Biotic drivers of seedling establishment in Neotropical savannas: selective granivory and seedling herbivory by leaf‐cutter ants as an ecological filter

Abstract

Summary 1.Herbivory has been shown to have prominent top-down effects on vegetation in Paleotropical savannas, where consumers of early stages of life history act as demographic bottlenecks. Such impact has been largely ignored in Neotropical savannas, however, despite insect consumption being linked to reduced recruitment of woody species. 2. We hypothesize that Atta leaf-cutter ants -the prevalent herbivores in the Neotropics -alter the establishment of woody plant seedlings in the Brazilian Cerrado by reducing seed availability and seedling survival. We also hypothesize that the demographic effects will be species-specific. 3. To test these hypotheses, we experimentally (i) compared seed removal in areas with and without Atta foraging and (ii) compared the survivorship of seedlings exposed to or protected from Atta foraging. Both experiments were performed with the same common Cerrado species which allowed us to evaluate the potential net effects of consumers at the population and community levels. 4. Overall seed removal rates in sites with Atta foraging were greater than those where ants were absent (59.2% and 39.2% of seeds removed, respectively). There were differences in removal for 10 of the 12 tested species, with per species removal 1.1-to 3.8-fold greater in areas with Atta foraging. On transplanted plants, 45% of seedlings exposed to Atta were attacked, resulting in a survival 7.6% lower than that of protected seedlings. Seedling survival was 11.8-31.5% lower in five species, with the largest differences in survival between treatments during the dry season. 5. Taken together, these results support the hypothesis that consumers can lead to demographic bottlenecks in Neotropical savanna plant species, with effects varying among life-history stages and between species. 6. Synthesis. Granivory by ants has been linked to reduced seedling recruitment of woody species in the Brazilian Cerrado. Our results show that leaf-cutter ants may largely limit early seedling establishment of woody species by reducing seed availability and seedling survival with differential species-specific effects. Atta ants may therefore be acting as an ecological filter, which coupled with potential selectivity in foraging on reproductive life-history stages, may ultimately influence the relative abundance of different species and hence the structure and composition of Cerrado vegetation.

Human disturbance promotes herbivory by leaf‐cutting ants in the Caatinga dry forest

“…LCA may affect plant assemblages directly through foliage removal and seed dispersal, and indirectly via decreasing topsoil fertility and increasing light levels in nest and foraging areas (see reviews in Leal et al 2014, Farji-Brener & Werenkraut 2015. The net outcomes of these effects negatively affect plant regeneration, reduce seed germination and seedling/sapling recruitment of shade-tolerant and disturbance-sensitive species and alter the dynamic and structure of tree assemblages (Wirth et al 2003, Corrêa et al 2010, Costa et al 2016.…”

Fire‐sensitive species dominate seed rain after fire suppression: Implications for plant community diversity and woody encroachment in the Cerrado

“…Seed addition decreased recruitment limitation for some species (Supporting Information Table S4; see also Ferreira, Bruna, & Vasconcelos, 2011), indicating that a higher seed production may benefit plant recruitment. Although harsh abiotic conditions and post-germination processes such as seedling herbivory may influence the likelihood of further seedling establishment (Costa, Vasconcelos, & Bruna, 2017;Hoffmann, Orthen, & Franco, 2004;Rocha-Ortega, Bartimachi, Neves, Bruna, & Vasconcelos, 2017), the massive seed rain of fire-sensitive species may increase the chances that some seeds reach safe sites for recruitment.…”

Integrating the host plant phylogeny, plant traits, intraspecific competition and repeated measures using a phylogenetic mixed model of field behaviour by polyphagous herbivores, the leaf-cutting ants

“…LCA also account for a prominent negative effect on forest regrowth by harvesting 12%-17% of annual forest reproduction (Cherrett 1986). Similarly, in savanna ecosystems they are known to act as an ecological filter affecting the structure and composition of vegetation, creating demographic bottlenecks by reducing seed availability and seedling survival (Costa et al . 2017).…”