Bracken, Ants and Extrafloral Nectaries. II. the Effect of Ants on the Insect Herbivores of Bracken

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Abstract
The hypothesis that the association between ants and the extrafloral nectaries of bracken is mutualistic was tested in return for nectar, ants exclude or significantly depress insect herbivore populations on bracken. In a paired comparison design ants were experimentally excluded from 1 member of pairs of fronds within 2 patches at Skipwith Common, North Yorks [England, UK]. The insect herbivore faunas of control and treatment fronds were then monitored throughout the season. Herbivore populations of bracken patches which had naturally different ant densities were compared; this was done at 2 sites in northern England. Although ant activity on treatment fronds was significantly reduced, there was no effect on the total number of species per front, nor on the population sizes of most species, compared to control fronds. Only 1 sp., the hemipteran Monalocorris filicis sp. (L.), was unequivocally more common on fronds from which ants were excluded. Two species were less common on treatment fronds, probably because the ant exclusion technique also tended to exclude these species. Between-patch comparisons showed no differences in total herbivore species richness on sites with a markedly different number of ants. Differences between the populations of individual species revealed varying patterns which were difficult to relate to ant density. For instance, the pattern of abundance, relative to ant density, for M. filcis was opposite to that found in the exclusion experiment. The effects of ant predation are feeble relative to other environmental or host-plant influences acting on M. filicis; for all other bracken herbivores, ant predation effects appear to be nonexistent. Alternative hypotheses for the bracken nectary-ant association are discussed. Apparently evolutionary adaptations (or expaptations sensu Gold and Vbra 1982) among the herbivores, in response to ant predation, may now place more of the bracken insects in enemy-free space. The association may be maintained by continuing mild selection if the ants are effective against nonadapted generalist species and potential colonists.