Highlights from the recent technical literature:
Savanna ants more resistant to fire than forest ants. Parr & Andersen. 2008. Fire resilience of ant assemblages in long-unburnt savanna of northern Australia. Austral Ecology. doi: 10.1111/j.1442-9993.2008.01848
Abstract: Tropical savannas and rainforests contrast in their flammability and the fire resilience of their associated species. While savanna species generally exhibit high resilience to burning, there is much debate about the fire resilience of forest-associated species, and the persistence of forest patches in a flammable savanna matrix. Where fire has been excluded, savanna tends on a trajectory towards forest, with an increase in forest-associated plants and animal species. This study tested the idea that given the high proportion of forest-associated taxa in long-unburnt savanna, the fauna of these areas would be expected to exhibit less resilience to fire than the fauna in frequently burnt savannas. The study investigated the immediate and short-term effects on ant assemblages of re-introducing fire into long-unburnt savanna in northern Australia. The ant fauna exhibited high resistance to fires, with no significant short-term change in mean abundance or species richness; instead, seasonality had a far stronger influence on overall ant activity. Fire caused dramatic declines in dominance of the patchily distributed forest-associated species Oecophylla smaragdina and Papyrius sp., but had no effect on overall dominance by open savanna species of Iridomyrmex. Dominance by Iridomyrmex pallidus declined, but this was compensated for by increases in I. reburrus, while two other species of Iridomyrmex showed no change. This indicates a high level of functional redundancy among dominant species of Iridomyrmex, which universally dominate open savanna communities, but not of dominant forest-associated species. Overall, our findings demonstrate a high degree of fire-resilience of the long-unburnt savanna ant fauna. Despite the occurrence of forest-associated species, the high proportion of savanna species persisting in this habitat means that long-unburnt savanna retains the general response characteristics of frequently burnt savanna.
Deforestation affects above-ground and below-ground army ants differently. Kumar & O’Donnell. 2008. Elevation and forest clearing effects on foraging differ between surface – and subterranean – foraging army ants (Formicidae: Ecitoninae). Journal of Animal Ecology. doi: 10.1111/j.1365-2656.2008.01483.x
1. Forest fragmentation often results in a matrix of open areas mixed with patches of forest. Both biotic and abiotic factors can affect consumer species’ ability to utilize the altered habitat, especially for species that range over large areas searching for prey.
2. Army ants (Formicidae: Ecitoninae) are highly mobile top predators in terrestrial Neotropical ecosystems. Army ant foraging behaviour is influenced by forest clearing at lowland sites, and clearing can reduce army ant population persistence.
3. Because high temperatures are implicated in hindering above-ground army ant foraging, we predicted that forest clearing effects on army ant foraging would be reduced at higher (cooler) elevations in montane forest. We also predicted that subterranean foraging, employed by some army ant species, would buffer them from the negative effects of forest clearing.
4. We quantified the foraging rates of above-ground and underground foraging army ants at eight sites along an elevational gradient from 1090 to 1540 m a.s.l. We asked whether these two foraging strategies cause a difference in the ability of army ants to forage in open matrix areas relative to elevationally matched forested habitats, and whether elevation predicts open area vs. forest foraging rate differences.
5. As predicted, army ants that forage above-ground had lower foraging rates in open areas, but the open area vs. forest difference declined with elevation. In contrast, underground foragers were not affected by habitat type, and underground foraging rates increased with elevation. Ground surface temperatures were higher in open areas than forested areas. Temperatures declined with elevation, and temperature differences between open and forested areas decreased with elevation.
6. We conclude that army ants that forage above-ground may be restricted to forested areas due to a thermal tolerance threshold, but that they are released from this limitation at higher elevations. We further suggest that underground foraging permits some army ants to persist within modified landscapes. Our findings have implications for the effects of habitat modification and climate change on these top predators.
Crowding in ant colonies increases energy use. Cao & Dornhaus. 2008. Ants under crowded conditions consume more energy. Biology Letters. doi: 10.1098/rsbl.2008.0381.
Abstract: Social insects live in colonies consisting of many workers, where worker interactions play an important role in regulating colony activities. Workers interact within the social space of the nest; therefore, constraints on nest space may alter worker behaviour and affect colony activities and energetics. Here we show in the ant Temnothorax rugatulus that changes in nest space have a significant effect on colony energetics. Colonies with restricted nest space showed a 14.2 per cent increase in metabolic rate when compared with the same colonies in large uncrowded nests. Our study highlights the importance of social space and shows that constraints on social space can significantly affect colony behaviour and energy use in ants. We discuss the implications of our findings regarding social insects in general.
Behavioral adaptations of ants should not be studied in isolation. Burd & Howard. 2008. Optimality in a partitioned task performed by social insects. Biology Letters. doi: 10.1098/rsbl.2008.0398
Abstract: Biologists have long been aware that adaptations should not be analysed in isolation from the function of the whole organism. Here, we address the equivalent issue at the scale of a social insect colony: the optimality of component behaviours in a partitioned sequence of tasks. In colonies of Atta colombica, a leaf-cutting ant, harvested leaf tissue is passed from foragers to nest workers that distribute, clean, shred and implant the tissue in fungal gardens. In four laboratory colonies of A. colombica, we found that the highest colony-wide rate of leaf tissue processing in the nest was achieved when leaf fragment sizes were suboptimal for individual delivery rate by foragers. Leaf-cutting ant colonies appear to compromise the efficiency of collecting leaf tissue in order to increase their ability to handle the material when it arrives in the nest. Such compromise reinforces the idea that behavioural adaptations, like adaptations in general, must be considered within the context of the larger entity of which they are a part.
Paratrechina pubens spreading in the Caribbean. Wetterer & Keularts. 2008. Population explosion of the hairy crazy ant, Paratrechina pubens (Hymenoptera: Formicidae), on St. Croix, US Virgin Islands. Florida Entomologist DOI: 10.1653/0015-4040(2008)91[423:PEOTHC]2.0.CO;2.
Abstract: The hairy crazy ant, Paratrechina pubens (Forel), is undergoing a population explosion on St. Croix, US Virgin Islands. Here, we evaluate the status of P. pubens on St. Croix. In 2002, residents of Calquohoun and surrounding areas in central St. Croix began reporting large infestations of P. pubens. In 2005 and 2006, we surveyed ants at >100 sites across St. Croix. We found 3 geographically discrete populations of P. pubens occupying >5% of the island: a main population centered on Calquohoun spread over 9 km2 and 2 smaller populations occupying <1 km2 each. Locals blamed P. pubens for crop damage due to high densities of plant-feeding Hemiptera that tended the ants. Surveys of trees in areas with and without P. pubens present indicated that P. pubens has a significant negative impact on arboreal-foraging ants. The distribution and chronology of P. pubens records on St. Croix suggest that this species is a recently arrived exotic. It is unknown whether P. pubens will become a more serious pest on St. Croix or whether populations will collapse to inconsequential levels.
Inquiline parasites are chemical mimics of ant males. Hojo et al. 2008. Chemical disguise as a particular caste of host ants in the ant inquiline parasite Nyphanda fusca (Lepidoptera: Lycaenidae). Proceedings of the Royal Society of London Series B. doi: 10.1098/rspb.2008.1064.
Abstract: The exploitation of parental care is common in avian and insect ‘cuckoos’ and these species engage in a coevolutionary arms race. Caterpillars of the lycaenid butterfly Niphanda fusca develop as parasites inside the nests of host ants (Camponotus japonicus) where they grow by feeding on the worker trophallaxis. We hypothesized that N. fusca caterpillars chemically mimic host larvae, or some particular castes of the host ant, so that the caterpillars are accepted and cared for by the host workers. Behaviourally, it was observed that the host workers enthusiastically tended glass dummies coated with the cuticular chemicals of larvae or males and those of N. fusca caterpillars living together. Cuticular chemical analyses revealed that N. fusca caterpillars grown in a host ant nest acquired a colony-specific blend of cuticular hydrocarbons (CHCs). Furthermore, the CHC profiles of the N. fusca caterpillars were particularly close to those of the males rather than those of the host larvae and the others. We suggest that N. fusca caterpillars exploit worker care by matching their cuticular profile to that of the host males, since the males are fed by trophallaxis with workers in their natal nests for approximately ten months.