Long-term fluctuations of an isolated population of the Pacific chorus frog (Pseudacris regilla) in northwestern Nevada

6 Pages·1993·2.3 MB·English

by  N H Weitzel

GreatBasinNaturalist.>3'4),pp.379-3W
LONG-TERM FLUCTUATIONS OF AN ISOL.\TED POPULATION
OF THE PACIFIC CHORUS FROG PSEUDACRIS REGILLA)
IN NORTHWESTERN NE\ADA
Norman H.WeitzeP andHowardR. Panik-
—
-\BSTR.^CT. InS0%oftheyearsbetween1975and19S9chorusfix>eswereabletoproduceoffspringthatcompleted
metamorphosisandbecamemembersofthenextgeneration. During3ofthese 15years 1975. 19S3.and 19S6i. floods
annihilated chorus frogs in the breeding pond. Between 1905 and 19S9 this geographicalK isolated peculation of
Pseudacris regilla. the Pacific chorus frog, located at the western edge ofthe Great Basin in northwestern Ne>ada.
bec-amereestablishedafter 10naturalphysicaldisturbanc-esincludingspringflashfloods,numerousIdll-offsb> sudden
drastic rises in uatertemperature, and stream dr\-ups. Ninet\ perce-nt ofthe frogs in this population were shadesof
greenwithblackdorsal specklesande>estrip<5 Predat-r;^vptcthebeltedkinsfisher^Megacerylealcyoni.westernter-
restrialgartersnake Thamnophiselegam, .' Felixdomestica).
Keywords: Pacificchorusfi'og, Pseudacris regilla. Gn •n reesUshlishment coloration, chonufrog
predators.
Pseudacris regilla •= Hyla regilla . the In recent years there has been much c-on-
Pacific chorus fi'og, is one ofthe most abun- cem over the decline ofpopulations ofmany
dant and widespread amphibians in the north- amphibian species 'Wake and Morowitz
western Nevada portion of the Great Basin. 1990 ; but as reported by Pechmann et al.
Yet. no studies ha\e been published on the 19911, supporting lon2-term c-ensus data are
chorus frog in this geographical area. Test generally unaxailable. This paper reports data
1S9S. Van Denburgh and Slexin '192L. and on population fluctuations from 1905 to 19S9
Cowles and Bogert 1936 reported that cho- in an isolated population ofthe Pacific chorus
rus frogs in southern Nevada were localized frog in northwestern Nevada. This study
and not especially abundant. Linsdale 1940' shows the resilience ofthis species to natural
recorded P. regilla populations in southern physical disturbances and predation. In addi-
Nevada. Banta 1961 studiedpopulations ofP tion, it shows that Pseudacris populations
regilla along the lower Colorado River in show wider fluctuations, this in agreement
southeastern Nevada and assumed other with Pechmann et al. 1991) and Caldwell
established populations were exterminated b\ 1987). Also, it is in agreement with Bradford
the rising waters ofLake Mojave. Outside the etal. '1992 . whoreported thatP regilla popu-
Great Basin. Brattstrom and Warren '1955 lations donotappeartobedeclining.
made obser\ations on the ecologx and beha\-
ior ofthe Pacific chorus frog in southern Cali- Study .\rea
fornia. Jameson (1956. 1957) reported on the
gro\\"th. dispersal, surxival. population struc- The study area is a semipermanent pond
Wtruuhrsiet,fnareon\gdaihnnodmtihKnergeWbriselsl1pa9om7ne5stjetssetoufVdatilheledeyPt.ahceiOfsripecagccohinon.-g l11o5cm0a0teamtd.o1T8nhekemepnosdnodu.ttoh291ofmmReianntolteahntgetahon,the\elarer\.iaeDtsiefopinrtoohmf
and calling in Pacifi>c chorus fi-ogs at Marion ranges from 0.1 to 1.2 m. Vegetation in the
Lake. British Columbia. Reynolds and pond includes common horsetail Equisetum
Stephens 19S4) reported on multiple ectopic arvense). water cress {Radicula nasturium-
limbs in a wild population of P. regilla near aquaticum). sedges, grasses, and algae, \ege-
Boise. Idaho. tation surrounding the pond includes pinyon-
-i1l>ilJOlRWhieesrtttNw\odtLLui.iie \\-,- '-se-CarsonCJt>.N>»a<l»SSOJl
379
380 Great Basin Naturalist [Volume 53
juniper on the north, east, and south sides of from 1975 to 1989. These males produced a
the canyon that contain the pond. To the west chorus that attracted other males. The num-
along tlie canyon are dense growths of Fre- ber ofmales increased from a few in late Feb-
mont Cottonwood {Populusfremonti), black ruaiy to an average of25 in mid-March. From
willow {Salix nigra), and white clover {Trifoli- mid-March to late April the number ofmales
um repens). decreased to fewer than 5. In addition to
attracting males, the chorus appeared to
Methods attract females. The breeding population of
males and females peaked in March (mean ±
From 1974 to 1989 censuses of chorus SD, 60 ± 3.8; range, 53-66; Fig. 1 and Table
frogs in or near the pond were conducted on 1). Thus, chorus frogs migrated to the pond
alternate evenings from 1600 to 2300 PST from late February to early May, and the male
throughout the breeding season, which ]:)reeding chorus extended over a 3-month
extended from February to April. During the period.
remaining nine months of the year, periodic
Reproduction and Metamorphosis
counts ofadult frogs were made.
Each census was conducted from an obser- Mating commenced soon after the frogs
vation site located 6 m from the pond. From entered the pond and continued until early
this point the total number offrogs in or near April. During this time no territorial disputes
the entire pond was counted. To increase the or fighting was obsei-ved. Egg laying occurred
accuracN of the counts, the pond was divided from mid-March to mid-May; eggs began to
into four regions: northeast, northwest, south- hatch into tadpoles in mid-April. The brown-
east, and southwest. Each census started with ish black tadpoles tended to localize in the
the northeast region. After all frogs were deeper (up to 1.2 m), downstream portion of
counted in this region, counts were then the pond. Tadpoles and near-transformed
made in northwest, southeast, and southwest individuals grewrapidly in the 12-18° C water,
sections. To increase the accuracy of the and by early October all frogs gradually exited
counts, a spotlight and binoculars were used. the pond. From early October to February
The procedine used to conduct a census was only an occasional chorus frog was seen or
the same each yearofthe study. heard in the pond. The transformed individu-
In addition to conducting annual popula- als probably matured in one season and
tion censuses, we also recorded life history returned to the pond as fully grown adidts the
information. For each year of the study the next yearas shown byJameson (1956).
date was recorded for the following activities: Ninety percent oftransformed adidts were
entering pond, chorusing, mating, egg laying,
light to dark green with dorsal black speckles
hatching, exiting of adults, completion of
and black e\e stripes. Brattstrom and W'arren
metamoiphosis, and exitingofjuveniles. (1955) found that green and brown phases
ortectTohhrerdopeoudngd1h,omuatnadbtohwveaetsettuhrdeyt,germaopireurntadetmaupnredera3wtamusrentoawrkateshn oincccuorlroerdofinP.abreoguitllaeqwuealremmniotbeorbss.erVvaeridat(iToensst
1898). Black eye stripes were always present
at thecenterofthe pond at adepth of15 cm. as reported b\' Brattstrom and Warren (1955).
Information concerning the study area,
Periodically, from Februar\ to July 1986-
ppornodv,idaenddbcyhoMirisi.s GfreoogrsgferoMmino1r9.05 to 1978 was 1989, 10 light green P. regilla were kept in a
brown aquarium, and within 1 h all individu-
als changed to a dark green phase. Even after
Results
120 h no green frogs tin-ned brown. Most dark
green frogs turned a lighter green in an open
Migration to the Pond
atiuariimi. Captive chorus frogs were not kept
Migration ofchoiiis frogs to the pond was aftei- 5 days because the\- did not feed. These
documented to 1905, and it probably observations supported those of Brattstrom
occurred manyyears priorto that (Mrs. Minor and Warren (1955) and Resnick and Jameson
personal communication). (1963), who reported that the primary green
Except for flood years, a few male chorus coloration and black eye stripes are genetical-
frogs migrated to the pond in late February ly determined, while the color phases and
1993] Pacific Chohls Fhoc i\ Nrn-ada 381
70 --
Sexually active
frogs
60 -• 3.8 Metamorphosed
tadpoles
10
en
o
u
fc 50
40 -•
-
30
20 --
-
10
O.Q JLj2 Q-O Q-O
Jan Feb Mar Apr May May Jun Jul Aug Sep Oct Nov Dec
Month
Fig. 1. Average numberoffrogs duringeach niontli lor 12ofthe 15years (1974-1989). CounLs were madeal night.
Topnumbersarethestandardde\iations.
spots are environmentally regulated. Season, and they generated rapidK moxing mudflows
locality, and, to some extent, age also affect that scoured the streanibed. In addition to
color (Resnick and Jameson 1963). Neither flash flooding, sudden drastic rises in pond
albinism described by Resnick and Jameson water temperature and drought conditions
(1963) nor limb abnormalities reported b\- also reduced the chorus frog population in the
Re\nolds and Stephens (19(S4) were obsened pond. In si.\ of the \ears. Irom 1974 to 1989,
in these populations oi P. regilla during the 15 in March and April, the pond water tempera-
years ofobsenation. ture rose from 7° C> to 24° C within 20 days.
This sudden rise in pond water temperature
Natural Plnsical Distuibances had a negati\e impact on tlu- chorus hog pop-
From 1905 to 1989 the chorus hog popula- ulation for those \ears. In three ol the \ears a
tion in the breeding pond was annihilated 10 light snowpack and sparsi* rainhill created
times. From 1975 to 1989, in 1975. 1983, and drought conditions; both Baile\ Oeek and
1986, the time of this stud\, the chorus frog the pond dried up.
population in the pond was eliminated three
Fredation
times b\ flash floods. For 12 ofthese 15 years
(80%), chorus frogs produced offspring that During the stuck three predaton- species,
completed metamorphosis. Hea\\- rainialls. the belted kingfisher iMviiacenjle alcijon),
along with melting snow at higher elevations, western terrestrial garter snake iThamuoplm
or prolonged downpours caused Bailey Creek elegans), and domestic cat (Felix domestica),
to become a rushing toirent, rising up to 1 m were obserxed takingchorus frogs.
aboxe normal pond level. Twice during the The belted kingfisher pre\ed on both lar-
stud\ \iolent thunderstorms were observed, vae and adults in the water. Throughout the
382 Great Basin Naturalist [Volume 53
Table 1. Numberofadultfrogsduringthestudyperiod,
Febnian-Mav1975-19S9.
Year
1993] Pacific Chohls Froc in Ne\ai>\ 383
Tabi.E3. Frosiacti\it\ associati'clwitli timeoi\carandrangeDl'hothamliit-ntandwaterft'inperalures.
Tempi-raturerange('C)
Frogactivih' Timeofvcar Watei Air
Enteredpond,chorusing,andmating Late Fehruan-earK April
Oxiposition
Hatcliingandtadpoledexelopnieiit
ExitofsexualK acti\eadults
Coiupk'tionotmetamorphosis
Exitofmi'tamorpliose'dtatlpoles
\'en few ornofrogsin thepond
.
3S4 Gre.\tB.\six Natle.\list [\blimie53
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ectopic limbs in a wild r>fHyla regilla. amphibian populations a global phenomenon?
GreatBasinNaturalist4-^ Reportofaworkshopsponsoredb> Boardon Biolo-
Test. F.C. 1S9S.Acontribution to tiieknowledgeofthe g>".NationalResearchCouncil. Irvine.California
\-ariations ofthe treefrog^ Hyla regilla. Proceedings Whitney. C. L..-\ndJ. R. Krebs. 1975. Spacingandcall-
ofthe United States Nature Museum 21 1156i; ingin Pacifictreefiiogs. CanadianJournalofZoolo-
47T-i92. gy-53: 151&-1527.
V.v\ Denblrgh, J.. ANDJ. R. Slemn. 1921. Alistofthe
amphibians and rept-'!e? rfNe^-.ida. with notes on
the species in the academy. Pro- Received21 October1991
ceedings ofthe Ca. ofScience 11: AcceptedSMay1993
27-^."