Variation in Insect Assemblage and Functional Groups along a Grazing Gradient in an Arid Environment

Abstract

Exotic livestock may affect plant and pollinator composition as well as their interactions. Changes in pollinator assemblage (e.g. composition) may alter pollination process and, therefore, plant reproduction. This study examined seven independent paddocks environmentally similar but with increasing livestock densities, in Patagonian Monte Desert. This allows investigating the response of different insect groups to livestock effect on vegetation along a grazing gradient. Water traps was used to capture insects (focusing on pollinators) and pollinator visitation frequencies were measured in five of the most abundant native plant species of the region. Regressions between livestock density and insect richness and abundance of the different functional groups of insects (dipterans, herbivores and pollinators) were performed. Insect assemblage was mostly composed, in decreasing order, by hymenopterans, coleopterans, dipterans and lepidopterans. Ordination analysis of insect composition showed a high correlation between the first axis and livestock density (r=0.95). In the first year of sampling, pollinators significantly decreased in richness and wasps increased in abundance with livestock density. In the second year of sampling, pollinator abundance increased and herbivores decreased as livestock density increased. Unimodal ordination analyses showed that plant species were significantly associated to insect composition and livestock density marginally explained the variation in insect composition that visited the five studied plant species across paddocks. Interacting factors such as livestock density and climatic conditions may partially explain the patterns observed in this work. Disturbances may alter the ecological scenarios, impoverishing the composition and abundance of flora and fauna. This study describes that changes in the density of exotic livestock can affect insect assemblage associated to pollination process differently through the years. Changes through time may determine population dynamic and their response to anthropogenic disturbances