8 NOTICIAS DE GALÁP AGOS No. 58

CURRENT STATUS, ANALYSIS OF CENSUS METHODOLOGY,AND CON SERVATI O N OF THE GALÁPAGO S PENGUIN,

SPHENISCUS MENDICULUS

By: Kyra 1. MilIs and Hernán Vargas

INTRODUCTION

The Galápagos penguin, Spheniscus mendiculus, is athreatened species endemic to the Galápagos Islands, andone of the least studied penguins in the world. Approxi-mately 90 percent of the popula tion of Galápagos penguinsinhabits the two western-most central islands ofFernandina and Isabela, which coincide with the mainpath of the Cromwell Current. This eastward-flowingundercurrent upwells as it meets the western edge of thearchipelago, lowering the surface temperature by severaldegrees and increasing the productivity in the surround-ingwaters (Houvenaghel1978). Since 1970 the populationof Galápagos penguins has been monitored via partial

Galapagos Penguin - Bahía Sullivan, Isla Santiago

and total censuses as a result of the joint efforts of theCharles Darwin Research Station (CDRS) and theGalápagos National ParkService (GNPS). Datafrom thesecensuses reveal a fluctuating popula tion tha t experienceda sharp decline during the El Niño event of 1982-83, anda slow increase since. The intent of this paper is to sum-marize the results of censuses from 1970 to 1995, and todiscuss the methodologies used to estima te the total popu-lation of Galápagos penguins. In addition, we presentnovel data on the expansion of the breeding range of thisspecies of penguin and implications for its preservation.

There are several factors that contribute to the lowreproductive rate of the Galápagos penguin, which mayexplain the slow recovery from declines caused by eventssuch as an El Niño. ApparentIy the Galápagos penguin isan opportunistic breeder, breeding when food is plentiful(Boersma 1974). However, once breeding has started theperiod is long, lasting approximately three months fromthe incubation of eggs through fledging of young. Fur-thermore, the mortality of juveniles is high (Boersma 1974).Past censuses indicate that the present population of pen-guins is still50 percent smaller than before the 1982-83 ElNiño, which caused a 77 percent decline (Valle 1983).Regular monitoring of the population of penguins is ofvital importance for the preservation of this species due toits restricted distribution, the effects caused by El Niñoevents, the threat of introduced animals, and the potentialimpacts of human activities such as tourism and fishing.

The CDRS and the GNPS are attempting to conductannual censuses of penguins and to standardize the meth-ods used. Standardized methods for censuses can decreaseinconsistencies caused by varying personnel, and/ or thetiming of censuses. Furthermore, standardized methodsfacilitate the analysis and monitoring of population trendsfrom one year to the next. A manual that presents meth-odology for censuses of penguins and flight1esscormorants was written in 1989 and has been reviewedand updated in 1993, 1994, and 1995 (Castro et al. 1995).

CENSUS METHODOLOGY

Because the flightless cormorant (Campsohaelius[Nannopterum] harrisi) and the Galápagos penguin havesimilar breeding distributions, both species are censusedsimultaneously. This decreases the time and eliminatesthe extra costs of conducting a separa te census for eachspecies. Even though both species are censused during a

May 1997 NOTICIAS DE GALÁP AGOS 9

single trip, this paper will mainly focus on the censusmethodologies for Galápagos penguins.

In conducting a census, several considerations areimportant, such as the particular behaviors and habits ofthe species involved, the time of the year, the time of dayto conduct the census, and the personnel involved. Eachof these factors may introduce error in the results if notplanned carefully.

Personnel involved in conducting any of these cen-sus es are two wardens from the GNPS and two scientistsfrom the CORSo All four people search for penguins bothon land and in the water, although one park warden andone scientist are the principal observers. The second parkwarden is in charge of managing the dinghy, and the sec-ond scientist keeps a written record of general weatherconditions and the number of penguins counted. Thecensuses are conducted from a small dinghy, approach-ing the coast as closely as possible. Counting begins in theearly morning, between 5:30 and 5:45, and ends between17:30 and 18:00, with an hour off at noon. Binoculars(7x25 and 8x40) are used to count penguins that are in thewater and on land, and penguins are identified, whenpossible, as juveniles or adults. The presence of moltingpenguins is also noted. Atmospheric and oceanographicconditions, including cloud cover, air and sea-surfacetemperatures and water clarity (measured with a Secchidisc) are recorded from the support ve ssel at regular in-tervals.

Oespite efforts to standardize counting methods, oneof the factors that may contribute to annual variability inefficiency is that each person searches for penguins in aslightly different manner, thereby introducing errorwhenpersonnel are changed. Annual variations in the appar-ent numbers of penguins counted often correlate withchanges in personnel, and may be due to individual dif-ferences and abilities in spotting penguins. Ouring thelast three censuses (1993, 1994 and 1995), two individualsparticipated in two out of the three censuses. AIso of greatimportance is the person who helms the dinghy. Skillfuland safe handling of the dinghy combined with knowl-edge of the census work and the character of the coastlinegreatly increases the numbers of penguins encountered.

The actual size of a population may be very differentfrom the number of individuals counted in a census.Breeding birds may be more sedentary while activelyforaging birds are more conspicuous. Flightless cormo-rants breed in small colonies on top of the lava. Thismakes them fairly conspicuous and relatively easy tocount. Their breeding activity is also obvious and easilyrecorded. On the other hand, Galá pagos penguins nest increvices in the lava. This makes them difficult to observewhen they are breeding, and the extent ofbreeding activi-ties is rarely determined during a census. Furthermore,during the times when a census is conducted, most pen-guins are foraging at sea, and are very easily missed byobservers. The most conspicuous penguins are those onland at the water' s edge.

Conducting censuses at the same time every year re-duces errors caused by comparing reproductive andnon-reproductive periods. When Galápagos penguinsbreed, one member of a breeding pair remains at the nestin the cave during the incubation period and the first fewweeks after the chick ha tches (Boersma 1974). Those pen-guins are not observed in a census. Past censuses ha veoccurred during different times of the year, varying fromone in January, two in August, two in late August/ earlySeptember, two in September, two in late September /early October, and three in October. Since 1993, person-nel from the CORS and the GNPS have set the date for thecensuses in the period between late August and earlySeptember, given that the majority of past censuses wereconducted during those two months.

ESTIMA TING POPULA TION SIZE

Because cormorants are more apparent, a greater per-centage of their populations are counted in a census. Thusa census may more directly reflect the actual size of theirpopulation compared to a census of penguins. Boersma(1974) attempted to account for this error in estimatingthe actual size of penguin populations by tagging indi-viduals from two specific popula tions and then conductingcensuses.

She established a method of estimating the total popu-lation size of penguins based on those mark-recaptureexperiments in 1970 and 1971. By comparing the numberof banded penguins sighted at two different periods (be-tween January and March, and between June and October1972), she concluded that penguins counted in a censusrepresent between 10 and 20 percent of the actual numberof penguins in the population. She then estimated a totalpopulation size between 11,000 and 23,000 penguins in1971. Using the same method, the range of estimates forthe population size in 1983 (the year when the El Niñooccurred) is from 2,000 to 4,000 individuals, and that for1995 (the most recent census) from 4,000 to 8,500.

Currently this method of estimation is questionable

. because of two potentially important differences in themethods that Boersma used in her analysis and those usedrecently. Boersma based the maximum and minimumlimits of her estimation on counts of penguins done be-tween 15:00 and 18:00, times when the penguins are mostlikely to be found on land. Current censuses are donebetween5:30 and 18:00. Therefore, in addition to the timesthat Boersma included in her estimates, current censusescover additional hours when many penguins are at seaforaging (between about 6:00 and 16:30). Thus, recentcensuses probably count a lower percentage of the totalpopulation of penguins than the 10 to 20 percent docu-mented by Boersma (Vargas 1995b).

A second factor that should be taken into account wheninterpreting estimates of the total population based onBoersma's method, is that her counts of penguins weredone in two protected areas, Elizabeth Bay (Isla Isabela)

10 NOTICIAS DE GALÁP AGOS No. 58

2.

0°Cabo Berkeley

Isla IsabelaBahía Banks 10 km

Penguin ZonesFernandina

1 -23 .F..T."-.-

4Isabela

5"

6."."''''~

7.1 11 I

7.2-7.3 II7.4 ....8 -9

Bahía Elizabeth

1°S

Cabo Rosa91°W

Figure 1. Zones used for dividing the census efforts on Islas Fernandina and Isabela. On Isla Isabela the darker regions indicaterelatively barren lava flows. Lava flows are not indicated on Isla Fernandina.

---

NOTICIAS DE GALÁP AGOSMay 1997

and Punta Espinosa (Isla Fernandina). Current censusesincludemanyareas that are exposed towaves, whichmakeit more difficult to spot penguins both on land and in thewater (Vargas 1995b). These factors, along with the longerperiod of the day used for censusing, reduce the successin spotting penguins. Hence, using Boersma' s 10 to 20percent as an index for estimating total population sizesmay not be accurate. If current censuses record a lowerpercentage of actual birds, then using Boersma's indiceswould under-estimate the total population. To estima tethe number of penguins present in the Galápagos moreaccurately, devising a new set of indices based on mul-tiple censuses of marked individuals will be necessary.

Because limited funds make conducting a total censusevery year difficult, Rosenberg et al. (990) used analysesof multiple regression to determine the best areas in thearchipelago for predicting the total population size. Hence,a partial census could potentially be conducted at a re-duced cost, and still be an accurate estima te of the totalnumber of penguins. Islas Isabela and Fernandina weredivided into nine zones (Figure 1), which had originallybeen established by Boersma in 1970-71. Rosenberg et al.used data from censuses in 1970 through 1986 to deter-mine which zone or zones were the most reliablepredictors, in other words, which zones had the greatestcorrelation with the total number of penguins observedin a census. They concluded that Zone 7 (on Isla Isabela)was the overall best predictor of the total penguin popu-lation, although Zone 1 (on Isla Fernandina) was also agood predictor. Both zones had the greatest abundance ofpenguins in the censuses. In years when funds are lim-ited, therefore, a census could be conducted in Zone 7,and Zone 1 could be included for more accuracy. Due tothe relatively large area that Zone 7 encompasses, Vargas0995a) further divided Zone 7 into four subzones (Figure1).

RESULTS

To date, 12 total and three partial censuses ha ve beenconducted.. The first systematic census of this species wascarried out in 1970 by P. D. Boersma, who reported a totalcount of 1584 individual s, and estimated a total popula-tionsizebetween6,OOOand 15,000 (Boersma 1974). Furthercensuses were conducted in 1971 (Boersma 1977), 1980(Harcourt1980a, 1980b), 1983 (Valle 1983),inJanuary1984(Valle 1984), in September 1984 (Valle and Coulter 1984),1985 (Valle 1985), 1986 (Rosenberg and Harcourt 1987),1989 (Castro 1989), 1993 (Mills 1993), 1994 (Soria et al.1994), and 1995 (Vargas 1995a). Partial censuses werecarried out in 1988 (Vargas 1988), 1990 (Paton and Valle1990),and 1991 (Palacios 1991). Themostdramaticpopu-lation fluctuation was recorded in the census of 1983,immediately following the warm-water event known asan El Niño. This census revealed a total count of 398individuals, or a 77 percent reduction in the populationfrom 1980 (Valle 1983). Population counts since then

11

suggest that the penguin population is still about 50 per-cent below its original numbers before the El Niño of1982-83 (Figure 2).

Since the decrease of 77 percent that occurred in 1982-83, a small but steady increase in numbers can be seen,with the 1994 census having the greatest number of pen-guins in thirteen years (Figure 2). One must keep in mind,however, thatthere is error between the counts thatmakescomparing these absolute numbers potentially mislead-ing. Nevertheless, given the similar methodology used inconducting the censuses, the margin of error is possiblysmall. The two major populations of Islas Isabela andFernadina are monitored separately beca use Isla Isabelahas introduced predators such as rats, cats, and dogs,which can ha ve serious effects on penguin populationswhile Isla Fernadina lacks them (Harcourt 1980b, Valle1986, Rosenberg and Harcourt 1987). Isla Isabela had alarger population count before 1980, after which IslaFernandinahad slightly greater numbers until1993 when,once again, Isla Isabela had a larger population (Figure 2).Variation in the numbers of penguins counted among thecensus zones appears to reflect the overall pattern of fluc-tuations at the population level (Table 1).

One indication of a recovering population is an in-creasing number of young or juveniles observed,indicating successful breeding. Before the 1980 census,adults and juveniles were not counted separately andtherefore numbers of juvenile are only known since 1980.Figure 3 shows the percentage of the total population thatwere juveniles since the 1980 census. Before the 1982-83El Niño the number of juveniles was low 0.8 percent in1980), and it remained below 10 percent unti11989. Theyear that shows the highest reproduction is 1993, with35.8 percent juveniles. The 1994 census showed a highproduction (23.6 percent juveniles), although somewhatlower than the previous year, and in 1995 juveniles com-prised 17.2 percent of the population.

DISCUSSION

It is not surprising that the censuses of penguins haverevealed a slow recovery since the 1982-83 El Niño event,given their high mortality of juveniles,low reproductiverate, and long breeding periodo Another limiting factor isa fluctuating food supply, caused by the spatial and tem-poral variation in the ocean currents that are directlyresponsible for the availability and abundance of pen-guins' prey. In addition to these factors, it is possible thathuman activity may ha ve adverse effects, especially inthose areas where it is great.

Contrary to many other species of Galápagos animal s,penguins are wary ofhumans and easily disturbed. There-fore, high levels of tourist and/ or fishing activity maypose serious threats to penguins, given their already lownumbers. Fortunately, most penguins inhabit the coastsof Islas Fernandina and Isabela, areas where tourism isrestricted to a few sites, although illegal fishermen have

D Fernandina fI Isabela~¡;;

'"~¡!J 9i iG

12

-t: 00O>..---00¡¡¡(5 70f-ü) 00Q)

c¡; [i)~

<.9 40Q)~+-' 3J'O

"E 3JQ)

~ 10Q)

o...O

c::O~ 00

::JCl.O [i)

o...¡¡¡(5 40f-Q)

.s 3J'O

~ 3JCO"E

~ 10.....Q)

o...

100

70

o

,~

1970

1931

1971

NOTICIAS DE GALÁP AGOS No. 58--

1720.

1980

463

3:-----

1983

665

~c~4

4~.

1984 1985

692.

873 844

6~

1984

. Santiago, Rábida, Baltra, Floreanaffi

Si

o

"'!!i

1986 1989 1993 1994 1995

Figure 2. Variation in the numbers of penguins censused. The bars represent the percentages of the total number counted in ayearthat were found in different areas. The dots represent the totals counted in differentyears as a percentage of theyearwith the greatestpopulation (1971). Dots from adjacent years are connected to indica te trends. The numbers above bars and dots are the actualnumbers of penguins counted that formed the percentages. Because there are missing years it is important to be cautious invisualizing growth or declines of the population of Galápagos penguins.

been found camping on these remo te coasts in recent years.Nevertheless, the most highly visited areas in the archi-pelago are also areas where small numbers of penguinsexisto Even though these populations are small, they maybe important for recruitment to the general population(Mills 1994).

Past studies (e. g., Boersma 1974, Harris 1982, and ref-erences therein) found the breeding range of the Galápagospenguin restricted to the coasts of the westernmost cen-tral islands (Fernandina and Isabela), adjacent to areaswhere the deep Cromwell Current upwells. Boersma andHarris did not find penguins breeding in the central is-lands, which are surrounded bywarmer water s, althoughthey did reportthe presence of juveniles and non-breedingadults in several of the central islands and Isla Floreana,a southern island. Recently, breeding was confirmed onIsla Bartolomé in 1993, 1994 and 1995 when chicks were

found (Mills 1993, Soria et al. 1994, Vargas 1995a), andreports from scientists and naturalist guides suggest thatbreeding may also be occurring on several other islandsas well (Isla Floreana [see Vargas et al. this issue), IslasSombrero Chino and Rábida), although there has been noconclusive evidence ofbreeding found on these islands inthe past three censuses. Prior to 1993, the central andsouthern islands were not included in penguin censuses,but since 1993 the total censuses have been organized toinclude these islands where the numbers of penguins maybe increasing (Figure 2).

Isla Bartolomé is one of the most visited sites in thearchipelago and therefore the potential for disturbance ofit's population of penguins is great. Small dinghies withgroups of tourists approach penguins in the water as wellas on land. It appears that these penguins are somewhataccustomed to this level of tourist activity, and allow a

May 1997 NOTICIAS DE GALÁP AGOS 13

Table 1. Counts of penguins per census zone by year.

Zone' Yearly Total Counted Average %

1970 1971 1980 1983 19842 19843 1985 1986 1989 1993 1994 1995

Fernandina1 41 381 373 86 50 102 134 123 132 92 163 84 147 162 36 66 144 11 21 56 41 23 65 40 17 30 46 53 83 32 151 73 6 22 17 17 28 32 3 19 40 44 586 281 257 67 174 68 165 161 151 109 142 138 192 21

Isabela5 75 15 75 5 10 8 18 37 46 35 96 38 38 46 144 239 197 45 34 59 38 21 20 36 37 33 75 87 568 619 456 93 154 79 232 175 212 212 178 246 269 308 51 298 63 18 14 41 20 27 38 87 199 220 90 109 O O 4 O O O O O O O O 1 0.42 0.05

Others10 NA NA NA NA NA NA NA NA NA 19 38 35 31 3

TOTAL 1584 1931 1720 398 463 435 665 584 692 662 873 844 904

1See Figure 1 for definition of zones.2 This 1984census was in January3 This 1984census was in September.

closer approach than penguins do on Isla Fernandina orIsla Isabela. However, many penguins on Isla Bartoloméshow signs ofbeing disturbed because they enter the wateruponapproach,ortheyswim away, if aIread y in thewater(Mills, personal observation). Even though GNPS regu-lations prohibit chasing or touching the animals, thesemIes are not always respected. At the moment, there is norestriction on how clase tourists can approach penguins.In the past, the potential disturbance of penguins causedby visita tia n on the central islands was not considereddetrimental tothepopulation becauseitwas believed thatthese penguins were immature and therefore non-breed-ers. However, because breeding penguins are more easilydisturbed, and we now know that penguins are breedingin the central islands, new tourist regulations may need tobe applied in these areas.

Introduced animal s may pose serious threats to thepopulation of penguins on islands such as Isla Isabela.The potentially most damaging introduced animals ap-pear to be rats, cats and dogs, w hich can eat penguin eggs,juveniles, or adults. Because these introduced animalspose serious threats to penguins and other endemic spe-des, the CDRS and GNPS hope to control their populations.Nevertheless, funding for these programs is scarce anderadication is difficult on some of the larger islands.However, localized control of populations of introducedanimals is feasible, and could be implemented in areasthat have the highest densities of breeding penguins andcormorants. Of particular concern in the past few yearshas been the possibility of introdudng alien animals toIsla Fernandina, the largest remaining island in the archi-

pelago with no introduced vertebrates. Because of therecent illegal fishing camps on this island, the likelihoodfor the introd uction of animals has increased, although sofar there has been no confirmation of any introductions.Because Isla Fernandina has high densities of breedingflightless cormorants and Galápagos penguins, the intro-duction of animals such as rats, cats, or dogs would bedevastating to these endemic seabirds. In addition, hu-man disturbances such as fishing offshore and camping

40237

c¡¡ 35o:: 3JoQ)

g'2Se~ 2J.....Q)

c...

en 15~

~ 10'c~ 5:J

""')

o31

3-=1980 1983 1985 1986 1989 1993 1994 1995

Figure 3. The population of juveniles as a percentage of the totalpopulation. The bars represent percentages and the numbersabove the bars are the numbers of juveniles counted.

NOTICIAS DE GALÁP AGOS-.-._-

14 No. 58

could have contributed to the decline in numbers of pen-guins that has been observed on Isla Fernandina since1993.

Since the 1982-83 El Niño there have been six El Niñoevents of weaker intensity. The 1982-83 El Niño appearsto be one of the most severe ever observed (Barber andChávez 1986). El Niño events in Galápagos are character-ized by increases in sea-surface temperatures and thesuppression of marine primary productivity (phytoplank-ton). This decrease in primary productivity creates acascade effect on the higher trophic levels, including thetop level of marine mammals and seabirds (Barber andChávez 1983, Cane 1983). Hence, in El Niño years thereis a reduction in the availability of prey for penguins.Now that penguins are known to be breeding on the cen-tral islands and because El Niño events may be morefrequent, the small, central island populations may becrucial for the maintenance of penguins in Galápagos.

Results from recent censuses have given us a reason tobe optimistic about the recovery of the population ofGalápagos penguins. The high percentage of juvenilesand the highest numbers of birds counted since the dra-matic decline following the 1982-83 El Niño, suggest thatthe penguin population is recovering, although slowly.As with any other population of animals, there will al-ways be natural fluctuations in the numbers of Galápagospenguins, but it is within our capacity to diminish thenegative effects of human activity. When natural fluctua-tions caused by events such as an El Niño are combinedwith fluctuations caused by human activity, the resultscan be devastating. Therefore, ifwe are successfulin elimi-nating the negative human effects, then this rare speciesof penguin will have a better chance at survival. Theregular monitoring of these populations through annualcensuses will foster the early observation of potentialdeclines.

ACKNOWLEDGMENTS

We would like to thank personne1 from the CharlesDarwinResearch Station and theGalápagos National ParkService who have been directly or indirectly involved inthe organization and planning of the censuses ofGalápagos penguins and the flightless cormorants. Wewould especially like to thank the Penguin Fund (Japan),who have helped finance and make the censuses possiblein the past few years. Our sincere gratitude to Georginaand Augusto Cruz and the crew of the Beagle III for thedonation of the vessel for the census trips of 1993 and1994. We also appreciate the contributions made byT AMEairlines in the form of reduced airfares to and from theGalápagos.

LITERA TURE ClTEO

Barber, R. T. and F. P. Chávez. 1986. Ocean variability in rela-tion to living resources during the 1982-83 El Niño. Nature319: 279-285.

Boersma, P. D. 1974. The Galápagos penguin: Adaptations for1ife in an unpredictab1e environment. Ph.D. dissertation,The Ohio State University, Columbus, Ohio.

Boersma, P. D. 1977. An ecological and behavioral study of theGalápagos penguin. Living Bird 15: 43-93.

Cane, M. A. 1983. Oceanographic events during El Niño. Sci-ence 222: 1189-1195.

Castro, 1. 1989. Report of the 1989 census of the flightless cor-morant and the Galápagos penguin around Fernandina andIsabela. Report to the Charles Oarwin Research Station andthe Galápagos National Park Service.

Castro, l., K. L. MilIs, L. Cayot, M. Soria, and H. Vargas. 1995.Operational Manual: Galápagos penguin and flightless cor-morant Census. Report to the Charles Darwin ResearchStation.

Harcourt, S. 1980a. Report of the 1980 census of the flightlesscormorant and the Galápagos penguin around Fernandinaand Isabela. Report to the Charles Darwin Research Stationand the Galápagos National Park Service.

Harcourt, S. 1980b. Report on a census of the flightless cormo-rant and Galápagos penguin. Noticias de Galápagos 32:7-11.

Harris, M. 1992. Birds of Galápagos. Sixth edition. HarperCollins, London.

Houvenaghel, G. T. 1978. Oceanographic conditions in theGalápagos Archipelago and their relationships with life onthe islands. In: R. Boje and M. Tomczak (eds.), UpwellingEcosystems, 181-202. Springer-Verlag, New York.

MilIs, K. L. 1993. Report of the 1993 annual census of theGalápagos penguin and flightless cormorant in theGalápagos Archipelago. Report to the Charles Darwin Re-search Station and the Galápagos National Park Service.

MilIs, K. L. 1994. Field Season Report 1994. Report to theCharles Darwin Research Station and the Galápagos Na-tional Park Service.

Palacios, J. 1991. Censo parcial de pingüinos y cormoranes.Report to the Charles Darwin Research Station and theGalápagos National Park Service.

Paton, S. and C. Valle. 1990. Censo parcial de pingüinos ycormoranes. Report to the Charles Darwin Research Stationand the Galápagos National Park Service.

Rosenberg, D. K. and S. A. Harcourt. 1987. Population sizes andpotential conservation problems of the endemic Galápagospenguin and flightless cormorant. Noticias de Galápagos45: 24-25.

Rosenberg, D. K., C. A. Valle, M. C. Coulter, and S. A. Harcourt.1990. Monitoring Galápagos penguins and flightless cor-rnorants in the Galápagos Islands. Wilson Bulletin 102(3):525-532.

Soria, M., K. L. MilIs, and L. Cayot. 1994. Report of the 1994annual census of the Galápagos penguin and flightless cor-morant in the Galápagos Archipelago. Report to the CharlesDarwin Research Station and the Galápagos National ParkService.

May 1997 NOTICIAS DE GALÁP AGOS 15

Valle, C. A. 1983. Report of the 1983 census of the Galápagospenguin and flightless cormorant in the Galápagos Archi-pelago. Report to the Charles Oarwin Research Station andthe Galápagos National Park Service.

Valle, C. A. 1985. Alteración de las poblaciones del cormoránno volador, el pingüino y otras aves marinas en Galápagospor efecto de El Niño 1982-83 y su subsecuente recuperación,p.245-258. [11:G. R. Robinsonand E.del Piño (eds.), Galápagos1982-83: A chronicle of the El Niño. Springer-Verlag, NewYork.

Valle, C. A. 1986. Status of the Galápagos penguin and flightlesscormorant populations in 1985. Noticias de Galápagos 43:16-17.

Vargas, H. 1995a. Informe del censo total de pingüinos ycormoranes en 1995. Report to Charles Darwin ResearchStation and Galápagos National Park Service.

Vargas, H. 1995/7. Comentarios sobre la estimación de laspoblaciones de pingüinos y cormoranes. Appendix 2, Re-port to Charles Oarwin Research Station.

Kyra 1. Mills, Department of Ecology and EvolutionaryBiology, University of California, Irvine, CA 92717-2525USA (klmills@uci.edu). Hernán Vargas, Charles Dar-win Research Station, Puerto Ayora, Isla Santa Cruz,Galápagos, Ecuador (hernanv@fcdarwin.org.ec).

MONOECY IN THE DIOECIOUS CROTON SCOULERI, ENDEMICTO GALÁPAGO S

By: André Mauchamp

INTRODUCTION

Establishment on islands is a challenge for propagulesarriving with the winds, the marine currents or borne byanimals. Survival capacities of the first individual plantsdetermine their fate in the short termo Whether they canform a viable population depends on a complex array oftraits, among which are the characteristics of the breedingsystem. It is generally considered that hermaphroditicself-compatible species have a better chance to establishon an island, since a single propagule can be sufficient tobuild up a population (Baker 1955). On the other hand,this may lead to low genetic variability, possibly negativein the long termo The contrasting proportions of differentbreeding systems on islands, about twenty percent dioe-cious in Hawaii and New Zealand (Bawa 1982) to lessthan three percent in Galápagos (Baker and Cox 1984),shows that a complex array of selective pressures deter-mine the reproductive traits of plants on islands. Theseobservations led to a considerable interest in the evolu-tion of breeding systems, particularly on islands (Baker1955,Carlquist1966, Bawa 1980, 1982, Givnish 1982, Bakerand Cox 1984).

Studies of the flora of the Galápagos Islands startedwith the very first scientific journeys to the archipelago(Hooker 1847, Stewart 1902) and culminated with thepublication of the Flora of the Galápagos Is/ands (Wigginsand Porter 1971). However, relatively few studies focusedon the processes of popula tion d ynamics or the reprod uc-tive biology of the Galápagos plants. The main studies ofbreeding systems show that self-compatibility is highlyrepresented in the islands (Rick 1966; Grant and Grant1981; and McMullen 1985, 1986, 1987). The dioeciousspecies were described in Wiggins and Por ter (1971) but

never specifically studied. 1present three cases of "lea k-age" in the breeding system of the dioecious Croton onthree of the Galápagos islands.

THE SPECIES

Croton scoll/eri Hook f. (Euphorbiaceae) is an endemicspecies of the Galápagos Archipelago. It is a highly vari-able taxa usually divided into four varieties, brevifo/illsMuell Arg.,darwilliiWebster,grandifo/ills Muell Arg. andscou/eri, according to the size of the lea ves and a few othermorphological traits (Wiggins and Por ter 1971, Lawessonet al. 1987). It is present on most of the islands of thearchipelago (45of a sample of 77islands, Snell et al. 1995).Species of the genus Croton are dioecious or monoecious(Mabberley 1987). Croton scoll/eri was considered exclu-sively dioecious (Wiggins and Por ter 1971). However,monoecy was recentIy observed in a few specimens by C.Huttel on Isla Española and nearby islets.

RESUL TS AND DISCUSSION

Monoecy was observed in three areas. Charles Huttelcollected monoecious specimens in the southern archi-pelago: on Isla Osborn (specimen Huttel # 2283), IslaGardner (Huttel # 2290) and on Isla Española (Huttel #2440). One monoecious population was found on IslaIsabela, on the southeastern slope ofV olcán Alcedo, about850 m elevation, and another on southern Isla Genovesa,a low, dry island in the north of the archipelago. Thepattern was the same for all three populations. Threetypes of plants could be found: female trees with onlyfemale inflorescences of ten to thirty flowers; male treesthat bear male inflorescences with a similar number of