Variation in ground insect diversity, composition and abundance across land use types in an African savanna, Zimbabwe

Abstract

Fragmentation of African savannas into such ecosystems as grazing land, woodland, fallow land and cropping land has deleterious effects on soil insect biodiversity. Soil insects are sensitive indicators of habitat fragmentation. They form an integral ecosystem functional component. As such, they are critical in ecosystem functioning. Pitfall traps were used to assess insect diversity, composition and abundance in four land use types (fallow, grazing, woodland and cropping) in an African savanna during months of February to October 2017. Pitfall traps were set up monthly in the four land use categories over a period of 7 days. Insects were collected twice every three days and counted and identified. Redundancy Analysis (CANOCO version 6.5) was used to determine soil characteristics that influenced insect community dynamics. A total of 2,138 insects from seven orders, 14 families, 48 genera and 57 species were collected. Orders Hymenoptera (46.46%) and Coleoptera (36.20%) comprised the dominant taxa across all land use types irrespective of season. The most dominant families were Formicidae (46.46%) and Tenebrionidae (27.19%) whose species occurred frequently across the four land use types. The carpenter ant, Camponotus maculatus, and the beetle, Zophosis boei, were the most abundant and dominant species in all land use types and occurred across the sampling period, being most abundant during the hot dry months. Highest insect abundances were recorded during the hot dry months, with grazing land recording the highest abundance (656) and woodland the lowest (418). The main difference among land use types was a seasonal shift in insect species associated with C. maculatus and Z. boei. Two assemblages comprising of sites from (i) woodland and cropping land were dominated by coleopteran species and (ii) fallow and grazing land by hymenopteran species mixed to a lesser extent with coleopteran species. Soil moisture, phosphorous, pH and initial nitrogen content were identified as influential variables in ground insect community dynamics (F ratio = 1.928, p = 0.002). Ground insect community dynamics was driven by the effect of land use on edaphic factors and season. The observation that a distinct soil insect community occurred in each land use category as a variant of seasonal associations with C. maculatus and Z. boe sets a basis for using ground insects as potential indicators of the impact of land use which contributes to the understanding and identification of indicators involved in adequate conservation and management.

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