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 characterist ic of interest, combined or not with assessment of treatment effects. The following analysis-of-variance approach to quantification of spati al heterogeneity is based on the adequate design of ecological field e xperiments, according to the type and the scale of heterogeneity of co ncern (at small scale, patches, one- or two-dimensional gradients). Th ere are no recipes for doing so and judgment must be exercised every t ime; the experimenter's knowledge about the experimental material, com bined with premanipulation or control, then, provides a useful prerequ isite. For patches and environmental gradients, in the presence of tre atment assignment, recommended designs require the blocking principle of grouping similar experimental units, which allows avoidance of spur ious treatment effects and inflated error mean square. Completely rand omized designs should only be used in the very particular case of spat ial homogeneity at large scale. Illustrations in ecological field expe rimentation are given and discussed.