Journal of Tropical Ecology www.cambridge.org/tro Research Article Cite this article: da Silva CHF, Arnan X, Andersen AN, and Leal IR (2019) Extrafloral nectar as a driver of ant community spatial structure along disturbance and rainfall gradients in Brazilian dry forest. Journal of Tropical Ecology 35, 280–287. https://doi.org/ 10.1017/S0266467419000245 Received: 31 January 2019 Revised: 17 July 2019 Accepted: 28 August 2019 Keywords: Aridity; climate change; dry forest; human disturbance; optimal foraging Author for correspondence: *Inara R. Leal, Email: irleal@ufpe.br © Cambridge University Press 2019. Extrafloral nectar as a driver of ant community spatial structure along disturbance and rainfall gradients in Brazilian dry forest Carlos Henrique Félix da Silva 1 , Xavier Arnan 1,2 , Alan N. Andersen 3 and Inara R. Leal 4, * 1 Programa de P´os-Graduação em Biologia Vegetal, Universidade Federal de Pernambuco, Av. Professor Moraes Rego s/n, Cidade Universitária, CEP: 50670-901, Recife, PE, Brasil; 2 CREAF, Cerdanyola del Vallès, Catalunya, Spain; 3 Research Institute for the Environment and Livelihoods, Charles Darwin University, Ellengowan Dr, Casuarina, Northern Territory, 0810, Australia and 4 Departamento de Botânica, Universidade Federal de Pernambuco, Av. Professor Moraes Rego s/n, Cidade Universitária, CEP: 50670-901, Recife, PE, Brasil Abstract Although extrafloral nectar (EFN) is a key food resource for arboreal ants, its role in structuring ground-nesting ant communities has received little attention, despite these ants also being frequent EFN-attendants. We investigated the role of EFN as a driver of the spatial structure of ground-nesting ant communities occurring in dry forest in north-eastern Brazil. We exam- ined the effects on this relationship of two global drivers of biodiversity decline, chronic anthropogenic disturbance and climate change (through decreasing rainfall). We mapped EFN-producing plants and ant nests in 20 plots distributed along independent gradients of disturbance and rainfall. We categorized ant species into three types according to their depend- ence on EFN: heavy users, occasional users and non-users. We found a strong relationship between ant dependence on EFN and nest proximity to EFN-producing plants: heavy-users (mean distance 1.1 m) nested closer to EFN-producing plants than did occasional users (1.7 m), which in turn nested closer to EFN-producing plants than did non-users (2.3 m). Neither disturbance nor rainfall affected the proximity of heavy-user nests to EFN-producing plants. Our study shows for the first time that EFN is a key driver of the spatial structure of entire communities of ground-nesting ants. Introduction Optimal foraging theory predicts that organisms seek to maximize net energetic yield in mini- mum foraging time (Grundel 1992, Oster & Wilson 1978, Stephens & Krebs 1986), and that this drives animal behaviour in terms of food selection and decisions about where, when and for how long foraging occurs (Pyke et al. 1977, Schoener 1971). According to optimal foraging theory (MacArthur & Pianka 1966), central-place foragers such as social insects optimize net energy gain by balancing trade-offs relating to colony location in a way that maximizes fitness (Covich 1976, Orians & Pearson 1979, Schoener 1979). Several studies have demonstrated that the for- aging of social insects can be optimized by the location of nests close to key food resources (Grundel et al. 2010, Kacelnik et al. 1986, Murray 1938, Potts et al. 2003). However, there have been few empirical studies addressing how the distribution of food resources influences spatial patterns of social insect colonies. Ants are ecologically dominant social insects in most terrestrial ecosystems, living in highly organized colonies where foragers retrieve food items to their nests, where they are stored, eaten or fed to offspring (Hölldobler & Wilson 1990, Traniello 1989). Plants provide many resources for ants as part of mutualistic interactions involving defence against herbivores (Bennett & Breed 1985, Bequaert 1922). Such resources can include nesting sites in the form of hollow thorns, stipules, leaf pouches, and chambers within epiphytic tubers (Janzen 1966, Rico- Gray & Oliveira 2007). However, most mutualistic interactions between plants and ants involve ant species that make their own nests, and therefore make their own decisions about where to locate their nests in relation to plant-based food resources such as leaves, nectar, seeds, honey- dew, or insects that live on vegetation (Holway & Case 2000, Kay 2002, Wagner & Fleur Nicklen 2010). Extrafloral nectar (EFN) is a carbohydrate-rich food resource produced by at least 3941 plant species (Weber & Keeler 2013, Zhang et al. 2015) for attracting ants, which helps protect plants from herbivores (Heil 2011, Rico-Gray & Oliveira 2007). Many specialist nectar-feeding ants have specialized digestive systems designed to exploit liquid carbohydrates, allowing intensified exploitation of such resources, and thus generating high fidelity with their host plants (Byk & Del-Claro 2011, Davidson 1997). Many studies have shown that the availability of EFN strongly https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0266467419000245 Downloaded from https://www.cambridge.org/core. IP address: 189.70.156.140, on 11 Oct 2019 at 20:09:09, subject to the Cambridge Core terms of use, available at