Abstract
BackgroundEcosystem 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.MethodsWe 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?ResultsWe 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.
Highlights
Species that transform habitats or create new ones are known as EcosystemEngineers (Jones et al 1994; Jones et al 1997), and they can have major impacts on population dynamics, community composition, and ecosystem function
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
Our study 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 the establishment of woody plants in the Cerrado vary with canopy cover or proximity to A. laevigata nests?
Summary
Species that transform habitats or create new ones are known as EcosystemEngineers (Jones et al 1994; Jones et al 1997), and they can have major impacts on population dynamics, community composition, and ecosystem function (reviewed in Kleinhesselink et al 2014; Wright & Jones 2006). Experimental studies disentangling the effects of engineers and gradients are rare, in part because they are challenging to design and implement at the landscape scale This makes surveys of biodiversity in landscapes where gradients and engineers overlap, coupled with measurements of ecologically relevant environmental parameters, an important tool for advancing conceptual and mathematical models of ecosystem engineering (Hastings et al 2007; Wright & Jones 2006). Like many other savanna biomes the Cerrado is a mosaic of plant physiognomies ranging from open grassland to forests (Oliveira-Filho & Ratter 2002) These vegetation types are often found in close proximity (Cardoso et al 2009), resulting in broad and continuous gradients in canopy cover that can have important implications for local plant biodiversity.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
