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Invasive ants generate heterogeneity in patterns of seed survival.

INTRODUCTION

Invasive species can have negative direct effects on ecological communities by reducing abundance and diversity of native species and disrupting ecosystem processes (e.g., Elton, 1958; Mooney, 2005; Vila et al., 2011). Furthermore, invasive species may also have important indirect effects on invaded communities by altering behavior and activity of native species (White et al, 2006). The capacity for invasive organisms to generate both direct and indirect effects is important to understand because these effects may work in concert, leading to strong, often unanticipated effects on native species such as increased apparent competition in addition to direct competition for resources (Orrock et al, 2015). Despite the potential for invasive plants and animals to generate strong direct and indirect effects (Orrock et al, 2010; Vila et al., 2011; Orrock et al., 2015), few studies have examined how the indirect effects of invasive species affect interactions among native species.

The invasion of North America by the red imported fire ant, Solenopsis invicta, has resulted in substantial ecological impacts, including decreased abundance and diversity of many fauna native to the southeastern United States (Porter et al., 1988; Porter and Savignano, 1990). Solenopsis invicta reduces native ant species richness and abundance and alters native ant behavior through competitive displacement (Porter and Savignano, 1990; Morris and Steigman, 1993; Gotelli and Arnett, 2000). Solenoposis invicta also affects the survival and distribution of many vertebrate species, including small mammals, birds, and herpetofauna (Killion et al., 1995; Ferris et al., 1998; Allen et al., 2004).

While the effects of S. invicta on animals are well documented, S. invicta may also play an important role in affecting the distribution and abundance of plants. This role might be especially acute in the context of affecting seed fate. For example, S. invicta can directly affect seed survival by consuming seeds that might otherwise be dispersed by native ants (Zettler et al., 2001; Ness and Bronstein, 2004). Solenopsis invicta may also alter patterns of seed survival by affecting the behavior or abundance of arthropod and rodent granivores. Small mammals manage the risk of foraging in the presence of S. invicta by altering their behavior (Smith et al, 1990; Killion et al., 1995; Ferris et al, 1998) and reducing foraging activity (Holtcamp et al., 1997; Orrock and Danielson, 2004), while abundance and diversity of native ants and other arthropods is decreased by S. invicta invasion (Porter and Savignano, 1990; Morris and Steigman, 1993; Gotelli and Arnett, 2000; Allen et al, 2001). Because ants preferentially consume small-seeded species and rodents prefer large-seeded species (Mittelbach and Gross, 1984; Orrock and Damschen, 2005), changes in ant and/or rodent granivory may generate changes in seed survival that lead to shifts in plant abundance and plant community composition (Brown and Heske, 1990; Hulme, 1998; Crawley, 2000; MacMahon et al, 2000; Orrock et al, 2006; Clark et al., 2007) Therefore, understanding how S. invicta affect patterns of granivory by native arthropods and rodents may be important for understanding the dynamics of plant species in invaded areas.

We present a study of the direct and indirect effects of S. invicta on seed survival by examining the relative influence of rodent and arthropod granivores and proximity to S. invicta mounds on predation of small and large seeds. Because initial S. invicta recruitment to bait decreases with distance from a mound (Hu and Ding, 2009), we expect S. invicta activity also decreases with distance from a mound and therefore hypothesize; (1) survival of large seeds (i.e., seeds too large to be consumed by ants) is greater near S. invicta mounds where S. invicta activity is higher and rodent foraging activity is lower relative to locations further from mounds, and (2) seed removal by native arthropods and rodents differs between locations adjacent to and away from S. invicta mounds, increasing spatial heterogeneity in seed survival.

METHODS

STUDY AREA AND DESIGN

Our study was conducted from 4 August to 5 September 2012, at the Savannah River Site (SRS), an 80,000 ha National Environment Research Park near Aiken, SC, U.S.A (33[degrees]21'46.3"N, 81[degrees]40'58.5"W). The most common forest types at SRS are forests dominated by longleaf pine (Pinus palustris) and loblolly pine (Pinus taeda) (Kilgo and Blake, 2005). We conducted our study with eight replicated experimental blocks, each consisting of a 15 nr plot of forest. We quantified the effect of S. invicta on seed removal by deploying four seed depots at each experimental block (n = 32 depots total). Seed depots consisted of plastic storage trays (17 X 11 x 10 cm) with 5.5 cm holes in two adjacent sides (Orrock and Danielson, 2004). These openings allowed access by all small rodent species common in our study area (most commonly Peromyscus gossypinus, the cotton mouse; Golley et al., 1965). Lids were attached to the top of trays in order to prevent loss of seeds via wind and rain. Lids also prevented foraging by avian seed predators, which, within this study area, have a negligible impact on seed predation compared to predation by arthropods and rodents (Orrock et al., 2003). Depots were deployed in pairs at two distances from a single active S. invicta mound: 0.1 m and 4.0 m. We expected S. invicta activity would be lower at 4.0 m from a mound, given a previous study demonstrated speed and amount of bait removal by S. invicta significantly decreases with increasing distance from a mound (after 1 h, removal of bait at 1.08-1.2 m was 10-31% lower than on the mound; Hu and Ding, 2009). We chose mounds that were active based on the appearance of ants when the mound was gently agitated. Within each depot pair, one depot allowed access by only arthropods while the other allowed access by both arthropods and rodents. Arthropod only depots had 1.25 [cm.sup.2] wire mesh secured over each entrance, which allowed arthropods to enter while excluding small mammals. Depots allowing both rodents and arthropods had unobstructed entrances.

Seeds of two native species, both found in our study area, were used in our seed removal study; one small-seeded species, Rubus cuneifolius (sand blackberry, 1.83-2.03 mm; 95% CI, n = 10), and one species with much larger seeds, Prunus serotina (black cherry, 5.75-5.91 mm; 95% CI, n = 10). Seeds were obtained from commercial suppliers (Sheffield's Seed Co., Locke, New York, U.S.A. and TheSeedman.com, Vancleave, MS, U.S.A). Prunus serotina is consumed primarily by rodents, yet ignored by S. invicta and other ants due to its large size and hard seed coat (Whelan et al., 1991; Orrock and Damschen, 2005), whereas Rulms cuneifolius is consumed by both arthropods and rodents (Orrock and Damschen, 2005). Neither R. cuneifolius nor P. serotina seeds bear elaiosomes. We scattered ten seeds of each species on the surface of a layer of commercial sand within each depot. Seed depots were left in the field for 20 d, after which, remaining seeds were counted. Seed removal was considered equivalent to seed predation as is supported by observations of signs of predation (seed fragments) in 25% of seed depots from which seeds were removed and by a study conducted at SRS demonstrating the negative relationship between seed removal and seedling recruitment (Orrock et al., 2003).

DATA ANALYSIS

Our design is a split-plot design, with distance to the S. invicta mound (0.1 m or 4.0 m) as the whole-plot treatment and depot access type as the split-plot treatment (i.e., one depot in the pair allowed access to arthropods, whereas the other depot in a pair allowed both rodents and arthropods). For our analyses of the proportion of seeds removed from seed depots, we used generalized linear mixed models, which treated exclosure type and distance from mound as fixed effects. We treated each site and each pair of depots within a distance from the mound as random effects to appropriately capture the split-plot nature of our design. For analysis of R. cuneifolius seed removal, we used a generalized linear mixed model with a binomial response distribution to evaluate the proportion of R. cuneifolius seeds removed. The analysis for R. cuneifolius fit a fully factorial model that included the interaction of depot access type and distance from a mound. For analysis of P. serotina, we did not evaluate the two-factor model because inspection of the data indicated the vast majority of P. serotina removal occurred within depots that allowed access to both arthropods and rodents. We conducted follow up analysis to evaluate P. serotina removal from only depots allowing both rodents and arthropods, in which we utilized a general linear mixed model with a Gaussian response distribution after transforming the dependent variable (the proportion of P. serotina seeds removed) using the logit transformation (Warton and Hui, 2011). Observations from four seed depots were excluded from analysis due to disturbance by wildlife (n = 3) and failure of rodent exclosure (n = 1). All analyses were performed in SAS version 9.3 (SAS Insitute Inc., 2012); for all analyses, we evaluated residual plots to see that model assumptions were not violated.

RESULTS

Averaged across all treatments, the proportion of R. cuneifolius removed from trays allowing access by both rodents and arthropods (0.49 [+ or -] 0.09 SE) was higher than the proportion of P. serotina removed (0.22 [+ or -] 0.10 SE; [t.sub.27.0] = -2.02, P = 0.053).

Prunus serotina removal was low across both exclosure treatments and locations relative to S. invicta mounds. The P. serotina removal that did occur was predominantly from depots that allowed access to both arthropod and rodent granivores (97% of seeds removed were taken from depots allowing both rodents and arthropods). When depots that allowed access by only arthropods were excluded from analysis, the proportion of P. serotina did not differ significantly between seed depot locations ([F.sub.1,6,0] = 114, P = 0.327, Fig. 1).

Removal of R cuneifolius seeds was affected by the interaction between exclosure treatment and seed depot location ([F.sub.1,24] = 8.73, P = 0.007, Fig. 1) as well as the main effect of exclosure treatment ([F.sub.1,24] = 7.24, P = 0.013). Across both seed depot locations, R. cuneifolius removal was 30% higher in depots that excluded rodents, compared to depots that admitted both rodents and arthropods (pairwise comparison, [t.sub.24] = -2.69, P = 0.013, Fig. 1). When rodents were excluded from depots, nearly twice the proportion of R. cuneifolius was removed at a distance of 4.0 m from a mound compared to the proportion removed 1.0 m from the mound (pairwise comparison, [t.sub.12.2] = --2.50, P = 0.028, Fig. 1). At 4.0 m from the mound, 40% more R. cuneifolius was removed from depots that excluded rodents compared to depots at 4.0 m which allowed both rodents and arthropods (pairwise comparison, [t.sub.24] = -3.54, P = 0.002, Fig. 1).

DISCUSSION

Our results suggest invasive ants generate spatial variation in seed survival, and this variation may be predictable based upon the size of the seed: small-seeded R. cuneifolius experienced nearly twice as much removal by arthropods when located far from a S. invicta mound than when located near a S. invicta mound, whereas the removal of largeseeded P. serotina was not affected by proximity to S. invicta mounds. The patterns of seed removal observed in our study are likely the outcome of either shifts in abundance or changes in foraging behavior of native arthropods near 5. invicta mounds in combination with the effects of rodent presence on the availability of small seeds to foraging arthropods. A potential implication of our findings is that, by altering spatial patterns of seed survival (Fig. 1), S. invicta may lead to changes in spatial patterns in the recruitment of plant species, just as granivores have done in other systems (Brown and Heske, 1990; MacMahon et al., 2000; Crooks, 2002).

The increase in the removal of small-seeded R. cuneifoius away from S. invicta mounds may result from an increase in native arthropod abundance and activity in response to decreased risk of S. invicta encounter. The speed and amount of bait removal by S. invicta decreases as distance from a mound increases (Hit and Ding, 2009) and the boundary between S. invicta colonies is often not occupied by S. invicta, but instead occupied by native ant species (Tschinkel, 2011). When colony size is controlled for, S. invicta possesses inferior foraging ability compared to two native ant species that employ similar foraging strategies (i.e., are omnivorous and use mass recruitment to a food resource), including Pheidok dentata, a species found in our study site: 5. invicta is inferior to P. dentata in both foraging distance traveled and number of individuals recruited to bait (Jones and Phillips, 1990). Furthermore, research conducted within our study area found S. invicta abundance to be negatively correlated with native ant abundance and diversity in pitfall traps (Resasco et al., 2014; Orrock, unpublished data). As such, the increase in the removal of small-seeded R. cuneifolius that we observed away from S. invicta mounds was likely a result of increased abundance and/or foraging activity of native arthropods in the areas between S. invicta mounds. In a study of seed removal by S. invicta, Seaman and Marino (2003) found where S. invicta was the only granivorous ant species present, removal of seeds of pigweed (Amaranthus retroflexus) did not differ with distance from a mound (up to 4.0 m). In our study sites, where 45 additional species of ants were present, including many granivorous species (although note that harvester ants, Pogonomyrtnex badius, were not found in any of our sites during our study; Resasco, unpublished data), we observed increased removal of smallseeded R. cuneifolius away from mounds. This further supports the conclusion that increased seed removal away from mounds may be a consequence of competitive displacement of native ants by S. invicta (Porter and Savignano, 1990; Morris and Steigman, 1993; Gotelli and Arnett, 2000), or a result of a shift in native ant foraging away from S. invicta mounds in order to avoid agonistic interactions. This shift in foraging activity may lead to altered composition and increased heterogeneity in the structure of the plant community. Although further research is needed to test whether the increased heterogeneity in seed survival observed results in heterogeneity in seedling recruitment, previous experimental manipulation of granivore activity has been linked to significant changes in plant community composition (Brown et al., 1979; Brown and Heske, 1990; MacMahon et al., 2000; Orrock et al., 2006).

[FIGURE 1 OMITTED]

Our finding of increased seed removal away from S. invicta mounds is in contrast with results from studies of removal of elaiosome-bearing seeds by S. invicta. Stuble et al. (2010) found increased removal of elaiosome-bearing seeds in areas with high S. invicta density, without a reduction in seed removal by native ants, and Ness (2004) found no difference in seed removal of elaiosome-bearing seeds between invaded and uninvaded areas. The difference in findings between our study, which evaluated seed species without elaiosomes, and studies of removal of elaiosome-bearing seeds may be because elaiosome-bearing seeds are removed faster and in greater amounts by S. invicta compared to nonelaiosome-bearing seeds (Cumberland and Kirkman, 2013). If this is the case, species with elaiosome-bearing seeds may benefit from S. invicta (Stuble et al., 2010), whereas the fate of species that produce seeds without elaiosomes may depend upon seed size and the distance to the nearest S. invicta mound. However, future study of the effect of distance to the nearest S. invicta mound using both seeds with and without elaiosomes would elucidate this potentially elaiosome-dependent pattern in seed survival.

Despite the well-documented negative effects of S. invicta on rodent behavior (Smith et al., 1990; Killion et al., 1995; Holtcamp et al., 1997; Ferris et al., 1998; Orrock and Danielson, 2004), we did not observe removal of P. serotina varying as a function of location relative to mounds. This may be an indication that rodent foraging is not affected by S. invicta, that the density of S. invicta is too low in our study site to affect rodent foraging, or that a more preferred seed species was abundant and available to rodents at the time of our study; however, this may alternatively be a result of generally low densities of granivorous rodents in the study area, perhaps because rodent densities within intact longleaf and loblolly forests can be low (Golley et al., 1965). The generally low rates of P. serotina removal provide support for the latter explanation, as removal of P. serotina by rodents has been much higher (>50%) in studies done in oldfield habitats in our study area (Orrock and Damschen, 2005) where rodent abundance is typically higher (Golley et al., 1965).

The observed increase in seed removal when rodents were excluded from seed depots away from S. invicta mounds suggests the potential occurrence of an interspecific interaction between rodents and native arthropods that reduces availability of small seeds to arthropods (e.g., interference competition, rodent burial of seeds). Rodents in our study area may consume arthropods (Gentry and Smith, 1968; Wolfe and Linzey, 1977) that would otherwise consume small-seeded R. cuneifolius. Alternatively, rodent foraging activity may displace sand, burying small seeds and protecting them from predation by arthropods, resulting in an inadvertent mutualism between rodents and small seeds; arthropods primarily forage for seeds on the soil surface (Reichman, 1979; Hulme, 1994) and burial has been shown to significantly decrease the probability that a seed will be removed by arthropods (Orrock and Damschen, 2007). Therefore, abiotic and biotic factors that lead to low rodent abundance, such as lire or increased predator abundance, may result in increased indirect effects of S. invicta on survival of small seeds.

Through its impact on native species interactions, S. invicta creates differences in seed survival that vary with proximity to a mound, increasing spatial heterogeneity. Because the eff ects of S. invicta on seed survival are species-specific, changes in seed survival mediated by S. invicta may lead to shifts in plant community composition. While our results indicate rodent presence leads to decreased removal of small seeds by arthropods, our study also highlights the need for future studies that focus on understanding the influence of seed morphology on seed predation by invasive and native arthropods, as well as the mechanisms through which rodent-arthropod interactions lead to changes in seed survival.

Acknowledgements.--Funding and support was provided by the USDA Forest Service, Savannah River, under Interagency Agreement DE-AI09-00SR22188, the Strategic Environmental Research and Development Program (Project RC-1695), and the Vilas Associates program at the University of Wisconsin-Madison. Valuable review, comments, and field assistance were provided by J. Herrmann, J. Ledvina, J. Stuhler, and the Orrock lab.

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JENNIFER L. CHANDLER (1) and JOHN L. ORROCK

Department of Zoology, University of Wisconsin, Madison 53706

AND

JULIAN RESASCO

Department of Ecology and Evolutionary Biology, University of Colorado, Boulder 80309

(1) Corresponding author: e-mail: jchandler3@wisc.edu

Submitted 9 February 2016

Accepted 15 June 2016
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Author:Chandler, Jennifer L.; Orrock, John L.; Resasco, Julian
Publication:The American Midland Naturalist
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Date:Oct 1, 2016
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