PLANT SIZE VARIATION AND VERTEBRATE HERBIVORY - WINTER-WHEAT GRAZED BY RABBITS

(1) To study the influence of the timing and duration of vertebrate grazing on the size structure of a plant population, the weight distributions of shoots, roots, ears and grains of populations of winter wheat under several regimes of rabbit grazing were analysed. (2) Shoot-weight distributions of ungrazed plants changed significantly over the course of the growing season, with size inequality between plants increasing rapidly at first, peaking in mid-winter, and remaining high thereafter. Variance: mean relationships for shoot-weight distributions conformed to Taylor's Power Law throughout the growing season, becoming Poisson by harvest time. (3) Continuous rabbit grazing reduced mean plant size and consistently increased size inequality. The largest plants in populations grazed in winter and then allowed to recover were typically larger than the largest plants from ungrazed populations. This was probably due to a reduction in competition from mortality caused by grazing. Total grain yield per unit area was never increased by grazing. (4) Weight distributions of grazed populations tended to show increased skewness and kurtosis compared with ungrazed populations. We did not find any evidence for bimodality, but platykurtosis was frequently observed.