Experimental Studies on Slug-Plant Interactions: II. The Effect of Grazing by Slugs on High Density Monocultures of Capsella Bursa-Pastoris and Poa Annua

SUMMARY (1) Monocultures of Capsella bursa-pastoris and Poa annua were sown at high density and grown in the presence and absence of the slug Agriolimax caruanae. The slug showed strong dietary preference for Capsella just as it had done in laboratory tests, and ate Poa only after starving for c. 66 h. (2) The populations of Capsella exhibited linear survivorship curves, individuals being lost from the populations at an approximately constant rate. (3) The rate of self-thinning and the growth made by survivors were such that populations conformed to the - 3/2 power thinning law, though stands of the two species had different carrying capacity. (4) Slugs behaved quite differently towards the two plant species. The) ate whole, or parts of, leaves of Capsella bursa-pastoris but rarely, if ever, killed the plants. They ate Poa annua (after the starving threshold) by chewing through shoots at ground level, 'felling' but not consuming the shoots. Many of the damaged plants of Poa died. (5) The effect of slugs was therefore generally to change the size of plants of Capsella but not their number and to change the number of plants of Poa rather than their size, at least in the early stages of the study. (6) There was some evidence that defoliation decreased the reproductive potential of the grazed monocultures. (7) The results are discussed in the context of the compensatory effects of competition and predation, the differential effect of herbivores on plants with different biology, and caveats for other work where grazing trials are involved.