INSTABILITY OF DEVELOPMENT AND FRACTAL ARCHITECTURE IN DRYLAND PLANTSAS AN INDEX OF GRAZING PRESSURE

Developmental instability has been used to monitor the well-being of n atural populations exposed to physical, chemical and biological stress ors. Here, we use developmental instability to assess the impact of gr azing on Chrysothamnus greenii and Seriphidium novum shrubs, and Oryzo psis hymenoides grass, common in the arid intermountain west of the U. S.A. Statistical noise in allometric relations was used as an indicato r of developmental instability arising from grazing-induced stress. Un palatable species that are not grazed (Chrysothamnus greenii) or speci es that are dormant during the winter-spring grazing period (Oryzopsis hymenoides) show lower allometric variability under high grazing pres sure. Palatable species (Seriphidium novum) exhibit high developmental instability under low and high grazing pressure. Grazing pressure imp osed by presumably co-adapted wild herbivores enhances developmental s tability in species habituated to moderate grazing, like Oryzopsis hym enoides, but stresses plants such as Chrysothamnus greenii that prefer disturbed environments. These grazing effects are probably due to the impact grazing has on competitive relationships and not to the direct action of the herbivore on the plants. (C) 1998 Academic Press Limite d.