Influence of native ants on arthropod communities in a vineyard

• 1 Ants can have a range of effects on arthropods in crops, including suppressing herbivores such as caterpillars. However, ants can also increase hemipteran densities while reducing natural enemy numbers. In vineyard ecosystem, the effects of native ants and their interactions with other arthropods are poorly understood.
• 2 An ant‐exclusion experiment was designed to test the impact of native ants on both canopy and ground arthropods concurrently. The potential influence of ants on predation and parasitism of light brown apple moth (LBAM) eggs, a grape pest, was also examined. Adult grapevine scale insects and earwigs under bark were counted after a season of ant‐exclusion.
• 3 Among 23 ground ant species collected, six were found to forage in the canopy, with two Iridomyrmex species being the most commonly encountered.
• 4 There was no difference in the abundance of most arthropod orders and feeding groups between ant‐excluded and control vines, although ground spiders were more abundant under ant‐excluded vines, despite increased ground ant foraging pressure. LBAM egg parasitism and predation were low and probably affected by weather and other arthropods. Ant exclusion did not reduce survival of scale insects, although the distribution and abundance of scale insects were negatively associated with earwigs.
• 5 In conclusion, native ants did not consistently suppress arthropod assemblages, including natural enemies, and they did not promote the survival of scale insects. Interactions among native ant species within a vineyard might minimize their effects on other arthropods, although this needs further study.

     

Strategic Management of an Invasive Ant‐scale Mutualism Enables Recovery of a Threatened Tropical Tree Species

Mutualisms between invasive ants and honeydew‐producing insects can have widespread negative effects on natural ecosystems. This is becoming an increasingly serious problem worldwide, causing certain ecosystems to change radically. Management of these abundant and influential mutualistic species is essential if the host ecosystem is to recover to its former non‐invaded status. This negative effect is particularly prevalent on some tropical islands, including Cousine Island, Seychelles. On this island, the invasive ant Pheidole megacephala has caused serious indirect damage to the threatened native Pisonia grandis trees via a mutualism with an invasive scale insect, Pulvinaria urbicola. We aimed to suppress the ant, thereby decoupling the mutualism and enabling recovery of the Pisonia trees. We treated all areas where ant pressure was high with a selective formicidal bait, which was deployed in custom‐made bait stations designed to avoid risk of treatment to endemic fauna. In the treated area, ant foraging activity was reduced by 93 percent and was followed by a 100 percent reduction in scale insect density. Abundance of endemic herbivorous insects and herbivorous activity increased significantly, however, after the decline in mutualistic species densities. Despite the native herbivore increase, there was considerable overall improvement in Pisonia shoot condition and an observed increase in foliage density. Our results demonstrate the benefit of strategic management of highly mutualistic alien species to the native Pisonia trees. It also supports the idea that area‐wide suppression is a feasible alternative to eradication for achieving positive conservation management at the level of the forest ecosystem.     

Impacts of an invasive ant species on roosting behavior of an island endemic flying‐fox

Introduced species can cause major disruptions to ecosystems, particularly on islands. On Christmas Island, the invasive yellow crazy ant (Anoplolepis gracilipes) has detrimental impacts on many animals ranging from the iconic red crabs (Gecarcoidea natalis) to the Christmas Island Thrush (Turdus poliocephalus erythropleurus). However, the full extent of its effects on the island's fauna is not yet known. In this study, we investigated the impact of the yellow crazy ants on the island's last native mammal: the Christmas Island flying‐fox (Pteropus natalis). This species has been described as a keystone species, but has recently experienced substantial population decline to the extent that it is now listed as Critically Endangered. We examined the impacts of the yellow crazy ants on the roosting behavior of the Christmas Island flying‐fox, and on its local and island‐wide distribution patterns. We showed that the crazy ants increased behaviors in the flying‐foxes that were associated with avoidance of noxious stimuli and decreased behaviors associated with resting. Roost tree selection and roost site location were not related to variation in the abundance of crazy ants on the island. Our results indicate that the crazy ants interfere with the activity budgets of the flying‐foxes. However, the flying‐foxes failed to relocate to ant‐free roost trees or roost sites when confronted with the noxious ant, suggesting that the flying‐foxes are either not sufficiently disturbed to override strong cultural attachment to roosts, or, are behaving maladaptively due to ecological naïveté.     

Experimental suppression of ants foraging on rainforest vegetation in New Guinea: testing methods for a whole‐forest manipulation of insect communities

1. Ants are extremely abundant in lowland tropical forests where they are important predators, plant mutualists, and herbivores. Their complex role in tropical plant–insect food webs can be best assessed by experimental manipulation of their abundance. Historically, ant exclusion experiments have had a small‐scale focus, such as single trees. Here, we test a new ‘whole‐forest' method of ant exclusion, using treated canopy bait stations, in a diverse tropical rainforest in New Guinea. 2. We conducted a 10‐month manipulative experiment in primary and secondary rainforests. In each forest type, a 625 m² treatment plot was isolated from the surrounding forest and 135 bait stations treated with fipronil, S‐methoprene, and hydramethylnon were placed in trees to suppress ants. Ant activity was monitored in the forest canopy and understorey with an additional 65 stations in treatment and control plots. 3. We achieved a dramatic decline in ant abundance in treatment plots compared with controls in both forest types, with an average decrease in ant numbers per station of 82.4% in primary and 91.2% in secondary forest. In particular, native dominant species Oecophylla smaragdina, Anonychomyrma cf. scrutator in primary forest, and invasive Anoplolepis gracilipes in secondary forest were greatly affected. In contrast, Tapinoma melanocephalum flourished in treatment plots, perhaps benefiting from reduced competition from other ant species. 4. Our study demonstrates that it is possible to selectively eradicate most of the foraging ants in a structurally complex tropical forest. We propose whole‐forest manipulation as a novel tool for studying the role of ants in shaping plant–insect food webs.     

Impact of the introduced yellow crazy ant Anoplolepis gracilipes on Bird Island, Seychelles

The introduced yellow crazy ant or long-legged ant Anoplolepis gracilipes was first reported in Seychelles in 1969 and now occurs on at least nine islands in the Central Seychelles. We describe the yellow crazy ant's effects on vegetation and invertebrate communities on one of these, Bird Island; in 2000, Anoplolepis (first reported in 1991) occurred there at densities at least 80 times higher than on other islands in the Central Seychelles. They were associated with high densities of coccid scale insects on foliage, especially of the native tree Pisonia grandis, in some instances causing tree death. Yellow crazy ants on Bird Island also significantly affected invertebrate communities on foliage and on the ground, both in terms of taxonomic composition and the density of specific taxa, apparently causing the local exclusion of some invertebrates.     

Negative Correlation between Ant and Spider Abundances in the Canopy of a Bornean Tropical Rain Forest

In tropical rain forests, high canopy trees have diverse and abundant populations of ants and spiders. However, accessing high trees and their fauna remains difficult; thus, how ants and spiders interact in the canopy remains unclear. To better understand the interspecific interactions between these two dominant arthropod groups, we investigated their spatial distributions at the canopy surface in a tropical rain forest in Borneo. We sampled ants and spiders six times between 2009 and 2011 by sweeping with an insect net at the tree crown surfaces of 190 emergent or tall (≥20 m in height) trees. We collected 438 ant individuals belonging to 94 species and 1850 spider individuals (1630 juveniles and 220 adults) belonging to 142 morphospecies (adults only) from a total of 976 samples. The fact that we collected four times more spider individuals than ant individuals suggests that fewer ants forage at the tree crown surface than previously thought. The number of spider individuals negatively correlated with the number of ant individuals and the number of ant species, indicating significant exclusivity between ant and spider spatial distributions at the tree crown surface. Niche‐overlap between the two taxa confirmed this observation. Although our data do not address the causes of these spatial distributions, antagonistic interspecific interactions such as interference behaviors and intra‐guild predation are ecological mechanisms that give rise to exclusive spatial distributions.     

Ecological effects of multi‐species,ant–hemipteran mutualisms in citrus

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The contribution of cacao agroforests to the conservation of lower canopy ant and beetle diversity in Indonesia

The ongoing destruction of tropical rainforests has increased the interest in the potential value of tropical agroforests for the conservation of biodiversity. Traditional, shaded agroforests may support high levels of biodiversity, for some groups even approaching that of undisturbed tropical forests. However, it is unclear to what extent forest fauna is represented in this diversity and how management affects forest fauna in agroforests. We studied lower canopy ant and beetle fauna in cacao agroforests and forests in Central Sulawesi, Indonesia, a region dominated by cacao agroforestry. We compared ant and beetle species richness and composition in forests and cacao agroforests and studied the impact of two aspects of management intensification (the decrease in shade tree diversity and in shade canopy cover) on ant and beetle diversity. The agroforests had three types of shade that represented a decrease in tree diversity (high, intermediate and low diversity). Species richness of ants and beetles in the canopies of the cacao trees was similar to that found in lower canopy forest trees. However, the composition of ant and beetle communities differed greatly between the agroforest and forest sites. Forest beetles suffered profoundly from the conversion to agroforests: only 12.5% of the beetle species recorded in the forest sites were also found in the agroforests and those species made up only 5% of all beetles collected from cacao. In contrast, forest ants were well represented in agroforests, with 75% of all species encountered in the forest sites also occurring on cacao. The reduction of shade tree diversity had no negative effect on ants and beetles on cacao trees. Beetle abundances and non-forest ant species richness even increased with decreasing shade tree diversity. Thinning of the shade canopy was related to a decrease in richness of forest ant species on cacao trees but not of beetles. The contrasting responses of ants and beetles to shade tree management emphasize that conservation plans that focus on one taxonomic group may not work for others. Overall ant and beetle diversity can remain high in shaded agroforests but the conservation of forest ants and beetles in particular depends primarily on the protection of natural forests, which for forest ants can be complemented by the conservation of adjacent shaded cacao agroforests.     

Impact of the invasive crazy ant Anoplolepis gracilipes on Bird Island, Seychelles

The crazy ant (Anoplolepis gracilipes) invaded Bird island, Seychelles, in the 1980s. In 1997, its range expanded and population densities increased. The impacts of this change were studied in 2001 using a combination of arthropod collecting methods. The ant population excluded larger invertebrates (principally the large ant Odontomachus simillimus and the crabs, principally Ocypode spp.). Cockroaches, however, remained abundant in ant-infested areas and tree-nesting birds (Lesser Noddy Anous tenuirostris) appear to be able to breed successfully in the presence of the crazy ant. The ants are only abundant in areas of deep shade which provide cool nesting areas, yet enabling them to forage in the open when ground temperatures fall. The expansion of the ants was correlated with the regeneration of woodland on the island. Recommendations are made for the management of the woodland which may reduce the impacts of the crazy ant.     

Escape to the high canopy—thermal deficiency causes niche expansion in a forest‐floor ant

The small formicoxenine ant Temnothorax saxonicus was known from about 40 localities in Central Europe nesting in anorganic substrates on floor of xerothermous forests whereas investigations of 198 tree canopies in 19 forest sites of the same region provided no indication for arboreal nesting or foraging. We present the first evidence for canopy‐nesting populations of T. saxonicus on old Quercus trees in 3 sites having maximum calibrated topsoil temperatures of 17.9 ± 0.3 °C which were significantly (P < 0.007) lower than 22.8 ± 2.0 °C measured in 5 sites with ground‐nesting populations. The thermal deficit on forest floor inhibits brood development in ground nests and caused a moving to canopy were maximum calibrated temperatures of the, now wooden, substrates are at least 26.1 °C for the whole canopy and 30.8 °C in more sun‐exposed spots. T. saxonicus competed here successfully with the obligatory canopy ants T. affinis and T. corticalis. The distributional data of this rope‐climbing study support former results that highest nest densities of small arboreal ants occur in temperate climate over the entire canopy mantle of single trees situated in open land or in park‐like environments but occur in the top of the canopy in tree stands with high degree of canopy closure.     

Response of the foraging behaviour of red wood ants (Formica rufa group) to exclusion from trees

Abstract 1 Because of the large numbers within a colony and their aggressive nature, red wood ants (Formica rufa group) have a potential to greatly influence the cold‐temperate forest ecosystem. Wood ants are omnivorous and hunt in trees as well as on the forest floor. 2 A field experiment in a mixed forest in central Sweden was carried out to examine (i) the foraging behaviour of wood ants on the forest floor and (ii) the impact of increased numbers of wood ants on the soil fauna. The foraging behaviour of wood ants was manipulated by excluding the ants from their food resources in the tree canopy, with the intention to increase ant activity on the forest floor. To estimate this activity, the number of trees with foraging ants, the numbers of ants going to and from their nests and the prey carried by home‐running wood ants were determined during the summer period. Pitfall traps were placed in the soil to determine effects on mobile soil invertebrates. 3 When excluded from local trees, wood ants searched other trees further away from the nests rather than searching more intensively for prey on the forest floor. By contrast to the initial hypothesis, more soil‐living prey were caught by ants in the control plots than in the plots where the local trees were not accessible to the wood ants. The proportion of soil‐living to tree‐living prey tended to be greater in the control plots. 4 In the treated plots (no access to the trees), wood ants had a negative effect on the activity of Linyphiidae spiders. There was little effect of wood ants on other soil invertebrates. 5 This study suggests that the role of wood ants as top predators in the forest soil food‐web in central Sweden is limited.     

Effects of planting method on the recovery of arboreal ant activity on revegetated farmland

Ecological restoration aims to re‐establish both biodiversity and ecological function in damaged ecosystems. Ants are important drivers of ecological functions and are early colonizers of restored ecosystems. Rates at which ants perform functions are thought to be fuelled by access to plant sugars. In revegetated farmland in south‐eastern Australia, I tested if ant activity on trees, which reflects use of arboreal sugars, follows a predictable trajectory of recovery towards a remnant‐like state. Additionally, I examined whether planting method alters this trajectory by comparing tube stock (TS), which results in low Eucalyptus densities, with direct seeding (DS), which results in high Eucalyptus densities. Replicate sites (n = 5) of young (planted between 1998 and 2001) and old (planted between 1989 and 1994) TS and DS revegetation, pastures and remnants were compared. Activity on trunks was significantly positively correlated with ant tending of Hemiptera in young and old revegetation. In DS plantings, activity and estimated liquid loads on Eucalyptus trees were low and rapidly approached that in remnants, while TS sites remained similar to high values observed in pastures with trees. Patterns for Acacia were less clear, reflecting consistent densities for this species between TS and DS. At the whole‐of‐field scale, planting methods did not differ. Importantly, although trajectories differed, neither TS nor DS sites approached the low activity or estimated liquid loads observed in remnants. Rates of ant use of arboreal sugars and associated sugar‐fuelled processes may thus take considerably longer to recover than the period covered by this study. This finding suggests planting method may affect the trajectory and outcome of revegetation for plant health, as well as sugar‐fuelled ecosystem functions performed by ants.     

Observations on the Ecology of Weaver Ants (Oecophylla smaragdina Fabricius) in a Thai Mangrove Ecosystem and Their Effect on Herbivory of Rhizophora mucronata Lam.

Ants of the genus Oecophylla are predators of other insects and are able to protect a variety of terrestrial plants against pest insects; however, observations on the ecology of these ants in mangrove forests are lacking. General observations on the ecology of Oecophylla smaragdina were carried out in a Thai mangrove forest to determine if these ants can protect their host plants in less favorable mangrove habitats. Leaf herbivory and the density of O. smaragdina ants were measured on Rhizophora mucronata trees at two sites. The results showed a negative correlation between ant density and herbivory. At both sites, the mean percent damaged leaf area was more than four times higher on trees without ants compared to “ant‐trees.” A significant negative correlation was found between tree mean percent leaf damage and the density of ants on the tree. Furthermore, on trees with ants, there was less herbivory on leaves close to ant nests compared to other leaves on the tree. Most damage was caused by chrysomelid beetles (62%) and sesarmid crabs (25%) and both types of herbivory were significantly reduced on ant‐trees.     

Sources of variation in the incidence of ant–aphid mutualism in boreal forests

• 1 The mutualism between wood ants of the Formica rufa group and aphids living in the canopy of trees is a widespread phenomenon in boreal forests, and it can affect tree growth. However, not all trees in the forest are involved in this interaction.
• 2 To assess the incidence of host trees involved in this ant–aphid mutualism and its spatial distribution in boreal forests, we inventoried sample plots with a radius of 10–15 m around wood ant mounds in 12 forest stands of two age classes (5–12‐year‐old sapling stands and 30–45‐year‐old pole stands) and two dominant tree species (Scots pine and silver birch) in Eastern Finland from 2007 to 2009.
• 3 The proportion of trees visited by ants out of all trees on the individual study plots were in the range 4–62%, and 1.5–39% of the trees on the plots were consistently visited by ants during all 3 years. The percentage of host trees increased with the ant mound base area on the plots. Trees visited by ants were larger and closer to the mound than trees not visited by ants. Within the group of visited trees, more ants were found on bigger trees and on trees close to the ant mounds.
• 4 Extrapolated from plot to stand level, we estimated that 0.5–6.6% of the trees were host trees in at least one of the three study years, and that only 0.01–2.3% of all the trees were consistently visited by ants during all 3 years. It is concluded that ant–aphid mutualism is a minor occurrence at the stand level.

     

Canopy Openness Enhances Diversity of Ant–Plant Interactions in the Brazilian Amazon Rain Forest

In closed‐canopy tropical forest understory, light availability is a significant determinant of habitat diversity because canopy structure is highly variable in most tropical forests. Consequently, variation in canopy cover affects the composition and distribution of plant species via creating variable light environments. Nevertheless, little is known about how variation in canopy openness structures patterns of plant–animal interactions. Because of the great diversity and dominance of ants in tropical environments, we used ant–plant interactions as a focal network to evaluate how variation in canopy cover influences patterns of plant–insect interactions in the Brazilian Amazon rain forest. We observed that small increases in canopy openness are associated with increased diversity of ant–plant interactions in our study area, and this change is independent of plant or ant species richness. Additionally, we found smaller niche overlap for both ants and plants associated with greater canopy openness. We hypothesize that enhanced light availability increases the breadth of ant foraging sources because variation in light availability gives rise to plant resources of different quality and amounts. Moreover, greater light availability promotes vegetative growth in plants, creating ant foraging ‘bridges’ between plants. In sum, our results highlight the importance of environmental heterogeneity as a determinant of ant–plant interaction diversity in tropical environments.     

Habitat complexity facilitates coexistence in a tropical ant community

The role of habitat complexity in the coexistence of ant species is poorly understood. Here, we examine the influence of habitat complexity on coexistence patterns in ant communities of the remote Pacific atoll of Tokelau. The invasive yellow crazy ant, Anoplolepis gracilipes (Smith), exists in high densities on Tokelau, but still coexists with up to seven other epigeic ant species. The size-grain hypothesis (SGH) proposes that as the size of terrestrial walking organisms decreases, the perceived complexity of the environment increases and predicts that: (1) leg length increases allometrically with body size in ants, and (2) coexistence between ant species is facilitated by differential habitat use according to body size. Analysis of morphological variables revealed variation inconsistent with the morphological prediction of the SGH, as leg length increased allometrically with head length only. We also experimentally tested the ability of epigeic ants in the field to discover and dominate food resources in treatments of differing rugosity. A. gracilipes was consistently the first to discover food baits in low rugosity treatments, while smaller ant species were consistently the first to discover food baits in high rugosity treatments. In addition, A. gracilipes dominated food baits in planar treatments, while smaller ant species dominated baits in rugose treatments. We found that the normally predictable outcomes of exploitative competition between A. gracilipes and other ant species were reversed in the high rugosity treatments. Our results support the hypothesis that differential habitat use according to body size provides a mechanism for coexistence with the yellow crazy ant in Tokelau. The SGH may provide a mechanism for coexistence in other ant communities but also in communities of other terrestrial, walking insects that inhabit a complex landscape.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Extrafloral nectar as a driver of arboreal ant communities at the site‐scale in Brazilian savanna

Despite years of study, it remains unclear if and to what extent the effects of extra‐floral nectaries (EFNs) on arboreal ants observed on individual trees scale up to larger spatial scales. Here, we address this issue in Brazilian savanna and tested three predictions: (i) Trees with EFN have higher richness of arboreal ant species than trees without; (ii) Arboreal ant species richness increases with the proportion of total EFN‐bearing trees at the site scale, due to a higher occurrence of non‐core ant species; (iii) Ant species composition changes with the proportion of EFN‐bearing trees at the site scale. We sampled arboreal ants in 32 plots with EFN‐bearing trees ranging from 0% to 60% of all trees. We sampled 72 ant species, from which 17 (mostly belonging to Camponotus, Cephalotes and Crematogaster) were identified as core species in at least one of the ant‐EFN networks in the 32 plots. Ant species richness was significantly higher on EFN‐bearing trees. We identified 11 ant species that preferentially occurred on EFN‐bearing trees, all of which were core partners in networks. Species richness at the site scales increased with the proportion of EFN‐bearing trees, regardless of tree density and richness; this pattern was due to a higher occurrence of non‐core ant species. Finally, species composition also varied with the proportion of EFN‐bearing trees. Therefore, we found that the presence of EFNs not only influences arboreal ants on individual trees but also has a substantial effect on the ant‐EFN network on a broader community scale. The increase in non‐core species site scale reveals that this interaction is unlikely to result in substantially enhanced protection services for EFN‐bearing plants.     

Effects of selective logging on the arboreal ants of a Bornean rainforest

We examined the effect of selective logging and corresponding forest canopy loss on arboreal ant diversity in a tropical rainforest. Arboreal ants were collected from an unlogged forest plot and from forest plots selectively logged 14 years and 24 years earlier in Danum Valley, Sabah, Malaysia, using a canopy fogging method. Selective logging was associated with a significant decrease in canopy cover and an increase in understory vegetation density relative to unlogged forest. Our study showed that selective logging in primary forest might not dramatically decrease total species number and overall abundance of arboreal ants; however, it may influence the species composition and dominance structure of the ant community, accompanied by an increase in abundance of shrub‐layer species and trophobiotic species. In view of the results of this study, management techniques that minimize logging impact on understory vegetation structure are likely to help maintain the conservation value of logged forests for arboreal ants. Our results also suggest that accurate assessment of the impacts on biodiversity should not be based only on measurement of species number and overall abundance, but also on analysis of species composition and community structure.     

Ants on oaks: effects of forest structure on species composition

The ant fauna of oak forest canopies in Northern Bavaria was studied by canopy fogging on 45 trees in August 2000 and May 2001. The study focused on a comparison of several different forestry management practices resulting in several types of canopy cover. Forests surveyed were: (1) high forest (high canopy cover), (2) coppice with standards (low canopy cover), (3) forest pasture with mostly solitary trees (very low canopy cover) and (4) transitional forest from former coppice with standards to high forest (approaching high canopy cover). This comprised a full gradient of canopy coverage. On the 45 oak trees sampled, a total of 17 ant species were found. Species composition was dependent on the different forestry management practices. The total number of species and the number of species listed in the Red Data Books of both Germany and Bavaria were much higher in the forest pasture and the coppice with standards, as compared to the high forest. The transitional forest was at an intermediate level. The highest number of ant species was found in the forest pasture. This can be explained by the occurrence of species of open habitats and thermophilous species. In the coppice with standards, forest dwelling and arboricolous species dominated, whereas the high forest showed much lower frequencies of arboricolous species like Temnothorax corticalis, Dolichoderus quadripunctatus and Temnothorax affinis. A multivariate analysis revealed that canopy cover (measured as “shade”, in percentage intervals of canopy cover) was the best parameter for explaining species distribution and dataset variation, and to a lesser extent the amount of dead wood, canopy and trunk diameter. Thus ant fauna composition was mostly driven by structural differences associated to the different forestry management practices. Many ant species clearly preferred the more open and light forest stands of the coppice with standards as compared to the dense and shady high forest.     

Collapse of an ant–scale mutualism in a rainforest on Christmas Island

Positive interactions play a widespread role in facilitating biological invasions. Here we use a landscape–scale ant exclusion experiment to show that widespread invasion of tropical rainforest by honeydew-producing scale insects on Christmas Island (Indian Ocean) has been facilitated by positive interactions with the invasive ant Anoplolepis gracilipes . Toxic bait was used to exclude A. gracilipes from large (9–35 ha) forest patches. Within 11 weeks, ant activity on the ground and on trunks had been reduced by 98–100%, while activity on control plots remained unchanged. The exclusion of ants caused a 100% decline in the density of scale insects in the canopies of three rainforest trees in 12 months ( Inocarpus fagifer , Syzygium nervosum and Barringtonia racemosa ), but on B. racemosa densities of scale insects also declined in control plots, resulting in no effect of ant exclusion on this species. This study demonstrates the role of positive interactions in facilitating biological invasions, and supports recent models calling for greater recognition of the role of positive interactions in structuring ecological communities.