Impacts of Large Mammalian Herbivores on Arthropod Assemblages within Fruits of a Rangeland-Invading Shrub (Solanum campylacanthum) in African Savannas

Abstract

Researchers have widely acknowledged the important role that ecosystem engineers play on the environment. By controlling the availability and quality of various resources, such as shelter, these animals greatly influence other organisms’ fitness (Jones et al., 1994). Here, we present the results of a study of the indirect effects of large mammalian herbivores on the assemblages of arthropods that live within dried fruit (“husks”) of a wide-spread rangeland-invading shrub (Solanum campylacanthum) in Kenyan savannas. We gave additional attention to the environment in which these assemblages were present, namely the plant itself and its composition of fruits. Using four levels of mammalian exclusion in the 6-year UHURU experiments (Goheen et al., 2013), we examined fruit-age compositions of every shrub sampled and collected fruit-husks to analyze the arthropod communities within them. Shrub volume, fruit abundance, and arthropod abundance all increased with successive exclusion of mammal species. However, the nature of the observed change suggests that different mammals influence plants, and therefore husk-dwelling arthropods, in varying ways. Of these, elephants had the most profound impact as arthropod assemblages were significantly less dense, diverse, and mature in areas where elephants were present. While medium sized mammals (e.g. impala) also impacted fruit and arthropod abundances, they did not alter arthropod density, diversity, or maturity. This difference between the elephant and impala suggests that varying methods of indirectly influencing arthropod assemblages exist between the two mammal species, with possible implications for the establishment of viable communities within fruits. As a whole, these results provide a novel view into this arthropod assemblage and offer insight into both direct and indirect effects that large mammalian herbivores have on the environment as ecosystem engineers.