Functional Ecology 2009, 23 , 196���202 doi: 10.1111/j.1365-2435.2008.01472.x �� 2008 The Authors. Journal compilation �� 2008 British Ecological Society Blackwell Publishing Ltd Body size predicts degree in ant���plant mutualistic networks S. A. Chamberlain* and J. N. Holland Department of Ecology and Evolutionary Biology, Rice University, Houston, TX 77005, USA Summary 1. The complexity of ecological communities can often hinder understanding their structural features. With the recent application of network theory, the structure of previously neglected mutualistic communities has begun to be elucidated. 2. Mutualistic communities have now been shown to follow particular power distributions in their degree, that is, the number of species interactions per species. However, predictive variables of degree and other structures of mutualistic networks remain largely unexplored. 3. Here, we show that body size of ants is positively correlated with their degree in mutualistic networks comprised of ant interactions with extrafloral nectar (EFN) bearing plants in the Sonoran Desert. This pattern in body size and the number of plant species with which ants interact occurred among all eight sampled communities, a relationship which was not contingent upon phylogenetic history among ant species. 4. These results indicate that further study of body size in ant���plant and other mutualistic networks may yield promising insights into the processes influencing their structure. Moreover, the degree���body size relationship for ant���plant mutualistic communities is consistent with that of predator���prey food webs, possibly suggesting similar underlying processes at work. Key-words: ant, allometry, body size, community structure, degree, extrafloral nectar, mutualism, network Introduction Understanding patterns in, and mechanisms for, the structure of ecological communities remains a central endeavour in ecology (Williams & Martinez 2008). With the application of network theory in recent years, ever greater recognition is being made of the structures of ecological communities, including previously neglected mutualistic communities (Bascompte et al . 2003 Jordano, Bascompte & Olesen 2003 Olesen et al . 2007 V��zquez et al . 2007). For example, mutualistic networks are now recognized to display non-random structural properties involving degree (number of links per species), nestedness (specialist species interacting with a subset of generalist species) and interaction strengths (Bascompte et al . 2003 Jordano et al . 2003 V��zquez 2005 Lewinsohn et al . 2006 V��zquez et al . 2007). Despite recognition of potential dynamic consequences of these and other structures for ecological networks (Dunne, Williams & Martinez 2002b Okuyama & Holland 2008), studies are just beginning to examine some of the biological factors that contribute to the structural prop- erties of mutualistic communities. Here, we examine whether body size, a predictive variable of predator���prey food web structures (Brose et al . 2004 Woodward et al . 2005 Otto, Rall & Brose 2007 Petchey et al . 2008), contributes to the number of species interactions (i.e. links) per species in mutu- alistic communities. Body size relationships among predators and prey vary systematically among habitats, scale positively with interaction strength and have been shown to contribute to community structures that may enhance stability of food webs (Brose, Williams & Martinez 2006 Otto et al . 2007). For example, predator���prey body size ratios of five real food webs fall within a predicted domain for community persistence (Otto et al . 2007). In food webs in which predators consume entire prey, body size has clear implications, as large prey are not readily consumed by smaller predators. The role of body size in mutualistic systems is less clear, as their consumer���resource interactions do not entail consumption of entire individuals, but rather resources produced by a mutualist to attract and reward its mutualistic partners (Holland et al . 2005). A key structural property of mutualistic networks is degree distribu- tion, non-random patterns in the number of species with which any given species interacts. At the community level, degree distribution often follows a power or truncated-power *Correspondence author. E-mail: schamber@rice.edu

Body size in ant���plant networks 197 �� 2008 The Authors. Journal compilation �� 2008 British Ecological Society, Functional Ecology , 23 , 196���202 distribution for mutualistic and predator���prey networks, indicating that many species interact with only a few species, and few species tend to interact with many species (Dunne, Williams & Martinez 2002a Jordano et al . 2003 Montoya & Sol�� 2003 Proulx, Promislow & Phillips 2005 but see Okuyama 2008). While food web analyses have shown that the degree of predators and prey can vary with their respective body sizes (Otto et al . 2007), little or no research has explicitly examined such degree���body size relationships for mutualistic communities. Nevertheless, other studies have shown that morphological matching between species in terms of body size variables (e.g. proboscis/flower matching) can be important to the structure of mutualistic plant���pollinator and plant���seed disperser networks (Jordano et al . 2003 Stang, Klinkhamer & van der Meijden 2006, 2007 Dalsgaard et al . 2008). In turn, this supports the suggestion that degree���body size relationships may be of use in understanding the structure of mutualistic networks. We quantified ant���plant mutualistic networks of eight communities in the Sonoran Desert to examine the extent to which the body sizes of ants correlates with their degree in mutualistic communities. Ant���plant mutualistic communities entail consumer���resource interactions between ants and plants in which ants consume extrafloral nectar (EFN) resources produced by plants and in turn protect and defend the plants from herbivores and other natural enemies (Holland et al . 2005 Bronstein, Alarcon & Geber 2006). We show that the number of plant species with which ants interact (i.e. ant species degree) increases with ant body size, and importantly, this relationship is not contingent upon phylogenetic rela- tionships among ant species. Methods We quantified community interactions among ants and EFN bearing plants at eight sites in the Sonoran Desert of Arizona (USA) and Sonora (Mexico) during spring and summer months of 2007 (Table 1). Contrary to many tropical ant���plant systems, those in the Sonoran Desert do not include plant production of food (lipid) bodies or domatia (ant housing Heil & McKey 2003). Most ant species were ground nesters, but some species ( Pseudomyrmex , Zacryptocerus ) were observed to nest within woody EFN-bearing plants (e.g. Prosopis ). Due to high ambient temperatures of the desert, most ant species were active only at night, except Forelius mccooki and F. pruinosis , which were active almost exclusively by day. These ant���plant communities add to the very few of such mutualistic networks thus far reported (Fonseca & Ganade 1996 Guimar��es et al . 2006 Bl��thgen et al . 2007). To document species interactions comprising these ant���plant networks, we established 3��6 ha plots (120 �� 300 m) randomly posi- tioned at each of the eight sites. We censused diurnal ( c. 16.00���18.30 h) and nocturnal ( c. 20.00���22.00 h) ant visitation to all EFN bearing plant species in each plot of each site. Each site was censused once to avoid inclusion of species with non-overlapping phenologies (Basilio et al . 2006 Medan et al . 2006). We censused an equal number of individuals ( c. 25) per plant species per plot to minimize biases associated with sample accumulation curves (e.g., see Gotelli & Colwell 2001). We censused each plant for 1 min each by day and by night, for a total of 2 min per individual plant. We recorded the abundance of each species of ant on all censused plants. Because the same individual plants censused by day were also censused at night, we combined both diurnal and nocturnal censuses for analyses. We calculated degree for each ant species at each site by counting the number of plant species with which ant species interacted. Although particular pairwise interactions between ant and plant species may vary in space and time, recent evidence from plant���pollinator communities suggests that structural properties such as degree are relatively invariant in time despite often large variation in the identity of particular species interactions (Petanidou et al . 2008). Body size for each ant species was estimated from 1 to 23 workers per ant species. Only minor workers were measured for dimorphic species (e.g. Camponotus ). Head capsule length of each worker was measured with a dissecting microscope (to 0��01 mm) equipped with an ocular micrometer, which was calibrated with a stage micrometer. Body mass was then estimated using allometric equations derived by Kaspari & Weiser (1999). The equation M = aL b , where L is the head capsule length, was used to convert head capsule length to body mass for each species, with subfamily specific constants for Dolichoderinae ( a = 3��870 �� 10 ��� 4 , b = 2��621), Formicinae ( a = 6��319 �� 10 ��� 4 , b = 3��493), Myrmicinae ( a = 5��1475 �� 10 ��� 4 , b = 3��361) and Pseudomyrmicinae ( a = 3��7024 �� 10 ��� 4 , b = 3��342) (Kaspari & Weiser 1999). A mean measure of ant body size was calculated for each species. We examined ant degree distributions, and their best model fits, for each of the eight sites. Due to difficulties in estimating the fit of models to degree distributions for small (e.g. Semilla Flats, where n = 12 species in the community) networks (Albert & Barab��si 2002 Guimar��es et al . 2007), we show degree distributions for all eight sites, but only test model fits to six of the eight sites. Using the r package bipartite , we examined the fit of exponential ( P ( k ) ��� exp( ����� k )), Table 1. Eight study sites sampled for ant���plant interactions and their respective latitude, longitude, altitude (meters above sea level), and geographic location in the Sonoran Desert Site name Latitude Longitude Altitude Location Seri flats (SF) 28��52���35��6��� N 111��57���17��8��� W 2 Sonora, MX Atta flats (AF) 28��52���54��6��� N 111��57���47��8��� W 7 Sonora, MX Teddy peak (TP) 28��57���23��7��� N 111��58���24��5��� W 63 Sonora, MX Staghorn saddle (SS) 28��58���28��1��� N 112��02���59��1��� W 182 Sonora, MX Alamo canyon (AC) 32��04���16��6��� N 112��43���41��4��� W 700 Arizona, USA Arches (AR) 32��02���18��2��� N 112��42���56��6��� W 790 Arizona, USA Semilla flats (SM) 32��11���31��6��� N 112��48���59��0��� W 498 Arizona, USA Cholla garden (CG) 32��10���54��6��� N 112��46���24��4��� W 542 Arizona, USA