STATUS OF BULLFROGS AND
NORTHERN LEOPARD FROGS AT FISU SPRINGS NATLONAL
WILDLU'E REFUGE, JUAB COUNTY, UTAR
1 2 2 2 Matthew n. McKell ) , Scott Peterson , Keiko Kobayashi , Ryoko Miyazato , Donovan
2 2 3 Sherratt , Michael P. Oonov-dil\and Ten:y D. Schwaner •
Running head: 'BULLFROGS AND LEOPARD FROGS AT FISH SPRlNGS
1Prcsent nddrc:;:;: Department ofZoo logy, Brigham Young University, Provo, Uf 84602.
2Dcpartment of Biology, Southern Utah University, Cedar City, ln' 84720 USA; telephone 435-586-7929, fax 435-865-8065.
3 Author to whom requests lor reprints should be sent. AnSTRACT.- Rutlfrogs (Rana catesbeiana) introduced into the western United
States prey on other frogs, apparently, sometimes leading to the extinction of native amphibians. This hypothesis is confounded by alteroativcs, including differences in habitat disturbance, other potential predators, and competitors, as well as the dl~cts of ultraviolet radiation, parasites and pollutants. In most cases, a test requires situations where variables can be controlled. ln tllis paper we describe aspects of the ecology and natural history of bullfrogs introduced to Fisll Springs National Wildlite Refuge, a IIU!jor habitat for migratory waterfowl, and their apparent efl:ccts on putative native populations of leopard frogs also found in the area. Generally, we Jow1d size-age structure, growth, heba>~.or and reproduction of bullfrogs and leopard frogs to be similar to other populations of these species. However, both species have restricted patterns of distribution, abundance and diet that could be rdated to habitat differences and/or to predation by bullfrogs on leopard frogs. Because buUfrogs are non-native and non essential to the ecology ofthe refuge, their systematic removal with continued monitoring ofleopard frog distributions and ahundance, could provide a controlied test oftbe predation hypothesis.
Key words: Rona catesheiana, R. pipiens, distribution, abundance, size, growth, age, ske/ctochronology, diet j
The bullfrog, Rana C/1tesbeiana, is not native west of the Rocky Mountains, but has been successlully introduced into every state in that region (Stebbins 1985). nullfrogs were introduced to Fish Springs National Wildliti.~ Refuge (FSNWR), Juab
County, Utah, in 1954, prior to the area being designated a federal wildlife refuge, to supply frog legs to local restaurants in nearby Salt Lake City (J. Banta pers. comrn.).
Fish Springs is located within the natur
!'SNWR (Tiovingh 1997). General ecological, life history and behavioral studies
(summarized in Bury and Whelan 1984, and Clarkson and DeVos 1986) indicated negative interspecific effects or bullfrogs on other ran.id frogs, including tadpole crowding that affected growth ofboth bullfrogs and other anurans (Licht 1967) and adult territoriality, possibly responsible for declines ofnativ e ranid populations in California
(Moyle 1973). Hrurummon (1982a) found lower abundances of leopard frogs in at<:ll~ of
Colorado inhabitw by buUfrogs. However, JcrUJings and Hays (1984) and Hayes and
Jennings ( 1986) cited the lack of experimcntaJ evidence and suggested causal factors other than bullfrog introductions for these d<:elines. Su~quently, Hecnar and
M'Closkcy (1996) documented increased abundances and distrib.utions ofnative ranids following the apparent extinction of bullfrogs from a national park in Canada. Similar studies 1bund reduced populations of red-legged frogs (Rona aurora) and ye11ow-legged frogs (R. boylii) in areas where bullirogs had been introduced (Kiesecker and Rlaustcilt
1997, 1998; Kupferberg 1997). Our study adds to tltis controversy by describing the distnbutions, abundances, sW:-age structure, gTO\vth, diet, and hahil~ of bullfrogs and
Jeopard frogs at FSNWR. The descriptive results suggest a test ofth e null hypothesis of 5
Each captured frog was sexed, mta:;ured for snout to vent length (SVL in mm), weighed (WT in g), and toe-clipped with a unique mark (after Ferner 1979). We used the
Peterson index (Canghley 1977) to ~tiroate population si7.es of bullfrogs in three transect~. Toe bones were prt:lit!rved in I 0 percent formalin cleaned of ~kin, muscle and tendons, de-minernlizcd overnight in 3 percent nitric acid, sectioned with a freeze microtome, stained with Ehrlich' ~ hematoxylin, and viewed a.nd photographed with a light microscope. We asswned lines of arrested growth (LAGs) formed during ~easonal inactivity and separating ~~a:;onal growth areas represented years since metamorphosis
(F.stcoo.u ct al. 1996). We computed growth rote~ as SVLt2-SVLtdt2-ts, where t, = time of initial capture and t2 =time in days for frogs recaptured 20 or more da~ since initial capture (Andrews 1982). We flushed stomachs with water from a 50cc syringe fitted with a plastic tube. Stomach contents were preserved in 10 percent formalin and later tran~l~m:d to alcohol for analysis. Stomneh items were sort~d into groups, identified,
:md measured for volume by water displacement in a graduated cylinder. Occurrence of a prey category was the number ofstomach:~ in which the item occurred divided by the total number of stomachs examined. Volume percentages were the total volume of a specific item divided by the total voiUlll\: for all samples.
Statistical comparisons followed Sokal and Rohlf(\981). We uscu tltt: Kruskal
Wallace test and Fcidman's method ofrandomized blocks to examine the abundances of fTogs in five transects over lour :sainpling periods. Spennnnn rank correlation tested the n.ssocwtio11 ofre lative abundances of bullfrogs and leopnrd frogs in the five transects.
Chi-square tested the assumption of even distributions of frogs in Transects 4 and 5, and the null model!: I sex ratio for adult~. Analysis of cov<.tri;mc.;e (ANCOV/1.) compared the 6 functional relationship of length and weight in adult males and ti:ma!t:s and linear regression determined the amount of variation in growth rate explained by body length.
RESULTS
Distnbutions
In February and March, we observed bullfrogs in springs directly connected to the main channel by permanent water flows (i.e., those m~ar transects 1 and 2, including
House.Spring), and in Waller Spring connected to the main channel by a long canal system with reduced or intcrmillcnt water during late summer. We saw larval (>80 mm total length), juvenile
Curle:;w, Shoveler, Egret, Ibis, Gadwall, Pintail and Harrison pools, or in roadside ponds with water temperatures below I o•c.
From June to August, male bullfrogs called from springs, canals coru1ecting springs to the main channel, pools and roadside ponds adjacent to the main chrumel. We obseJ:Ved clasping pairs and egg masses only itl the main channel and canals connecting springs and adjacent pooL~ along Transects 1-4. Bullfrogs were absent from North
Spd.ng, Deadmatl Spring, and the northernmost pools (Harrison, Pintail, Gadwall Ibis), at all times during the study. Leopard frogs became active in March-April. We observed them along the maio ch.am1el and in dense vegetation at the edge ofc Abundances We counted a total of 77 1 bull !Togs and 165 leopard frogs in systematic surveys along tr.msects, monthly from April to July. For both species, the relative numbers of frogs (adjusted to observations per one kilometer t.ransect, Tabk I) did not differ among sampling dates (Kruskal-Wallis tests: bullfi:ogs, H - 1.58, df- 3, P = 0.66; leopard frogs, II = 4. 73, df .. 3, P- 0.19); among transects, however, we found highly significant differences in relnt.ive nbundauces of both species (Friedman's method for randomized blocks: bullfrogs, x2 = 11.8, df= 4, p < 0.025; leopard 6·ogs, x_2 = 23.5, df- 4, I' < 0.001). Total counts of bullfrogs and kopard !Togs per transect (Fig. 2), adjusted to observations pc::r km, show a significant negative relationship (Spearman rank correlation = -1.00, p < 0.001). In Transects I and 2, we marked 82 bull !Togs between April and July, and r~:eaptured only two frogs; however, eighteen of82 adult buUfrogs (SVL > 110nun) captured between August and October were recaptures. In Transect 5, we marked 15 bullfrogs and recaptured none between April and July; however, seven of 11 adults captured between August and October were recaprurcs. Applying a simple Petersen estimate (Caughlcy 1977) to these data gave population estimates and 95 percent confidence limits of358 ± 138 bullfTog.~ fur Transects 1 and 2, and 23 ± 81in Tr.msect5 (a difference in average densitic~ of 179 vs.l2 frogslkm, respectively). Although we marked 68 leopard frogs between April and July (51 !Tom Transect 5), and captured 52 (SVL > 60 mm SVL) between August and October (33 in Transect 5), only 3 were n:o.:aptured, all in Transect 5. These data do not give accurate density results (95% wnfidence limits exceed the population estimate by a fuctor of2, for Transect 5) due to the small number.; ofrecaptures. We arbitrarily designated bullfrogs as smaU, mcditun or large, in our April 1999, direct count, survey of the five transects. Later, when bullfrogs were caught and measured, small frogs were fuund to be juveniles in their second growing sea~on, post metamorphosis; medium and large bullfrogs represented adult~ ofvarious ages. Small bullfrogs C{)mprised 18 percent of totals in TrrulSccts 1-4, but were abt;t:nt in Transect 5. Frequency of medimn bullfrogs declined from 60% to 17 %and large individuals increa~ed fi:om22% to lG %, directionally, from Transects 1-5 (Fig. 3A). Only one medium and two large leopard frogs were found in Tr~cts 1-2; all three size classes were represented in Transects 3-5, each with about equal proportions of the total number observed (rig. 3B). ReproductioJJBody Size/Sexual Size Dimorphism We observed large bullfrog tadpoles near the springs during February-May. However, very small bullfrogs (<50 mm), believed to be post-metamorphic juveniles of the year, were not observed Wltil JWJe. Adult males vocali7.ed during .Tune-August, und we observed ·five cla~ping pairs in Transects I and 2 in June-July; during this srune time, sheet-like ma~s of gelatinous material with embedded eggs were found attached below the water to emergent (usually dead) vegetation. Bimonthly size-frequency distributions ofall captured bullfrogs (Fig. 4) do not give any clear indication of age cohorts, except tor the abseJJce ofyoung of the year in April-May (the smallest frog observed was 75 mm SVL) and their presence in June-October (i.e., fi·ogs 34-75 mm SVL). We captured adult males and females in about equal numbers during each sampling period (2 x 3 contingency table, -l = 1.8, df- 2, P - 0.40). Forty-nine percent ofl78 captured adults were males and 51% were females, accepting the null hypothesis of a I: I sex mtio (x2 ~ 0.01, df-1, P= 0.91). 9 Puhli~hed reports for bullfrog~ in northern and mid-western populations indicate Lhdr maturity at 85-125 nUD SVL (Bm·y and Whelan 1985). Tn our survey, the smallest mature male bullfrog (identified by swollen 4111 and s•• fingers and a ycllo~h green throat) wa~ 110 nun SVL; the smallest gravid female WAS 120 mm SVL. Assuming both sexes in the F'ish Springs population mature at II 0 mm S VL, average body ~iGe was 145. 1 :1: 1!>.7 nun SVL (mean ~ s, range = II 0-195, n = 88) for mature males and 138.8 :1: 22.11 mm SYL (mean± s, nmge = I I 0-195, 11 = 67) for mature temales (ncith~.:r gravid nor 5pent). Adult trrnlt: bu!Urogs are slightly heavier than adult (non-gravid, non-:;p~.:nl) females (ANCOVA: male mean± s; 233 ± 92.9 g, female mean± s ; 226 ± 116.3 g; slopes, <.If ~ 1,151, F; 3.52, P > 0.05; means, df= I, 152, F= 16.06, P < 0.001; Fig. 5). Gravid female bullfrogs were heavier than spent females with similar SVL (ANCOVA: gr.tvid mean :1: s, SVL = 147.8 :1: 13.4, WT- 283.8 ± 45.3 g; ~J>enl mean ± s, SVL =151.7 .h 8.5, Wf = 223 ± 26.4 g; slopes, df- 1,1!>, F = 0.41, P > 0.50; means, df= 1,20, F = 77.4, P < 0.001). Average difference in mean body weight betweengr.tVid and spent females wM 60.5 g. Mature male leopard frogs began voclllizalions in late March. We observed (but could not capture) clasping pairs in April. Atlhis time numerous egg ma.o;ses were fouod attached to underwater stems in Transect 4, where reeds and grasses were abundant along the margins of the canal. Leopard frog tadpole.~ were observed in this same area during May. Leopard frogs <60 mm SVL were not oh$erved in March-May. Although bimonthly :.ize-frequency distributions (Fig. 6) do not clearly indicate age cohort:>, we speculated lhat leopard frogs with SVL <60 mm, captured in June-October, were young oJ'the year. None of the leopard frogs captured in April-May were female. Initially, we bad difficulty distinguishing mature fi:lll!llcs and juveniles by external examination, and HI mature males could only be idcnti (icd by vocalization or presence of a nuptial pad in males. Thus, the preponderance of males in the Jun~-July samp)e may be an artifact of tlus problem. The smallest mature males rangt:d 60-65 mm SVL. Although adult females (n ~ 34) were noticeably fewer than adult males (n ~5 7) in Tnm$ect 5 during August October (the largest sample), we found no deviation from a I: I sex ratio (t2 = 2.95, df ~ I, P = 0.09). Weights ofmatur e females (SVt -75-97 mm) do not differ rrom those of mature molCJ; (SVL = 67- 105 mm) of similar size (ANCOVA: male mean :t s, SVL = &1.7:!: 6.0 llllll, wr = 42.6 ± 10.7 g, n - 33; female mean I s, SVL = &3.8 ± 7.4 mm, wr - 47.7 ± 15.0 g, n = 17; slopes, df- 1,46, F= 3.31, P > 0.05; means, df- 1,47, F= 0.98, .P > 0.50; Fig. 7). Growth/Age Structure We recaptured only 21 bullfrogs at least 20 days after they were first marked. Although sample sizes vvere small, growth ratt: is a decrea~ing function ofave roge body length between captures, and SYL explained 64 percent of grov.th rate in males and 71 percent in females (GR male-;- 0.86-005SVL, R2 = 0.64, P < 0.001, n = 12; GR ft:male ~ 0.96-0.00SSVL, H? = 0.71,1' < 0.001, n = 9). Adult female growth rate (mean = 0.21 mm/d) was almo:;l twice-; the rate (mean = 0.13 mm/d) for adult male~ (ANCOV A: ~lopes, df= 1,17, F- 0.006, P > 0.75; means, df- 1,18, F= 7.11, P < O.Q2S), probahly hecause male growth rille is slowed due to earlier maturity (Howaru 1981). We analy:c.ed histological sections of the second phalanges from a variety of fu1gers and toes for 36 bullfrogs (SVT. 34-195 mm, n "' II juveniles, 12 males and l3 fumales) and 53 leopard frogs (SVL 38-100 nun, n- 16 juveniles, 22 males and 15 Jcrnales). Lines of arrested growth (LAGs) appeared as dark narrow rings separating lighter area~ of hone (Fig. SA). J)ouble LAGs J;eparated hy a narrow area oflightt:r bone I I wefe observed in 6 percent (n ~ 2) of bullfrog and 2 percent (n ~ I) of leopard frog section~ (Fig. llR); these were i.ntccprctctl ns n single LAG, representing frogs that briefly resumed growth during a seasonal hi~mation (Leclair and Ca~tanet 1987). Endosteal resorption lines (ERL, Fig. 8C), characteristic ofprogres.~ive erosion ofthe growth area between ..:mlostcal and the first LAG, were observed in 47 percent (n- 17) of bullfrog and 53 percent (n - 28) ofleopard frog sections. However, resorption did not hinde•· Otlr ubility to interpret LAGs or the overall pattern of growth in any bone section. In all sections, the uistance between successive LAGs decreased, usually outward from the :;ccond LAG (Fig. 80), indicating that growth rates were fastest for frogs in their first and :;ccond years and slower thereafter. Bul U\-og~ and leopard frogs probably mature in the second year, although post-metamorphic bullfrogs that over-wintered as tadpoles are at least one year older than young oftbe year leopard frogs. Bullfrogs, therel.ore, l.ive up to 8 yr (Fig. 9) and leopard frogs up to 4 yr at fSNWR (Fig. I 0). Diets Seventy-four of 198 bullfrog stomachs (37%) and 42 ofl42leopard frog stomachs (30%) contained prey items. Both species conswned a variety ofarthropods. each taking prey from 9 of the~ 14 broad calt:gories (Table 2). Seventy percent of bullfrog stomachs and 88% of those for leopard frogs contained beetles; however, on closer examination of these prey items, 72% of all beetles taken by bullrrogs (n - 86) w.,;re aq uatie forms (i.e., Amphizoidae and Dytiscidae) and 80% of those taken by leoparu frogs (n =56) were terrestrial forms (i.e., Carnbidae and Elateridae). fiily-nine pt:rcent of all odonates consumed by bu ll frog~ (n- 66) \verc larvae whereas tht: only odonate taken by a leopard frog wa.~ on adult dragoolly. All tht: isopods (Onisc11s sp.) taken by bullfrogs live in or near water. Viets of bullfrogs also contained fewer flies, 4 no ~ignificant interaction between bullfrogs and leopard frogs at the rc1ugc. We also compare our analysis ofbullfrogs in a semi-lotic, canal-like enviromnent to that of Clarkson and DeVos's ( 1986) study of hull frogs in the lower Colorado River. STUDY AREA The refhgc is situated in Fi~h Springs Flat, at the southernmost end of the Great Salt.l.ake Desert. Warm water springs (near 24°C) arising from the NE base of the Fish Springs Range t1ow into a series of lllllll-ma<.k canals that empty into several large pools (Fig. 1). Water in the main canal rtulS north, parallel to the line of springs, before turning east, northeast and north to the northern boundary of the refuge. /1. mad parallels the entire length of the nrui.n canal. Width, depth and current velocity ofthe main canal varied 4-8 rn, 1-2 rn, and 1-3 mlhr, respectively. The area immediately around the springs and canals is dominated by common reed, Phragmites auatralis, buln1shes, Scirpus americanus, and saltgrass, Disiichlis spica/a (Rolen 1964). MATERIALS AND METHODS We visited FSNWR (42'30"N, 113'40"£::) monthly February-August and in October 1999. initial sightings in .February and March suggested that bullfrogs were concentrated ncar the springs and adjacent canals. We selected five sections of the main canal (Fig!.), each separated by culverts, as Transects 1-3 ( = 1.0 km), 4 (= 0.6 km), and 5 (= 1.6 km). With headlight~, we counted and hand-captured bullfrogs on both sides of the canal at night from canoes. We hand captured leopard frogs at night and during the day, along the same canals and in adjacent ponds. The capture position ofa :frog was noted within 0.1 km subsections ofeach transect hy co1werting mileage from a car odometer on the road adjacent to the canal. 12 gra~~hnppers, butterflies and 100ths, true bugs, mantid~ and other terrestrial orders than those of leopard frogs. ~y volume, leopard frog :;tomachs contained mostly terrestrial beetle~ and spiders (65.5%); bullfrogs contained aquatic beetles and odonatc larvae (30%), but the largest percentage of total prey volume (63%) was non-arthropod prey, including 10 snails, I bird, 5 muskrats, 2 leaches, I $nuke and 5 frogs; one frog Wlt~ carulibaliz£d and 4 other:; were leopard frogs. DISCUSSION i\ harsh environment limits the distribution and migration of amphibian species at Fish Springs (see Solen 1964, for a detailed description of the &ea). Salt-desert conditions surround permanent waters with no namral outllows from ponds fed by the springs. Dry sur:fuce soils and thick deposits of salts formed by evaporation of mineral rich waters further limit overland migration. If leopard frogs are native to the area, they arc most certainly a relic population isolated ll·om others for about 14,000 yr (Hovingh 1997). In most re:.-pects, introduced bullfrogs and putative native Jeopard frogs at FSNWR are similar to other populations ofthese species. In northern areas, leopard Ji·ogs tolerate cool temperatures, breed early (March-April) and their tadpoles nu.:tarnorphose in the ~arne year. Bullfrogs require warmer air and water temperatures and breed late (May-July), oo larvae usually cannot fully develop in one season (Werner 1994). Warm waters ofFish Springs may allow bullfrog tadpoles to over-winter and buller adults from seasonal freezing temperatures (bullrrogs also survive at !ugh elevation in warm spring waters in Colorado; 1-Jammcrson 1982b), but frogs (and their prey) are restricted to the immediate area ofthc springs by very dry and cold conditions, particularly in winter and early spring. Terrestrial for.sging leopard frogs cannot 13 withstand freezing air temperatures during wiutcc (no leopard frogs were active before March) and probably hibernate underground and/or underw·atcr during winter (e. .g., Emery et al. 1972, and Hine et al. 1981). In sununer, several canal~ dry up and pools shrink in size, further restricting dispersal of frogs away from permanent water. The original site of commercial r~::aring ofbullfrogs was just W ofTransect 2, in what is now called ~{; pring (see Fig. 1 ). Records indicate that eggs were rAatll/17 collected in sununer ru1d larvae reared in enclosures. We asswne that bullfrogs dispersed away from the area of initial introductions. Cold-intolenmt bullfrogs (Lot shaw 1977) must retreat in winter from shallow ponds and ditches that freeze-over to the warm water springs and adjacent canals. Their absence or low numbers in isolated springs (e.g., at Walter Spring only 5 were ob~erved throughout the study) probably reflects limited migr.ttion along the canals; only one of27 (4%) of recaptured bullfrogs wa~ taken in a transect other than the one in which it wa~ initially caught. We believe that bullfrogs in Transect 5 and Walter Spring are transients. Far fewer bullfrogs, all adults, were found in these areas; although males in Tran._<;ect 5 and at Walter Spring vocalized, clasping pairs, egg masses or tadpoles were not observed. Breeding populations contain several size classes, particularly young ofth .e yenr and juveniles; these size classes were much less ' numerous away from Transects J and 2 and missing from Transect 5 and Walter Spring. Both the dccliuc iu bullfrog abundances from TrrulSect 1-5 rutd the negative correlation between bullfrog ru1d leopard frog abundances along the same transects cruu1ot be explained by temperature alone. The dillerence in aver.tge water temperatures for Transects I and 5 in March was only 5°C. Microhabitat di!Terence~ (a gradient of sorts), although not mea~ured, do exist between transects. Undercut canal b-anks in Transects I and 2 provide an exposed or shaUowly covered shelf upon whk:h mru1y adult 14 bu llfrogs were ob:.erved at night. Although mo ~t bullfrogs prctcrred are (.Emlcn 1968), may more easily defend open patches of bank, and these ~ilc:; rnay also be u~d lor thermoregulation (T .illywhite 1970). By contrast, the C Our body size distribution for the Fish Spring population (34-195 mm SVL) i!l similar to tbe range (47-179 mm) for bullfrogs introduced along the lower Colorado River (Clarkson and Oe Vos 1986) and for other native populations (e.g., Schroeder and BaskcU1968 , Table 3; Howard 197.8, Fig. 6). Average density of bullfrogs in Transects lnnd 2 (179/km) i~ 20 t ime:~ higllc:r than along the lower Colorado River (i.e., 9.1/km, Clarkson and De Vo ~ 1986) and almost twice that of bullfrogs along the shoreline ofa 6.9-ht:etare Jake in Illinois (180.7/km, Durham and 'Rennett 1963). Bullfrog tadpoles transform at 25-60 mm SVL (Collins 1979). At FSNWR, we did not sec bullfrogs smaller . than 78 mm SVL until June and we observed the smnllest frogs (34-78 nun SVL) in July- O~:tober. Average growth rate for the smallest recnpn1rcd juvenile frogs (SVL- 94-115 mm, n = J) was 0.56mm/d. At this rate, a 34 OlJl1 SVL bullfrog tbaltrdruiformed in July or June would grow to I 02 mm or 86 mm SVL, rcspcctivdy, by October; a distinct peak in the frequency ofju venile bu iJfrogs between 75-96 mm SVL was obtained in the August-October samples (fig. 4). Average growth rate for recaptured sub-adult bullfrogs (SVL = 124-129, n- 4) was 0.26rnmld. At this rate, bullfrogs entering n second post- metamorphic year at 86-'102 mm SVL would easily reach observed minimal adult size ( 116-120 mm SVL) by the end of the active season and be ready to breed in the third post-metamorphic year. These estimates, based on very small samples, are supported by age-size estimates from exam.ination of toe bone sections (Fig. I 0). Leopard frogs grow to betwcen60-70 nun SVL in their fu-st post-metamorphic year and breed in the;: following and subsequent years (Fig. 9); growth, maturity and rnaximurn ages of the FSNWR population are essentially the same as those in conspec.ific populations in southwestern Quebec (Leclair and Castanet 19ll7). A plethora of studies (see Bury and Whelan 1985) agree that bullfrogs lake a large variety of prey itelliS., virtually anything lh<::y can swallow, including their own species. Dullfrogs do eat leopard frogs at Fish Springs, and a negative correlation in the distributions ofthe two species along the transects (Fig, 2) may in part reflect an inability of leopard frogs (if native to the area) to survive predation in area~ where bullfrogs are most abundant. Dense reeds and other vegetation at the edges ofTransects 3-5 and around pools and ponds may provide protection fur leopard frogs (they are especially hard to catch during the day in this habitat). Alternatively, dense vegetation may increase the number ofprey species and their abundance; we particularly noted numerous spiders (e.g., Araneidae, Clubionidae, and Lycosidac) in these areas. However, differences in diet between the two ::;pecies probably do not reflect babitat difl:crenccs in prey aburtdanccs, but dillerences in the loragi.ng strategies (sit-and-wait for bullfrogs v. terrestrial foraging lor leopard frogs). Ifleopard frogs are hiding from bullfrogs in dense vegetation in Transects 3-4, removal of bullfrogs from these areas rnay result in an increase in nUIJlbers of leopard frogs. ACKNOWLEDGF.MF.NTS H> We thank J. Banta, Refuge Manager, the U.S. Fish and Wildlife Service, and the Utah Division of Wildlife Resources for financial and logistic support, and L. Schwaner and the Spring 1999 herpetology class at Southern Utah University for their assist~mce in the field . LITERATURE CITED Andrew~, R. 1982. Patterns of growth in reptiles. Pages 273-320 in C. Gans and F.H. Pough, editors, Biology of the Reptilia, Vol. B. Academic Press, New York. I3ury, R.I3. and J.J\. Whelan. 1985. Ecology and management ofthe bullfrog. Resource Publication I 55. U.S. Fish and Wildlife Service, Washington, D.C. Caughley, G. 1977. Analysis of vertebrate populations. Wiley, New York. Clarkson, R.W. and J.C. DeVos. 1986. The bullfrog, Rana catesbeiana Shaw, in the lower Colorado River, Arizona-California. Journal of Herpetology 20:490-509. Collins, J.P. 1979. Intrapopulation variation in the body size at mctamotphosis and timing of metamorphosis in the bullfrog, Ra.na catcsbeirum. Ecology 60:738-749. Durham, L. and G.W. Bennetl. 1963. Age, growth and ht,ming in the bullfrog. Journal of Wildlife Management 27:107-1?3. Emlen, S.T. 1968. Territoriality in the bu!Hrog, Rtma catesbeiuna. Copeia 1968: 240-243. Emery, A.R., Berst, A.Il. andK Kodaim. 1972. Under-ice observations of wintering sites of!eopard frogs. Copeia 1972:123-126. Esteban, M., Garcia-Paris, M. and J. Castanet. 1996. Use ofhone histology in estimating the age ofrrogs (Rana perezi) from a warm tempewte climate arelj. Canadian Journal of Zoology 74: 1914-1921. I I Ferner, J.W. 1979. A review ofma rking techniques for amphibians and reptiles. Herpetological Circular No.9. Society fur the Study ofAmphibians and Reptiles, University ofKansas, Lawrence. Hammcrson, G.A. 19S2a. Bullfrog eliminating leopard frogs in Colontdo? Herpetological Review 113:115-116. ______1982b. Amphibian~ and reptiles in Color&do. Colorado Division of Wildlife Publication. DOW-M-I-27-82. Hayes, M.P. and M.R. Jennings. 1986. Decline of ranid frog species in western North America: Are bullfrogs (Rana ca/esbeiana) responsible? Journal of Herpetology 20:490-509. Hine, R.L., Les, B.L. and B.F. Hellmich. 1981. Leopard frog populations and mortality in Wisconsin, 1974-76. Technical Bulletin No. 122 Department ofNatural Resources, Madison. Hovingh, P. 1997. 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Conservation Biology 12:776-787. Kupferberg, S.J. 1997. Bullfrog (Rana catesbeiana) invasion of a Califurnia rivc.r: the role oflai'Vlll competition. Ecology 78:736-745. LeClair, R., Jr. and J. Castanet. 1987. A skelctochronological assessment ofage and growth in the frog Rana pipi11n11 Schrcbt:r (Amplubia, Anur.:t) from southwestern Quebec. Copeia 1987:361-369. Licht, L.E. 1967. Growth inhibition in crowded tadpoles: Intra:;pecific and interspecific efiects. Ecology 48: 736-745. Lillywhite, B.D. 1970. Behavioral temperature regulation in the bullfrog, Rana catesbeiana. Copeia 1970:158-168. Lotshaw, D.P. 1977. Temperature adaptation and effects ofthermal acclimation in Rana sylvatica and Rana catesbeiana. Comparative Biochemistry, Physiology and Comparative Physiology 56:5287-294. Moyle, P.B. 1973. Effects of introduced bullfrogs, Rana catesbeiana, on native frogs of the San Joaquin Valley, California. Copeia 1973:18-22. Schroeder, E.E. and T. S. Baskett. 1968. Age estimation, growth rates, and population stn1ctorc in Missouri bullfrogs. Copcia 1968:583-592. Sokal., RR. and FJ. Rohlf. 1981. Biometry. Freeman, New York. Slebbin:;, R.C. 1985. A field guide to western reptiles and amphibians. Houghton Mifflin, Doston, MA. Werner, E.E. 1994. Ontogenetic scaling of competitive relations: size-dependent eiTecls and responses in two anuran larvae. Ecology. 75:197-213. Table.; 1. Direct counts offrog~ in five transects along the main canal at Fish Springs National Wildlife Refuge, Juab County, Utah, takc.;n monthly, from April to July, 1999. Nwnbers in brackets are adjusted to I km. Number of frogs/trruJSCCl Transect km Apr May Jun Jul 1 1.0 260 18 64 72 2 1.0 71 24 48 44 Bullfrogs 3 1.0 26 34 19 9 4 0.6 17 (28) 10 (16) 13 (22) 17 (27) 5 1.6 6 (4) 9 (6) 9 (6) 14 (9) I 1.0 1 0 0 0 2 1.0 2 0 0 0 Leopard frogs 3 1.0 25 3 ' 4 11 4 0.6 6 (10) 0 0 16 (27) 5 (1.6) 14 (9) 3 (2) 13 (8) 67 (42) r:Jz_ 70 l 97 60 •bullfrogs 0 leopard frogs 414 50 :g 40 ·-I) 1:: II) :J 0" ~ 30 1 43 I 187 20 I - I I 22 -- ' ' 75 10 -1 I 57. 38 I I 1 -2 - I 0 - 1 2 3 4 5 Transects c::; JA..) ~ vl-L. P~ s ( f.JB) L >.c r' ftT~!.P fi>'.()G S -...:...... - 1'-JO. ";, rvo. 126 5 If{ /l () () \ vJ } ~ 1 '0 0 () L ~8 2.2. I I <.·() 5 11. It ~ 0 3o t{l. I )0 -1..<1 'il I f' . ·"' " " ' .. ,., ...... ' ....J.."t .... '" ...... ·' ...... -' 7-. .. -·-·-· _.s~ - - - - __ __ . '·.. · -- .. 1..?: .... -.. ....$. . -... 0 __ .,0.-...... - .... ".g .. --·-· - -~.. ---""" s - ...... 11'1.. . 1 I f. ...... ·q . .. -. .2:1 .. . 5 ___ g3 . . i. ,g ' ~3...... - ---· ---- ·· ---·- - .. . -····· - . ···-···- .. . --··· ..... ,, . .-.-...... -- (r ~- ~- c.j .," BULLFROG SIZE--FREQUENCY DISTRIBUTIONS I '' '' ' '' APRlL-MAY '' I' • rvlales I' : :'' 6 • 4 • Fema les I ' • 2 L------L.! • • u .•. 0 - I ' '' '' '' 6 i ' ' '' '' JUNE-JULY 4 ! '' I 201 '' '' 2 I' ! / '' 0 ; I / AUGUST-OCTOBER 5 34 I • I I • I I 1- I ., 0 I I 1 I I I I I I I 21-25 .' 46-50 71-75 96-100 121-125 146 - 150 171-175 196-200 ' tf;.r 600-1 • 0 • 500 males 1 0 0 females • 0 ~ juveniles • 400 IXgr avid o·B spent x. • • ~ ><-xo t • i3JCl x•~· ~ . ~~ X 200 :~ •••• :1( 0!~-.i~f :+: ~ "'' ®~ 100 - l .: t;~ "' ,:•i * "'"' 0 +-to .. _ .. """ .... I - ~~~~----,--r-~ 30 50 70 90 110 130 150 170 190 210 Snout-Vent Length (In mm) r:; If Leopard Frog Size-Frequency Distributions 6 5 Ma l es F e m al es April/May 4 [;Ju ven i les 3 105 2 ~ 1 0 7 --, June I July c 6 " - 5 ='l,l • ... 4 0" 3 'l,l 2 ~"" 1 0 August/October 8 ' - 6 4 36 2 j ~ 0 2 1 - 3 1 " 4 1 " 5 1 - 6 1 - 7 1 - 8 1 - 9 1 - 1 0 1 - 1 1 1 - 2 5 3 5 4 5 5 5 6 5 7 5 8 5 9 5 1 0 5 1 1 5 Snout-Vent Length (mm) \\. • ~ ....0 • • 0 - ....0 ltJ- 0 • ~ o •• 0) ~ ct E ~· E ott+ 1: [)p-. 0 co -.r:: C') -c Ql ·.4 ....-' .I... 1: 0 (I) ... 1"- ::;- ::I -0 ··u 1: Cl') (/) - ,----.--- --,------.--- ~---,---.--- 0 0 0 0 0 0 0 0 0 (J) (() 0 ...... 0 0 0) B I 0 -E a:> E t: -...... &; ~ C) t: ,....0 (1,) ....I 1:: -(1,) >0 ..... :I 0 0 I <0 ·c 1 0 - lO MIll E > I ti g ·~ ~ I 0 l ' ~ N ...- o (J~ U! a6e :l!4dJOWe'l ~· 0 ~ ~ .....co ~~ ~ ~· 0 .....m . 8 ~ ~ •.. '"> 0 ~ .....~ •• ~'" ... -E •• • . 0 E N..... t: )~ ·-~ j .t:..... C) 0 t: /. 0 Q) ·'1 ..- ..J "i ] ~ .....c \ Q) ~ >, ~ 0 ..... co :::l .. 0 t: ~ en ~ ~ 0 .. • tD (/) ~ (/) Q) = .. Q) Iii c ~ -ro E Q)> • E ~ .2, • • +D~ • ~ 0 N 0 m ~ ~ ~ N ~ 0 (J~ 0!4dJowe}aW·lSOd=) sEnf1 JO JaqwnN Table 2. Stomach contents of74 bullfrogs and 42 leopard frogs from Fish Springs National Wildlife Refuge, Juab CoW1ty, Utah. Percentage occurrence i~ the number ofstomach~ over the total examined that contained the prey item; volume percentage is the amount (in ml) over the total volume of all pcey categories. • = aq\mtic groups, + ~ items prcscut but, individually, ofminor importance (<1.0%). Percent Occurrence Volume Percentage Prey bullfrog leopard frog bullfrog leopard frog Annelids (leaches) 2.7 + Mollusks (snails)* 10.8 1.0 + + Arthropods Myriopodli 36.5 + Arachnida Araneae 32.0 + Araneidae 4.2 3.1 Clubionidae 8.5 : 2.4 Lycosidac 6.4 4.4 Philodroolidae 2.1 + Salticidae 2.1 + Tetragnathidae 1.0 + Pseudoscorpionida Chemctidae 1.0 + Insecta Coleoptera Amphizoidae• 27.0 + Carabidae 4.0 38.1 + 47.5 Chrysomelidae 2.1 + Cicindelidae 10.2 2.1 + 3.1 Curculionidae 2.7 + Dyli:;cidae• 24.0 16.4 Elateridae 1.4 7.4 + 1.6 Halipidae• 4.0 + Heteroceridae 1.0 + Hydropilidae• 6.6 1.0 + 1.0 Languriidae 2.1 + Staphylinidae 1.0 + Unidexrtified 11.9 1.0 Diptera Chironomidae 2.1 + Culicidae• 1.7 + Muscidae 2.1 + Simu!idae 1.4 1.0 + + Unidentified 3.2 + Hemiptera Belostomatidac 4.0 + Lygacidac 1.0 + Micidae 1.0 + Unidentified 1.4 + Homoptera Coccidae 5.3 1.0 Hymenoptera Formicidae 1.4 6.4 + + Lepidoptcr Mantodca Mantidae 1.4 1.0 + 1.2 Mecoptcra Panorpidae• 2.1 + Odonata LibeUulidae* 69.0 1.0 11.0 8.5 Orthoptera Acrididae 5.4 3.2 + 4.2 Gryllidae 1.0 .. + Vertebrates Amplu'bia (frogs)• 6.8 4.0 Reptilia (gartersnalce)* 1.4 2.0 Aves (bird) 1.4 + Mammalia (muskrats)• 6.8 57.0 I' I' I I ' I' •I I I I I I ! I I I pvc~ , Ov Spnngs 0 2 Kilometers N Roads