研究结果还驳斥了“放牧草坪是过度放牧的产物，因此极不可取”这一预设观点。与高大丛生草原相比，该草原类型的蜘蛛多样性与物种丰富度均更高。该证据进一步支持了如下结论：放牧草坪本身即为稳定的群落类型，并非其他任何草原类型的子集。因此，旨在根除放牧草坪的维尔德草原（veld）管理决策，对该保护区而言是不利的，因为与该草原类型共生的动植物群落将会消失，进而引发级联效应（cascading effects）。此外，本研究表明，外来入侵植物物种入侵所造成的栖息地改造，会对特有动物群落（endemic fauna）产生有害影响。飞机草（C. odorata）入侵会导致栖息地结构趋于单一。因此，与猎物多度相比，栖息地结构异质性是决定蜘蛛多样性的首要因素，因为植物高度与形态结构会驱动蜘蛛的定殖过程。因此，移除外来入侵杂草可使生态系统逐步恢复至受干扰前的状态，其结构与功能均可得到重建。群落组成模式（assemblage patterns）可作为衡量生态修复（ecological rehabilitation）效果的监测终点；因此，对照样地与清除样地的群落组成模式无显著差异，这意味着仅通过清除入侵植物而无需额外管理干预，生态系统即可实现修复。本研究为现有关于放牧强度、草原类型、短期与长期入侵以及外来入侵植物清除对蜘蛛群落（spider communities）影响的有限研究补充了新的证据。