Presidential Address: A Positive Distrust in Simplicity - Lessons From Plant Defences and From Competition Among Plants and Among Animals

Abstract

1. The address is concerned with the degree of complexity needed in a satisfying ecological theory. 2. Using the incidence of spines, it is argued that previous hypotheses intended to explain the development of anti-herbivore defences are inadequate, and a new 'scarcity-accessibility' hypothesis is proposed instead. 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, for example, particularly slow-growing and particularly fast-growing plants, or the deciduous species in one system and the evergreen in another. The new hypothesis is shown to forecast effectively the distribution of poisonous chemicals in plants. 3. Parallels and differences between studies on competition among plants and competition among animals are considered briefly. 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 in a little detail, especially for semi-desert perennials. It is argued that the 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. It is emphasized that 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,. 4. A plea is made that ecologists should respond positively when the simplest theories are found to be wanting; such theories should be firmly discarded and replaced by more complex theories that are more realistic but still as simple as is compatible with the known complexities of nature.