African Acacia population dynamics : the impact of large herbivores, rodents and bruchids

Abstract

Large mammals have little impact upon Acacia flowers and thus do not affect pod production via flower consumption. Giraffe consumed most mature pods whereas ground feeding kudu, impala, steenbok and grey duiker took fewer. Despite seed loss to digestion, it is estimated that more Acacia seeds can potentially germinate following seed ingestion by large herbivores compared to uningested seeds. Seed ingestion maybe potentially advantageous to seed germination. In contrast rodents may have little or no impact as Acacia seed consumers. Large herbivores dispersed Acacia seeds into open habitats ideal for seed germination thereby enabling the future recruitment of Acacia populations. Whereas, in the absence of large herbivores, seeds were mainly dispersed in the shade. Since Acacia seedlings are shade intolerant, dispersal by herbivores maybe advantageous. Bruchid beetles are important in controlling the quantity of Acacia seeds which are viable. The intensity of bruchid predation increases throughout the seed season corresponding to bruchid development and subsequent emergence. Large herbivores show no selection against bruchid infested seeds, however, seeds eaten and egested by a variety of large herbivores had a lower bruchid infestation than uningested seeds. Thus large herbivores may decrease the survival and densities of bruchids thereby encouraging the spread pf viable Acacia seeds. Ungulates predominantly browsed upon the shoots of mature trees and seldom browsed on saplings or immature trees. Some mature trees were overbrowsed and their canopies showed signs of shape modification. Overall, at the current game densities on the reserve, ungulate browsing increased Acacia shoot growth and productivity. Suggestions are given in the thesis on how the research information can be used to manage and conserve Acacia