STATUS OF BULLFROGS AND

NORTHERN LEOPARD FROGS AT FISU SPRINGS NATLONAL

WILDLU'E REFUGE, JUAB COUNTY, UTAR

Matthew n. McKell1) , Scott Peterson2

, Keiko Kobayashi2, Ryoko Miyazato2, Donovan

Sherratt2, Michael P. Oonov-dil\and Ten:y D. Schwaner2•3

Running head: 'BULLFROGS AND LEOPARD FROGS AT FISH SPRlNGS

1Prcsent nddrc:;:;: Department of Zoology, 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 of the 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

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.).

j

Fish Springs is located within the natur<d r.mge oft.he northern leopard frog, Rana pipiens

pipiens (Stebbins 1985). Whether this species v.>as al~o introducw or is native to the area

is unclear; a natural population of leopard frogs exists in Snake Valley just southwest of

!'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 of native 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 of native 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 of the 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 1 0 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 of stomach:~ 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 of randomized blocks to examine the abundances of

fTogs in five transects over lour :sainpling periods. Spennnnn rank correlation tested the

n.ssocwtio11 of relative 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

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

6

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 <md adult bullirog$ in $prings adjacent to Transects I and 2, and the

main charUlel, where water temperatures were > l9°C; bullfrogs were not seen in Avocet,

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 <md

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 of c<mals <md

adjacent pools in Transects 1-5, when water temperatures were >l5°C. Leopard frog~

inhabited the easternmost margins of Egret Pool and Curlew Pool and adjacent roadside

ponds (Fig. I). However, we rarely ob~erved leopard frogs in the springs or in canals

connecting the springs with the main channel in Transects 1 and 2.

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.; of recaptures.

'I

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

of all captured bullfrogs (Fig. 4) do not give any clear indication of age cohorts, except

tor the abseJJce of young 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).

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

9

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 ~57) 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 of mature 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 of averoge 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 of the 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. BulU\-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 of arthropods.

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 bull 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,

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

4

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 of the 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 of a :frog was

noted within 0.1 km subsections of each transect hy co1werting mileage from a car

odometer on the road adjacent to the canal.

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

12

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 of these 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 of Transect 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 of th.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<ll! of extensive

reed~ in a !otic habitat along the lowt.'I Colorado River in Arizona-California, they were

rarely observed actually in the reeds and more often found at night on short reaches of

open bank (Clarkson and DeVos 1996). Adult males, whose territoriality is well known

(.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<mal in Transects 3-5 was

narrow and shallow with increa.~ingly dense, emergent grasses am! rcc.xb. Sl~;t.-p-sided

banks in Transect 5 rarely featured bare undercut areas, but most bullfrogs observed in

that transect were on those few shelves.

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

BaskcU 1968, Table 3; Howard 197.8, Fig. 6). Average density of bullfrogs in Transects

lnnd 2 (179/km) i~ 20 time:~ 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 of a

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 of juvenile 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 of Transects 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 of prey 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 oft he 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 of hone 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 of marking techniques for amphibians and reptiles.

Herpetological Circular No.9. Society fur the Study of Amphibians and Reptiles,

University of Kansas, 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. Amphibians in the easte111 Great Basin (Nevada and Utah, USA); a

gcog,:aphical study with paleo zoological models and conservation implications.

Herpetological Natural History 5:97-134.

Howard, R.D. 1978. The evolution ofmntiog strategies in bullfrogs, Rana catesbeiana.

Evolution 32:850-871.

Hecnar, S.J. and R.T. M'Closkey. 1996. Changes in the composition of a ran.id frog

community following bullfrog extinction. American Midland Naturali~t 137:

145-150.

Jennings, M.R. and M.P. Hayes. 1984. Pre-1900 overharvest of the California red-legged

frog ( Rana aurora draytoniz): the inducement for bullfrog ( Rana catesbeiana)

introduction. Herpetologica 41 :94-103.

Kicsecker, J.M. and A.R. Blaustein. 1997. Population differences in responses of

red-legged frog:; (Rana aurora) to introduced hullrrogs. Ecology 78:

1752-1760.

18

____ . 1998. Effects of introduced bullrrogs and s.rmllmouth bass on microhabitat

use, growth, and survival of native red-legged frogs (Rana aurora). 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 of age 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 of thermal 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 of frog~ 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.

Bullfrogs

Leopard frogs

Transect km

1 1.0

2 1.0

3 1.0

4 0.6

5 1.6

I 1.0

2 1.0

3 1.0

4 0.6

5 (1.6)

Number of frogs/trruJSCCl

Apr

260

71

26

17 (28)

6 (4)

1

2

25

6 (10)

14 (9)

May

18

24

34

10 (16)

9 (6)

0

0

3

0

3 (2)

Jun

64

48

19

13 (22)

9 (6)

0

0

' 4

0

13 (8)

Jul

72

44

9

17 (27)

14 (9)

0

0

11

16 (27)

67 (42)

r:Jz_

70 l

60 414

•bullfrogs 0 leopard frogs

97

50

:g 40 ·-I) 1:: II) :J 0"

43 I 187 ~ 30 1 -20

I - - I I 22

' ' 75 10 -1 I 57.

38 I I -1 -2

I -0 1 2 3 4 5

Transects

\

5 vJ

L

5

s .. ~- -

.. - .. . - · .L

5..

c::; JA..) ~ vl-L.P~ s -...:......

1'-JO. ";,

If{ /l

} ~ 1 '0 ~8 2.2.

11. It 3o t{l. -1..<1 'il

5 ~ to ':l.f! .. t c 'fg

. . ·~ --..

( f.J B) L >.c r' ftT~!.P fi>'.()G S -rvo. 126

() ()

0 ()

I I <.·()

~ 0

I )0

I f'<l

- . . .. - ........ II - _'ly ____

"' .G:> ... .2. ':! ··-· ... ~ .. J4

- ·-··· ·- - · r o ___ __ . 4 . . --~c__ _

. ·"' " " ' .. ,., .... ..... . ' .... J.."t.... '" .. ........ ·' ............ -' 7-. .. -·-·-·

.... $.. - ... s - ........... 11'1 .. .

- ---· ---- ··-- -·- - .. . -····· -

0

1

5

_ .s~- - - - ___ _ . '· .. ·-- .. 1..?: . ... -..

__ .,0.-....

I f. ­___ g3

.

... .... . - .... " .g .. --·-· --~ .. ---""" q . .. . .2:1 . . . .. ... . ... · -.,g ~3 .. ... . . i ' .

. ···-···- .. . --··· ..... ,, . .-.-... ....... . --

" ., (r ~-~- c.j

BULLFROG SIZE--FREQUENCY DISTRIBUTIONS

6 4 2 0 -

• rvlales • Females

L--------

I ' ' ' ' ' ' ' ' I

: ' I

' ' ' I

' ' : • •

APRlL-MAY

! u .•. L. • • I ' ' ' ' ' ' ' ' 6 i

4

2

0

5 34

; I

/

!

' ' ' ' ' ' ' I ' ' ' ' ' I

! ' '

0 I • I I • I I 1- I ., I I 1 I I I I I I I

JUNE-JULY 201

/

AUGUST-OCTOBER

21-25 .' 46-50 71-75 96-100 121-125 146 - 150 171-175 196-200 '

600-1

500

400

i3J Cl

~ 200

100 -

~males 1

females juveniles

IX gravid spent

X

~

"'' ®~ l .: t;~ "' ,:•i * "'"'

0

o·B x. •

~ ><-xo t x•~· . ~~

:~ •••• :1(

0!~-.i~f :+:

• • 0

• •

0 • 0

•

tf;.r

0 +-to .. _ .. """ .... I -~~~~----,--r-~

30 50 70 90 110 130 150 170 190 210

Snout-Vent Length (In mm)

c = 'l,l ... ... 0" 'l,l

"" ~

r:; If Leopard Frog Size-Frequency Distributions

7 6 5 4 3 2 1 0

6 5 4 3 2 1 0

8 6 4

2

0

--, "

'

j 2 1 -2 5

[;

Ma l es F e m al es Ju ven i les

-36 ~

3 1 " 4 1 " 3 5 4 5

• -

5 1 - 6 1 - 7 1 -5 5 6 5 7 5

April/May 105

~

June I July

August/October

8 1 - 9 1 - 1 0 1 - 1 1 1 -8 5 9 5 1 0 5 1 1 5

Snout-Vent Length (mm)

\\. •

~

0 (J)

•

0 (()

• -ltJ

<J .

• ~ o ••

ct ~· ott+ [)p-.

·.4 .I ... ... ··u

<n (/)

.~ (/) Q) .... Cll «i c

«i E Q) .... :> E Q) ;:! ._ ·~ •1 • 0 •

- ,----.--- --,-- ----.--- ~---,---.---

0 0 0 0 <D 1.[) ~ (")

(6 U!) l46!<~M

0 N

0 .....

0 .... ....

0 - 0 ....

0 0)

~

E E 1:

0 -co .r:: -C')

c Ql -' .... 1:

0 (I)

1"- ::;--::I 0 1:

Cl')

0 <D

l 0 1.[)

0

· '

B I

~

I

1

M E -

I Ill >

l ti g ·~ ~ I

~ N ...- o

(J~ U! a6e :l!4dJOWe'l<IWJSOd =) S~JV'l JO JaqWnN

0

0 0 ......

0 0)

0 a:>

0 ,....

0 <0

0 lO

0 '<t

-E E t: -

.&; ..... C)

t: (1,)

....I -1:: (1,)

> 0 ..... :I 0 ·c

<I)

~~

~·

~ ~ ~

~~

~· . 8 ~

• .. ~ ~

'">

'" •• ~ ... )~ • • •

j /. ·'1 ~ "i \ ] ~

~ .. ~

~ • (/) ~

.. ~ (/) Q) = ..

Q) Iii c • • - E Q) ro > E ~ .2, • ~ +D~ •

m ~ ~ ~ N ~

(J~ 0!4dJowe}aW·lSOd=) sEnf1 JO JaqwnN

.

~

0

0 0 N

0 co .....

0 m .....

0 ~ .....

0 N .....

0 0 ..-

0 co

0 tD

0 N

0

-E E t: ·-~ .t: ..... C) t: Q) ..J ..... c Q)

>, ..... :::l 0 t: en

Table 2. Stomach contents of74 bullfrogs and 42 leopard frogs from Fish

Springs National Wildlife Refuge, Juab CoW1ty, Utah. Percentage occurrence i~ the

number of stomach~ 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, of minor 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<l 1.4 2 .1 + +

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