A POSITIVE DISTRUST IN SIMPLICITY - LESSONS FROM PLANT DEFENSES AND FROM COMPETITION AMONG PLANTS AND AMONG ANIMALS

Using the incidence of spines, it is argued that previous hypotheses intended to explain the development of antiherbivore defences are inadequate, and a new "scarcity-accessibility' hypothesis is proposed. The variables that need to be considered are productivity, accessibility and proportion of the landscape covered, architecture, seasonal behaviour relative to neighbours, nutrient concentration relative to neighbours and kinds of herbivore present. The new hypothesis accounts for completely opposite kinds of plants being armed, eg particularly slow-growing and particularly fast-growing plants, or the deciduous species in one system and the evergreen in another. The new hypothesis is effectively forecasting the distribution of poisonous chemicals in plants. Parallels and differences between studies on competition among plants and competition among animals are considered. The term "inhibition' is used to cover deleterious (non-parasitic) effects of one organism on another. Allocation to roots and its significance for water uptake is considered, especially for semi-desert perennials. Intensity of inhibition arising from competition for water, averaged over time, is likely to increase as rainfall decreases until a point is reached where plants hardly compete with each other. Alternating periods of inhibition arising from competition for water are seen as alternating with periods of inhibition arising from competition for nutrients or light, not just in semi-deserts but in forests and grasslands too. For both plants and animals competitive inhibition of some sort can paradoxically be especially intense both where resources are in very poor supply and where they are in very rich supply. -from Author