Consequences of herbivory by native ungulates for the reproduction of a savanna tree

Abstract

Summary Herbivores may negatively impact plant populations by reducing the survival, growth and reproduction of individual plants. In African savannas, browsing by large mammalian herbivores has been shown repeatedly to reduce adult survival and growth of members of the genus Acacia, the dominant trees in these systems. However, the potential costs of mammalian herbivory to Acacia reproduction are largely unexplored, and have never been investigated experimentally in African savannas. Using a long‐term, large‐scale field experiment, we demonstrate that, in the absence of native herbivores, individual trees were twice as likely to reproduce, and those that reproduced produced a greater biomass of seeds. In addition, spine length (an indicator of past herbivory) was correlated with reduced reproduction of Acacia drepanolobium, the dominant tree across large areas of East Africa. Browsing by native herbivores triggers the production of longer spines, an induced resistance, and spine length was significantly and negatively related both to the occurrence and magnitude of reproduction. Induced resistance appeared to mitigate the negative effects of herbivory where large herbivores were present: trees with long spines reproduced at levels comparable to trees with similarly long spine lengths in the absence of browsers. Large mammalian herbivores kill and suppress the growth of adult Acacia, and therefore often are regarded as critical in maintaining the co‐dominance of trees and grasses in African savannas. Our results provide the first experimental evidence that large mammalian herbivores can suppress reproduction in Acacia, thus highlighting an additional pathway through which these herbivores might impact population dynamics of this important and widespread genus. In savanna systems, declining populations of large herbivores may trigger a series of complex demographic responses for trees, with potentially strong consequences for the structure and function of savannas.