How leaf-cutting ants impact forests: drastic nest effects on light environment and plant assemblages

Abstract

Leaf-cutting ants (Atta spp.) have become a topical issue in Neotropical ecology, particularly because they are reaching hyper-abundance due to escalating levels of fragmentation in recent years. Yet, despite intensive research on their role as dominant herbivores, there is still insufficient documentation on the impacts of their large, long-lived nests on plant assemblage structure and ecosystem functioning. Our study aimed at investigating the magnitude, nature, and spatial extent of nest influence by assessing 11 attributes of ant nest, canopy structure, light environment and sapling assemblage for 20 colonies in four plots along nest-understorey gradients in a large remnant of Atlantic forest. We also monitored the performance of seeds and seedlings of Chrysophyllum viride, an abundant shade-tolerant species. Previously unrecognized canopy gaps above ant nests (0.04-87.9 m(2)) occurred in 95% of all colonies surveyed. Overall, canopy openness and light availability at least doubled in ant nest plots compared with distant understorey plots. These drastic changes in the light environment paralleled those in plant assemblage: sapling density almost tripled (mean +/- SE: 0.42 +/- 0.1 saplings m(-2)) and sapling species richness doubled (0.16 +/- 0.02 species m(-2)) in distant plots, as did shade-tolerant species. After a 1-year period, only 33 +/- 15.6% of the seeds germinated and all seedlings died on nests, whereas seed germination reached 68 +/- 5.1% in distant plots and 66.4 +/- 7.6% of their seedlings survived after 12 months. Therefore, plot location was the most significant explanatory variable for predictable and conspicuous changes in the light environment and structure of sapling assemblages. Our findings greatly extend knowledge on the role played by leaf-cutting ants as ecosystem engineers by demonstrating that ant nest-mediated disturbance promotes environmental modifications in tens of meters around nests and is thus, strong enough to drive plant recruitment and consequently alter both the floristic and functional signature of plant assemblages.

Ecological engineering by a native leaf-cutting ant increases the performance of exotic plant species

“…While other studies have suggested that leaf-cutting ants may act as ecological engineers affecting vegetation patterns (Farji-Brener and Illes 2000;Wirth et al 2003;Correa et al 2010), few works have experimentally tested this hypothesis, and none have specifically linked that idea with the likelihood of biological invasions. Using a representative number of plant species of the study area, we experimentally demonstrated that exotics gain *100% more leaf and root biomass than natives when both kinds of species grow in RDs, suggesting that leaf-cutting ant nests may promote the invasion of exotic plants.…”

“…Although leaf-cutting ants have been proposed as ecological engineers (Wirth et al 2003;Correa et al 2010), studies of their effects under this conceptual framework, including their potential impact on exotic plant species, are still lacking (e.g., absent in the review of Hastings et al 2007). Earlier evidence suggests that engineering activities by leaf-cutting ants may promote biological invasions.…”

“…Leaf-cutting ants are probably one of the most notable examples of ecological engineers because of their high capacity to alter the environment where they live (Correa et al 2010). Ants move an extraordinary quantity of soil to construct and maintain their nests, cut a large amount of vegetation, and deposit large piles of organic waste around the nest area (Farji-Brener and Illes 2000).…”

Field colonies of leaf‐cutting ants select plant materials containing low abundances of endophytic fungi

“…Leaf-cutting ants (genera Acromyrmex and Atta) are major defoliators in the Neotropics, removing at least 2% of annual leaf production and impacting the distribution of resources in habitats where they occur (Farji-Brener 2001, Herz et al 2007a, Corrêa et al 2010. The ants collect tissue from a diverse group of plants and therefore interact with a vast number of endophytes.…”

Leafcutter Ant Nests Inhibit Low-Intensity Fire Spread in the Understory of Transitional Forests at the Amazon's Forest-Savanna Boundary

“…are considered conspicuous herbivores in the neotropics [1,2]. Their role in the ecosystem, however, goes well beyond their herbivory because their construction and maintenance of nests causes diverse impacts to soil [3,4] with consequences for recruitment dynamics, [5][6][7][8][9] nutrient access [10,11], and growth of nearby vegetation [12].…”