Bracken

Abstract: (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.

Abstract: We tested the hypothesis that ants, attracted to the foliar nectaries of bracken fern (Pteridium aquilinum) confer protection to the plant in South Africa. Using ant—exclusion experiments with unmanipulated and artificially augmented ant densities, we were unable to detect a positive effect of ants on bracken, as neither the number of herbivores nor the level of herbivory was significantly greater on ant—free fronds. We then examined possible explanations for these results by performing a number of laboratory experiments using the ant Cremtogaster peringueyi and two lepidopteran species, the major bracken—feeding herbivores. We showed that their eggs were susceptible to ant predation, but gained protection from their oviposition sites. We also demonstrated that bracken can benefit when densities of ants are high, such as when fronds are infested with honeydew—producing homopterans, as the lepidopteran larvae were more vulnerable to attack under these conditions. We propose that low ant density on the fronds is the primary factor limiting the occurrence of mutualism between bracken and ants in nature.