Interactions between Grazers and a Cyanogenic Grass, Cynodon plectostachyus

Abstract

Grasses exhibit many traits that have been attributed to selective pressures by grazers. These include vegetative growth from an intercalary meristem, prostrate growth form, perennial lifestyle, high nutritional content, and inducible secondary compounds. However, alternative selection pressures, that do not include grazers, have been proposed for these traits. Extreme manifestations of all these traits were found in a grass from African savannas (C. plectostachyus (K. Schum.) Pilger.), which is remarkably well adapted to extreme herbivory: 1) Heavy grazing and trampling by large mammals increases the relative abundance, and by inference the fitness, of C. plectostachyus. 2) The grass exhibits marked morphological plasticity, depending on the intensity of previous grazing and trampling, but productivity is unaffected. 3) C. plectostachyus is exceptionally nutritious compared with other co-occurring grass species. 3) Both vertebrate and invertebrate (Spodoptera exempta, Lepidoptera, Noctuidae) grazers feed preferentially on species in the genus Cynodon. 4) Defoliation is more potent than are abiotic factors that induce cyanogenesis in C. plectostachyus, suggesting that the trait evolved as a herbivore deterrent. Circumstantial evidence suggests that, after defoliation by army worm larvae (S. exempta), levels of cyanide in the leaves of C. plectostachyus were high enough to kill cattle, but S. exempta larvae are not affected by cyanide. Therefore, interactions between grass and grazer can be beneficial or antagonistic, depending on the degree of defoliation. The combination of extreme tolerance of intense herbivory, and the complexity of interactions with grazers suggests (but does not prove) that large herbivores have strongly influenced the evolution of C. plectostachyus. However, the relationship between this grass and grazeres is not obligatory, at least in ecological time, and cannot be termed mutualistic in the strict sense that each is exclusively dependent on the other.