We investigated responses of ant communities to habitat complexity, with the aim of assessing complexity as a useful surrogate for ant species diversity. We used pitfall traps to sample ants at twenty‐eight sites, fourteen each of low and high habitat complexity, spread over ca 12 km in Sydney sandstone ridge‐top woodland in Australia. Ant species richness was higher in low complexity areas, and negatively associated with ground herb cover, tree canopy cover, soil moisture and leaf litter. Ant community composition was affected by habitat complexity, with morphospecies from the genera Monomorium, Rhytidoponera and Meranoplus being the most significant contributors to compositional differences. Functional group responses to anthropogenic disturbance may be facilitated by local changes in habitat complexity. Habitat complexity, measured as a function of differences in multiple strata in forests, may be of great worth as a surrogate for the diversity of a range of arthropod groups including ants.

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We examined the responses of a beetle assemblage to habitat complexity differences within a single habitat type, Sydney sandstone ridgetop woodland, using pitfall and flight-intercept trapping. Six habitat characters (tree canopy cover, shrub canopy cover, ground herb cover, soil moisture, amount of leaf litter, and amount of logs, rocks and debris) were scored between 0 and 3 using ordinal scales to reflect habitat complexity at survey sites. Pitfall trapped beetles were more species rich and of different composition in high complexity sites, compared with low complexity sites. Species from the Staphylinidae (Aleocharinae sp. 1 and sp. 2), Carabidae ( Pamborus alternans Latreille), Corticariidae ( Cartodere Thomson sp. 1) and Anobiidae ( Mysticephala Ford sp. 1) were most clearly responsible for the compositional differences, preferring high complexity habitat. Affinities between general functional groupings of pitfall-trapped beetles and habitat variables were not clear at a low taxonomic resolution (family level). The composition and species richness of flight-intercept-trapped beetles were similar in high and low complexity sites. Our study demonstrates that discrete responses of the various functional groups of beetles are strongly associated with their feeding habits, indicated by differing habitat components from within overall composite habitat complexity measures. Although habitat preferences by beetle species may often reflect their foraging habits, clarification of the causal mechanisms underpinning the relationships between habitat complexity and beetles are critical for the development of general principles linking habitat, functional roles and diversity.

Ant communities are thought to consist of a competitive hierarchy of interacting species, with an assemblage of subordinate species being structured by a dominant species. Mensurative and behavioral studies suggest a significant role for competition in structuring ant communities, although there are few experimental studies to support this contention. We examined the effect of the dominant ant Iridomyrmex purpureus on the ant fauna of sandstone outcrops in southeastern Australia. We conducted a mensurative survey using transect counts to compare ant assemblages at eight outcrops with, and eight without I. purpureus. Using cages, we then successfully excluded the dominant ant from four outcrops and compared assemblages at these exclusion sites with those at sites with and without I. purpureus and with procedural control sites over a period of 12 months. We conducted behavioral studies comparing I. purpureus with six other common species in terms of their abilities to locate, recruit to, and defend bait. While initial surveys indicated both positive and negative effects of I. purpureus on the activity of several subordinate species, exclusion of I. purpureus resulted only in changes in the abundance of other species of Iridomyrmex, which are behaviorally and ecologically similar to the dominant species. Iridomyrmex purpureus was faster at discovering bait than other species, but not always better at recruiting to the bait. It interfered with the foraging of all species tested; however, it displaced other Iridomyrmex from bait most often. While longer‐term studies may provide more definitive results, exclusion of the dominant species over one year had strong effects only on ecologically similar species. Despite its behavioral dominance and association with several other species in the mensurative surveys, I. purpureus was not shown to cause any other major changes in ant community structure.

Abstract.  1. Many moth and butterfly larvae are gregarious early in development, but become solitary in late instars. This ontogenetic variation in behaviour is probably the result of temporal changes in the costs and benefits associated with gregariousness. This study provides observational and experimental evidence that, in one particular moth species, a series of different ecological factors influence larval behaviour at different times during development. 2. Field observations show that young caterpillars of the limocodid Doratifera casta form large aggregations while foraging, but that mature larvae are largely solitary. 3. A field experiment revealed that individual first to third instar larvae in larger groups develop more rapidly, but that group size had no detectable influence on survival. The developmental advantage associated with gregariousness is affected by host plant species, but not by predator exclusion, suggesting that group living in these cryptic early instar larvae promotes feeding facilitation, but does not provide individuals with protection from natural enemies. 4. Laboratory experiments revealed that aposematic fourth instar caterpillars in large groups were less likely to be attacked by a generalist insect predator than those in small groups. 5. Field observations provided no evidence that group living affects body temperature, suggesting that microclimatic factors do not favour gregariousness in this species. 6. It is concluded that gregariousness in D. casta confers at least two different advantages on larvae at different stages early in development, but that these advantages disappear, or are outweighed by costs associated with intraspecific competition, in final instars.