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

Abstract

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. Copyright