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Western North American Naturalist Volume 63 Number 3 Article 9 8-6-2003 Dispersal of sagebrush-steppe seeds by the western harvester ant (Pogonomyrmex occidentalis) John F. Mull Weber State University, Ogden, Utah Follow this and additional works at: https://scholarsarchive.byu.edu/wnan Recommended Citation Mull, John F. (2003) "Dispersal of sagebrush-steppe seeds by the western harvester ant (Pogonomyrmex occidentalis)," Western North American Naturalist: Vol. 63 : No. 3 , Article 9. Available at: https://scholarsarchive.byu.edu/wnan/vol63/iss3/9 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Western North American Katuralist 63(3), ©2003, pp. 358-362 DISPERSAL OF SAGEBRUSH-STEPPE SEEDS BY THE WESTERN HARVESTER ANT (POGONOMYRMEX OCCIDENTALIS) John F. ~1u1P ABSTRACT.-Like many seed-harvesting ants, the western harvester ant (Pogonomynnex occidentalis) can act as both a seed predator and a seed disperser. Dispersal results when seeds are dropped en route to the nest, are left in nest gra. naries when colonies die or are abandoned, or are removed from granaries and discarded. in nest middens. This study e.xamined the density and species identity of seeds discarded in harvester ant nest middens and compared them \\ith those found in nearby soils. Nineteen species ofseeds were recovered from middens, compared \"ith 13 species in S.m reference areas and 9 species in adjacent disk areas. Total density ofsound seeds was nearly 3 times higher in middens than at 5 m from the nest and nearly 50 times higher than in disk soils. Moreover, 4 ofthe 6 most common species overall were significantly more abundant in middens. One species, Munro globemalJow (Sphaeralcea l1Iunroana), was recovered from nearly 50% of middens but was not found in the other 2 areas. These findings suggest that the western harvester ant is a potentially important disperser ofsome sagebmsh-steppe plant species. Key words: seed dispersal, harvester ants, Pogonomyrmex, alll·,Jlont interaction, shrub-steppe, granivonJ. By definition granivores are predators that phytic fungus were more likely to be discarded consume seeds and cause the death ofindivid­ than uninfected seeds. Intrinsic seed attributes, ual plants (Brown and Ojeda 1987); however, such as nutritional quality (Kelrick et al. 1986) they commonly lose or reject seeds that they and seed coat thickness (Hissing and Wheeler have collected and cache others that are never 1976), may also influence a seed's chances of recovered. Thus, granivores may be both preda­ being discarded. Ultimately, individuals of some tors and dispersers of a pmticular plant species species dispersed in this manner may be more (Janzen 1971, Archer and Pyke 1991, Byrne abundant near nests and have increased repro­ and Levey 1993). Even when granivores are ductive output relative to individuals growing primarily seed predators, their rare dispersal in surrounding areas (Janzen 1971, Rissing services may be highly significant from a 1986). plant's perspective (Levey and Byrne 1993). This study quantified the extent to which the Harvester ants (Pogonomyrmex spp.) are western harvester ant (Pogonomyrmex occiden­ talis) discards intact the seeds of shrub-steppe specialist seed predators that can also disperse plants upon which it forages. Specifically, the seeds in several ways (MacMallOn et al. 2000). study was concerned with determining den­ Seeds that foragers collect (at distances ranging sity, species identity, and physical attributes of from < 1 m to >20 m) and return to the nest seeds found in colony nest middens that are may eventually germinate and grow ifthe colony maintained near nest entrances of P. occiden­ dies or emigrates. More typically, dispersal taUs colonies. occurs when foragers drop seeds en route to the nest or when nest workers discard seeds STUDY SITE from nest stores into middens (relUse piles) outside the nest (Hissing 1981, Kelrick et al. The study site is located in the sagebrush­ 1986, Wolff and Debussche 1999, Detrain and steppe 8 km southwest ofKemmerer in Lincoln Tasse 2000). A variety of seed attributes may County, Wyoming, on lands of the Pittsburg influence the probability that a seed, once har­ and ~1idway Coal Mining Company. Climate vested, will be discarded. For example, Knoch is cold and arid, with mean monthly tempera­ et al. (1993) found that tall fescue (Fesfuca tures ranging from -SOC (January) to l7°C anmdinacea) seeds infected with an endo- (July). '.>Iean annual precipitation of99.6 cm is IDepartment ofZooIog}'. 2505 Unin~rsi~' Circle. Weber Stale Universil)', Ogden. UT 8-¥lO!l. 358 2003] HA..RVESTER ANTS AND SEED DiSPERSAL 359 highly variable and falls mainly as snow (Par­ (1981) "apparently viable" and to Young et a1.'s menter and MacMahon 1983). Vegetation at (1983) "potentially germinable" seed. I used the site is dominated by big sagebrush (Artemi­ only sound seeds in the analyses. Seeds were sia tridentata) and rabbitbrush (Chrysotham­ identified to species using a reference collec­ nus viscidiflorus). Other shrub species include tion previously established for the site. The winterfat (Ceratoides lanata), bitterbrush (Pur­ term "seeds" is used broadly here to refer to shia tridentata), gray horsebrush (Tetradymia the dispersal units, or diaspores, ofeach species, eaneseens), and Gardner's saltbush (Atriplex most ofwhich are actually fruits. gardneri). Dominant understory plants are 3 Differences in total seed densities among perennial grasses, needle-anci-thread grass the 3 sampling locations were analyzed with a (Slipa eomata), Indian ricegrass (Oryzapsis i-way ANOVA. A Tukey-Kramer test was used hymenoides), and bluegrass (Poa spp.), and one to test for painvise differences (Wilkinson et annual grass, cheatgrass (Bromus tectorum). al. 1992). All other statistical tests involved Density of harvester ant colonies at the site is comparisons of individual species in the mid­ >30 . ha-] (Parmenter and MacMahon 1983). den and 5-m samples because disk samples contained too few seeds of most species to be METHODS included in the analyses of individual species. Densities of the 4 most abundant species were Soil samples were collected at 30 harvester compared using t tests. Chi-square goodness­ ant colonies in July 1994. To determine whether of-fit tests were used to compare total number density and species composition of seeds in of midden and 5-m samples containing the 6 colony middens differed from those in adja­ most common species of seeds. cent soils and in the colony foraging area, I took samples from 3 locations at each colony: a RESULTS midden located near the nest mound entrance, a point on the disk (= denuded area surround­ I recovered 19 species of seeds from mid­ den samples, 13 species from 5-rn samples and ing the mound) located 1 In from the entrance, and a point well beyond the disk located 5 m 9 from disk samples. Soil seed densities were nearly 3 times higher in middens than in the from the mound center. Thus, I collected a 5-m reference areas and nearly 50 times total of 90 samples. Disk and 5-m samples higher than in disk samples (Fig. 1). were taken in randomly chosen directions. Though seed densities in littered, undershrub locations at this site are much higher on aver­ age than those in open, intershrub spaces (Mull and MacMahon 1996), I made no attempt to 1000" stratify the 5-m samples to account for this dif­ ~ N T ference. Thus, reported seed densities for the e ~ ~ 5-m samples are a composite of undershrub "C 7S0(} '" and interspace samples. '"~ • Samples were extracted with a cylindrical 0 sampling core having a cross-sectional area of ~= >, SOO(} 40 cm2. Samples were returned to the labora­ •• -~ tory and placed in a freezer until processing. = T Each sample was sieved to remove the fine "C'" "C 2S0(} mineral portion «0.25 mm). Seeds were then '" recovered from the remainder of the sample CJJ'" while it was viewed under a dissecting micro­ ~ 0 ~ scope. Recovered seeds were classified as either Midden Disk Sm sound or unsound. Sound seeds were firm under the pressure of forceps and were not discolored or otherwise visibly degraded; un­ Fig. 1. Mean soil seed densities in 3 sample types taken sound seeds were not firm under pressure and at 30 harvester ant colonies. Seed densities differed signif­ icantly among sampling locations (F = 31.56, P < 0.0001). were either discolored or degraded. As used All 3 pairwise comparisons were significant (P < 0.05) here, "'sound seed" is comparable to Roberts' based on a Tukey-Kramer test. Error bars are 1 Sf' 360 WESTERN NORTH AMERICAN NATURALIST [Volume 63 Four of the 6 most abundant species in soil sities (Tables 1, 2) are either common in the samples occurred more frequently in middens seed pool at the site (B. tectorum, A. deserto­ than in the 5-m reference areas (Table 1). Most rum, 0. hymenoides, and C. poroiflora) or may notably, seeds of Munro globemalJow (Sphaer­ be difficult for harvester ants to process and alcea munroana) were found in nearly half of consume once they have been collected (5. the midden samples but were not present in munroana and L. redowskii). Rejection of pre­ any ofthe 5-m samples. viously collected seeds is a behavior that has Three of the 4 most abundant species were been frequently obsen1ed in harvester ants. present in significantly higher densities in Explanations proposed to account for the re­ middens (Table 2). Midden densities ofthe 4th jection ofviable seeds by harvester ants, though species, B. tectorom, were not significantly varied and sometimes speculative. include the different from those in 5-m reference areas addition of more preferred species to nest but were still much higher than those in adja­ stores, differential rates of infection \vith fun­ cent disk areas (mean = 32.5, Sx = 19.6 seeds gal endophytes, seed novelty, and "mistakes" .

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