Edge effects decrease ant-derived benefits to seedlings in a neotropical savanna

Abstract

Edge effects may lead to changes in mutualistic plant-animal interactions, such as seed dispersal, that are critical to plant regeneration. However, research into edge effects is neglected in the Brazilian cerrado, the largest neotropical savanna. We evaluated the consequences of edge effects in the cerrado for the regeneration of Erythroxylum pelleterianum (Erythroxylaceae), a shrub that benefits from seed dispersal by ants. We compared air temperature, relative humidity, and vapor pressure deficit, as well as the frequency and outcome of ant-diaspore interactions between cerrado edges and interiors. The inner portion of cerrado was likely to be moister than its borders, but seed production and germination did not differ between edge and interior of cerrado. Ants removed more seeds near fragment edges than at the interior. However, Myrmicinae ants dominated ant-fruit interactions at edges. These ants are likely to provide few benefits to the seeds. Seedlings of E. pelleterianum growing close to Ponerinae ant nests showed higher survival than seedlings growing away from nests in the interior of cerrado, but such effect disappeared near edges. Widespread seedling mortality due to a higher evaporative demand at edges may partially account for this effect. Furthermore, Ponerinae's nests also showed a lower residence time near edges, decreasing possible benefits derived from ant colony activity such as nutrient enrichment and protection against insect herbivores. Edge effects could change the structure and dynamics of vegetation in cerrado fragments, due in part to the collapse of the mutualistic interactions demonstrated here.

Assessing the Impact of Deforestation of the Atlantic Rainforest on Ant-Fruit Interactions: A Field Experiment Using Synthetic Fruits

“…Indeed, our experiment with synthetic fleshy fruits showed that the decreased occurrence of a particularly beneficial ant group (i.e., large ponerines) in the disturbed forest fragments corresponded with a decline in the ants' potential dispersal benefit to plants (i.e., less frequent dispersal and shorter displacement distance of ‘fruits’). The present study indicates that the deforestation process of the Atlantic rainforest (i.e., fragmentation and other human-induced disturbances) negatively affects the potentially mutualistic interaction between ground-dwelling ants and small to medium-sized fleshy fruits, corroborating other studies on ant-fruit interactions in human-disturbed habitats [20], [31], [33], [34]. The potential decrease in the benefits resulting from opportunistic ant-fruit interactions adds up to the already pessimistic scenario faced by vertebrate-dispersed plants [4], [11], [69], since frugivores are among the first to disappear from human-disturbed forest fragments [9], [10].…”

“…[20], [61], [65]). Pheidole species are commonly reported at fallen fleshy diaspores in tropical ecosystems (e.g.…”

Not all in the same boat: trends and mechanisms in herbivory responses to forest fragmentation differ among insect guilds

“…Cagnolo et al 2006Cagnolo et al , 2009Krauss et al 2010;Marini et al 2012) and reduced plant population size (Piotti 2009;Leimu et al 2010;Vranckx et al 2012) as fragment area decreases. Also, altered abiotic conditions at the edges, including higher temperature, greater light incidence and lower relative humidity (Chen et al 1993;DaviesColley et al 2000;Christianini and Oliveira 2013), could affect plant seed germination (Ashworth and Martí 2011) and tree mortality (Murcia 1995;Laurance et al 2000). Plant species with reduced populations may sustain lower herbivory levels according to the resource concentration hypothesis (Root 1973;Connor and McCoy 2001).…”

Crop size influences pre-dispersal seed predation in the Brazilian Cerrado

“…plantations, as well as firebreaks and pasturelands, and is not subjected to fire since ca. 30 years (see Christianini & Oliveira 2013).…”