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The effect of grazing on ant (Hymenoptera: Formicidae) diversity in the south Okanagan grasslands Heron, Jennifer M.

Abstract

Ants play a significant role in almost all terrestrial ecosystems as soil engineers, seed dispersers, scavengers and predators. Ants can adapt to a variety of environments and recover from habitat disturbance. Ant distributions are influenced by climate and its influence on soil temperature. Temperate ant species are generalist foragers, scavengers and predators, rely on soil properties to build their colonies, and their presence is often governed by intra- and inter- specific relationships. In this study we captured thirty-one species, one of which is not described. Five species were new records for British Columbia. Moderately grazed sites showed the both the highest and the lowest total number of species per site yet the lowest average number of ants per trap per site. Perhaps moderate levels of grazing permit a higher number of species to coexist in an area thus supporting the intermediate disturbance hypothesis. The results from this study indicated grazing might have an effect on the ant biodiversity in the south Okanagan. Sample values for biodiversity indices reveal that non-grazed sites are at a lesser index value than moderately or heavily grazed sites. Although tested results were not significant; this could be due to high between site variability and a low number of replicates. Regression analyses showed a significant relationship between ant numbers and the percent bare soil, and the Shannon Wiener diversity index and plant richness were also significantly related. When ants were grouped according to a behaviour profile (based on the inter- and intraspecific aggressiveness towards other species) or nest building profile (where or how species build their nests), a pattern between grazing history and ant biodiversity and abundance was not apparant. It is unrealistic to view the entire bunchgrass ecosystem as one continuous unit in terms of invertebrate biodiversity. The microhabitat structural heterogeneity throughout the region is as complex as the endemic flora and fauna that inhabit the ecosystem. With grazing comes the alteration of plant structure and composition and as a result, changes in the microhabitat structure are more frequent than a change in the overall landscape structure. Invertebrate assemblages are more reliant on the microhabitat structure in a small area, and ant abundance and diversity is no exception. In a complex habitat, ants are less likely to forage beyond the necessary limits of their closest food source, and therefore non-grazed sites may not have been sampled as effectively as moderately or heavily grazed sites. Alternatively there is also a possibility that with intermediate disturbance a greater diversity and abundance of ants can persist in the south Okanagan where otherwise they may not. Anthropogenic disturbances affect every terrestrial ecosystem and there is no doubt that much of the south Okanagan grasslands will continue to be grazed by livestock. Intermediate levels of grazing may create a mosaic of successional stages to the benefit of many of the ant species.

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