Effects of Grazing by a Chrysomelid Beetle, Gastrophysa Viridula, on Competition Between Rumex Obtusifolius and Rumex Crispus

Abstract

The effects of clipping (simulated grazing) and grazing by a chrysomelid beetle, G. viridula, on plants of 2 spp. of Rumex were investigated. Although G. viridula feeds on both R. obtusifolius and R. crispus in the field, the preference is for R. obtusifolius. Moderate levels of grazing or clipping had no significant effects on plants of either species when competing intraspecifically. In the conditions of the experiments, R. obtusifolius grew better when competing interspecifically with R. crispus than when competing intraspecifically, while the reverse was found for R. crispus. This advantage of R. obtusifolius was removed by moderate levels of clipping or grazing. Clipping removed the competitive disadvantage of R. crispus but moderate grazing enhanced it. Heavy grazing significantly reduced leaf area, leaf dry weight, root dry weight and whole plant dry weight of R. obtusifolius and R. crispus grown alone. R. obtusifolius responded to heavy grazing by an increase in the root:shoot ratio, while R. crispus responded by a decrease in this ratio. There was a significant interference interaction between interspecific competition and heavy grazing in R. crispus. This was especially marked below ground. The possible relevance of these findings to observations on the distribution of the 2 Rumex spp. in the field is discussed. There may be circumstances in which grazing by G. viridula may affect the distribution of R. crispus, and that this is more likely to be the case if it is growing in the presence of R. obtusifolius.