Why Fast-Growing Plants Do Not Bother about Defence

Abstract

In an influential paper Coley et al. (1985) launched their resource-availability hypothesis (RAH) for plant defence. They predicted that under conditions of slow growth, plant species should invest more in defence against herbivores than under more favourable conditions with rapid growth. In their model Coley et al. (1985) assumed that herbivory does not depend on plant size but rather that a fixed amount is eaten, which can be reduced by the concentration of the defence chemical produced. The assumption of size-independent herbivory is awkward as an herbivore might eat more from a large plant. Moreover, large plants may attract more herbivores, especially if these herbivores are small. For instance, Windig (1993) found that flea beetles ate holes in leaves of Senecio jacobaea in proportion to plant size. A consequence of a fixed amount of herbivory in Coley's model is that very small plants are eaten completely unless they are defended, which provides an immediate advantage for high defence in small or slow-growing plants. Because Coley et al. asserted that their model result depends on the extent to which the assumption of sizeindependent herbivory is true, the RAH may be criticized on this point. Recently two models were formulated which assumed that herbivory is a fraction of the biomass. Yamamura and Tsuji (1995) applied optimal control theory to the problem of allocation to defence in an environment with a limited season for plant growth. Iwasa et al. (1995) considered the plant as a structured population of individual leaves and calculated optimal allocation of a defence chemical to each leaf as a function of its age. In addition to other interesting results both papers repeated the prediction of the RAH that fastgrowing plants should defend less. In contrast to Coley's assertion, the RAH does not rest on the assumption of size-independent herbivory. Here I point out why this result emerges and make some suggestions how to proceed in testing the RAH. As Coley et al. (1985) did, I calculate the growth of plants without herbivory and their loss of biomass due to herbivory for types that differ in the defence investment D. D (in g.g-1) is the fraction of photosynthetic products that the plant allocates to defence. The remainder (1-D) is allocated to primary growth. Defence is costly because it diverts assimilates from growth, reducing the inherent growth rate of the plant. I define C as the primary biomass of the plant (g), i.e. the biomass of all structures that contribute to growth, excluding the biomass in defence chemicals. The maximum photosynthetic production G is expressed in g sugars.g biomass-'.day-'. 6 is the conversion efficiency of sugars to primary biomass in g biomass.g sugars-' (Lambers and Rychter 1989). Hence, 6G is the relative growth rate of an undefended plant in a herbivore-free environment ('maximum inherent growth') in g.gj.day'. The relative growth rate which is realised (r) depends on the difference between growth rate without herbivores and the rate of herbivory (Fig. 1). In equation: