Reduction of oak fecundity by low-density herbivore populations

Abstract

Explanations for the low levels of defoliation in natural woodlands^1–6 have assumed that Insect herbivores have a negligible effect on plant fitness and demography^7,8, perhaps because of plant compensation^9–11. Levels of defoliation between 5 and 15% are typical, and are attributed variously to the impact of natural enemies, inclement weather and low food quality in keeping herbivore populations at low densities^12,13 (but see refs 14, 15). Here I report the results of a 4-yr study in which matched pairs of oak trees, Quercus robur, were sprayed regularly to exclude all herbivorous insects; despite the fact that the unsprayed trees lost only 8–12% of their leaf area, the sprayed trees consistently produced more seeds (from 4.5 to 2.5 times the numbers on unsprayed plants). It is not known whether this substantial reduction in seed production actually leads to a reduction in plant recruitment. It does seem, however, that plant compensation is not fully effective against low-density herbivore populations and, therefore, that slight differences in the level of defoliation between plants may cause important differences in plant fitness.