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Basiotis, Katherine A.
The effects of invasive cogongrass (Imperata cylindrica) on the threatened gopher tortoise (Gopherus polyphemus)
h [electronic resource] /
by Katherine A. Basiotis.
[Tampa, Fla.] :
b University of South Florida,
ABSTRACT: The gopher tortoise (Gopherus polyphemus) is critical to upland communities and considered a keystone species. A recent threat to gopher tortoise habitat is the invasive cogongrass (Imperata cylindrica), which spreads rapidly, eliminating native vegetation. This study consisted of three experiments to investigate the effects of the cogongrass on a population of gopher tortoises. A feeding experiment revealed that individuals readily ate native vegetation, but would not eat cogongrass. A tracking experiment showed that there was a significantly different mean angle of movement between individuals whose home ranges were outside cogongrass compared to those that overlapped cogongrass, indicating that the presence of cogongrass disrupts normal movement patterns. An orientation experiment showed that individuals outside cogongrass oriented in a direction that would take them to their home burrow, while individuals inside cogongrass showed no preferred directional orientation. Cogongrass effectively eliminates the gopher tortoises' food source and habitat, and disrupts orientation. The experiments indicate that a cogongrass infestation has the capacity to eliminate populations of gopher tortoises if its spread is not checked.
Thesis (M.S.)--University of South Florida, 2007.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
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Document formatted into pages; contains 40 pages.
Advisor: Henry Mushinsky, Ph.D.
t USF Electronic Theses and Dissertations.
The Effects of Invasive Cogongrass ( Imperata cylindrica ) on the Threatened Gopher Tortoise ( Gopherus polyphemus ) by Katherine A. Basiotis A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Biology College of Arts and Sciences University of South Florida Co-Major Professor: Henry Mushinsky, Ph.D. Co-Major Professor: Earl McCoy, Ph.D. Peter Stiling, Ph.D. Date of Approval: April 6, 2007 Keywords: feeding experiments, habitat us e, orientation, geotax is, exotic species Copyright 2007, Katherine Basiotis
Acknowledgements I would like to thank a ll the members of the USF herpetol ogy lab and the staff at Teneroc Fish Management Area for their assistance, my professors for their guidance, and my friends and family for their support.
i Table of Contents List of Tables ii List of Figures iii Abstract iv Introduction 1 Objectives 4 Methods 5 Study Species 5 Study Site 9 Feeding 11 Habitat 12 Orientation 14 Results 17 Feeding 17 Habitat 17 Orientation 18 Discussion 21 Literature Cited 33
ii List of Tables Table 1: Diversity indices calculated from qua drat vegetation sampling. 27
iii List of Figures Figure 1: Map of study site with cogongrass perimeters delineated. 28 Figure 2: Reciprocal rates of feeding on thr ee types of vegetation. 29 Figure 3: Angles of movement and mean vector s of tortoise paths. 30 Figure 4: Regressions of distance traveled onto distance from the cogongrass. 31 Figure 5: Angles of movement and mean vector s of displaced tortoises. 32
iv The Effects of Invasive Cogongrass ( Imperata cylindrica ) on the Threatened Gopher Tortoise ( Gopherus polyphemus ) Katherine Basiotis ABSTRACT The gopher tortoise ( Gopherus polyphemus ) is critical to upland communities and considered a keystone species. A recent thr eat to gopher tortoise ha bitat is th e invasive cogongrass ( Imperata cylindrica ), which spreads rapidly, elim inating native vegetation. This study consisted of three experiments to investigate the effect s of the cogongrass on a population of gopher tortoises. A feeding expe riment revealed that individuals readily ate native vegetation, but would not eat cogongras s. A tracking experiment showed that there was a significantly different mean angl e of movement between individuals whose home ranges were outside cogongrass compar ed to those that overlapped cogongrass, indicating that the presence of cogongrass disrupts normal movement patterns. An orientation experiment showed that individuals outside cogongrass orie nted in a direction that would take them to their home burrow, while individuals insi de cogongrass showed no preferred directional or ientation. Cogongrass effec tively eliminates the gopher tortoisesÂ’ food source and habitat, and disrupt s orientation. The experiments indicate that a cogongrass infestation has the capacity to eliminate populat ions of gopher tortoises if its spread is not checked.
1 Introduction Invasive species are a threat to worldwid e biodiversity, second only to habitat loss (Allendorf and Lundquist 2003). The effects of invasion vary from none to severe. If a species doesnÂ’t pass the intr oduction phase, then the effect of the invasive on the community is non-existent. Species that do b ecome invasive can have severe ecological and economical consequences. In the Un ited States alone, an estimated 50,000 nonnative species cause estimated economic losses of over 125 billion USD per year (Allendorf and Lundquist 2003). The ecological c onsequences include a loss of diversity, extinction of native species, as well as physic al and chemical changes in the abiotic environment (Vitousek et al 1996). Adventive species must go through four stag es to be considered invasive. The first is introduction, or the arrival of memb ers of the species to a new area. Not all introduced species survive in a new environm ent, but if the area is not immediately unsuitable the second stage, escape and subseq uent colonization or reproduction, occurs. The third stage is naturalizat ion or establishment, where the species can maintain its population in the new area without human assist ance (Di Castri 1989). The final stage is spread or invasion, in which the establis hed population of the introduced species branches out and colonizes new areas. In th e end, only about 2.5% of adventive species reach the invasion stage (Di Castri 1989).
2 Many species colonize new areas natu rally, although humans have greatly increased the natural expans ion rate of species. Nonnative plant species may be introduced accidentally, via the nursery plant trade, as contaminants of agricultural seed, or as stowaways or hitchhikers on global travelers. Species are also introduced deliberately, for agricultural or biol ogical control purposes (Primack 1998). Any introduced species has the potential to become invasive, but some traits appear to be characteristic of successful invasives. Invasi ve species tend to have large geographical ranges and be more tolerant of variable physical conditions (Sakai et al 2001). Often invasive animals are generalis t feeders, with a broad diet (Sakai et al 2001). Good invasive species have a high reproductive ra te (Crawley 1986). Any species already intentionally cultivated by humans has an advantage and may escape cultivation and spread. Propagule pressure (t he number of individua ls introduced and the number of independent introdu ctions) is the most important factor for predicting the establishment success of a non-native species (Allendorf and Lundquist 2003), but other factors can be critical as well, such as th e presence of mutualists (Crawley 1986) or the absence of competitors and predators (Mack et al 2000). Certain characteristics of communities are as sociated with a higher susceptibility to invasion. Disturbed comm unities and those in fragment ed habitat may be more susceptible to invasion, and communities with lo w diversity are also believed to be more open to invasion because of a lack of func tional groups leaving open niches (Hobbs and Huenneke 1992, Sakai et al 2001). Lower species diversit y also increase s the likelihood that the invaded community will lack the e quivalent of the intr oduced speciesÂ’ usual predators, parasites, or competitors that normally regulate its population growth (Mack et
3 al. 2000). Islands have more invaders than continents, possibly because of a paucity of native island species leaving niches unoccupied (Elton 1958, Gordon 1998). Recently the Ecological Society of Amer ica (ESA) issued recommendations for the federal government to improve the ma nagement of non-native invasive species. These include focusing prevention efforts on pathways already known to be major sources of non-natives, screening potential li ve imports for invasive potential before allowing entry, and improving monitoring of cu rrently established invasives. The ESA report urges the allocation a nd use of emergency funding for rapid responses to newly established non-native species, when elimination is much easier to achie ve. In situations where eradication is not possi ble, funds should be available to develop and implement Â“slow-the-spreadÂ” strategies (Lodge et al 2006). Currently an invasive grass, cogongrass ( Imperata cylindrica ), is spreading throughout Florida. Cogongrass has been docum ented in 34 of the 67 counties in Florida (Wunderlin 2000), including areas that are occupied by gopher tortoises ( Gopherus polyphemus ). The gopher tortoise is considered a keystone species, critical to upland communities, because of the numerous commensal species its burrows support (Eisenberg 1983). The gopher tortoise is listed as threatened in Mississippi and Louisiana, and west of the Tombigbee and Mobile Rivers in Alabama (USFWS 1992), and is currently a Â“species of special concernÂ” within Florida (Mushinsky et al 2006), although an uplisting to Â“threate nedÂ” status is expected onc e a new management plan is approved. Therefore, the cogongrass inva sion has the potential to harm the gopher tortoise and its associates, and their habitats, both direc tly and indirectly.
4 The cogongrass invasion probably is encro aching on the tortoisesÂ’ habitat and eliminating their food source by out-competing na tive vegetation. Such a direct effect on a native herbivore is unusual. Invasive non-na tive plants in Florida have altered habitat structure, soil erosion, water table depth, a nd nitrogen fixation, and have competed with native plants for light, nutrients, and wa ter (Gordon 1998). Invasive plants have indirectly affected grassland bird densities by changing habi tat structure and reducing the abundance of arthropod prey (Scheiman et al 2003, Dudley and DeLoach 2004), and the development of American toad ( Bufo americanus ) tadpoles was negatively affected by the presence of the invasive we tland plant purple loosestrife ( Lythrum salicaria ) (Brown et al 2006). The cogongrass invasion may be a rare example of a non-native plant directly and negatively impacting a protected native herbivore. Objectives This study evaluates the relationship between an expanding monoculture of cogongrass and a population of gopher tortoises in cen tral Florida. The apparent threat of the invasive grass to this population, and, by extension, to other populations throughout the range of the gopher tortoi se, is assessed by determini ng whether gopher tortoises consume the less-nutritious cogongrass when preferred na tive vegetation is no longer available, determining whether tortoises us e areas of cogongrass w ithin their potential home ranges, and comparing the navigational ability of tortoises traveling within the cogongrass to those traveling in adjace nt, non-invaded habitat.
5 Methods Study Species The gopher tortoise ( Gopherus polyphemus ) is found in open-canopy areas with sandy well-drained soils in the southeastern co astal plain from the extreme south of South Carolina to southeastern Loui siana and peninsular Florid a (Auffenberg and Franz 1982, Ernst et al. 1994). Individuals are long-lived and reach sexual maturity at various ages depending on their location, with size, rather than age, as a better predictor of maturity. Females reproduce at approximately 24 cm carapace length (CL) throughout much of the speciesÂ’ range (McCoy et al. 1995, Mushinsky et al. 1994). The effective rate of reproduction is approximately five hatchlings per mature female per 10 years, assuming annual egg laying (Ernst et al. 1994), with sometimes low hatchling success (Epperson and Heise 2003). The gopher tortoise digs extensive burrows for shelter from the elements and predation (Douglass and Layne 1978, Mushinsky et al 2006), and thus requires deep, well-drained soil for burrow construction. A gophe r tortoise spends a majority of time in a burrow (Mushinsky et al 2006), and most activity is conducted in a home range surrounding the burrow. Sizes of adult fema le home ranges vary from 0.08 to 0.56 ha, while those of adult males vary from 0.45 to 1.27 ha (Diemer 1992, McRae et al 1981, Mushinsky et al. 2006).
6 Grasses (family Poaceae), especially broadleaved grasses, are a large component of a gopher tortoiseÂ’s diet, particularly duri ng the winter when leaf y forbs are unavailable (Garner and Landers 1981, MacDonald and Mushinsky 1988). Forbs, especially legumes, increase in importance as grasses b ecome more fibrous in the summer and fall (Garner and Landers 1981). Mo st water is obtained from fo od, increasing the importance of succulents in the diet (Garner and Lande rs 1981), and sometimes from drinking from the burrow apron when raining (Ashton and Ashton 1991). Humans are the leading cause of adul t mortality in gopher tortoises (Ernst et al. 1994). Since the 1950s the greate st threat to the gopher tortoi se has been habitat loss (Auffenberg and Franz 1982, Mushinsky et al. 2006). High-quality habitat for the tortoises is also prime habitat for housi ng development in Florida, and the human population of Florida has grown rapidly by as much as 23% per year, increasing development pressure and habitat loss (U .S. Census Bureau 2002). Auffenberg and Franz (1982) predicted a 68% decline in gopher tortoise habitat by 2000. The decline occurred even more rapidly, and by th e 1990s only 4.2% and 12.3% of prime gopher tortoise scrub and sandhill habitats, resp ectively, remained in Florida (Mushinsky et al 2006). The encroachment of the cogongrass wi ll compound this problem of rapid habitat loss. As habitat is lost and a gopher tortoise population is compressed into small areas of high population density, the size and age struct ures of the population change. The large reproductive individuals emigrate, limiting the future hatching and recruitment of small individuals in that population. The remaini ng medium-sized individuals become stressed and stop reproducing. Although the population pers ists because of the longevity of the
7 species, the population will ev entually become extinct because of the lack of reproduction and recruitment (McCoy et al 1995, Mushinsky et al 2006). Cogongrass ( Imperata cylindrica ) spreads rapidly and is co nsidered one of the ten worst weeds worldwide (Coile and Shilli ng 1993). Cogongrass is a fast-growing C4 grass, able to survive in hot, dry climates The leaves have rough serrated edges and contain silica bodies, discour aging insect herbivory (C oile and Shilling 1993). Cogongrass is poor forage material for livestoc k, and has fewer nutrients than the native vegetation often ingested by the tort oises (Hubbard 1944, Kearl 1982, Garner and Landers 1981). Cogongrass in its native Asia decreased in nutrient content as it matured (Kearl 1982) and contained 3.7% crude protei n, 0.5% crude fat, 8.7% crude fiber, and 10.8% nitrogen free extract (Hubbard 1944). In comparison, the tortoisesÂ’ typical dietary components of Pityopsis, Galactia and Tephrosia contain much higher amounts: 5.417.2% crude protein, 2.0-4.4% fat, 19.9-35.0% crude fiber, and 46.4-57.5% nitrogen free extract (Garner and Landers 1981). Th e seasonally selected wiregrass ( Aristida beyrichiana ) contains 3.1-8.9% crude protein, 1.1-1.5% fat, 3 5.4-39.4% crude fiber, and 49.3-56.7% nitrogen free extract (G arner and Landers 1981). Cogongrass, which can grow to more than one meter in height, was introduced to Florida and propagated for soil stabilization and forage in 1939; it rapidly expanded to cover 1000 acres in less th an ten years (Shilling et al 1997). Infestations along roads can expand laterally 25 to 40 centimeters per year (Willard et al 1990), and swards in firemanaged sandhill spread up to 2.6 meters per year (Lippincott 1997). Cogongrass exhibits allelopathy (Coile and Shilling 1993) and is highly competitive for water (Shilling et al 1997). The amount of light at gr ound level is signi ficantly less in
8 cogongrass than in native sandhill vegeta tion (Lippincott 1997) indicating that cogongrass competes with native plants for light as well as water. Cogongrass grows rapidly into dense monocultures and elimin ates native vegetation (Coile and Shilling 1993), and is resistant to c ontrol attempts (Shilling et al 1997). Cogongrass loses water content as it mature s, decreasing from 73% moisture at 114 days growth to 39% at maturity (Kearl 1982) In contrast, nativ e grasses contain 74.276.4% moisture at the beginning of the gr owing season (March through May) and 60.369.7% at the end (September to November), maintaining the main water source for the tortoises (Garner and Landers 1981). Dehydr ation may become an issue for the gopher tortoise if cogongrass eliminates the native grasses, because most of their water is obtained from food (Garner and Landers 1981). Like the gopher tortoise, cogongrass pr efers open-canopy areas, but cogongrass alters the sandhill fire regimes that mainta in the open canopy areas (Hobbs and Huenneke 1992, Lippincott 2000). Because the standing mass of cogongrass is so large, the fuel load is very high, increasing fire intensity. The density of the cogongrass, or the fuel packing ratio, is also high, increasing the rate of fuel combustion. The lack of moisture content in mature cogongrass increases the prob ability of a fire and the length of the fire season (Brooks et al 2004). The invasion of cogongrass al so changes the fire type. The height of the cogongrass increases vertical fuel continuity, leading to crown fires instead of the usual surface fires (Brooks et al 2004). Gopher tortoises may be killed directly by more frequent and more intense fires, and the change in fire regime will alter the plant community, lowering diversity and potent ially harming the tortoise population (Lippincott 2000).
9 Study Site My study was conducted at the Teneroc Fi sh Management Area in Polk County, Florida (2806Â’00Â”N, 8151Â’00Â”W). Historically composed of sandhill, pine flatwoods, and scrub habitats, the area was mined for phos phate in the 1950s. Phosphate mining is a strip mining process that severe ly disturbs the soil. The la nd was reclaimed as pasture, and in 1988, 116 gopher tortoises were reloca ted to a 280 hectare section of pasture (Macdonald 1996). Cogongrass was not present at the time of rele ase, but now is encroaching on the site, alrea dy occupying about 50% of the open area as estimated by examining aerial maps (SWFWMD 2004). To characterize the study site further, the vegetation diversity was examined to document the spread of the cogongrass. Th e perimeter of the cogongrass monoculture was mapped in July 2003, August 2004, July 2005, and August 2006 using a Trimble Global Positioning System. These maps of the cogongrass monoculture were used to determine the rate of expansion across the site by using the ruler tool in ArcMap to measure the distance between consecutive year sÂ’ perimeters at five randomly chosen points along each perimeter. The cogongrass expanded each year (Figure 1). From 2003 to 2004, the perimeter advanced a mean distance of 2.48m (SD=1.48), while the perimeter grew a mean distance of 1.24 m (SD=0.25) between 2004 and 2005. From 2005 to 2006, the cogongrass expanded a mean distance of 1.96m (SD=1.05). Above-ground biomass was estimated as a measure of the space cogongrass occupies. Five 10x10 centimeter areas of c ogongrass were clipped at ground level, and the length of the largest blade was measure d. Clippings were dried in a drying oven at approximately 60 C, with periodic weighing until the mass for each clipping was
10 constant. The clippings averaged 1.10m (S D=0.095) in length and had a mean dry mass of 26.42g (SD=8.9). The above-ground biomass of the cogongrass was estimated to be 26420 kg/ha based on the mean mass of the 10x10cm clippings, which is over twice the biomass of a wheat field (Thornton et al 2006). The cogongrass occupies space horizontally and vertically, crowdi ng and shading out other plants. Cogongrass is reported to exclude other ve getation (Coile and Shilling 1993). To determine if vegetation diversity was lower in invaded areas of the site, I compared vegetation composition between locations without cogongrass and locations with cogongrass. A randomly thrown 1x1 meter quadr at was used to estimate the percent cover of each species of plan t, using a field guide by Tayl or (1992) for identification purposes. Dead herbaceous vegetation, bare ground, and unidentified but distinct grass species categories were also used. Six quadrat s contained cogongrass, and fourteen did not, and were considered cogongrass-absent quadrats. Percent cover values were summed for each species across all quadrats with in each of the two groups. An expected distribution of species in a single quadrat was calculated by averaging the percent cover of each species in the cogongrass-absent quadrats and bootstrapping 95% confidence intervals with 10000 iterations. Confidence intervals for diversity indices were bootstrapped with 10000 repeats, and th e Shannon, Simpson, and HillÂ’s N1 and N2 diversity indices were calcul ated, as well as HillÂ’s modified evenness index, for both the cogongrass-absent group and the cogongrass-pr esent group. Twenty-two species of plants were found in the vegetation quadrats. Twelve were identified to species, three were unidentified but distinct forb species, and seven were un identified but distinct grass species. Species richness ranged from two to seven, with a median of four species per
11 quadrat. Bare ground had the hi ghest percent cover in the co gongrass-absent quadrats. As expected, observed percent cover values fe ll within the 95% confidence intervals for the cogongrass-absent quadrats, and were mu ch lower in cogongrass-present quadrats, falling outside of the confidence interval. Pl ant diversity and evenness decreased in the presence of cogongrass, beyond the 95% confidence interval (Table 1). Feeding To determine if the gopher tortoise will consume cogongrass, I conducted a feeding experiment in the field. I expected the tortoises to eat native vegetation readily, including wiregrass, a less-preferred native grass (Gar ner and Landers 1981, MacDonald and Mushinsky 1988, Mushinsky et al 2003), and not to eat cogongrass at all. I conducted preliminary experiments at the Ecolog ical Research Area of the University of South Florida. Individuals were captured in bucket traps placed outside of active burrows (Auffenberg and Franz 1982), and were al so opportunistically hand-caught. Once captured, tortoises were placed in a 3x1.5m encl osure constructed of aluminum flashing. The enclosure contained either natural vege tation (including wiregrass, goldenaster [ Pityopsis graminifolia ], bahia grass [ Paspalum notatum ], and blazing star [ Liatris spp. ]) or only wiregrasss. Wiregrass is ingested by gopher tortoises when it is the dominant vegetation and more preferred f oods such as golden aster and Liatris spp. are unavailable (MacDonald and Mushinsky 1988). Five of the wiregrass feeding experiments were conducted during the winter (November-Februar y) when tortoises are more likely to positively select wiregrass as a food source (Garner and Landers 1981). The tortoises exposed to native vegetation served as a cont rol, determining how long it would take for a tortoise to eat in captivit y, and those exposed to wiregr ass only served as another
12 control, determining how long it would take for a tortoise to eat a less preferred food item. At Teneroc, captured tortoises were placed in a 4x4m holding pen with natural vegetation (including wiregrass camphorweed ( Heterotheca subaxillaris ), Liatris spp. and Galactia spp .), or only the invasive cogongrass which was transplanted into the pen in pots to avoid contamination and count er-sunk into the grou nd to mimic natural occurrence. I observed each to rtoise until it began to eat, and noted the time. Some feeding trials were terminated before a ny vegetation was consumed because of time constraints, the approach of inclement weathe r, sunset, or signs of heat stress in the tortoise. Feeding rate was calculated as the reciprocal of the time to first bite. If no vegetation was consumed, time was infinity an d the feeding rate was zero. The feeding rate was graphed for each of the three types of vegetation presented to the tortoises in a box and whisker plot in SigmaStat. Because the data were not normally distributed, a nonparametric Kruskal-Wallis Test was perfor med and pairwise comparisons were made using a Mann Whitney U Test to determine if there was a difference in selection among the three vegetation types. Habitat To determine whether or not the gopher to rtoises used the cogongrass as habitat, individuals were powder-tracked. I expected that the tortoi ses would have significantly smaller proportions of trail oc curring within cogongrass comp ared to outside cogongrass. After trapping a gopher tortoise, a mesh ba g filled with non-toxic fluorescent powder (Radiant Color) was temporar ily attached with duct tape to the posterior carapace
13 (Blankenship et al 1998), as was a cocoon thread bobbin (Wilson 1994). I returned one to two days after release to follow the trails of powder and thread. Trails were flagged and then mapped with a Trimble GPS. Th e experimental group consisted of tortoises caught at burrows within 50 me ters of the cogongrass, so that the cogongrass could be considered to be within each individualÂ’s possible home range. A 50 meter radius gives an area of 0.785ha, which falls within the va lues of home range size for adult male gopher tortoises and is slightly larger than a female tortoise Â’s home range (see above; Mushinsky et al 2006). As a control, this experime nt was repeated with tortoises whose burrows were located over 50 meters from the cogongrass. The home ranges of these tortoises most likely do not overlap the cogongrass. To determine if either the control or the experimental groups exhibited directional movement, angle of direction from point of re lease (at the home burrow) to the end of the trail was measured with a prot ractor on maps of the mapped trails printed from ArcMap and the Rayleigh Test for uniformity was performed using Orianna software. The Watson-William Test was performed by hand following Zar (1999) to determine if there was a difference between the two groupsÂ’ mean angles of movement. The original analysis would have measured trail lengt h within each type of habitat (cogongrasspresent vs. cogongrass-absent) relative to habi tat availability, calculated as the percent area covered by the habitat type within a 50m radius of the tortoiseÂ’s home burrow. These data would be used to determine propor tional habitat use with SmithÂ’s measure of niche breadth (Krebs 1989), but control gr oup tortoises captured 50 meters or farther from the grass mostly had east-west paths, in troducing a directional bias that precluded the intended analysis of habitat use.
14 Orientation To determine if the presence of cogongra ss interferes with the gopher tortoisesÂ’ navigational abilities, an orie ntation experiment was perfor med. Low sinuosity (index of straightness: ratio of distance true traveled to straight line displacement distance; Emlen 1969, Connor 1996), a high rate of movement, a nd directional movement are indicators of orientation. I expected th e tortoises to become disorien ted in the cogongrass and have significantly higher sinuosity values and slower, non-direct ed movement. A captured tortoise was displaced 30 to 50 meters from its burrow and its speed and directness of travel to another burrow were observed. Tort oise paths were flagged and mapped using a Trimble GPS and analyzed in ArcMap. Multiple readings on each individual at different angles of displacement were attempte d in order to rule out any directional bias, although some individuals (n=7) escaped into burrows before a second displacement could occur. Three of these tortoises were displaced once into habitat without cogongrass, while four were displaced once into cogongrass. Some individuals were displaced twice into habita t without cogongrass (n=9), one was displaced twice into cogongrass, and some were displaced once in to habitat without cogongrass and once into the cogongrass monoculture (n=5), for a total of 22 individuals and 37 tr ials. The type of habitat a tortoise was released into de pended on its home burrowÂ’s proximity to the cogongrass. If the home burrow was within 50 meters of the cogongra ss, the tortoise was displaced into cogongrass. If the home burrow was more than 50 meters away from the cogongrass perimeter, the tortoise was displaced into non-invaded habitat. The travel time in minutes was observed a nd recorded for each individual, and the true distance traveled was recorded with GPS as the paths were mapped. Straight-line
15 distance traveled and the di stance from the cogongrass of the point of release was calculated in ArcMap with the ruler tool. Sinuosity of moving to a burrow was calculated, as well as rate of movement (true distance divided by time), rate of displacement (straight distance divided by tim e), and sinuosity per unit time (index of straightness divided by time). Regressions of straight-line distance tr aveled onto distance from cogongrass and of actual distance traveled onto distance fr om cogongrass were performed using SPSS to determine if a significant relationship existe d. The two slopes were compared using a t Test (Zar 1999) to determine if the latter slope was significantly larger than the former, which would indicate that tortoises placed in the cogongrass had to move farther to achieve the same displacement. One-way ANOVAs were used to compare rate of movement, rate of displacement, sinuosity, and sinuosity per unit time between the group of tortoises displaced within the cog ongrass and the group displaced beyond the cogongrass to determine if the former had a more sinuous path, indi cating disorientation. For sinuosity per unit time, results were also calculated with a data set created by randomly discarding one trial of individuals who were displaced during both trials outside of the cogongrass to avoid pseudoreplication. To examine the movement patterns of th e tortoises, angles of movement from point of release to the chosen burrow were cal culated using printed maps of the paths and a protractor, with the card inal direction north as 0 In two cases, tortoises had not found a burrow after 45 minutes of movement, and th e orientation experiment was terminated. In these cases, the point of recapture was used as the endpoint of the path. The Rayleigh Test for uniformity and the V Test (expected =0 ) were performed to see if movement
16 outside of the cogongrass was directional, while movement within the cogongrass was uniform. If the distribution was bimodal, th e Hodges-Ajne Test was substituted for the Rayleigh Test. Angles of movement were al so calculated placing the home burrow at 0 and the Rayleigh Test for uniformity and the V Test (expected =0 ) were performed to determine if tortoises placed outside of the cogongrass tended to move towards their home burrows and tortoises placed within the cogongrass did not have a preferred direction. To examine the effect of the presence of cogongrass on the tortoisesÂ’ ability to navigate using geotactic cues, slopes of the terrain were marked with a GPS trail labeled sloping (uphill or downhill) or level, and these trails were added to the map of tortoise trails in ArcMap. The slope of each tortoise path was determined, as well as the slope from the point of release to the home burro w. The number of tortoises moving in the same vertical direction (uphill, downhill, or level) as their burrows was compared to the number of tortoises moving in a different ve rtical direction as their burrows for the within-cogongrass group and the beyond-cogon grass group using a 2x2 contingency table and a G Test (Zar 1999). The sample sizes we re too small for sufficient statistical power using the G Test, so a Fisher Exact Test was performed on the contingency table using SigmaStat software to determine if the pr oportions of individual s in each cell were different from those expected from random occurrence.
17 Results Feeding Seven of the eight tortoises exposed to native vegetation began to eat within 45 minutes. Sabatia sp. Galactia sp. and unidentified grasses we re consumed. No feeding was observed in one individual in the nativ e vegetation pen and the experiment was ended after 57 minutes because of signs of h eat stress in the tortoise. In all eight individuals offered cogongrass, a nd ten of the eleven individu als exposed to wiregrass, no feeding was observed, even during the winter months. One tortoise offered wiregrass began to eat in 62 minutes. The Kruskal-Wallis Test indicated a significant difference in feeding rates among the three vegetation types ( 2=20.1, p<0.001, n=28). The MannWhitney U Test for cogongrass vs. wiregras s was not significan t (U=40, p=0.778, n=19), but the feeding rates for natu ral vegetation were significan tly smaller than those of wiregrass (U=6, p<0.001, n=20) and cogongra ss (U=4, p=0.001, n=17), indicating that gopher tortoises will readily eat preferred vegetation in captivity, are reluctant to consume wiregrass but will consume it under certain circumstances and will not eat cogongrass (Figure 2). Habitat The majority of trails led from the point of release to another burrow. Four of the eighteen individualsÂ’ trails we re truncated because of broke n thread and indiscernible powder trails. None of the tortoisesÂ’ pow der and thread trails entered the cogongrass
18 monoculture. The mean direction of m ovement for tortoises with home ranges overlapping the cogongrass monoculture was =221 (SD=93, n=8), and for tortoises whose home ranges were entirely ou tside of the cogongrass monoculture, =53 (SD=78, n=10) (Figure 3). Although the Rayleigh Test showed that neither group exhibited significant directionality (z=0.564, p=0.584, n=8, and z=1.557, p=0.215, n= 10, respectively), the Watson-William Test reveal ed that the two means were significantly different from each other (F(1), 1, 16=8.476, p<0.02, n=18), indicating that tortoises using habitat adjacent to the cogongrass move towa rd the southwest, aw ay from the cogongrass (see Figure 1), and tortoises occupying a home range entirely beyond the cogongrass move northeast, or toward the cogongrass. The latter individuals may move northeast because they are bounded by the dirt road that runs along the south edge of the site. Orientation Only two tortoises returned to the burrow at which they were captured during the orientation experiments. The others entered different burrows usually within 30 minutes. Two tortoises placed inside of the cogongr ass walked for 73 and 61 minutes without finding a burrow, and were returned to their home burrows. Many tortoises placed inside the cogongrass found worn paths through the gr ass and followed them to a burrow, and many walked in circles or back and forth ove r the same path. Individuals often paused for up to five minutes. True distance traveled and straight-line distance traveled were not significantly related to the distance of the initial release point from the cogongrass (r2=0.074, p=0.103, and r2=0.041, p=0.229, respectively, n=36) (Figur e 4). The results of the t Test comparing the slopes of the two re gressions were not significant (t(2),70=0.349, p>0.5,
19 n=36), indicating that tortoi ses displaced within the c ogongrass do not move a greater distance to achieve the same displacement. The rate of displacement (straight-distance traveled/time) was not significantly different between tortoises displaced outside of the cogongrass ( x =0.599, SD=0.700, n=26) and tortoises displaced into the cogongrass ( x =0.992, SD=0.660, n=26; F=2.523, p=0.121, n=37). The rate of movement (true distance traveled/time) was significan tly different (F=12.472, p=0.001, n=37), with tortoises moving faster inside the cogongrass ( x =0.488, SD=0.246, n=11) than outside ( x =0.259, SD=0.146, n=26). Mean sinuosity was not significantly different between tortoises placed outside of the cogongrass ( x =2.06, SD=1.235, n=26) than those placed inside the cogongrass ( x =1.97, SD=0.487, n=11; F=0.049, p=0.826, n=37). Sinuosity per unit time was not significantly different between tortoises displaced inside the cogongrass ( x =0.139, SD=0.107, n=11) and tortoises displaced outside the cogongrass ( x =0.424, SD=0.691, n=11; F= 1.828, p=0.185, n=37). Significant differences were found when the tortoises disp laced outside of the cogongr ass were randomized (F=4.690, p=0.04, n=28), with a higher sinuosity per unit time outside of the cogongrass ( x =0.261, SD=0.165, n=17) than inside ( x =0.139, SD=0.107, n=11), indicati ng that tortoises were moving as efficiently if not mo re so inside the cogongrass. The angles of movement of tortoises pl aced inside the cogongrass were bimodal, and randomly distributed according to the H odges-Ajne Test (m=3, p=0.81, n=11) with a mean angle of 23 (angular deviation=0.59, n=11). Tortoises placed outside the cogongrass exhibited directional movement when the entire data set was used (z=3.459, p=0.03, n=26), with a mean vector of 38 (S D=81, n=26), which is not significantly different from 0, according to the V Test (u=2.066, p=0.019, n=26), but when the data
20 were randomized to avoid pseudoreplicati on, the results were no longer significant (z=1.233, p=0.296; u=1.194, p=0.118, n=17) (Figure 5). The study site has variable topography. In general, th e terrain slopes uphill from south to north, and begins to level off insi de the cogongrass monocultu re at the north end of the site. From the west to east direction, the terrain consists of a central hill with two valleys on either side. This variation in elevation may have an effect on how the gopher tortoises navigate. Eighteen of the twenty-two tortoises displaced outside of the cogongrass moved along a path with the same vertical direction (uphill, downhill, or level) as the path that woul d take them to their home burrow. Only six of thirteen tortoises displaced within or at the edge (less than one meter outside) of the cogongrass moved along a path with the same vertical di rection as the return path to their home burrow. The results of the Fisher Exact Test on the contingency table were marginally significant (p=0.057, n=35), indicating that tort oises displaced into the cogongrass were equally likely to move uphill or downhill, rega rdless of which direction their burrow was, while the tortoises displaced into natural habitat were more likely than expected by chance alone to take a path with the same slope as the path that would return them to their burrow.
21 Discussion The results indicate that the gopher to rtoises do not consume cogongrass and do not use the cogongrass monoculture as habita t. IndividualÂ’s movement patterns were affected by their proximity to the cogongrass, and individualÂ’s orientation abilities were affected by the presence of cogongrass. Although most native vegetation was cons umed readily and the native wiregrass sparingly, cogongrass was not consumed at all. Observations of tortoises consuming wiregrass, as well as the presence of wiregr ass in scats, have been reported in the literature (Garner and Landers 1981, M acDonald and Mushinsky 1988, Mushinsky et al. 2003). As cogongrass decreases or eliminates plant diversity, it eliminates the tortoisesÂ’ food sources, leaving them with a poor fora ge material, low in nutrients and water content. The gopher tortoises present at the stu dy site appear to avoid the cogongrass monoculture, rarely entering and then only al ong well-worn paths, some of which were created by human vehicles. Lippincott (1997) al so noted active tortoise burrows inside a monoculture of cogongrass, connected to ad jacent sandhill by narrow, trampled paths, usually less than ten meters in length. The tortoisesÂ’ movement pa tterns change when their home range includes the cogongrass. Tort oises with home ranges within 50m of the edge of the cogongrass had a mean angle of movement of 221, which corresponds to the cardinal direction southwest, whereas tort oises with home ranges beyond the cogongrass
22 had a mean angle of movement of 53, or no rtheast. The cogongrass monoculture covers the northeast part of the field site, so tortoises with home ranges overlapping the cogongrass monoculture tend to move away fr om the monoculture, while other tortoises tend to move toward the monoculture. C onnorÂ’s (1996) results suggested that gopher tortoises avoid areas that di ffer greatly from preferred open-canopy habitat, such as overgrown fire-suppressed plots. Gopher tort oises at Teneroc may also avoid the dense cogongrass because it eliminates their normal view of the horizon and landmarks, making navigation difficult. Although the tortoises moved faster thr ough the cogongrass when placed in it, the sinuosity of path was not significantly differe nt. In fact, a tort oiseÂ’s path was less sinuous if it was within the cogongrass, which may be because of a lack of obstacles to move around. In habitat without cogongrass, tortoises move around bushes and forbs that are in their path. Inside the cogongrass, th ere is little habitat heterogeneity, and the tortoises tended to walk along any ex isting pathways they encountered. The increase in directness of path wi thin the cogongrass may be explained by a lack of landmarks. Studies suggest that gopher tortoises orient themselves by using visual landmarks and, to a lesser extent, a sun comp ass (Gourley 1974, Connor 1996). Cogongrass is very dense and uniform relati ve to native sandhill vegetation, and may obscure the tortoisesÂ’ usual landmarks while blocking any new visual cues. While inside the tall cogongrass, tortoises cannot view the horizon or the sky. Without any cues to cause them to readjust their direction, the tortoises will continue to walk in the same direction in which they star ted, resulting in a low sinuosit y of path. Gourley (1974) found that individuals lacking visual cues conti nued to orient in a pa rticular direction, but
23 not toward their home range. Connor (1996) found that displaced tortoises would not move into overgrown, unburned plots and in stead began walking along the perimeter of the plots, resulting in low sinuosity of path. The results indicate that gopher tortoises us e geotaxis, as well as visual cues, to navigate. Geotaxis, or grav ity orientation, is a response to gravitational cues (Jander 1963). Individuals can right themselves wh en upside down, and can sense an uphill or downhill slope. Movement uphill is negativ e geotaxis, and movement downhill is positive geotaxis (Murphy 1970). If an individu al tortoise moves downhill as it leaves its burrow to forage, it must move uphill to retu rn to its burrow. Other species of turtles have exhibited geotaxis: the painted turtle (Chrysemys picta), spiny softshell (Apalone spinifer), and the easte rn box turtle (Terrapene carolina) (DeRosa and Taylor 1982), and the wood turtle (Glyptemys insculpta) (Tuttle and Carroll 2005). The habitat use data showed that many paths taken by tortoises were along the east-west axis. The ground slopes down from the central part of the site to the east and to th e west, which supports the idea that gopher tortoises follow geotactic cues to navigate while outside of their burrows. The gopher tortoises that were re leased into native vegetation exhibited a tendency to orient along the same slope as the path that would return them to their home burrows. This tendency was not apparent in individuals released into the cogongrass, indicating that the tortoisesÂ’ ability to navigate using geot actic cues is affected by the invasive grass. The tortoises may misinterpr et the geotactic cues, possibly because of a difference in topography where the cogongrass ha s invaded. Another possibility why the gopher tortoises were unable to orient them selves properly inside the cogongrass is because the cogongrass obscures the view of the sky and the horizon, eliminating cues
24 from the sun compass as well as visual landmark cues. The exact relationship between the geotactic and visual cues used by the gopher tortoise is unknown, and warrants further research. The cogongrass has the potential to have a negative influence on the gopher tortoise population beyond the ways investigated in this study. Gopher tortoises are dependent on fire to maintain their upland habitats as open-canopy. Cogongrass maintains a large standing biomass, altering sandhill fire regimes by increasing fine-fuel load and increasing both maximum temperatur e and height of fire (Lippincott 2000). Consequences of this change in disturbance regime include direct mortality of the tortoises, and a decrease in plant divers ity, reducing the amount of forage material available to the tortoises. Changes in the fr equency and intensity of natural disturbances, such as fire, can facilitate othe r invasions (Hobbs and Huenneke 1992). A congener of the gopher tortoi se, the desert tortoise (Gopherus agassizii), also is threatened by invasive grasse s, particularly the bunchgrass Bromus rubens (Brooks et al. 2004). In the Mojave Desert non-native plan ts compete with nativ e annuals, which can be 95% of the desert tortoise Â’s diet, lowering diversity and biomass, thereby decreasing the availability of forage material (Brooks and Berry 2006). The bunchgrass also threatens to alter the fire regime in the Mo jave Desert, which will have a negative effect on the native plants and animals of the region (Brooks et al. 2004). As the area of their preferred scrub and sandhill habitat rapidly declines, gopher tortoises are being forced to occupy the suboptimal surrounding habitat that is being invaded by the cogongrass; the gopher tortoise in Florida will face a new threat in its natural habitat. Human disturbance has led to much relocation of gopher tortoises
25 throughout Florida. Over 5000 relocation permits have been issued by the Florida Fish and Wildlife Conservation Co mmission, and often suitable av ailable relocation sites are lacking (Holder et al. 2007). Concerns about the spread of upper respiratory tract disease (URTD) and other diseases complicate the relocation process (Mushinsky et al. 2006). The possibility of losing local genetic vari ation and outbreeding de pression is also a concern (Schwartz and Karl 2005); tortoises ma y only be relocated to sites less than 50 kilometers north or south of their native site (Holder et al. 2007). Often the tortoises are relocated to fragmented and/ or disturbed sites such as reclaimed phosphate-mined land. Phosphate mining has disturbed more than 1180 square kilometers of FloridaÂ’s 138000 square kilometers of land area (FIPR 2004), much of it in central Florida. These fragmented and disturbed habitats are more susceptible to invasion (Hobbs and Huenneke 1992, Mack et al. 2000), leaving many gopher tortoi se populations vulnerable to cogongrass as well as other invasi ve species which may prove harm ful. Invasive fire ants (Solenopsis invicta) in southern Mississippi caused 27% of gopher tortoise hatchling mortality (Epperson and Heise 2003) and have been documented to kill 70% of Florida red-bellied cooter (Pseudemys nelsoni) hatchlings during pipping or shortly after hatching (Allen et al. 2001). In fiscal year 2005-2006, fede ral, state, and local governments expended over nine million USD to control invasive species in Florida (DEP 2006). Invasive species are altering communiti es and ecosystems throughout the world. Like the desert tortoise in the Mojave Desert, the gopher to rtoise is threatened by an invasive grass. The grass is eliminating th e gopher tortoisesÂ’ hab itat and food source, and is disrupting the tortoisesÂ’ ability to naviga te back to the shelter of their burrows. Cogongrass is present in much of central Florida (Wunderlin 2000) especially on the
26 disturbed areas of phosphate mines. Cont rolling the spread of the cogongrass is necessary to prevent the elim ination of this population of relocated gopher tortoises and most likely many other populations throughout the speciesÂ’ range.
27 Table 1: Diversity indices calculated from quadrat vegetation sampling. Cogongrass-absent quadrats Bootstrapped 95% confidence interval Cogongrass-present quadrats ShannonÂ’s HÂ’ 2.38 2.30-2.42 0.92 HillÂ’s N1 10.76 10.00-11.26 2.50 HillÂ’s N2 8.40 7.64-9.05 1.65 SimpsonÂ’s 10.88 0.87-0.89 0.40 HillÂ’s modified evenness 0.76 0.73-0.79 0.44
28 Figure 1: Map of study site with cogongr ass perimeters delineated. The cogongrass extends from the perimeters to the north and east. --2003 --2004 --2005 --2006
29 Figure 2: Reciprocal rates of fe eding on three types of vegetation. Vegetation Type NaturalWiregrassCogongrass Feeding rate (bites/minute) 0.0 0.2 0.4 0.6 0.8 1.0 1.2
30 Figure 3: Angles of movement and mean v ectors of tortoise paths. (A) TortoiseÂ’s home range includes the cogongrass. (B) To rtoiseÂ’s home range is entirely outside of the cogongrass. North=0. A) B) 0 90 180 270 0 90 180 270
31 Figure 4: Regressions of di stance traveled onto distance from the cogongrass. A) Regression of true distance traveled onto distance of point of release from the cogongrass monoculture. B) Regression of straight-line dist ance traveled onto distance of point of release from the cogongrass monoculture. A) B)
32 Figure 5: Angles of movement and mean vectors of displaced tortoises. A) Tortoises displaced into the cogongrass. B) Tortoises displaced outside or at the edge of the cogongrass. North=0. A) B) 0 90 180 270 0 90 180 270
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