SPATIAL HETEROGENEITY AND THE DESIGN OF ECOLOGICAL FIELD EXPERIMENTS

Experimental design should be accommodated to spatial heterogeneity in nature as well as indoors, whether it is a nuisance or a characteristic of interest, combined or not with assessment of treatment effects. The following analysis-of-variance approach to quantification of spatial heterogeneity is based on the adequate design of ecological field experiments, according to the type and the scale of heterogeneity of concern (at small scale, patches, one- or two-dimensional gradients). There are no recipes for doing so and judgment must be exercised every time; the experimenter's knowledge about the experimental material, combined with premanipulation or control, then, provides a useful prerequisite. For patches and environmental gradients, in the presence of treatment assignment, recommended designs require the blocking principle of grouping similar experimental units, which allows avoidance of spurious treatment effects and inflated error mean square. Completely randomized designs should only be used in the very particular case of spatial homogeneity at large scale. Illustrations in ecological field experimentation are given and discussed.