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Seed Preferences of Three Harvester Ants of the Genus Pogonomyrmex (Hymenoptera: Formicidae) in the Monte Desert: Are They Reflected in the Diet?

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Seed Preferences of Three Harvester Ants of the Genus Pogonomyrmex (Hymenoptera: Formicidae) in the Monte Desert: Are They Reflected in the Diet? ECOLOGY AND POPULATION BIOLOGY Seed Preferences of Three Harvester Ants of the Genus Pogonomyrmex (Hymenoptera: Formicidae) in the Monte Desert: Are They Reflected in the Diet? 1,2 1 GABRIELA INE´ S PIRK AND JAVIER LOPEZ DE CASENAVE Ann. Entomol. Soc. Am. 104(2): 212Ð220 (2011); DOI: 10.1603/AN10093 ABSTRACT Harvester ants play an important ecological role as seed consumers in arid areas. We performed choice experiments to study preferences of Pogonomyrmex rastratus (Mayr), Pogono- myrmex mendozanus (Cuezzo & Claver), and Pogonomyrmex inermis (Forel) (Hymenoptera: For- micidae) for seeds of six native species (three grasses, two forbs, and a shrub) in the central Monte desert, Argentina. We hypothesized that ant diet composition would reßect ant preferences. Thus, based on previous ant diet studies, we expected that 1) the three species would prefer grass to nongrass seeds, 2) P. inermis would have a lower preference for nongrass seeds than P. rastratus and P. mendozanus and 3) relative abundance of seeds in the diet would be positively associated with relative preference. In general, relative abundance of seeds in the diet was positively associated with relative preferences. Ants preferred grass seeds, but P. inermis did not have a lower preference for nongrass seeds. We also analyzed the relationship between preferences for seeds of the six species and their size and morphology, and we found higher preferences for seeds of intermediate size but no relationship with seed morphology. The overall match between seed preferences and diet composition could increase the chances of ants affecting the abundance and composition of some seed resources in the Monte desert, with important community implications. RESUMEN Las hormigas cosechadoras de semillas cumplen un papel ecolo´gico de gran importancia en las zonas a´ridas. Realizamos experimentos para evaluar las preferencias de Pogonomyrmex rastratus (Mayr), Pogonomyrmex mendozanus (Cuezzo y Claver), y Pogonomyrmex inermis (Forel) (Hyme- noptera: Formicidae) por semillas de seis especies nativas (tres pastos, dos dicotiledo´neas herba´ceas, y un arbusto) en el Monte central, Argentina. Nuestra hipo´tesis fue que la composicio´n de la dieta de estas hormigas es el reßejo de sus preferencias. Por lo tanto, basa´ndonos en trabajos previos sobre la dieta de estas especies, esperamos que 1) las tres especies preÞrieran semillas de pastos sobre las dema´s semillas, 2) P. inermis presentara una menor preferencia por las semillas de dicotiledo´neas herba´ceas y arbustos que P. rastratus y P. mendozanus, y 3) que la abundancia relativa de semillas en la dieta estuviera asociada positivamente con las preferencias. En general, la abundancia relativa de semillas en la dieta estuvo asociada positivamente con las preferencias. Las hormigas preÞrieron semillas de pastos, pero P. inermis no presento´ una menor preferencia por semillas de dicotiledo´neas herba´ceas y arbustos. Analizamos tambie´n la relacio´n entre las preferencias por las semillas de las seis especies estudiadas y su taman˜ o y morfologõ´a y encontramos una mayor preferencia por semillas de taman˜ os intermedios, pero ninguna asociacio´n con la morfologõ´a. Finalmente, la coincidencia entre las preferencias y la dieta podrõ´a incrementar la probabilidad de que las hormigas afecten la abundancia y composicio´n de de semillas en el desierto del Monte, con importantes implicancias para la comunidad. KEY WORDS antÐseed interaction, choice experiments, desert communities, diet, granivory Harvester ants are a common component of arid and ecological role as seed consumers (Ho¨ lldobler and semiarid ecosystems, where they play an important Wilson 1990, MacMahon et al. 2000). Several studies have shown that they can inßict severe seed losses, 1 Grupo de Investigacio´n en Ecologõ´a de Comunidades de Desierto, with a greater impact on the more preferred species Departamento de Ecologõ´a, Gene´tica y Evolucio´n, FCEyN, Univer- (Brown et al. 1979, Reichman 1979, Crist and Mac- sidad de Buenos Aires, Piso 4, Pab. 2, C. Universitaria, C1428EHA Mahon 1992). This could eventually alter the rela- Buenos Aires, Argentina. tive abundance of plant species, causing changes in 2 Correspondingauthor:LaboratorioEcotono,INIBIOMA,CONICET- Universidad Nacional del Comahue, Pasaje Gutie´rrez 1125, 8400 the structure of plant communities (Inouye et al. 1980, Bariloche, Rõ´o Negro, Argentina (e-mail: [email protected]). Samson et al. 1992, Espigares and Lo´pez-Pintor 2005). 0013-8746/11/0212Ð0220$04.00/0 ᭧ 2011 Entomological Society of America March 2011 PIRK AND LOPEZ DE CASENAVE:SEED PREFERENCES OF HARVESTER ANTS 213 Although harvester ants collect a wide range of diet composition of these ants mirrors their seed pref- seeds, most experimental studies have shown that they erences, we expected that 1) the three species would have marked preferences for certain species (Kelrick prefer grass to forb and shrub seeds; 2) P. inermis et al. 1986, Gross et al. 1991, Crist and MacMahon 1992, would have a lower preference for forb and shrub Reyes-Lo´pez and Ferna´ndez-Haeger 2002, Nicolai et seeds than P. rastratus and P. mendozanus; and 3) al. 2007). In many of these studies, grass seeds are relative abundance of the six offered species in the diet among the most preferred, although seeds of other of P. rastratus, P. mendozanus, and P. inermis should be families are sometimes chosen [e.g., Purshia tridentata positively associated with their relative preference. (Pursh) DC. [Rosaceae], Kelrick et al. 1986; Trifolium We also hypothesized that these ant preferences subterranean L. [Leguminosae], Reyes-Lo´pez and would be associated with seed size and morphology. Ferna´ndez-Haeger 2002]. Several seed traits have We expected 1) a positive association between ant been found to inßuence forager choices; among them, seed preferences and seed size and 2) a higher pref- nutritional quality (Kelrick et al. 1986, Crist and Mac- erence the more elongated the seeds. Mahon 1992), concentration of secondary compounds (Carroll and Janzen 1973, Whitford 1978, Davidson Materials and Methods 1982, Buckley 1982), size (Davidson 1977, Kelrick et al. 1986, Crist and MacMahon 1992, Willott et al. 2000, Study Area. The study took place at the Biosphere Heredia and Detrain 2005), and morphology (Pulliam Reserve of N˜ acun˜ a´n (34Њ 03ЈS, 67Њ 54Ј W), located in and Brand 1975, Azca´rate et al. 2005). Regarding size, the central portion of the Monte desert. The main ant preferences have been positively correlated with habitat is the open woodland of Prosopis flexuosa DC. seed size in several studies (Kelrick et al. 1986, Crist where individuals of this species and of Geoffroea and MacMahon 1992), probably because of the higher decorticans (DC.) B. are scattered within a matrix of energy reward they offer. For seed morphology, there perennial tall shrubs (Ͼ1 m height, mostly Larrea is evidence that elongated seeds that bear conspicuous divaricata (DC.) Coville, but also Condalia micro- awns, hairs, and other projections are preferentially phylla Cav., Capparis atamisquea Kuntze, Atriplex harvested by several ant species (Pulliam and Brand lampa Gillies ex Moq., and Larrea cuneifolia Cav.), low 1975, Azca´rate et al. 2005) because they could be shrubs [Lycium spp., Junellia aspera (Gillies ex Hook), easier for ants to handle than rounded seeds with and Acantholippia seriphioides (A.Gray) Moldenke], smooth outlines. and perennial grasses [e.g., Trichloris crinita (Lag.) Pogonomyrmex rastratus (Mayr), Pogonomyrmex Parodi, Pappophorum spp., Sporobolus cryptandrus mendozanus (Cuezzo & Claver), and Pogonomyrmex (Torr.) Gray, Aristida spp., Digitaria californica inermis (Forel) (Hymenoptera: Formicidae) are (Benth.) Henr., Setaria leucopila (Scribn. & Merr.) K. three sympatric harvester ant species that occur in the Schum.]. Annual forb cover (e.g., Chenopodium papu- central Monte desert, Argentina. P. mendozanus (de- losum Moq., Phacelia artemisioides Griseb., Parthenium scribed in Cuezzo and Claver 2009) was referred to as hysterophorus L.) is highly variable from year to year. P. pronotalis in our previous publications based on The climate of N˜ acun˜ a´n is dry with hot summers and Claver and Fowler (1993). All three species have small cold winters. Mean annual temperature is 15.9ЊC colonies of 300Ð1,100 individuals, with Ϸ70% adult (1972Ð2004) and mean annual rainfall is 333.5 mm workers (Nobu´ a Behrmann et al. 2010). Their activity (1972Ð2004), with high interannual variation. Seven- season spans from mid-spring to early autumn (Oc- ty-Þve percent of the annual rainfall occurs in spring toberÐApril), and they exhibit diurnal activity with and summer (OctoberÐMarch), and seed production temporal changes responding mainly to soil temper- of almost all plants is restricted to summer. For a atures (Pol and Lopez de Casenave 2004). Their diet complete description of the study area, see Lopez de (as determined by quantiÞcation of the type and abun- Casenave (2001). dance of seeds brought to the nest by foragers) con- Sampling Design. Seed preferences of P. rastratus, sists mainly of grass seeds and only P. rastratus and P. P. mendozanus, and P. inermis were assessed in the mendozanus occasionally harvest forb and shrub seeds Þeld by performing choice experiments under a paired (Pirk et al. 2004, 2009; Pirk and Lopez de Casenave comparisons design (David 1988, de Vries 1998). Ant 2006). Seed size seems to be an important trait for colonies were offered one pair of seeds of different choice by these ants. Seeds of intermediate size prevail species at a time in all possible species combinations. in the diet, and a slight size match between seed size This design was chosen over cafeteria experiments and ant body size was detected, with P. mendozanus (Gross et al. 1991, Crist and MacMahon 1992) because carrying larger seeds than P. rastratus, followed by P. when more than two seeds were offered, individual inermis, matching body-size differences of the ant foragers generally removed one of them without eval- species (Pirk and Lopez de Casenave 2010). If the uating the complete set.
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