Nonequilibrial community structure of sawflies on arroyo willow.

We use Wiens' (1984) nonequilibrium-equilibrium continuum concept to evaluate aspects of community structure for sawfly herbivores (Hymenoptera: Tenthredinidae) that attack arroyo willow, Salix lasiolepis. The sawfly community on arroyo willow shares many characteristics of nonequilibrium communities: unsaturation, biotic decoupling, lack of density dependence, and loose emergent patterns. Species abundances exhibit highly significant differences among clones (P<0.001) and among years (P<0.001) and exhibit significant and complex clone-by-year interactions both for absolute and relative abundances. Organizing forces, such as interspecific competition and regulation by natural enemies, are largely absent from the arroyo willow system. Sawfly densities exhibit significant positive covariation among willow clones and appear to be affected by largely stochastic abiotic forces, primarily winter precipitation.