Oecologia DOI 10.1007/s00442-011-1982-4 COMMUNITY ECOLOGY - ORIGINAL PAPER Fast food in ant communities: how competing species Wnd resources Jessica M. C. Pearce-Duvet · Martin Moyano · Frederick R. Adler · Donald H. Feener Jr. Received: 5 August 2010 / Accepted: 14 March 2011 © Springer-Verlag 2011 Abstract An understanding of foraging behavior is species cannot exploit both high numbers and high discov- crucial to understanding higher level community dynamics; ery capacity as a strategy. These results highlight that while in particular, there is a lack of information about how forager number is crucial to determining time to discovery diVerent species discover food resources. We examined the at the community level and within species, individual for- eVect of forager number and forager discovery capacity on ager characteristics inXuence the outcome of exploitative food discovery in two disparate temperate ant communities, competition in ant communities. located in Texas and Arizona. We deWned forager discov- ery capacity as the per capita rate of resource discovery, or Keywords Community ecology · Forager density · how quickly individual ants arrived at resources. In general, Forager discovery capacity · Formicidae · Resource resources were discovered more quickly when more forag- discovery ers were present; this was true both within communities, where species identity was ignored, as well as within spe- cies. This pattern suggests that resource discovery is a mat- Introduction ter of random processes, with ants essentially bumping into resources at a rate mediated by their abundance. In contrast, Foraging for food is a necessity faced by almost all animals, species that were better discoverers, as deWned by the pro- and diVerences in how species accomplish this task inXu- portion of resources discovered Wrst, did not have higher ence niche segregation (Emlen 1966; MacArthur and Pianka numbers of mean foragers. Instead, both mean forager 1966), contribute to species coexistence (e.g., Brown and number and mean forager discovery capacity determined Davidson 1977; Pulliam 1985; Kotler and Brown 1988), and discovery success. The Texas species used both forager stabilize food webs (Kondoh 2003; Beckerman et al. 2006). number and capacity, whereas the Arizona species used Consequently, understanding foraging behavior is crucial to only forager capacity. There was a negative correlation understanding higher level processes (Werner 1992; Becker- between a species’ prevalence in the environment and the man et al. 2010), especially when considering species coex- discovery capacity of its foragers, suggesting that a given istence in communities of generalists that use the same resource base (Tilman 1982; Kneitel and Chase 2004). In general, much more is known about how species diVer in Communicated by Phil Lester. eYciency during resource harvest (Brown 1989; Ziv et al. 1993; Brown et al. 1994) as opposed to resource discovery J. M. C. Pearce-Duvet · M. Moyano · F. R. Adler · D. H. Feener Jr. (Westoby et al. 2002), even though resource discovery is the Department of Biology, University of Utah, W 257 South 1400 East, Salt Lake City, UT 84112, USA rst step in the foraging process and, as such, may reveal more about the intensity of competition and its inXuence on Present Address: species interactions (Vincent et al. 1996; Calcagno et al. J. M. C. Pearce-Duvet (&) 2006; Adler et al. 2007). Estación Biológica de Doñana, CSIC, c/Americo Vespucio s/n, 41092 Sevilla, Spain Ant communities represent an ideal study system for e-mail: [email protected] examining the dynamics of foraging behavior, as most 123 Oecologia species compete Wercely for the same pool of plant and scout number to explain discovery dynamics within local insect matter (Pontin 1963; Brown et al. 1979; Davidson ant communities. Second, we ascertain whether diVerences et al. 1984; Davidson 1985; Davidson et al. 1985; Hölldobler in discovery ability among species could be explained by and Wilson 1990), and their behavior is directly observable. scout number, thus making community discovery dynamics A trade-oV between the ability to arrive at resources Wrst an emergent property of the species’ traits, or whether indi- and the ability to control them during harvest has been pro- vidual foragers have diVerent discovery capacities. Third, posed as a mechanism for species coexistence (Vepsäläinen we consider the eVect of abiotic factors, such as time of day and Pisarski 1982; Fellers 1987; Morrison 1996; Davidson and desiccation risk, on discovery at the community and 1998; Holway 1999; LeBrun and Feener 2007; Feener et al. species level. 2008). As is true in other systems, past work has focused almost exclusively on resource dominance (Fellers 1987; Savolainen and Vepsäläinen 1988; Savolainen et al. 1989; Materials and methods Andersen 1992; Morrison 1996; Andersen 1997; Cerdá et al. 1998b; LeBrun 2005), to the detriment of understand- Study sites and systems ing how food is found. In ants, as in other generalist foragers, the challenge is to In the summer of 2005, we collected data on resource dis- locate diverse food resources that are both unpredictably covery at six plots in Texas and Wve plots in Arizona. Sam- located and renewed (Harkness and Maroudas 1985; Jones pling took place during the peaks of ant community activity et al. 2001), and it appears discovery is not directed by in each location: May–June in Texas and July–August in resource signals (Pearce-Duvet and Feener 2010). As social Arizona. Three of the Texas plots were located at the insects, they can enhance the probability of encountering Brackenridge Field Lab (latitude »30°17ЈN, longitude food indirectly via both the number of foragers employed »97°46ЈW, elevation »145 m), and three were located at as well as through those foragers’ individual traits. It has the Stengl “Lost Pines” Biological Station (latitude long been assumed that resource discovery relies on »30°04ЈN, longitude »97°10ЈW, elevation »140 m). The species-speciWc forager number (Johnson et al. 1987; JaVe Brackenridge plots were characterized by sugarberry (Cel- and Deneubourg 1992; Kaspari 1993b; Dechaume- tis laevigata), Ashe’s juniper (Juniperus ashei), and cedar Moncharmont et al. 2005; Avgar et al. 2008). In particular, elm (Ulmus crassifolia), while the Stengl plots were domi- ant colonies are thought to contain a Wnite number of forag- nated by varying mixtures of blackjack oak (Quercus mari- ers that are divided into scouts which search independently landica) and loblolly pine (Pinus taeda). The Arizona plots for food and recruits which are summoned to collect or were located in mixed forests containing Chihuahua pine Wght for food, and this mechanistic trade-oV is thought to (Pinus leiophylla var. chihuahuana), grey oak (Quercus underlie species co-existence (Johnson et al. 1987; Feener grisea), and alligator juniper (Juniperus deppeana); three 2000). Yet, the relationship between forager number and plots were located on the eastern side of the Chiricahua resource discovery has not been examined explicitly. It is Mountains, near Portal (latitude »31°54ЈN, longitude also unknown if scout number is a species-speciWc trait that »109°14ЈW, elevation »1,700 m), and two plots were accounts for discovery ability or if individual foragers of established on the western side of the range, on the El certain species have a greater capacity for discovery. Coronado Ranch near Pearce (latitude »31°51ЈN, longi- How abiotic factors aVect resource discovery is also tude »109°22ЈW, elevation »1,700 m). Based on species largely uncharacterized, although their inXuence on observations at baits and from pitfall traps, Texas and Ari- resource dominance is well described (Cerdá et al. 1997, zona communities were equivalently rich, with a mean of 1998a, b; Bestelmeyer 2000; Santini et al. 2007; Lessard 21 species in Arizona (range = 20–22) and 22 species in et al. 2009). In North American and Mediterranean ant Texas (range = 17–27) (t = 0.5, df = 5.3, P = 0.6; corrected communities, many species have been found to trade oV for unequal variance). There was extensive genus but lim- between behavioral dominance and thermal tolerance, with ited species overlap. behavioral subordinates being more likely to forage during thermally extreme times (Cerdá et al. 1997, 1998a, b; Experimental design Lessard et al. 2009; Wiescher et al. 2011) (although see Santini et al. 2007). These results generate a hypothesis for Each plot measured 50 m by 50 m and was divided into 25 discovery dynamics: good discoverers, which are often behav- evenly spaced stations. Stations were located 10 m apart ioral subordinates, should show greater abiotic tolerance. and individually marked by a survey Xag. In order to mea- This study examines the simple but unexplored question sure forager density, we placed a circular hoop encompass- of how ants Wnd food, using observations in two temperate ing 0.25 m2 at each station on all plots and visually scanned woodland communities. First, we examine the ability of the area inside the hoop for 5 min. The identity and the 123 Oecologia number of ants present in the hoop were recorded; ants on rejection, thus resulting in more power than in traditional recruitment trails or which were clearly returning with food multiple comparison methods (Garcia 2003). were not counted. We felt conWdent about accurately esti- We examined overall discovery dynamics in each com- mating ant presence, as visual surveys provide reliable esti- munity using survival