On the ecological role of insects in Australian eucalypt forests

Abstract

If heterotroph grazers act as rate regulators of autotroph processes, the general intensity of grazing probably reflects the necessity for more or less intense rate-regulation. The high rates of insect grazing in eucalypt forests imply that considerably more regulation of eucalypts is necessary than in other forests. After examining the biological adaptations of eucalypts and reviewing nutrient cycling in eucalypt forests, the hypothesis is proposed that the impoverished soils and erratic Australian climate have led to the development of very efficient autotrophic exploitation of the resource base, with associated sensitive and rapid growth responses to favourable conditions and a correspondingly sensitive and intense level of insect grazing. The rate-regulatory activity of the insects ensures that the resource base is not depleted, and it maintains a proportion of the available nutrients in rapid circulation. In addition to the observed pattern of intense insect grazing in eucalypt forests, it is suggested that a two-pathway nutrient cycling system has been developed, with pathways more clearly defined than the anastomosing nutrient-cycling systems in forests elsewhere. A slow nutrient cycle is maintained by the trees while a smaller pool of nutrients is rapidly recycled in the litter and understorey. Insects skim off nutrients from the slow cycle and pass them into the faster cycling pool. This idea is important in relation to the frequency of fire in eucalypt forests and the large inputs into the system of nitrogen from the leguminous shrubs which regenerate after fire.