Ant Density Research Articles

Different-sized oak trees are equally protected by the aphid-tending ants

Theoretical models predict that the ecological impacts of ants on plants will vary with the size of the plant, but experimental evidence supporting this hypothesis is lacking. Focusing on aphid-tending ants, Lasius fuliginosus, and host oak trees, Quercus liaotungensis, we surveyed the density of aphids on the leaves of different-sized trees and the relationship between the densities of aphids and ants on the leaves. We then evaluated the impacts of ants on small (diameter at breast height (DBH) < 5 cm) and large trees (DBH > 25 cm). The field survey showed that the density of ants on leaves was determined by the density of aphids and the latter was similar among different-sized trees; through an ant-exclusion treatment, we found that the impacts of ants on plants, aphids, caterpillars, and galls did not differ by the size of the tree. L. fuliginosus had a significant protective effect for Q. liaotungensis, but this effect did not convert to enhanced fruit production during the 2-year study period. Considering that size can be used as an indicator of a plant’s ontogenetic stage, our results indicate that with similar densities of aphids and, in turn, ants on their canopy, trees at early and later ontogenetic stages are equally protected by aphid-tending ants.

Differential response of ants to nutrient addition in a tropical Brown Food Web

In tropical ecosystems, access to nitrogen (N) and phosphorus (P) limits the decomposition rate of leaf-litter. Leaf-litter ants are abundant in this microhabitat and present a wide variety of diets. Our aim was to study the response of various ant trophic groups to an increased nutrient availability which boosts the decomposition of their habitat and selectively affects the abundance of their prey. A 6-month nutrient addition experiment (CN, CNP) was performed in a tropical montane forest of the Ecuadorian Andes. The density of ants, of other predators (e.g. arachnids, beetles) and of their potential prey (mesofauna, ranging from 0.1 to 2 mm) was measured in treatments and control plots. The litter volume in fertilized plots decreased significantly. Collembola and total mesofauna density were enhanced by the CNP addition. Ants responded differentially according to their trophic group: despite increased prey availability, predatory species in general and collembolan hunters in particular were negatively affected by both treatments. Other ant trophic group densities did not change. By contrast, the density of Dermaptera increased with the treatments. A complementary isotopic approach allowed us to trace carbon fluxes through the food web. Our results suggest that the nutrient input enhanced the litter decomposition rate, leading to reduction of habitat size. They also suggest that predatory ants in tropical leaf-litter food webs are limited by habitat size rather than by prey availability, and that these ants are more affected by habitat loss than their prey, other ant trophic groups and other macrofauna taxa.

Harvester ant ( Messor barbarus (L.)) density as related to soil properties, topography and management in semi-arid cereals

Harvester ants are important weed seed predators in semi-arid cereal fields. Densities of harvester ants vary considerably across fields and so do concomitant weed seed predation rates, however, causes for variability are largely unknown. Understanding the factors that affect ant density may help to identify areas where high natural weed seed control is likely and management factors that could help to enhance ant densities where they are low. Messor barbarus ant density was approximated by estimates of nest density and nest size, and these were correlated to different edaphic, topographic and crop management factors in 34 no-till and 6 minimally tilled cereal fields. The number of years since conversion to no-till explained 12.6% of variability in ant nest density. No other variable could explain nest densities. M. barbarus ant and nest densities increased with increasing number of years of no-till, reached a peak at 11–12 years, after which densities decreased again. Overcompensation by a large cohort of newly established queens could provide an explanation for this observation. Other factors not included in this study, in particular food (seeds) availability, may play an additional role in determining ant density.

Ant-Plant Mutualism in Hawai‘i? Invasive Ants Reduce Flower Parasitism but also Exploit Floral Nectar of the Endemic Shrub Vaccinium reticulatum (Ericaceae)

Ants had been absent from the Hawaiian Islands before their human introduction. Today they cause severe alterations of ecosystems and displace native biota. Due to their strong demand on carbohydrate-rich resources, they often exploit floral nectar of native Hawaiian plant species with largely unknown consequences for the plants' reproduction. We examined effects of flower-visiting invasive ants on reproduction of the endemic shrub Vaccinium reticulatum (Ericaceae) in Hawai‘i Volcanoes National Park. Ant densities in flowers were high and floral nectar was excessively exploited, which may lead to a reduced visitation rate of pollinators. However, the ants' presence on flowers strongly reduced flower parasitism by caterpillars of the introduced plume moth Stenoptilodes littoralis and thus decreased the loss of flowers and buds. This is, to our knowledge, the first documented mutualism between invasive ants and an endemic plant species in Hawai‘i. Developed fruits of this partly self-incompatible plant, however, bore relatively low proportions of viable seeds, irrespective of the experimentally controlled visitor spectrum of the flowers. This may indicate that ants do not function as pollinators and that effective pollinators (probably Hylaeus bees) are scant or absent.

Spatial movement optimization in Amazonian Eciton burchellii army ants

Foraging army ants face a problem general to many animals—how best to confront resource depletion and environmental heterogeneity. Army ants have presumably evolved a nomadic lifestyle as a way to minimize re-exploitation of previously foraged areas. However, this solution creates a challenge for an army ant colony: foraging by this colony and others creates a shifting landscape of food resources, where colonies should theoretically avoid their own previous foraging paths as well as those of other colonies. Here, we examine how colonies exploit this resource mosaic, using some of the optimality arguments first proposed and tested by Franks and Fletcher (1983), but with much larger data sets in a new location in SW Amazonia. Our data supported Franks and Fletcher’s (1983) model for systematic avoidance of raided areas during the statary phase, as well as a hypothesis of distance optimization between successive statary bivouacs. We also test and find significant evidence that foraging raids turn in opposite directions from the previous day’s directional angles more frequently than what would be expected if turning angles were distributed at random, which acts to move a colony away from recently exploited areas. This implies that colonies follow a straighter line path during the nomadic phase as opposed to a curved one, which acts to maximize distance between statary bivouacs. In addition to intra-colony movement optimization, we examine evidence for inter-colony avoidance from more than 330 colony emigrations and suggest that colony-specific pheromones are not necessarily repulsive to other colonies. Lastly, we compare our results with those of similar studies carried out at Barro Colorado Island (BCI), Panama. Despite a higher density of army ants in the SW Amazon region, colonies spend less time emigrating than their counterparts at BCI, which suggests a higher prey density in SW Amazonia.

Global diversity in light of climate change: the case of ants

AbstractAim To use a fine‐grained global model of ant diversity to identify the limits of our knowledge of diversity in the context of climate change.Location Global.Methods We applied generalized linear modelling to a global database of local ant assemblages to predict the species density of ants globally. Predictors evaluated included simple climate variables, combined temperature × precipitation variables, biogeographic region, elevation, and interactions between select variables. Areas of the planet identified as beyond the reliable prediction ability of the model were those having climatic conditions more extreme than what was represented in the ant database.Results Temperature was the most important single predictor of ant species density, and a mix of climatic variables, biogeographic region and interactions between climate and region yielded the best overall model. Broadly, geographic patterns of ant diversity match those of other taxa, with high species density in the wet tropics and in some, but not all, parts of the dry tropics. Uncertainty in model predictions appears to derive from the low amount of standardized sampling of ants in Asia, in Africa and in the most extreme (e.g. hottest) climates. Model residuals increase as a function of temperature. This suggests that our understanding of the drivers of ant diversity at high temperatures is incomplete, especially in hot and arid climates. In other words, our ignorance of how ant diversity relates to environment is greatest in those regions where most species occur – hot climates, both wet and dry.Main conclusions Our results have two important implications. First, temperature is necessary, but not sufficient, to explain fully the patterns of ant diversity. Second, our ability to predict ant diversity is weakest exactly where we need to know the most, the warmest regions of a warming world. This includes significant parts of the tropics and some of the most biologically diverse areas in the world.

Spider web guilds in cacao agroforestry – comparing tree, plot and landscape‐scale management

AbstractAim Owing to their role as insect predators, web‐building spiders can be important biological control agents within agricultural systems. In complex tropical agroecosystems such as agroforests, management determines plant architecture, vegetation composition and associated ant density, but little is known on how these attributes, together with landscape context, determine spider web density. We hypothesized that all three spatial scales and the presence of Philidris ants significantly contribute to the explanation of spider web density with web types being differently affected.Location In 42 differently managed cacao agroforestry systems in Sulawesi, Indonesia.Methods We surveyed the distribution of five spider‐web types on 420 cacao trees to determine how these relate to habitat variables and a numerically dominant ant species at three different spatial scales, comparing tree, plot and landscape features. We fitted linear mixed‐effects model, selected the best model subset using information‐theoretic criteria and calculated the model‐averaged estimates. We used non‐metric multidimensional scaling (NMDS) to determine and visualize guild level responses to the effects of the tree, plot and landscape‐scale variables.Results The five spider guilds preferred different features of cacao tree architecture. Most frequently recorded webs belonged to the line‐ and orb‐web type. At the tree scale, overall web density was positively related to canopy openness. At the plot scale, a higher number of shade trees was related to a higher web density. At the landscape scale, the altitude determined the distribution patterns of web‐building spiders. Presence of Philidris ants was positively associated with density of orb webs, while no pattern was found for other web types.Main conclusions Results suggest spider web density could be increased by pruning of cacao trees while keeping shade trees at high density in cacao plots. The results emphasize the need to consider scale dependency of crop management and web‐guild‐specific responses that may be related to different functional roles of spiders as a high‐density predator group in agroforestry.

Initial Assessment of the Impact of the Recent Invader,Solenopsis invictaBuren, on Resident Ant Assemblages in Matamoros, Mexico

Abstract. The red imported fire ant, Solenopsis invicta Buren, is exotic to Mexico. It has been detected recently at least 3 km south of the Texas-Mexico border, in Matamoros, Tamaulipas, Mexico. Previous studies in the USA have indicated that the arrival and establishment of the red imported fire ant has resulted in reductions of density and diversity of resident ant populations; S. invicta also modifies the associations with and among other ant species. This initial ecological study was conducted from 2007 to 2009 in Matamoros, Mexico, to assess the impact of the red imported fire ant on relative density, diversity, and interspecific associations of other resident ants. The goal of the study was to determine the impact of the imported fire ant infestation on remaining resident ants in two habitat types: (agricultural fields and natural habitats) in adjacent S. invicta infested and non-infested areas. Pitfall traps were used to estimate ant abundance and diversity. A total of 31,838 ants (12,515 in the n...

Consequences of ants and extrafloral nectar for a pollinating seed‐consuming mutualism: ant satiation, floral distraction or plant defense?

Non‐pollinating consumers of floral resources, especially ants, can disrupt pollination and plant reproductive processes. As an alternative food resource to flowers, extrafloral nectar (EFN) may distract and satiate ants from flowers, thereby reducing their antagonistic effects on plants. Yet, EFN may actually attract and increase ant density on plants, thus increasing the disruption of pollination and/or their defense of plants. In this study, we tested the effects of ants and EFN on pollinating seed‐consuming interactions between senita cacti and senita moths in the Sonoran Desert. Prior study of senita showed that EFN can distract ants from flowers, but consequences for plant–pollinator interactions remain unstudied. In our current study, ant exclusion had no effect on pollination or oviposition when moths were abundant (&gt;85% flower visitation). Yet, in an ant by EFN factorial experiment under lower moth abundance (&lt;40% visitation), there was a significant effect of ant exclusion (but not EFN or an ant × EFN) on pollination and oviposition. In contrast with our predictions, ant presence increased rather than decreased pollination (and oviposition) by moths, indicating a beneficial effect of ants on plant reproduction. While ant density on plants showed a saturating response to continuous experimental variation in EFN, in support of ant satiation and distraction, the probability of pollination and oviposition increased and saturated with ant density, again showing a beneficial effect of ants on plant reproduction. Ants showed no significant effect on fruit set, fruit survival, or fruit production of oviposited flowers in the ant exclusion experiment. Ants did not affect the survival of moth larvae, but there was a marginally significant effect of ants in reducing wasp parasitism of moths. We suggest that EFN may not only distract ants from disrupting plant–pollinator interactions, but they may also enhance plant–pollinator interactions by increasing pollination and reducing wasp parasitism. Though often considered dichotomous hypotheses, ant distraction and plant defense may be synergistic, though the mechanism(s) for such positive ant effects on plant–pollinator interactions needs further study.

Fire Ant Response to Management of Native Grass Conservation Buffers

Native grass conservation buffers established for grassland birds require periodic disturbance, but disturbance of vegetation and soil may increase abundance and activity of imported fire ants (Solenopsis sp.). This is a concern because fire ants are invasive, often respond positively to disturbance and can be a major cause of nest losses in grassland birds. We experimentally tested if fire ant mound density and foraging activity increased after burning and disking in native grass buffers. In 2008, fall disking increased mound density and foraging activity during the first growing season post-disturbance, but burning did not. In 2009, disking had no effect, but effects of disking the previous season persisted into the second growing season. Prescribed fire, which tends to maintain or increase perennial grass cover, may be a better option than disking for managing native grass buffers where fire ant densities are high.

Association of ant nests with successional stages of biological soil crusts in the Tengger Desert, Northern China

Biological soil crusts are important cover in arid desert landscapes, yet their importance as habitats to secondary producers such as ants is relatively unknown. This study was conducted to determine if the presence and development of biological soil crusts on dune surfaces stabilized by revegetation facilitates ant establishment and survival. We measured topsoil properties and crustal features during different successional stages, which were characterized by cyanobacteria and algae, lichens, and mosses, respectively. The species richness and abundance (nest density) of ants were closely associated with silt content, soil organic matter, nitrogen and soil moisture, as well as topsoil temperature. However, ant nest distribution was largely dependent on the biomass and thickness of crusts and topsoil. These findings provide evidence that the recovery or development of biological soil crusts on dune surfaces in the Tengger Desert could favor and maintain higher ant species diversity. Therefore, the disturbance of crusts would result in a reduction of ant species richness and abundance in desert systems.

Long-Term Field Trial to Control the Invasive Argentine Ant (Hymenoptera: Formicidae) With Synthetic Trail Pheromone

Previous short-term experiments showed that trail following behavior of the Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), can be disrupted by a high concentration of synthetic trail pheromone component (Z)-9-hexadecenal. In this study, a long-term field trial was conducted in 100-m2 plots of house gardens in an urban area of Japan to see whether the control effect on Argentine ants can be obtained by permeating synthetic trail pheromone from dispensers. The dispensers were placed in the experimental plots during the ant's active season (April-November) for 2 yr with monthly renewal. To estimate Argentine ant population density, foraging activity of Argentine ants in the study plots was monitored by monthly bait surveys. Throughout the study period, Argentine ant foraging activity was suppressed in the presence of the dispensers, presumably via trail forming inhibition. In contrast, the level of foraging activity was not different between treatment and no-treatment plots when the dispensers were temporarily removed, suggesting that treatment with pheromone dispensers did not suppress Argentine ant density in the treatment plots. Population decline may be expected with larger-scale treatment that covers a significant portion of the ant colony or with improvement in the potency of the disruptant.

Combined Effect of Hemipteran Control and Liquid Bait on Argentine Ant Populations

The invasive Argentine ant, Linepithema humile (Mayr), has become a worldwide problem capable of inflicting significant ecological and economic injury on urban, agricultural, and natural environments. The mobility of this pest ant has long been noted, rapidly moving nests to new food resources and then away as resources are depleted. This ant, like many pest ant species, has a special affinity for honeydew excreted by phloem-feeding Hemiptera. We investigated the effect of various hemipteran control strategies on terrapin scale densities and measured their indirect effect on local Argentine ant densities and foraging effort. We then determined whether this indirect treatment strategy improved the performance of an ant bait. We predicted that Argentine ants would move nests away from trees treated for Hemiptera and then move nests back when a liquid bait was offered, followed by a decline in ant numbers due to intake of the toxicant. A horticultural oil spray and soil application of the systemic insecticide, imidacloprid, had no effect on terrapin scale numbers. However, trunk-injected dicrotophos caused a reduction in scale and a decline in local Argentine ant nest density and canopy foraging effort. We also recorded a reduction in local Argentine ant ground foraging when large amounts of liquid bait were applied, and we found no evidence that combining dicrotophos with liquid ant bait performed better than each treatment alone. We suggest that a strategy of combined hemipteran control plus application of liquid ant bait can reduce local Argentine ant densities, when both components of this system are highly efficacious.

Efecto de los cortafuegos sobre el ensamble de hormigas (Hymenoptera, Formicidae) en una región semiárida, Argentina

En las regiones áridas y semiáridas los bordes de los caminos o cortafuegos pueden afectar variables micro-climáticas las cuales, a su vez, alteran la abundancia de las hormigas que nidifican en el suelo. Se estudió la densidad de nidos en ambientes con diferentes características edáficas (suelos sueltos y compactados), y de cobertura de vegetación (monte cerrado, pastizal y suelo desnudo). El área de estudio se encuentra en el sur del Caldenal (sudeste de La Pampa), tiene 12 ha clausuradas al pastoreo con seis unidades experimentales en cada una de las cuales se seleccionaron tres sitios con cobertura leñosa (monte), con cobertura herbácea (pastizal) y con el 80% de suelo desnudo (cortafuegos). En cada sitio se registraron la temperatura superficial, y la humedad, el pH, y el grado de compactación del suelo. La densidad de nidos se evaluó colocando tres transectas (80 m x 5 m) al azar por cada unidad experimental. La temperatura del suelo fue mayor en los cortafuegos y la compactación del suelo fue mayor en los ambientes de monte y pastizal. El ensamble de hormigas estudiado no mostró diferencias (p&gt;0,05) de nidificación entre los ambientes. En cambio, Acromyrmex striatus (Roger, 1863) se encontró principalmente en los cortafuegos donde los suelos sueltos con mayor porosidad permiten mayor intercambio gaseoso e infiltración de agua. La construcción de cortafuegos favorece el establecimiento de especies cortadoras de hojas que por ventajas competitivas podrían afectar negativamente la composición de la comunidad de hormigas y las comunidades vegetales.

Indirect effects of ant eradication efforts on offshore islets in the Hawaiian Archipelago

Invasive species eradication and control are considered vital components of the conservation, restoration, and management of many native ecosystems. Invasive ants, which are notoriously difficult to eradicate, can cause catastrophic changes in ecosystems and are aggressive colonists. Here we report the eradication and control of two widely distributed invasive ants and subsequent unanticipated effects on arthropod and avian communities. We used a paired experimental design that included 1 year of baseline data collection, to test the effects of the formicide hydramethylnon on abundances of two ant species on two pairs of offshore islets. Pheidole megacephala was eradicated from the treated islet in pair 1 and was not detected during 2003–2008. On pair 2 Solenopsis geminata numbers declined, but the species remained present. Target ant densities remained high on untreated islets. Application of hydramethylnon reduced numbers of alien cockroaches (Order: Blattaria), but we did not detect effects on other non-target arthropods. The eradication of P. megacephala was followed by dynamic compositional changes in the ant community, including the apparent colonization by three species (S. geminata, Tetramorium bicarinatum and Anoplolepis gracilipes) previously undetected on the islet. One of these, A. gracilipes, underwent a rapid range expansion during 2006–2008 which corresponded with reduced seabird nesting success. We conclude that hydramethylnon can be used effectively to eradicate P. megacephala. However, ant eradications can have detrimental effects on ecosystems and the potential for subsequent colonization of sites by other ant species that may be more harmful and more difficult to eradicate needs to be considered.

Argentine Ant Invasion Associated With Loblolly Pines in the Southeastern United States: Minimal Impacts But Seasonally Sustained

Invasive ants are notorious for directly displacing native ant species. Although such impacts are associated with Argentine ant invasions (Linepithema humile) worldwide, impacts within natural habitat are less widely reported, particularly those affecting arboreal ant communities. Argentine ants were detected in North Carolina mixed pine-hardwood forest for the first time but were localized on and around loblolly pines (Pinus taeda), probably because of association with honeydew-producing Hemiptera. We explored the potential impacts of L. humile on arboreal and ground-foraging native ant species by comparing interspersed loblolly pines invaded and uninvaded by Argentine ants. Impacts on native ants were assessed monthly over 1 yr by counting ants in foraging trails on pine trunks and in surrounding plots using a concentric arrangement of pitfall traps at 1, 2, and 3 m from the base of each tree. Of floristics and habitat variables, higher soil moisture in invaded plots was the only difference between plot types, increasing confidence that any ant community differences were caused by Argentine ants. Overall patterns of impact were weak. Composition differed significantly between Argentine ant invaded and uninvaded trunks and pitfalls but was driven only by the presence of Argentine ants rather than any resulting compositional change in native ant species. Native ant abundance and richness were similarly unaffected by L. humile. However, the abundance of individual ant species was more variable. Although numbers of the arboreal Crematogaster ashmeadi (Myrmicinae) declined on and around invaded pines, epigeic Aphaenogaster rudis (Myrmicinae) remained the most abundant species in all plots. Argentine ant densities peaked in late summer and fall, therefore overlapping with most native ants. Unexpected was their continued presence during even the coldest months. We provide evidence that Argentine ants can invade and persist in native North Carolina forests, probably mediated by pine-associated resources. However, their localized distribution and minimal impact on the native ant fauna relative to previously described invasions requires further resolution.

Laboratory evaluation of aggregation, direct mutual interference, and functional response characteristics of Pseudacteon tricuspis Borgmeier (Diptera: Phoridae)

Studies of parasitoid behavior yield information on behavioral strategies, and are crucial tests of theoretical models. The studies reported here provided insights into behavioral and functional responses of the red imported fire ant ( Solenopsis invicta Buren) parasitoid, Pseudacteon tricuspis (Borgmeier) that were previously unknown. Laboratory studies were performed to quantify aggregative responses of P. tricuspis adults to variable host ant densities, determine effect of direct mutual interference between multiple ovipositing P. tricuspis females confined with host S. invicta, elucidate the effect of confining one or two additional males with premated females on progeny sex ratios, and determine the form of the functional response of individual ovipositing P. tricuspis. Consistent with theory and field observations, P. tricuspis tended to aggregate at host patches containing greater numbers of ants. Some evidence of direct mutual interference was found, as per capita oviposition success declined when more than one female was confined and weak, but significant, reductions in searching efficiency were found when more than two P. tricuspis females are simultaneously ovipositing. The presence of one or two males did not appear to affect ovipositional efficiency of solitary premated females, but normally male–biased sex ratios of progeny trended toward a 1:1 ratio when the number of males was increased from zero to two. None of the linear parameters in the logistic models were significantly different from zero suggesting that P. tricuspis had constant, type 1, attack rates regardless of host density, at least under our laboratory experimental design.

Modeling, analysis and simulation of ant-based network routing protocols

Using the metaphor of swarm intelligence, ant-based routing protocols deploy control packets that behave like ants to discover and optimize routes between pairs of nodes. These ant-based routing protocols provide an elegant, scalable solution to the routing problem for both wired and mobile ad hoc networks. The routing problem is highly nonlinear because the control packets alter the local routing tables as they are routed through the network. We mathematically map the local rules by which the routing tables are altered to the dynamics of the entire networks. Using dynamical systems theory, we map local protocol rules to full network performance, which helps us understand the impact of protocol parameters on network performance. In this paper, we systematically derive and analyze global models for simple ant-based routing protocols using both pheromone deposition and evaporation. In particular, we develop a stochastic model by modeling the probability density of ants over the network. The model is validated by comparing equilibrium pheromone levels produced by the global analysis to results obtained from simulation studies. We use both a Matlab simulation with ideal communications and a QualNet simulation with realistic communication models. Using these analytic and computational methods, we map out a complete phase diagram of network behavior over a small multipath network. We show the existence of both stable and unstable (inaccessible) routing solutions having varying properties of efficiency and redundancy depending upon the routing parameters. Finally, we apply these techniques to a larger 50-node network and show that the design principles acquired from studying the small model network extend to larger networks.

Nest density and distribution of subterranean ants in an evergreen broadleaf forest in Japan with special reference to Amblyopone silvestrii

AbstractThe density and distributions of nests of the subterranean ants were studied by excavation of thirty 1‐m2 quadrats in an evergreen broadleaf forest in central Japan. A total of 20 ant species were collected and the densities of Amblyopone silvestrii and Paratrechina flavipes were highest (both 0.83 nests/m2). The distribution patterns were analyzed by the Standardized Morisita Index (Ip) for the five species that had mean densities of more than 0.5 nests/m2. Of them, A. silvestrii and Hypoponera sauteri showed random distributions, while nests of Paratrechina flavipes were significantly overdispersed and those of Carebara yamatonis and Myrmecina nipponica were weakly clumped. These distributional patterns were discussed in connection with the ecology of the ants. In particular, the social organization of A. silvestrii was investigated based on the distributional pattern and the composition of collected colonies.

Factors affecting the vulnerability of cane toads (Bufo marinus) to predation by ants

Experimental evidence on the determinants of prey vulnerability is scarce, especially for vertebrates in the field. Invasive species offer robust opportunities to explore prey vulnerability, because the intensity of predation on or by such animals has not been eroded by coevolution. Around waterbodies in tropical Australia, native meat ants (Iridomyrmex reburrus) consume many metamorph cane toads (Bufo marinus, an invasive anuran). We document the determinants of toad vulnerability, especially the roles of toad body size and ant density. Larger metamorphs were attacked sooner (because they attracted more ants), but escaped more often. Overall, smaller toads were more likely to be killed. Ant densities influenced toad responses, as well as attack rate and success. Data on the immediate outcomes of attacks underestimate mortality: more than 73% of apparent ‘escapees’ died within 24 h. Because mortality during this period was independent of toad size, predation was less size selective than suggested by immediate outcomes. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 738–751.