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Fire Ants, Cattle Grazing, and the Endangered Florida Grasshopper Sparrow
James W. Tucker, Jr., Gregory R. Schrott, and Reed Bowman

Southeastern Naturalist, Volume 9, Issue 2 (2010): 237–250

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2010 SOUTHEASTERN NATURALIST 9(2):237–250 Fire Ants, Cattle Grazing, and the Endangered Florida Grasshopper Sparrow James W. Tucker, Jr.1,*, Gregory R. Schrott1, and Reed Bowman1 Abstract - We measured densities of Solenopsis invicta (Red Imported Fire Ant) mounds at sites occupied by Ammodramus savannarum floridanus (Florida Grasshopper Sparrow), a federally endangered subspecies, at Avon Park Air Force Range (APAFR), Kissimmee Prairie Preserve State Park (KPPSP), and Three Lakes Wildlife Management Area (TLWMA). Our objective was to compare densities of fire ant mounds among areas with active cattle grazing programs (two areas of native dry prairie habitat at APAFR and a tamed pasture at KPPSP) and areas without active grazing programs (native dry prairie habitat at KPPSP and TLWMA). Densities of fire ant mounds differed among the five areas examined and were greater in areas with active grazing programs than in areas without active grazing programs. We measured densities of fire ant mounds inside and outside of a cattle exclosure, but the total numbers detected were insufficient for analysis. We also placed bait stations inside and outside the exclosure. Fire ants were detected at 50% fewer bait stations inside the exclosure, but these differences were not significant. Introduction Solenopsis invicta Buren (Red Imported Fire Ant) was introduced into the southeastern United States between 1933 and 1945 and was found in 670 counties/parishes in 11 states and Puerto Rico by 1995 (Callcott and Collins 1996). Mounting evidence suggests that Red Imported Fire Ants may have strong negative effects on many native North American wildlife species (Allen et al. 2004, Wojcik et al. 2001). For example, Stake and Cimprich (2003) found 31% of depredated nests of the endangered Vireo atricapilla Woodhouse (Black-capped Vireo) in Texas were destroyed by fire ants. Ammodramus savannarum floridanus Mearns (Florida Grasshopper Sparrow) is a federally endangered subspecies endemic to dry prairies of south-central Florida. The numbers and occupied range of this species have declined from historic levels because of habitat loss and degradation (Delany and Linda 1994, Delany et al. 2007b, Shriver and Vickery 1999). Florida dry prairie is an endemic landscape restricted to south-central Florida (Bridges 2006). The dry prairie landscape is maintained by frequent, lightning-season fires followed by temporary, seasonal flooding (Platt et al. 2006) and consists of large, open expanses of prairie dominated by a diversity of grasses and forbs interspersed with low-growing shrubs (Orzell and Bridges 2006). 1Archbold Biological Station, Avian Ecology Lab, PO Box 2057, Lake Placid, fl33862. *Corresponding author - 238 Southeastern Naturalist Vol. 9, No. 2 Most dry prairie habitat has been lost to agriculture, including conversion to improved (i.e., tamed) pasture, often by plowing and/or roller chopping followed by planting of non-native grasses (Delany and Linda 1994, Delany et al. 1985, Shriver and Vickery 1999). As recently as 2005, five breeding aggregations (i.e., spatially distinct groups) of the Florida Grasshopper Sparrow were known from only 3 protected populations (Fig. 1): Avon Park Air Force Range (APAFR; 3 aggregations, Polk and Highlands counties), Three Lakes Wildlife Management Area Figure 1. Location of Florida Grasshopper Sparrow populations at Avon Park Air Force Range (two aggregations, Delta Trail/OQ Range to the west and Charlie/Echo Ranges to the east), Kissimmee Prairie Preserve State Park, and Three Lakes Wildlife Management Area. The location of a cattle exclosure on OQ Range at Avon Park Air Force Range also is shown. 2010 J.W. Tucker, Jr., G.R. Schrott, and R. Bowman 239 (TLWMA; 1 large aggregation, Osceola County), and Kissimmee Prairie Preserve State Park (KPPSP; 1 large aggregation, Okeechobee County). One breeding aggregation at APAFR (Bravo/Foxtrot Range) was recently extirpated, with sparrows last observed there in 2005 (Tucker et al. 2008, in press). Although annual estimates have varied, populations at TLWMA (1991–2005) and KPPSP (1998–2005) have appeared relatively stable, whereas the population at APAFR declined dramatically between 1999 and 2003 and has since remained at critically low numbers (Tucker et al. 2008, in press). A primary difference in management practices among populations is that native dry prairie habitat, the primary habitat occupied by Florida Grasshopper Sparrows, is grazed by cattle (1 cow-calf pair/8 ha) at APAFR, but those at TLWMA and KPPSP are not. Allen et al. (1997) examined the distribution of Red Imported Fire Ants at APAFR and found evidence of negative impacts on Florida Grasshopper Sparrows. These impacts included a negative spatial relationship between fire ant and Florida Grasshopper Sparrow abundance, and between fire ant and native invertebrate (the major food source for breeding Florida Grasshopper Sparrows) abundance; however, a multiple regression analysis that examined the additive effects of fire ant and native invertebrate abundances on Florida Grasshopper Sparrows did not yield significant results (Allen et al. 1997). Because Red Imported Fire Ants are most prevalent in disturbed habitats (King and Porter 2007; Todd et al. 2008; Tschinkel 1988, 1993) and because of the previous work of Allen et al. (1997), we conducted a study to compare densities of Red Imported Fire Ant mounds among sites actively grazed by cattle with those at sites not actively grazed. We also conducted studies at APAFR that compared abundance of fire ants inside and outside of a cattle exclosure to examine whether disturbances by cattle contributed to higher abundances of fire ants, and therefore to possible negative impacts on Florida Grasshopper Sparrows. Methods Study areas and site selection We used point-count data collected to monitor breeding populations of Florida Grasshopper Sparrows to aid in selection of sampling points for fire ants within each management area. The basic method used for monitoring Florida Grasshopper Sparrows by point counts at all 3 areas was described by Walsh et al. (1995). Briefly, at each area, individual counting points were arranged in a grid pattern spaced about 400 m apart. We included only counting points located within dry prairie habitat and ≥100 m from private property in our selection of sampling points. Overall, we considered 163 counting points at APAFR, 129 at KPPSP, and 136 at TLWMA in our selection of sampling points in native dry prairie habitat. An additional 32 counting points at KPPSP were located in a tamed pasture (Fig. 1), and we 240 Southeastern Naturalist Vol. 9, No. 2 also selected a separate subset of these points for comparison with results from native dry prairie. All study sites were managed with prescribed fire on a 2–3 year rotation, and all historically were grazed by cattle. Cattle last grazed the dry prairie at TLWMA in 1987 and KPPSP in 1996, but continue to graze the dry prairie at APAFR at stocking densities of 1 cow-calf pair/8 ha (Delany et al. 2007a). The previous landowner of KPPSP retained grazing and management rights of the tamed pasture, and cattle continue to graze this pasture at stocking densities of 1 cow-calf pair/4 ha. Furthermore, this tamed pasture was treated by roller-chopping every 2 years and was last roller-chopped 17 months before we sampled it. Except for the tamed pasture at KPPSP, which was roller-chopped and planted with Paspalum notatum Fluegge (Bahia Grass) before establishment as a State Park in 1996, study sites were relatively undisturbed (see discussion) and were similar in vegetation structure and composition. Although written records are lacking, aerial photographs of Delta Trail/OQ Range at APAFR show that a portion of that area was lightly roller-chopped sometime between 1979 and 1986 (Steve Orzell, Environmental Flight, APAFR, pers. comm.). We acquired ArcView shapefiles of site boundaries, roads, and all pointcount locations for each management area and overlaid these onto digital orthophoto imagery of the individual areas. Using ArcView GIS (version 3.3; ESRI, Redlands, CA), we randomly selected 40 counting points at each of 4 sites: 2 sites (Delta Trail/OQ Range and Charlie/Echo Range; hereafter, Delta/OQ and Echo) at APAFR and 1 site each at KPPSP and TLWMA. We also randomly selected 9 counting points from the tamed pasture at KPPSP for comparison with native dry prairie. We did not include Bravo/Foxtrot Range at APAFR in this study, because habitat in that range is outside the dry prairie landscape and was historically a forested site that was cleared of trees in the 1920s and has been maintained as treeless by near annual burning since establishment as an active bombing range in the 1940s (Delany et al. 1999). Thus, we selected a total of 160 counting points (40 from each of 4 different sites) in native dry prairie habitat and 9 counting points in tamed pasture for sampling fire ants. To measure density of fire ant mounds, we randomly located ten 10-m transects within a 100-m radius of the center of each counting point. We used ArcView to identify 10 randomly located points to serve as the starting points for each transect, and we generated random compass bearings to dictate orientation of the transects. We used hand-held GPS units with WAAS corrections to navigate to the starting point of each transect, and we used a measuring tape and handheld compass to identify transects on the ground. Fire ant and vegetation sampling We perceived visual obstruction by vegetation as the most likely factor to influence detection probabilities of fire ant mounds, so we measured visual 2010 J.W. Tucker, Jr., G.R. Schrott, and R. Bowman 241 obstruction at each transect using a modification of Robel’s pole (Robel et al. 1970). The “modified pole” consisted of a board that was 1.5 m tall, measured 1.8 cm x 4.0 cm in thickness and width, and was painted with alternating red and white bands that were 10 cm in height. The “modified pole” was held vertically in the center (i.e., at the 5-m interval) of each transect and an observer recorded the lowest 10-cm interval that was ≥50% visible from a height of 1.5 m from the 2 endpoints of the transect. The average of these 2 readings was used as a measure of visual obstruction for the transect. We sampled fire ants and measured visual obstruction between 26 September 2005 and 12 January 2006. We used distance methods (Buckland et al. 2001) to survey for fire ant mounds along each transect. Forbes et al. (2000) recommended distance sampling to estimate density of fire ant mounds in grassland habitats. We walked the center of each transect and measured the perpendicular distance to the center of each active fire ant mound that we observed. We used program DISTANCE (Buckland et al. 2001) to estimate the density of fire ant mounds. We reasoned that visual obstruction would be the most influential factor affecting detection probability, so the only covariate we examined in estimation of detection probability was our measure of visual obstruction. We used a global estimate of detection probability and post-stratified by site to estimate densities (active mounds/ha) of fire ant mounds for Delta/OQ, Echo, KPPSP, TLWMA, and the tamed pasture at KPPSP. We concluded differences in densities of fire ants between sites when 95% confidence intervals around the density estimates did not overlap. Confidence intervals were estimated by the program DISTANCE using a log-based approach with degrees of freedom calculated by Satterthwaite’s procedure (Buckland et al. 2001). Compared to standard methods of significance testing (e.g., ANOVA), examining overlap in confidence intervals yields conservative results (Schenker and Gentleman 2001), but avoids problems associated with violation of assumptions for normality and equal variances. Cattle exclosure studies To further examine the effects of active cattle grazing on densities of fire ants, we conducted 2 studies sampling fire ants inside and outside a 13.7-ha cattle exclosure constructed in 1995 on OQ Range at APAFR (Fig. 2). The first study employed distance sampling along 10 transects located randomly inside and outside the cattle exclosure. Each transect was 50 m in length, and we limited transects to >10 m from the exclosure fence to avoid edge effects. We restricted transects outside the exclosure to areas within 300 m of its boundary that were similar in soil type, hydrology, and vegetation composition to that inside the exclosure, and that had no additional disturbances except for active cattle grazing. We measured the perpendicular distance from the transect to the center of each active fire ant mound that we observed. 242 Southeastern Naturalist Vol. 9, No. 2 The second study utilizing the cattle exclosure employed stations baited with frankfurter meat (Porter and Tschinkel 1987). Bait stations consisted of an 18.9-gram piece of frankfurter placed at 19-m intervals along 2 transects inside the cattle exclosure (i.e., ungrazed transects) and 3 transects outside Figure 2. Cattle Exclosure located at OQ Range of Avon Park Air Force Range and the location of transects used in distance sampling and placement of bait stations in the studies comparing fire ant densities between actively grazed and ungrazed dry prairie habitat. Crosshatching identifies patches of prairie recovering from past (>10 yrs) disturbance. 2010 J.W. Tucker, Jr., G.R. Schrott, and R. Bowman 243 the exclosure (i.e., grazed transects; Fig. 2). The territory size of a fire ant colony is relatively small (Adams 1998, Tschinkel et al. 1995), and distances between our bait stations probably were sufficient to ensure independence. We examined 40 bait stations inside the exclosure and 40 bait stations outside the exclosure. Ungrazed transects ran north–south, were located at 100 m and 200 m from the western edge of the exclosure, and contained 20 bait stations each. One grazed transect with 20 bait stations was located 100 m west of the exclosure. We used 2 other shorter transects, each containing 10 bait stations, outside the exclosure to avoid disturbed areas (see Fig. 2). One shorter transect was located 200 m west of the exclosure and paralleled the longer transect, but extended about 100 m further south. The other shorter transect was located 100 m south of the exclosure and 500 m east of the other short transect (Fig. 2). Baits were placed on 15-cm diameter paper plates, anchored to the ground with a 16-penny nail, individually numbered for identification, and marked with a wire flag for relocation. We recorded the time of establishment for each bait station. After about 2 hours of exposure, we checked the bait stations; recorded the station number, time checked, and presence/ absence of fire ants; and removed the paper plate, bait, nail, and wire flag from the area. For analysis, we calculated the proportion of bait stations with fire ants present and 95% confidence intervals (95% CI) around these binomial proportions (Zar 1984:378–379). We concluded significant differences if 95% confidence intervals did not overlap. We used a two-sample t-test to compare length of time that bait stations inside and outside the exclosure were exposed during the trials. Results For all sites, we detected a total of 330 fire ant mounds along the vegetation transects. However, graphical analysis of the distance measurements suggested that distances beyond 1000 cm were outliers, so we used right truncation (Buckland et al. 2001) at 1000 cm for the analysis. After right truncation, the analysis included a total of 320 distance measurements. The data best fit a hazard rate model with no adjustments (Buckland et al. 2001). A model that included visual obstruction as a covariate (AICc = 4022.9) in estimating the detection probability was more parsimonious than a model that did not include a covariate (AICc = 4045.2). After accounting for the effect of visual obstruction (β = -0.024, SE = 0.005), our estimate of detection probability was 0.252 (95% CI = 0.226–0.279). Densities of fire ant mounds (active mounds/ha) were greater at Delta/OQ (mean = 74.0, 95% CI = 57.8–94.7), Echo (mean = 46.7, 95% CI = 35.1–62.1), and the tamed pasture (mean = 97.2, 95% CI = 62.1–152.0) than they were at KPPSP (mean = 11.9, 95% CI = 7.2–19.6) and TLWMA (mean = 4.5, 95% CI = 2.2–9.0) (Fig. 3). We surveyed 500 m of transect (i.e., n = 10 transects) both inside and outside the cattle exclosure at OQ Range (Fig. 2) on 31 August 2007. During these surveys, we detected only 2 active fire ant mounds outside the exclosure (i.e., in actively grazed habitat) and no active fire ant mounds inside 244 Southeastern Naturalist Vol. 9, No. 2 the cattle exclosure. These numbers were insufficient for analysis, so we conducted a second study on 1 February 2008 that utilized bait stations systematically located along transects inside and outside the cattle exclosure at OQ Range (Fig. 2). During this study, we recorded fire ants present at 8 bait stations inside the exclosure (P = 0.20, 95% CI = 0.091–0.356) and at 12 bait stations outside the exclosure (P = 0.30, 95% CI = 0.166–0.465). Thus, fire ants were recorded at 50% more stations in the grazed area than in the ungrazed area, but this difference was not significant at the 95% confidence level. Time between placement and checking of bait stations was similar (t-test adjusted for unequal variances: t = 0.654, df = 70.52, P = 0.515) for stations inside (mean = 130.2 min., SD = 5.1 min.) and outside (mean = 130.8 min., SD = 3.6 min.) the exclosure. Discussion Densities of fire ant mounds differed among sites and were greater at sites actively grazed by cattle (i.e., Delta/OQ, Charlie/Echo, and the tamed pasture at KPPSP) than at sites without active grazing (i.e., dry prairie habitat at KPPSP and TLWMA; Fig. 3). Fire ants occur mostly in disturbed areas (Hill Figure 3. Density (± 95% confidence intervals) of fire ant mounds estimated from vegetation transects in the tamed pasture at Kissimmee Prairie Preserve State Park (Pasture-Kiss) and native dry prairie habitat at Delta Trail/OQ Range (Delta/OQ), Charlie and Echo Ranges (Charlie/Echo), Kissimmee Prairie Preserve State Park (Kissimmee), and Three Lakes Wildlife Management Area (Three Lakes). The tamed pasture at Kissimmee Prairie Preserve State Park and the dry prairie at Delta/OQ and Charlie/Echo are all grazed by cattle, but the dry prairie at Kissimmee and Three Lakes are not grazed. 2010 J.W. Tucker, Jr., G.R. Schrott, and R. Bowman 245 et al. 2008; King and Porter 2007; Stiles and Jones 1998; Todd et al. 2008; Tschinkel 1988, 1993), and disturbance from cattle grazing might explain the distribution of fire ants that we observed. The highest density of mounds was found in the tamed pasture at KPPSP, which has a higher grazing intensity than any of the APAFR sites as well as frequent disturbance from roller chopping. Although our 2 studies utilizing a cattle exclosure at OQ Range failed to find statistical differences supporting our hypothesis concerning cattle grazing, the size of the exclosure limited the number of samples we could collect and therefore our ability to detect differences. We acknowledge a potential shortcoming of our study was recording presence/absence of fire ants on the baits rather than collecting and counting numbers on the baits (e.g., Porter and Tschinkel 1987). However, using 2 different methods, we did find more fire ants in the grazed area than inside the cattle exclosure as predicted by our hypothesis. Allen et al. (1997) found a small negative impact of fire ants on Florida Grasshopper Sparrows at APAFR, and we recently (16 June 2007) documented likely depredation of a Florida Grasshopper Sparrow nest by fire ants at APAFR (Fig. 4). When checked on the expected hatch date, this nest contained 3 dead, recently-hatched chicks and 1 pipped egg, all covered with fire ants. This was the only nest out of 18 that we have monitored at APAFR that was depredated by fire ants. Marianne Korosy (University of Figure 4. Florida Grasshopper Sparrow nest at Avon Park Air Force Range that was depredated by Red Imported Fire Ants on 16 June 2007. Vegetation and top of the domed nest were pushed back to photograph nest contents. Photo by James Tucker. 246 Southeastern Naturalist Vol. 9, No. 2 Central Florida, Orlando, fl, pers. comm.) also documented likely depredation of a Florida Grasshopper Sparrow nest at KPPSP on 9 June 2007. This nest was discovered 2 days post hatch and contained 3 healthy chicks. The nest was checked 3 days later and the chicks had recently died (estimated <12 hrs) and were covered with fire ants. Negative effects of fire ants on Florida Grasshopper Sparrows probably are more diverse than just reduced nesting success. For example, Mueller et al. (1999) found survival of Colinus virginianus L. (Northern Bobwhite) chicks was negatively correlated with abundance of fire ants in an experimental study in Texas. Survival of chicks to 21 days after-hatch was almost 3 times greater (60.1% vs. 22.0%) for nests in areas treated with an insecticide to eliminate fire ants than nests that were not treated. The difference in survival was attributed to mortality resulting from fire ant stings within the first day after hatching (Mueller et al. 1999). Pedersen et al. (1996) found presence of fire ants altered behavior of Northern Bobwhite chicks ≤6 days old. Chicks exposed to fire ants spent more time responding directly to fire ants and less time foraging, sleeping, and moving than chicks not exposed to fire ants; furthermore, Pedersen et al. (1996) documented that fire ants usually stung chicks on the eyes, legs, or toes and, although the physiological responses of chicks varied, reactions often included eyelids swelling shut and legs or feet swelling to prevent normal movement. Giuliano et al. (1996) found that fire ant stings reduced survival and weight gain of Northern Bobwhite chicks. In addition to direct effects, fire ants probably have indirect effects that adversely influence Florida Grasshopper Sparrows. For example, fire ants reduce abundance and diversity of arthropods (Porter and Savignano 1990), an important food source for Florida Grasshopper Sparrows. Allen et al. (1997) found a negative spatial correspondence between fire ants and abundance of native invertebrates on dry prairies at APAFR. In conclusion, we believe the drastic population decline in Florida Grasshopper Sparrows at APAFR (Tucker et al., in press) probably resulted from a multitude of both direct and indirect factors, only part of which were related to cattle grazing and fire ants. Major differences in management of dry prairies among populations at APAFR, KPPSP, and TLWMA include active cattle grazing and military activities at APAFR. Although disturbances from military activities might be suspected as a primary factor contributing to the decline of Florida Grasshopper Sparrows, the evidence does not support this conclusion. For example, dry prairie at Delta/OQ receives very little disturbance from military activities and Echo is an active target range, yet sparrow populations have declined on both ranges. Most Florida Grasshopper Sparrows remaining at APAFR are located on Echo, and disturbances on Echo mostly are localized to areas surrounding targets. We believe active cattle grazing at APAFR may be a strong factor contributing to the population decline of Florida Grasshopper Sparrows. The direct impacts of cattle on nesting success (e.g., trampling 2010 J.W. Tucker, Jr., G.R. Schrott, and R. Bowman 247 [Renfrew et al. 2005], direct predation [Nack and Ribic 2005]) probably have minor effects compared to cumulative effects of indirect factors (e.g., reduced nesting cover resulting in lower nest success [Sutter and Ritchison 2005, Walsberg 2005], reduced biomass of invertebrate prey [Sutter and Ritchison 2005], and reduced overwinter survival resulting from decreased production of grass seeds [Gonnet 2001] and increased predators associated with fencing [Perkins and Vickery 2001]). We hypothesize that another indirect effect of cattle grazing on Florida Grasshopper Sparrows at APAFR includes increased abundance of fire ants which reduces nesting success directly through depredation (Tucker et al. 2008), and indirectly through reduced survival of fledglings (Giuliano et al. 1996, Mueller et al. 1999, Pedersen et al. 1996), and possibly adults, and reduced abundance of invertebrate prey (Allen et al. 1997, Porter and Savignano 1990). Acknowledgments We greatly appreciate assistance from Peg Margosian at APAFR, Paul Miller at KPPSP, and Steve Glass and Tina Hannon at TLWMA for providing point-count data and GIS coverages to us. Roberta Pickert was instrumental in setting up the GIS projects we used in this study. Logistic support from staff of Environmental Flight at APAFR, Charlie Brown and Paul Miller at KPPSP, and Steve Glass at TLWMA was critical to the success of this study. We gratefully acknowledge the hard work of Jacob Patchen, Risha Sparhawk, Darcy Stumbaugh, and Laurel Temmen for collecting fire ant and vegetation data and Jen Benson for assistance with validating GPS coordinates of counting points. This study was inspired and greatly benefited from discussions and ideas of many individuals at meetings of the Florida Grasshopper Sparrow Working Group. Funding for this study was provided by the US Department of the Interior, Fish and Wildlife Service, Agreement No. 401815G016. We also gratefully acknowledge APAFR for supporting this project via serving as our base camp and providing financial support for James Tucker via funding from the US Army Medical Research and Materiel Command under Cooperative Agreement No. W81XWH-06-2-0026 with Archbold Biological Station. 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