Colony structure and spatial distribution of food resources in the polydomous meat ant <Emphasis Type="Italic">Iridomyrmex purpureus</Emphasis>

Polydomous social insects may reduce the costs of foraging by the strategic distribution of nests throughout their territory or home-range. This efficiency may most likely be achieved if the resources are relatively stable in place and time, and the colonies and nests are distributed in response to the location of the resources. However, no study has investigated how the distribution of food sources influences the spatial patterns of nests within polydomous colonies under natural conditions. Our two year study of 140 colonies of the Australian ant Iridomyrmex purpureus revealed that the decentralization of nests within colonies is associated with the distribution of trees containing honey-dew producing hemiptera. We show there is a positive correlation between the maximum distance between trees containing hemiptera and the maximum distance between nests within a colony. In addition, we demonstrate the mechanism by which this pattern may arise; new nests are built nearer to trees containing hemiptera than existing nests. Further, the distance between trees containing hemiptera and the nearest nests was negatively correlated with the length of exploitation of that tree. Finally, we show that most food is delivered to the nearest nest after which other ants redistribute it between the nests. Combined, these data suggest that foraging efficiency may be an important selection pressure favouring polydomy in I. purpureus. Received 6 April 2006; revised 29 September; accepted 4 October 2006.     

Generalist caterpillar prey are more palatable than specialists for the generalist predator Iridomyrmex humilis

Summary Experiments are described which test the hypothesis that more host-specific species of caterpillars should be less aceptable to a generalist predator than polyphagous species. Caterpillars of all species were tested in paired choice tests with the Argentine ant, Iridomyrmex humilis. Experiments were replicated ten times, videotaped and later analyzed. Brightly colored specialist species (normally considered to be aposematic) were clearly the least palatable, while more cryptic specialists were also significantly less acceptable than generalists overall. Leaf-tying species were considered separately; all were highly palatable independent of host range. The results indicate that among caterpillars that do not construct leaf shelters, those with a wide post range are more acceptable than those with a narrow host range. This is consistent with the notion that generalist predators provide selection pressure favoring narrow host range in their herbivorous prey.     

Asian weaver ants,Oecophylla smaragdina,and their repelling of pollinators

The Asian weaver ant, Oecophylla smaragdina, is known to have outstanding predatory power. This ant can protect the host plants from attacks of phytophagous insects and therefore has been used for biological control in the tropics. We present evidence for a possible negative effect of Oecophylla on the performance of host plants. Our observation in a fruit orchard of rambutan in Sumatra suggested that the presence of Oecohylla nests on the trees statistically significantly lowered the flower-visiting rate of flying insects, involving the major pollinator Trigona minangkabau. The visiting rate of Oecophylla workers to each flower shoot of rambutan significantly negatively correlated with the visiting rate of flying insects. Empirical evidence of such an inhibitory effect on flower-visiting of pollinators cased by aggressive ants has been scarce so far.     

Movement and spatial organization of small mammals following vertebrate predator exclusion

Predation directly changes the demographics of prey, generally through a numerical decrease. An indirect effect of predators is alteration of movements and spatial patterns of prey. The relationship between these direct and indirect effects can be tested by excluding predators. Home range size of white-footed mice (Peromyscus leucopus) decreased and home range overlaps increased in the absence of predators. Home range size and overlaps of meadow voles (Microtus pennsylvanicus) did not change despite an increase in vole density. P. leucopus had significantly lower interspecific home range overlaps with M. pennsylvanicus than intraspecific overlaps. The changes in P. leucopus spatial behavior may be an indirect effect of predator exclusion resulting from the increase in M. pennsylvanicus densities.     

Emergent multiple predator effects in an experimental microbial community

Studies on multiple predator effects have typically focused on metazoans. Here we examined the effects of multiple consumers on bacteria. Specifically, we investigated the separate and combined effects of two common bacterivorous ciliates, Colpidium striatum and Paramecium caudatum, on a simple bacterial assemblage. Non-additive multiple predator effects emerged for all bacterial species significantly affected by grazing, where bacterial responses in the two-consumer treatment cannot be predicted by summing the separate effects of the two consumers. Some species showed risk reduction (observed responses less than expected), whereas others showed risk enhancement (observed responses larger than expected). We attributed risk alteration to the interference between the two consumers. Contradictory to theoretical predictions, total bacterial abundance in the two-consumer treatment did not differ from single-consumer treatments and consumer-free controls, due largely to risk reduction and compensatory responses within the bacterial assemblage. Decomposition of particulate organic matter was greater in the two-consumer treatment than the Paramecium single-consumer treatment, but did not differ among other treatments. These results suggest that the presence of multiple consumers may have unexpected impacts on the structure and functioning of bacterial communities.     

Coordinated group ambush: A new predatory behavior inAzteca ants (Dolichoderinae)

Summary This article describes a new technique used by ants (Azteca cf. lanuginosa) in the Brazilian Cerrado to capture large mobile insects. Large numbers of ants position themselves along a leaf margin in sit-and-wait groups and ambush prey by simultaneously attacking insects that land on the upper surface of the leaf. The workers are able to seize insects such as beetles, bees, and butterflies, most of them being two times longer than the ants.     

Phylogeography of Asian weaver ants, Oecophylla smaragdina

The phylogeography of Oecophylla smaragdina was studied using the mitochondrial cytochrome b gene (Cytb), cytochrome oxidase subunit I (COI), and nuclear long-wavelength opsin gene (LW Rh). Weaver ants were collected from 35 localities and from one to nine colonies per locality. Neighbor-joining trees inferred from 647 bp of Cytb and 1,026 bp of COI using Oecophylla longinoda as an outgroup indicated that the haplotypes of O. smaragdina were clearly separated into seven groups: group 1 of India excluding West Bengal; group 2 of Bengal, Indochinese Peninsula, Malay Peninsula and Greater Sunda Islands, including Lombok and Sumbawa; group 3 of the Philippines; group 4 of Flores; group 5 of Sulawesi; group 6 of Halmahera; and group 7 of New Guinea and Australia. This grouping was also supported by a strict consensus tree derived from maximum parsimony and maximum likelihood trees. In addition, two haplotypes of LW Rh were found in O. smaragdina: one in group 2 and another in all the other groups. Comparison to haplotypes in other hymenopteran species suggests that group 2 is younger than other groups of O. smaragdina. The clustering of the seven groups was coincident with geological evidence of the distribution of continents, islands, and seas during glacial periods.     

Induced defences in an endangered amphibian in response to an introduced snake predator

Introduced species have contributed significantly to the extinction of endemic species on islands. They also create new selection pressures on their prey that may result in modified life history strategies. Introduced viperine snakes (Natrix maura) have been implicated in the decline of the endemic midwife toad of Mallorca (Alytes muletensis). A comparison of A. muletensis tadpoles in natural pools with and without snakes showed that those populations subject to snake predation possessed longer tails with narrower tail fins but deeper tail muscles. Field and laboratory experiments showed that these changes in tail morphology could be induced by chemical and tactile cues from snakes. Populations of tadpoles that were subject to snake predation also displayed clear bimodal size-frequency distributions, with intermediate-sized tadpoles missing from the pools completely. Tadpoles in pools frequented by snakes developed faster in relation to their body size than those in pools without snakes. Variation in morphology between toad populations may therefore be caused by a combination of size-selective predation and tadpole plasticity. The results of this study indicate that the introduction of alien species can result in selection for induced defences, which may facilitate coexistence between predator and prey under certain conditions.     

Chlamydial endocytobionts of free-living amoebae differentially affect the growth rate of their hosts

Bacteria closely related to chlamydiae live and multiply as endocytobionts within free-living amoebae, making these amoebae potential vehicles of new emerging bacterial pathogens of humans. Hartmannella vermiformis containing endobiotic Neochlamydia hartmannellae grew more rapidly than those without endobionts, whilst Acanthamoeba sp. harbouring the Parachlamydia-related endocytobiont UWE25 multiplied more slowly than those without endobionts. The cause for the opposite effect of chlamydial endocytobionts on the growth of their host cells remains unknown.     

Dispersed central place foraging in Australian meat ants

Summary Australian meat ants often inhabit colonies with widely dispersed nest holes, and this study examines how resource is harvested and distributed in a colony ofIridomyrmex sanguineus Smith (Formicidae: Dolichoderinae). The three principal types of foragers (tenders, honeydew transporters, scavengers) exhibited nest hole fidelity, where harvested resource was consistently delivered to the same nest hole by each foraging individual. Australian meat ants thus use a harvesting system based on dispersed central place foraging. Evidence of frequent larval transport among nest holes, age polyethism developing in the direction of foraging, and the tendency for nest-associated workers to accept new nest holes more readily than foragers, suggests that workers develop fidelity to the particular nest hole in which they eclose. Coupled with larval transport, nest hole fidelity may allow a colony with widely dispersed nest holes to adjust its structure to more efficiently harvest a resource distributed unevenly in space or time.     

Food-web modification by an invertebrate predator in the Great Salt Lake (USA)

Summary During unusually wet years the salinity of the Great Salt Lake (Utah) decreased from above 100 g/L to 50 g/L. This allowed the predaceous insect Trichocorixa verticalis to invade the pelagic region of the lake and reach a mean summer density of 52/m³. Concurrent changes in the pelagic ecosystem were: a decrease in the dry biomass of the previously dominant filter-feeding brine shrimp Artemia franciscana from 720 to 2 mg/m³, the invasion of three other zooplankton taxa, a 10 × decrease in community filtration rate, a 20 × increase in chlorophyll a concentration, a 4 × decrease in water clarity and perhaps a decrease in soluble nutrients. Trichocorixa abundance was also inversely correlated with the abundance of Artemia along a salinity gradient in the lake's estuary. In a 9-d microcosm experiment Trichocorixa preyed on nauplii and decreased the total density of Artemia from 103 to 6/L. The reduction in Artemia allowed protozoans to increase 10–100 ×. Changes in chlorophyll and clarity were consistent with those observed in the lake. These results suggest that invertebrate predation may be an important factor structuring simple food webs such as those found in moderately saline lakes.     

Impacts of Argentine ants on avian nesting success

Biological invasions can have severe and widespread impacts on ecological communities. A few species of ants have become particularly damaging invaders but quantitative data of their impacts on many taxa is still lacking. We provide experimental evidence using artificial nests baited with quail eggs that the invasive Argentine ant (Linepithema humile) can be a significant avian nest predator – Argentine ants recruited to more nests and in higher abundance than the native ant species they displace. However, at a site invaded by Argentine ants, we monitored over 400 nests of a ground-nesting species, the Dark-eyed Junco (Junco hyemalis), and found that less than 2% of nests failed as a result of Argentine ant predation/infestation. A review of the literature also suggests that Argentine ants may not be a serious threat to bird nests relative to other predators or parasites. However, invasive ants with the capability of overwhelming prey though stinging (specifically the red-imported fire ant, Solenopsis invicta), may have a higher impact on avian nesting success. Received 14 January 2005; revised 28 April 2005; accepted 12 May 2005.     

Diet does not affect intercolonial fighting in Leptothoracine ants

Summary An experiment examined the effects of diet on brood rearing in colonies ofLeptothorax ambiguus andL. longispinosus and on fighting between colonies of the two species. Colonies fedDrosophila and honey reared more pupae than colonies fed the Bhatkar and Whitcomb (1970) diet. However, diet had no effect on intercolonial fighting.     

The urban fire ant paradox: native fire ants persist in an urban refuge while invasive fire ants dominate natural habitats

In contrast to the widespread extirpation of native fire ants (Solenopsis geminata) across southern US following the invasion by imported red fire ants (S. invicta), some residential areas of Austin form unexpected refuges for native fire ants. Ironically, these urban environments provide refuges for the native fire ants while adjacent natural habitats have been overrun by invasive fire ants. Resistance to invasive fire ants in these urban areas occurs mainly in older residential properties constructed prior to the S. invicta invasion, while more recent construction has allowed establishment by S. invicta. The invasive ability of S. invicta is often attributed to escape from parasitoids and efficient dispersal of polygyne multiple queen colonies. Here we also show the importance of landscape parameters in the invasion process, where low levels of disturbance and continuous plant cover in older residential areas form possible barriers to colonization. Dense leaf cover (high NDVI) was also found to be associated with native ant refuges. Long term residential land ownership may have resulted in lower recent disturbance levels and increased plant cover that support refuges of native fire ants.     

Dominant fungi in the rhizosphere of established tea bushes and their interaction with the dominant bacteria under in situ conditions

Species of Penicillium and Trichoderma were found to dominate the rhizosphere of established tea bushes in a detailed study conducted from various tea growing locations in India. Penicillium erythromellis, P. janthinellum, P. raistrickii, Trichoderma pseudokoningii and T. koningii were found to be closely associated with tea roots. While seasonal fluctuation was observed in the case of Penicillium spp., the population of Trichoderma spp. showed less variation during the year. Both species were sensitive to low temperatures. In general, fungi associated with the tea rhizosphere were found to prefer a mesophillic temperature range (15 °C to 35 °C). The dominant species of Penicillium and Trichoderma also exhibited tolerance to lower temperatures, i.e., 5 to 10 °C on agar plates. Most fungi were able to grow in a wide range of pH (4 to 12). Lowering of soil pH in the rhizosphere of tea bushes was positively correlated with the age of the bush and may have affected the development of a specific microbial community in the rhizosphere.The populations of Penicillium and Trichoderma species were inversely correlated with the populations of two most dominant rhizosphere bacteria, Bacillus subtilis and B. mycoides. Both Bacillus species have been shown to have antagonistic activity against these two fungi under in vitro conditions. The present study demonstrates the existence of a similar antagonism under in situ conditions in the rhizosphere of established tea bushes.     

Evaluating biological control of fire ants using phorid flies: effects on competitive interactions

The effects of the parasitic phorid fly, Pseudacteon tricuspis Borgmeier, on the competitive interactions between the red imported fire ant, Solenopsis invicta Buren, and a native North American ant, Forelius pruinosus (Roger), were investigated in the laboratory. P. tricuspis is a highly host-specific endoparasitoid of S. invicta workers that is currently being reared and released as a biological control agent of S. invicta in the US. We tested the effect of P. tricuspis on the colony growth rate of S. invicta when S. invicta was forced to compete with F. pruinosus for a protein resource (freeze-killed crickets) in laboratory competition arenas. In addition to colony growth rate, we quantified the effect of the phorid flies on the foraging rate of S. invicta. Though S. invicta significantly reduced its foraging rate in the presence of the phorid flies, we did not detect an effect of the flies on colony growth rate. Possible explanations for these results include behavioral compensation by S. invicta for the presence of the flies. We present these laboratory results in light of a literature search indicating that laboratory tests of biological control agent efficacy are good predictors of field efficacy. We conclude that P. tricuspis alone is unlikely to suppress S. invicta populations in the field by reducing their competitive ability.     

Trace fossils of the arthropod Camptophyllia from the Westphalian (Carboniferous) rocks of Lancashire, UK and their palaeoenvironmental context

Two arthropod trace fossils are described and analysed from the Carboniferous Lower Westphalian (C. communis and basal A. modiolaris chronozones) coal-bearing strata of Lancashire. The biserial trackway Diplichnites triassicus consists of five overlapping en echelon sets of 7–9 tracks preserved as epichnia and hypichnia in lacustrine siltstones. The trackway suggests subaqueous in-phase walking by a multi-segmented producer with a body length of 35–40 mm, width 17–22 mm, and 7–9 appendages. Curved, clustered, or laterally repeated, hypichnial lobes with transverse striations on the base of ripple cross-laminated sandstone are identified as Rusophycus versans. This trace fossil is interpreted as shallow resting or furrowing burrows of a homopodous arthropod, 30–60 mm long, 15–30 mm wide, and probably the same kind of arthropod as produced D. triassicus.A review of contemporary arthropod body fossils from Lagerstätten in Lancashire favours the onisciform, or Arthropleura like arthropod Camptophyllia as a potential producer of both of these trace fossils in a lacustrine palaeoenvironment.This study integrates the analysis of sediments, trace fossils and body fossils for reconstructing the arthropod biota and ecology in Westphalian lacustrine and crevasse splay fluvial palaeoenvironments.     

Habitat structure mediates top–down effects of spiders and ants on herbivores

Differences in structural complexity of habitats have been suggested to modify the extent of top–down forces in terrestrial food webs. In order to test this hypothesis, we manipulated densities of generalist invertebrate predators and the complexity of habitat structure in a two-factorial design. We conducted two field experiments in order to study predation effects of ants and spiders and, in particular, of the wasp spider Argiope bruennichi on herbivorous arthropods such as grasshoppers, plant- and leafhoppers in a grassland. Predator densities were manipulated by removal in habitats of higher and lower structural diversity, and the effects on herbivore densities were assessed by suction sampling. Habitat structure was changed by cutting the vegetation to half its height and removing leaf litter.We found a significant negative effect of this assemblage of generalist predators on plant- and leafhoppers, which were 1.6 times more abundant in predator removal plots. This effect was stronger in low-structured (cut) than in uncut vegetation. Densities of the most abundant planthopper Ribautodelphax pungens (Delphacidae) were 2.2 times higher in predator removal plots. Furthermore, adult plant- and leafhoppers responded more strongly than juveniles and epigeic species more strongly than hypergeic species. The presence of predators had a positive effect on plant- and leafhopper species diversity. In a second field experiment, we tested the exclusive impact of Argiope bruennichi on its prey, and found that its effect was also significant, although weaker than the effect of the predator assemblage. This effect was stronger in grass-dominated vegetation compared to structurally more complex mixed vegetation of grasses and herbs. We conclude that habitat structure and in particular vegetation height and architectural complexity strongly modify the strength of top–down forces and indirectly affect the diversity of herbivorous arthropods.