Roads increase population growth rates of a native leaf-cutter ant in Neotropical savannahs

Are There Edge Effects on the Genetic Diversity of the Trap-Jaw Ant Odontomachus chelifer (Formicidae: Ponerinae) in a Neotropical Savanna Fragment? A First Assessment

Habitat fragmentation is considered an important threat to biodiversity, increasing species exposure to edge effects. The Brazilian Cerrado savanna is considered a biodiversity hotspot and has been converted to small, isolated fragments due to human activities. Ant communities and colony survivorship are known to be affected by edge effects in Cerrado, but to date there is no information on the genetic diversity of ant colonies at the edge of fragmented areas. Here, we investigate if colony genetic diversity and structure of Odontomachus chelifer (Latreille) ants (Hymenoptera: Formicidae) are subject to edge effects in a Cerrado reserve in southeast Brazil. Using microsatellites, we evaluated the number of breeders (queens and males) and the genetic diversity in O. chelifer colonies located in the interior versus edge of a Cerrado fragment. All O. chelifer nests had multiple queens, which presented a low mating frequency. The number of breeders and most estimates of genetic diversity did not differ between colonies at the edge versus interior of the fragment. Genetic structure was not influenced by nest location as well. However, we detected a small and positive increase in the observed heterozygosity in colonies located at fragment edges. High heterozygosity is thought to be particularly important in fast-changing environments, such as edges, providing an advantage for genetic diversity. Further investigation is needed to assess in greater detail how habitat loss affects O. chelifer biology. Our study is a first step toward elucidating edge effects on genetic diversity of ant colonies, a topic still poorly explored in tropical environments.

Unexpected costs of extended phenotypes: nest features determine the effect of fires on leaf cutter ant's demography

A key principle of the extended phenotype concept is that the benefit of the structures that an animal builds exceeds its cost. However, some contexts may enhance the costs of structures that often represent a benefit, reversing their adaptive nature. In leaf-cutting ant nests, thatched mounds are extended phenotypes that offer a stable microclimate for the growth of the fungus culture. We hypothesized that fires will affect the species that build external, easily flammable thatch mounds (Acromyrmex lobicornis) more than colonies that build subterranean nests in the less-flammable bare ground (Amoimyrmex striatus). We use a stochastic matrix demographic model parameterized with 4 years of data in pre- and post-fire scenarios. Before fires, Ac. lobicornis showed higher stochastic population rate (λs) than Am. striatus. However, fire frequency every 2 years completely reversed this trend, showing population decline only in Ac. lobicornis. Small nests were the stage that most contributed to λs and the most sensitive in all the species and fire scenarios. This illustrates a novel effect of disturbances; the reversion of the adaptive nature of extended phenotypes, which may have strong consequences on population dynamics and assemblage structure through the invert of dominance relationships.

The benefits of being big and diverse: early colony survival in harvester ants

In sessile organisms such as plants and benthic invertebrates, founding propagules typically suffer extremely high rates of mortality due to both extrinsic and intrinsic factors. Many social insect species share similarities with these groups, but factors influencing early colony survival are relatively unstudied. We used a field experiment to measure the importance of environmental quality relative to intrinsic colony properties in the harvester ant, Pogonomyrmex occidentalis, by monitoring the survival of 584 experimental colonies. We measured survival of transplanted colonies over four months in each of three years (2014-2016) at a site in western Colorado. Colony survival was primarily determined by colony features. Multiple mating by the queen and larger colony size at the time of transplant increased survival, but queen size, maternal lineage and the composition of plant species in the vicinity of the colony did not. Food supplementation increased survival significantly when natural food was scarce, but was not consistently beneficial, in contrast to predictions. Our results emphasize the general importance of rapid growth and early attainment of large size in the survival of sessile species. However, attributes specific to ants that are a consequence of their sociality also strongly affected survival. Colonies with multiply-mated queens were more likely to survive over a wide range of circumstances, highlighting the importance of this trait even at the early stages of colony life.

Protected areas and unpaved roads mediate habitat use of the giant anteater in anthropogenic landscapes

The drastic reduction of the Brazilian Cerrado has transformed this savanna hotspot into vast swaths of commodity-based agriculture fields, mainly soybean, sugarcane, and beef-production pasturelands. The resulting habitat loss and fragmentation are the principal factors underlying population decline of native species inhabiting the Cerrado, particularly those with a high demand for space, low population density, and specialized diet, such as the endangered giant anteater (Myrmecophaga tridactyla). Although the species has been studied in protected areas, we know much less about its ability to endure in disturbed landscapes. Here, we analyzed camera-trapping data to estimate a proxy of habitat use (ψ^; occupancy) and detection probabilities of the giant anteater, identifying environmental covariates influencing these parameters in landscapes with intensive agriculture and commercial forestry. We found this species using about half of the study area (model average ψ^ = 0.51, CI = 0.40–0.62), with two predictors strongly influencing habitat use: protected areas and unpaved roads. In turn, detection probability correlates positively with area of open Cerrado and negatively with area of settlements. The species is more likely to use unpaved roads inside protected areas (ψ^ = 0.90, CI = 0.47–0.75), compared to off road sites in the surrounding areas (ψ^ = 0.19, CI = 0.10–0.34). Our findings indicate that giant anteaters are dependent on nature reserves and native vegetation areas existing on private properties, whose protection is regulated by the Brazilian Native Vegetation Protection Law. Given the relative paucity of state-owned protected areas in the Brazilian Cerrado, increasing the adherence of rural owners to this law is, therefore, key for the conservation of the giant anteater. The intense use of unpaved roads might reflect travelling and/or foraging optimization, a behavioral response that, nevertheless, may compound this species’ susceptibility to suffer mortality from roadkill.

Responses of ant communities to disturbance: Five principles for understanding the disturbance dynamics of a globally dominant faunal group

Ecological disturbance is fundamental to the dynamics of biological communities, yet a conceptual framework for understanding the responses of faunal communities to disturbance remains elusive. Here, I propose five principles for understanding the disturbance dynamics of ants-a globally dominant faunal group that is widely used as bioindicators in land management, which appear to have wide applicability to other taxa. These principles are as follows: (1) The most important effects of habitat disturbance on ants are typically indirect, through its effects on habitat structure, microclimate, resource availability and competitive interactions; (2) habitat openness is a key driver of variation in ant communities; (3) ant species responses to disturbance are to a large degree determined by their responses to habitat openness; (4) the same disturbance will have different effects on ants in different habitats, because of different impacts on habitat openness; and (5) ant community responses to the same disturbance will vary according to ant functional composition and biogeographical history in relation to habitat openness. I illustrate these principles using results primarily from studies of ant responses to fire, a dominant agent of disturbance globally, to provide a common disturbance currency for comparative analysis. I argue that many of the principles also apply to other faunal groups and so can be considered as general ecological "laws." As is the case for ants, many impacts of habitat disturbance on other faunal groups are fundamentally related to habitat openness, the effects of disturbance on it and the functional composition of species in relation to it.

Do an ecosystem engineer and environmental gradient act independently or in concert to shape juvenile plant communities? Tests with the leaf-cutter ant Atta laevigata in a Neotropical savanna

Background

Ecosystem engineers are species that transform habitats in ways that influence other species.While the impacts of many engineers have been well described, our understanding of how their impact varies along environmental gradients remains limited. Although disentangling the effects of gradients and engineers on biodiversity is complicated-the gradients themselves can be altered by engineers-doing so is necessary to advance conceptual and mathematical models of ecosystem engineering. We used leaf-cutter ants (Atta spp.) to investigate the relative influence of gradients and environmental engineers on the abundance and species richness of woody plants.

Methods

We conducted our research in South America's Cerrado. With a survey of plant recruits along a canopy cover gradient, and data on environmental conditions that influence plant recruitment, we fit statistical models that addressed the following questions: (1) Does A. laevigata modify the gradient in canopy cover found in our Cerrado site? (2) Do environmental conditions that influence woody plant establishment in the Cerrado vary with canopy cover or proximity to A. laevigata nests? (3) Do A. laevigata and canopy cover act independently or in concert to influence recruit abundance and species richness?

Results

We found that environmental conditions previously shown to influence plant establishment in the Cerrado varied in concert with canopy cover, but that ants are not modifying the cover gradient or cover over nests. However, ants are modifying other local environmental conditions, and the magnitude and spatial extent of these changes are consistent across the gradient. In contrast to prior studies, we found that ant-related factors (e.g., proximity to nests, ant changes in surface conditions), rather than canopy cover, had the strongest effect on the abundance of plant recruits. However, the diversity of plants was influenced by both the engineer and the canopy cover gradient.

Discussion

Atta laevigata in the Cerrado modify local conditions in ways that have strong but spatially restricted consequences for plant communities. We hypothesize that ants indirectly reduce seedling establishment by clearing litter and reducing soil moisture, which leads to seed and seedling desiccation. Altering soil nutrients could also reduce juvenile growth and survivorship; if so these indirect negative effects of engineering could exacerbate their direct effects of harvesting plants. The effects of Atta appear restricted to nest mounds, but they could be long-lasting because mounds persist long after a colony has died or migrated. Our results support the hypothesis that leaf-cutter ants play a dominant role in Cerrado plant demography. We suggest the ecological and economic footprint of these engineers may increase dramatically in coming decades due to the transformation of the Cerrado by human activities.

Leaf-cutting ant populations profit from human disturbances in tropical dry forest in Brazil

Anthropogenic disturbance often results in the proliferation of native species of particular groups that leads to biotic homogenization. Leaf-cutting ants are an example of such winner organisms in tropical rain forests, but their response to disturbance in dry forests is poorly known. We investigated Atta colony density in areas of tropical dry forest in Brazil with different distance to roads and vegetation cover. Atta colonies were surveyed in 59 belt transects of 300 × 20 m, covering a total area of 35.4 ha. We found 224 Atta colonies, 131 of which were active and belonged to Atta opaciceps (87 colonies, 2.45 ha−1), A. sexdens (35 colonies, 0.98 ha−1) and A. laevigata (9 colonies, 0.25 ha−1). The density of active colonies sharply decreased from 15 ± 2.92 ha−1 in the 50-m zone along roads to only 2.55 ± 1.65 ha−1 at distances up to 300 m. The reverse pattern was observed for inactive colonies. Active Atta colonies preferentially occur in areas with low vegetation cover, while inactive colonies prefer areas with high vegetation cover. We demonstrate for the first time that anthropogenic disturbances promote the proliferation of leaf-cutting ants in dry forest in Brazil, which may affect plant regeneration via herbivory and ecosystem engineering as demonstrated for rain forests.