Species Richness of Plant Parasites and Growth Form of Their Hosts

Trees tend to have more pest species than do shrubs, which in turn have more than herbs, per unit area of geographical distribution. These patterns are shown by species/area analyses of the insect associates of 215 genera of British plants and the fungal parasites of 323 United States plant species. Within these growth-form categories some comparisons show no discernible differences in pest species richness: Angiosperm trees have approximately the same number of fungal parasites as gymnosperms, and monocot herbs have about the same number of fungal parasites as do herbs in general, in the United States. British sedges, grasses, all monocots, composites, and bog plants do not support discernably different numbers of insect species than herbs in general, per unit host range. However, other comparisons within the herb-shrub-tree categories do show differences: Ferns support fewer insect species than other herb genera in Britain, and weedy herbs support fewer fungal parasite species than climax and subclimax herbs in the United States, per unit range of host species. We propose that the greater size and morphological complexity of trees than shrubs, and than herbs in turn, accounts for these patterns in pest species richness; larger more complex hosts present a greater diversity of pest niches. If one makes the assumption that these plant categories represent a spectrum of resource constancy to parasites, then our results might be interpreted to support the idea that parasite pressure will be slightly greater upon more "apparent" host species. Much residual variation remains about the species/area lines even after the effects of plant growth form have been subtracted. The residual variation might be explained by underreporting of pests, by differences in commonness of plant taxa of equal geographic range, and by variation in the effectiveness of plant morphology and plant chemistry in protecting the plant from pests.