Effects of Harvester Ants on Plant Species Distribution and Abundance in a Serpentine Grassland
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Oecologia (1997) 112:237±243 Ó Springer-Verlag 1997 Mark J.F. Brown á Kathleen G. Human Effects of harvester ants on plant species distribution and abundance in a serpentine grassland Received: 31 March 1997 / Accepted: 6 May 1997 Abstract Seed harvesting ants can have important ef- weeding or ``gardening'' (Kleinfeldt 1978; Jolivet 1990), fects on the composition and structure of plant com- altering nutrient availability (Gentry and Stiritz 1972; munities. We investigated two eects of Messor andrei, Beattie and Culver 1977; King 1977; Petal 1978; Rissing the black seed-harvesting ant, on a serpentine grassland 1986; Treseder et al. 1995; Wagner 1997), pollinating plant community in northern California. First, to de- ¯owers (Hickman 1974; Wyatt 1981; review in Peakall termine if selective seed predation by ants aects plant et al. 1991) or by dispersing and harvesting seeds community composition, we excluded harvester ants (Berg 1972; Beattie and Lyons 1975; Brown et al. 1979; from 1-mediameter circular plots of grassland. Abun- Westoby et al. 1982; Bond and Slingsby 1984; Davidson dances of all species on these plots and on control plots et al. 1984, 1985; Samson et al. 1992). were measured before and after exclosure. Second, to Seed-harvesting ants can aect plant communities determine if M. andrei nest mounds aect plant com- through at least two dierent processes. Selective har- munity composition, we compared plant species abun- vesting of particular seed species can aect the relative dances on and o nest mounds. M. andrei deposit large and absolute abundance of plant species (Brown et al. amounts of organic matter on their nest mounds over a 1979; Carroll and Risch 1984; Davidson et al. 1985; foraging season, so mounds may alter the edaphic en- Hobbs 1985; Risch and Carroll 1986; Andersen 1987; vironment. The exclusion of seed-harvesting activity did Samson et al. 1992), while the production of mounds not cause changes in the plant community. Nest mounds provides enriched edaphic environments that can sup- had a strong eect on plant communities: there were port plant communities that dier from the surrounding many more grasses and fewer forbs on ant mounds, vegetation (Hobbs 1985; review in Beattie 1989; Danin although at least one forb, Lepidium nitidum, produced and Yom-Tov 1990; Nowak et al. 1990). In addition, twice as many seeds when it grew on nest mounds. We plants that grow on mounds may enjoy increased seed found that nest mounds formed islands of higher-tem- production (Tevis 1958; Rissing 1986; review in Beattie perature soil in the serpentine grassland. 1989; Dean and Yeaton 1993). Messor andrei, the black seed-harvesting ant, is the Key words Seed-harvesting ants á Ant mounds á primary granivore in serpentine grasslands in northern Serpentine grassland á Plant abundance á Messor andrei California (Hobbs 1985). M. andrei ants harvest seeds selectively (Hobbs 1985; M.J.F. Brown, unpublished work), and hence might in¯uence plant abundances in Introduction this community. A study conducted at a site in northern California (Jasper Ridge Biological Preserve) suggested Ants in¯uence the structure and composition of that seed predation by M. andrei decreased the abun- plant communities. They can protect plants directly dance of preferred species in areas immediately adjacent from herbivores or from competition with other plants to nests (Hobbs 1985). Hobbs suggested that these local (Janzen 1966, 1967, 1969; Bentley 1977). Ants also aect eects might be representative of similar reductions plant community composition or dynamics by selective across the entire grassland, regardless of proximity to ant nests. In this study, we test this hypothesis and ask whether seed-harvesting aects plant abundances in the serpentine grassland community. M.J.F. Brown (&) á K.G. Human Department of Biological Sciences, M. andrei produces large nest mounds composed Stanford University, Stanford, CA 94305-5020, USA of excavated soil and seed cha which is deposited e-mail: [email protected]; fax: 415 723 6132 throughout the year. Hobbs (1985) demonstrated that 238 mounds support plant communities that dier from around each ant exclosure and pseudo-exclosure plot: (1) center, (2) those on the surrounding serpentine soil. The abundance inside, 2 cm from the shaded inside edge, and (3) outside, 10 cm from the edge. Soil temperatures were measured in April 1996 with of dierent species in the serpentine grassland commu- a Licor soil probe thermocouple (model 6000-09TC) and an Omega nity has changed signi®cantly over the last decade digital readout (model 450-AET). We took measurements early in (Hobbs and Mooney 1995). We repeated Hobbs' study, the morning of a clear day to maximize the likelihood of discov- comparing plant communities on and o ant mounds, to ering dierences between the shaded interior edges of exclosures and the unshaded centers and outside edges. To determine if soil determine if mounds have a consistent eect on plants temperatures diered among the locations, we used a one-way despite changes in the surrounding grassland commu- ANOVA, with a priori contrasts between center and outside, and nity. We also examined how ant mounds aect the seed interior edge and outside (Sokal and Rohlf 1981). In addition, set of one forb which was consistently present both on to minimize the impact of potential microclimatic eects, plant and o mounds. community censuses were taken at the center of plots where they were least likely to be shaded by the exclosure cylinder. Methods Biotic eects Study Site The serpentine plant community is dependent upon the annual seed rain for next year's seedlings. There is no carry-over in the seed bank from one year to the next (Hobbs and Mooney 1985). Furthermore, Our study took place between 1994 and 1996 on a serpentine though ants harvest seeds, they rarely drop seeds anywhere other grassland site at Jasper Ridge Biological Preserve, a 450-ha reserve than on their mounds (M. Brown, personal observation). Thus the in San Mateo County, northern California (122°12¢W, 36°25¢N, distribution of next year's seedlings on the open grassland depends elevation c. 180 m). Serpentine soils are shallow and nutrient poor, on the present year's plant distribution and corresponding seed rain and plant communities are spatially and temporally heterogeneous shadow. Hobbs and Mooney (1985) showed that the seed rain (Hobbs and Mooney 1985). The plant community consists pri- shadows of most species on the serpentine grassland are extremely marily of native annual forbs, a few native perennial bunch-grasses, local. Most plants drop seeds within 12.5 cm of the stem. Those few and some native and exotic annual grasses (McNaughton 1968). grass species that have a larger seed rain shadow (up to 25 cm) are tall enough to disperse seeds over the exclosures. Thus, exclosure cylin- ders are unlikely to interfere with seed dispersal patterns, except Ant exclosures possibly by increasing seed densities at the inside edge of cylinders. To avoid this problem, plant communities were sampled in the To determine the eect of ant exclusion on the plant community, centers of the plots, away from the edges. we set up three types of experimental plot in March 1994: (1) ant exclosure plots, (2) pseudo-exclosure plots, and (3) open plots. Pseudo-exclosure plots and open plots were controls for the eects Harvesting eects on the plant community of exclosures and the eects of harvesting respectively. Ant exclo- sure plots were ringed with painted tin cylinders, 20 cm high and The serpentine grassland community is extremely heterogeneous. 1 m in diameter, sunk 2.5 cm into the soil. Cylinders were slippery Because most plants in the community are small, it is dicult to and tall enough that harvester ants could not climb over them. sample a large enough area to account for spatial heterogeneity and Pseudo-exclosure plots were surrounded by similar cylinders, ex- still obtain an accurate count of plants within the area. To increase cept that semi-circles were cut from the bottom edges, so that ants the likelihood of discovering dierences due to ant exclosure, we could pass through easily. Occasional observations con®rmed that used two sampling methods, percent cover and density. Percent harvester ants did indeed forage within pseudo-exclosure plots. cover, which incorporates both spatial distribution and abundance, This treatment enabled us to investigate the eects of the cylinders was measured by counting the number of 5 cm by 5 cm squares in a on the plant community. Open plots were of the same size and 50 cm by 50 cm grid in which plants of a species were present shape as the other treatments but were not ringed with cylinders. (Hobbs and Mooney 1995). Density, a smaller-scale measure of We set up seven replicates of each of the three types of exper- abundance, was measured by counting the number of plants of imental plot, for a total of 21 plots, in an area of serpentine each species in three 10 cm by 10 cm squares in each plot. Density grassland. After excluding areas with large rocks, recent gopher measurements served as both an independent measure of the plant disturbances, or ant mounds, plot positions and treatments were community and a check for percent cover results. assigned randomly. All plots were separated by at least 10 m. As a In 1994 we sampled the percent cover and density of all plant measure of potential seed-harvesting activity, we counted the species within the 21 plots in April, May, June and September, number of active M. andrei nests within 10 m of each plot in April using the 50 cm by 50 cm sampling grid placed in the center of each 1994, September 1994, and April 1996.