Table Of Content28 Journalofthe Lepidopterists' Society
-JournaloftheLepidopterists'Society
61(1).2007,28-31
WHAT DO MONARCH POPULATION TIME SERIES TELL US ABOUT EASTERN ANDWESTERN
POPULATION MIXING?
RobertVandenbosch
DepartmentofChemistry,UniversityofWashington,Seattle,WA98195,e-mail: [email protected]
ABSTRACT.Timeseriesfortheannualvariationinthesizeofthecentral Mexicanoverwinteringpopulation,theCalifornia(western)sum-
merbreedingpopulation(4thof|ulvCount)andtheCaliforniacoastalwinteringpopulation(ThanksgivingCount)areexamined.TheCalifor-
niasummerandfollowingwinteringpopulationsarefoundtocorrelatewiththesizeofthepreviouswinter'scentral Mexicopopulationinsup-
portofthehypothesisdiatmonarchsmigratingnorthinthespringfrom Mexicocontributetothewesternpopulation
Additionalkeywords: Danausplexippus,Annualcensus.Timeseries,Climatedependence, Mexico,California
The monarch (Danaus plexippus) butterfly has long Brower and Pyle (2004) have summarizedthe evidence
been of interest because of its remarkable migratory for some interchange between the eastern andwestern
pattern. There are two North American populations of populations.
monarchs; one that summers east of the Rocky Thepurpose ofthis studyistoexaminecorrelationsin
Mountains and migrates to and overwinters in the year-to-year Monarch fluctuations at the eastern and
mountain forests in central Mexico, and another western population overwintering sites and at western
population west of the Rocky Mountains that winters population breeding sites for clues as to whether the
along the central and southern California coast and western population is supplementedby Monarchs from
disperseseastandnorthinthesummer(Urquhart 1987, die eastern population overwinteringareain Mexico.
Brower 1995a).
Methods and Data Sources
The recognition ofthis pattern has arisen and been
supported by extensive mark/recapture results, This study makes use ofthree annual census counts.
particularly for the eastern population (Monarch The first is the 4th ofJulv Butterfly Count, the longest
Watch). Unlike birds, but similar to the migratory running monarch butterfly monitoringprogram. Itwas
Painted Lady (Vanessa cardui) butterfly, any particular initiated bythe Xerces Societyin 1975 and is presently
individual ofthe eastern population does not make the under the auspices of die North American Butterfly
complete trip from the wintering grounds to the Association (NABA). Butterflies within a 15-mile
northernmost summer grounds. Instead the returning diametercirclearecounted. Therearefivecountcircles
overwintered individuals breed along the Gulf Coast west of the Rocky Mountains for which counts have
and their offspring continue the migration northward been performed everyyear during the last decade, and
(Malcolm et al.1993, Knight et aZ.1999). Additional at which Monarchs are seen on a regular basis. These
breeding generations complete the northeastern are Cosumnes River (38 21 N, 121 27 W), Mt. Diablo
recolonization (Brower 1996). Thetemporalvariationin (37 57 N, 121 52 W), Willow Slough (38 34 N, 121 44
die recolonization rate has been shown to be W), Dardanelles (38 22 N, 119 45 W) and SanJoaquin
remarkablystable fromyeartoyear(Davis andHoward, Co. (38 9 N, 121 18 W). The number of butterflies
2005). The return migration to the overwintering areas observed has been divided by the number of party-
is made by the final fall generation that enters hours reported in an attempt to correct for differences
reproductive diapause (Urquhart 1960) and flies to inobservereffortandcoveragefromyeartoyear. Issues
Mexico (Urquhart and Urquhart, 1978). relatedtothese counts havebeendiscussedbySwengel
The extent to which the two populations may mix is (1990). Many counts have observers that have
largely unknown. Some mark-release-recapture results participatedin aparticularcountcircle foranumberof
(Monarch Watch) have trajectories that hint at mixing. years, helping to make observed changes in butterfly
Directions ofmovement ofindividuals in the westhave numbers from year to year more accurate. One count
alsobeen interpretedas indicating migration paths that circle, Willow Slough, CA, has been counted by the
might lead to population mixing (Pyle 1999). However same observer, Art Shapiro, forallbutone ofthelast27
mitochondrial DNA sequencing indicates very little years (NABA 1999)! The count results are presently
heterogeneity between the eastern and western North published yearly by NABA (4 Delaware Rd.,
Americanpopulations, andevenbetweenthe Northand Morristown, NJ 07960).
SouthAmerican populations (A. Broweretal. 2004). L. The second census, the Monarch Program
Volume 61, Number 1 29
Thanksgiving Count ofthe overwintering population in
California, is patterned after the 4th ofJuly Butterfly K
Count, takes place during a 2-week period around ffi
Thanksgiving. Approximately 100 overwintering sites in >I>
K
15 California counties are surveyed. The largest <
numbers ofMonarchs are between San Francisco and \PL.
LosAngeles. Thefirstcountwasin 1997. Theresultsfor EZ- 5 10 15 20
!O=> MEXICO (HECTARES)
the seven years 1997-2003 have been compiled by O 800000 - THANKSGIVING vs. 4th of JULS
Dennis Frey and Shana Stevens and are posted on the
Monarch Programwebsite (www.monarchprogram.org/
tagging.htm). The results from die first fouryears have
been discussed by Frey and Schaffner (2004). The
1997-2003 data, as well as 4th ofJuly Butterfly Count 5 10 15 20 2!
data for the period 1977-1999 has been examined by 800000 4th of JULY (COUNT/PH)
Koenig (2006). He looked for spatial synchrony, the 600000 ! THANKSGIVING vs. MEXICO ;
extenttowhichthepopulationsvaryinsizefromyearto —
Eh 400000
yearinunisonoverconsiderablegeographicaldistances.
A modest (correlation coefficient about 0.3) but gO 200000£- ,£<<_
statistically significant correlation was found for the :, 5 ,110, ,115, ,210,
Thanksgivingcounts overdistances as large as 1000km. MEXICO (HECTARES)
With onlv three years overlap with his 4th ofJulv data
set he did not compare trends between die two data Fig. 2. Regressionplotsforthedireepairsofvariables.
sets.
of monarchs winter in high-altitude oyamel forests in
The third count, the Monarch Butterfly Biosphere
central Mexico. Garcia-Serrano et al. (2004) have
Beserve census, estimates the size ofthe overwintering
describedamonitoringprograminitiatedin 1993 under
population in Mexico. The eastern breedingpopulation
the auspices of the Monarch Butterfly Biosphere
Beserve. All known colonies within and outside die
reserve are monitored. Batherdian attemptingtocount
individual butterflies, the surface area occupied bv
colonies was measured. The data collection takes place
during the last twoweeks in December at atimewhen
the colonies have consolidated and are least active. In
January an estimate of the mortality at each site was
assessed. (This mortality7 estimate reflects bird and
mouse predation as well as mortality- due to unknown
causes. Itdoes notcapturethe mortalityassociatedwith
winterstorms thatoccurlaterin diewinter).
Estimates of the areas occupied bv overwintering
monarchs are from updates appearing on die Monarch
Larvae Monitoring Program (www.mlmp.org) and
Journey North (www.learner.org/jnortli/tni/monai-cli/
PopulationMexico.html)websites. Since I aminterested
in estimating the influence ofthe size of the wintering:
population on the following summer and fall monarch
1994 1996 1998 2000 2002 2004 2006 numbers, I have reduced bv 75% the Decembercount
YEAR
for the 2001-2002 winter due to die severe winter
Fig. 1. Comparisonofwintering(eastern)populationin Mex- storm in January of 2002 (Brower, Kust et ah 2004).
Cicaoli(ftoorpn)i,aCTalhiafnokrsngiiav(iwnegstCeorunn)tsu(bmomtetorm)p.opTuhleatiMoenxi(cmainddploep)ualnad- There was also a significant winter storm effect on die
tionsizeisexpressedashectaresoccupied.Theplacementofdie 2003-2004 winter population but as estimates for die
bars reflectsthetimeofyearatwhichthecensus isperformed, mortality differ greatly (Monarchwatch: www.monarch
pegl.otttheedbaatrf1o9r96t.h9e.1T9h9e6—149th97oMfeJxuilcyandawtiantiesritnhgepospuumlatoifonthies watch.org/update/2004/0216.litml) Ihavenotattempted
counts/partyhourfor5Californiasites. acorrection forthis season.
30 Journalofthe Lepidopterists' Society
The largest area occupied by monarchs since the Taule 1. Monarch counts from three kinds of censuses. The
initiation ofthis project was about 20 hectares (ha) in wintering census in Mexico are characterized by the number of
hectares occupied, and are listed for the year in which Jan. ofthe
Dec. 1996. Using previous estimates of butterfly particularwinteroccurs. The 4th ofJuly results are the sum ofthe
densities of approximately 10 million butterflies/ha monarch's per party hour for 5 California sites. The Thanksgiving
(Calvert & Brower 1986, Brower 1995b) would suggest counts arethenumberofmonarchsobservedsummedoverallsites
surveyed.
a population of 200 million. More recent density
estimates (Broweretal. 2004; Calvert 2004) wouldlead Year Mexico 4thofJuly Thanksgiving
to a much larger population total. During some years 1994 3.03
howeverthe areaoccupiedbymonarchswas onlyabout 1995 7.8 3.08
l/lO*ofthis. 1996 12.6 14.61
Comparison of MexicanWintering Population, 1997 20.97 17.81 631140
California SummerPopulation, and California 1998 5.77 13.99 353272
Wintering Population Time Series 1999 5.56 2.71 156659
2000 9.05 8.64 214198
Population estimates for the Mexican wintering
2001 2.83 2.08 98418
population are available for about a decade and forthe
2002 2.35 2.33 65375
Californiawintering population ("Thanksgiving" count)
forsevenyears goingbackto 1997. There are five count 2003 8.0 0.31 111909
circles in California where monarchs were regularly 2004 11.12 8.69
Linearregressionperlbnnedonthesedatagivethefollowing
observed during the 4th of July Count and for which
relations:
data are available over the last decade. The count per 4thJuly=0.84• Mexico+0.60
party hour values for these sites have been added and Thanksgiving=3,120'4thJuly+ 1960
Thanksgiving=2810° Mexico+ 1410
compared with the Mexican wintering population and
TheserelationsareillustratedbythestraightlinesinFig.2
the Californiawinteringpopulation estimates in Fig. 1.
The moststrikingobservation from this figureisthatthe correlations are: firstly, there is some climate variation
California summer population and the California effect common to both populations, and, secondly, that
winteringpopulation exhibitamaximum in thesummer the western population is supplemented by monarchs
and fall following thewinterwith the largestpopulation from the Mexican overwintering population. The first
in Mexico, in spiteofthegeneraltenetthatthe Mexican possibility does not seem likely as it would seem to
wintering population remigrates to eastern and central require the springandsummerclimatein the easttobe
NorthAmericaviathegulfcoastratherthantothewest favorable the year before a favorable spring and
coast. summer climate in the west. This requirement arises
Multiple regression has been used to explore the because the peaks in the summer 4th ofJulycount and
correlation between the Mexican winteringpopulation, the California Thanksgiving count lag the peak in the
the 4th of July summer counts, and the California Mexican count byone halfand oneyear, respectively. A
wintering population (Thanksgiving counts). The data second explanation is that some fraction ofthe eastern
usedforthis analysis aregiveninTable 1, alongwiththe Mexican winteringpopulation may migrate to the west
regression equations obtained. The results of the in the spring and contribute to the western population
regression analysis are displayed in Fig. 2. The sum of in the followingsummerandfall. Such ahypothesiswas
the count/party-hour for die five California 4th ofJuly put forward independently by L. Brower and S.
counts is found to be strongly correlated with the Gauthreauxin 1996 (see Browerand Pyle, 2004). Since
previous winter's Mexican population, R2=0.57, the easternwinteringpopulation is about two orders of
p=0.019. The Thanksgiving count is also strongly magnitude larger than the western wintering
correlated with the previous winter's Mexican population, itwouldrequireonlyaverysmallfractionof
population, R2=0.78, andp=0.008. As expectedfromthe the Mexican overwintering population to noticeably
two previous correlations, the Thanksgiving count influence the western breeding and subsequent fall
correlates well with the 4di of July count/party-hour winteringpopulation. BrowerandPylemakethefurther
sum, R2=0.89, p=0.001. Thus well over half of the suggestion that the long-term survival of the western
fluctuation irom year-to-year in the western 4th ofJuly population of monarchs may depend on occasional
count and the western Thanksgiving count can be replenishment from die Mexican winter population.
explained in terms ofdie size ofthe Mexican wintering This mechanism mayalsohave enabledan expansionof
population. diewesternpopulationtoexploitchangesincoastaltree
Two possible explanations for these strong species distributions in historical times. The western
Volume 61, Number 1 31
population's wintering grounds are presently primarily Davis, A. K, and E. Howard. 2005. Spring recolonization rate of
in Eucalyptustrees,whichare notnativetothe U.S. and monarch butterflies in eastern North America: new estimates
fromcitizen-sciencedata. Lepid. Soc.59:1-5.
were introduced by settlers in the 1850s (Lane 1993). Frey, D., andA. Schaffner,J2.004. Spatial andtemporalpatternof
Lane has reviewedthehistorical evidence forthe use of monarch overwintering abundance in western North America.
othertree speciesbyoverwintering Monarchs. Pages 167-176In K. S.Oberhauser, M.J. Solensky,editors.The
Monarchbutterfly:biologyandconservation. Cornell University
Press,Ithaca,NY.
Summary
Garcia-Serrano, E„ L. Reye,andB.X. M.Alvarez,2004. Loca-
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The time correlation ofthe year-to-year fluctuations tionsandareaoccupiedbymonarchbutterfliesoverwinteringin
Mexicofrom 1993to2002. Pages 129-133In K. S.Oberhauser,
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inferreddiatthewesternpopulationis supplementedby Journey North, http://www.learner.org/jnortli/tm/monarch/Popula-
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contributions from the eastern population. This Knight, A., L. P. Brower, andE. H. Williams, 1999. Springremi-
inference, based only on time series correlations, was grationofdiemonarchbutterfly,Danausplexippus(Lepidoptera:
possible independent ofthe mechanism responsible for Nymphalidae)innorth-centralFlorida;estimatingpopulationpa-
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Koenig,W. D.,2006. Spatial synchronyofmonarchbutterflies.The
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