Nectar secretion on fern fronds associated with lower levels of herbivore damage: field experiments with a widespread epiphyte of Mexican cloud forest remnants.

BACKGROUND AND AIMS: The oldest group of plants in which nectar secretions have been observed are the Polypodiopsida (ferns sensu lato). Nectaries have been reported in a dozen extant genera. The function of these nectaries has been investigated in several fern species, and in some circumstances has been demonstrated to have an antiherbivore role, attracting and maintaining biotic defence (ants and/or other predatory arthropods). This study documents foliar nectaries in Pleopeltis crassinervata, a widespread Central American epiphyte growing on a variety of trees in cloud forest areas of Veracruz, Mexico. This is a new record for this genus and species. METHODS: As previous experimental work on epiphytic species of Polypodium has demonstrated a protective role of ants for developing fronds, we conducted similar experiments (using nylon nail polish to cover nectaries rather than excluding ants with bands of sticky resin as in earlier work). The fronds of Pl. crassinervata developed over 6 weeks, at which time damage was assessed. The experiment was simultaneously conducted on a sympatric species lacking nectaries, Polypodium furfuraceum. Herbivore placement experiments were conducted with large and small caterpillars on both of these ferns. KEY RESULTS: Fronds with nectaries covered suffered greater damage from herbivores over the course of their development, compared with fronds that had uncovered nectaries functioning normally. The parallel experiment on Po. furfuraceum showed no difference between manipulated and control fronds. Six species of ants (Brachymyrmex minutus, Crematogaster formosa, Paratrechina longicornis, Solenopsis geminata, S. picea and Wasmannia auropunctata) were observed visiting nectaries of Pl. crassinervata; most were effective in removing herbivore larvae placed on the fronds. CONCLUSIONS: The long evolutionary history of ferns may explain why some previous studies of fern nectaries have shown little or no benefit to ferns from nectary visitors, as any coevolved herbivores are those resistant to ant defence. The results suggest that ants protect Pl. crassinervata fronds against herbivory. The presence of nectaries, and the relationship with ants, may contribute to this fern's widespread occurrence and persistence in the face of disturbance, though many other factors also play a role. Ant defence may be more likely to benefit a widespread species of disturbed habitats that encounters a wide range of non-adapted herbivores.

The ants of North and Central America: the genus Mycocepurus (Hymenoptera: Formicidae).

We provide a review of the North American ants (north of Colombia) of the ant genus Mycocepurus, including keys to the workers and females, illustrations and distribution maps. The distribution of M. tardus is extended to Nicaragua and Costa Rica. The female of M. curvispinosus is described.

The relative contributions of termites and microarthropods to fluff grass litter disappearance in the Chihuahuan Desert.

We tested the hypotheses that both subterranean termites and soil microarthropods are important in the disappearance of fluff grass (Erioneuron pulchellum) litter on the soil surface by an experiment designed to separate termite and microarthropod effects. Subterranean termites (Gnathamitermes tubiformans) removed more than 50% of the fluff grass litter in one year.Since there was no difference in mass loss of fluff grass with microarthropods present or excluded, they had no effect on decomposition of fluff grass litter. Microarthropod densities increased during the first 3 months then slowly decreased. The densities of microarthropods in fluff grass litter were too low to have a measurable effect on decomposition even if we assumed that the microarthropods consumed litter equivalent to their body weight each day.

A comparison of the energy budgets of three species of Pogonomyrmex harvester ants (Hymenoptera: Formicidae).

This investigation compares the energy budgets of three species of Pogonomyrmex harvester ants along an altitudinal transect in southern California. All three demonstrate similar seasonal patterns, with high foraging activity and high respiratory costs in mid-summer and little or no activity during winter. Respiration, predominantely metabolism of the workers, was estimated to account for 84-92% of the energy assimilated by the nests. The remaining 8-16% of the energy was invested in the production of new individuals. However, total production was not correlated with food input. It is suggested that worker care of the brood may be the most important determinant of brood production, and thus food may not be a direct limiting resource in harvester ants.A higher percentage of production is invested in workers than in reproductives in all three species. The species usually partition similar proportions of energy between the production of males and females, but since females are larger, more males are produced. Although the species are in different habitats and have very different numbers of workers per nest, the numbers of sexuals produced per nest are similar. The sex-ratio appears to be ecologically determined. Nests that were provided with additional food invested more energy in the production of males. Control nests, nests which had food removed and older nests invested more in the production of females or invested equally in the production of the two sexes.

The effect of predation of western widow spiders (Araneae: Theridiidae) on harvester ants (Hymenoptera: Formicidae).

Spiders have been shown to be important predators on ant populations, although ant mortality may be low. Western widow spider (Latrodectus hesperus) predation on Pogonomyrmex rugosus ant nests is slight, less than 0.2% of the nest population per day. Yet, the ants respond by halting foraging temporarily and may even close the nest entrace with pebbles. If spiders are removed from such nests, the ants become active within 24 h and within five days foraging activity reaches higher levels than at any previous time of the year.Ants have at least four responses to spiders predation: 1) Moving the nest entrace; 2) posting of "guards"; 3) attacking the spiders; 4) halting foraging. Pogonomyrmex rugosus practices the fourth response, although it results in losses of food intake into the nest. This is apparently the only option available. If the ants continue to forage, the spider density may increase, resulting in heavy mortality of the ants.