Vacant Niches and Unsaturated Communities: a Comparison of Bracken Herbivores at Sites on Two Continents

(1) Communities of herbivorous insects feeding on the above-ground parts of bracken (Pteridium aquilinum (L.) Kuhn) were compared at matched open and woodland sites in the north of England (Skipwith Common) and the Sacramento Mountains, New Mexico (Sierra Blanca). (2) General supporting surveys of herbivores were also conducted throughout Britain and the south-western U.S.A. (Arizona and New Mexico). (3) Twenty-seven species of insects feed on the above-ground parts of bracken fronds in Britain, with another eight possibly or occasionally doing so. Only five species were found on bracken in New Mexico, with two more in Arizona. (4) This difference in faunal richness between Britain and the south-western U.S.A. is consistent with predictions based on standard species-area relationships, given the areas within which bracken grows in the two regions. (5) The small pool of species in Arizona and New Mexico means that local communities of bracken herbivores in this region are also species-poor. Compared with communities in Britain, bracken in New Mexico has a large number of apparently vacant niches, i.e. totally unutilized, or underutilized plant-parts. (6) Despite containing a markedly impoverished number of species, there is no clear evidence either for niche-expansion, or for density compensation in the New Mexico communities. Two possible exceptions to this generalization are enigmatic. (7) These results for bracken are probably representative of phytophagous insect communities in general, in that such communities appear rarely to be saturated with species, and are not structured to any major extent by interspecific competition.