Mauchamp, André

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May 1997 NOTICIAS DE GALÁP AGOS 15
Valle, C. A. 1983. Report of the 1983 census of the Galápagos
penguin and flightless cormorant in the Galápagos Archi-
pelago. Report to the Charles Oarwin Research Station and
the Galápagos National Park Service.
Valle, C. A. 1985. Alteración de las poblaciones del cormorán
no volador, el pingüino y otras aves marinas en Galápagos
por efecto de El Niño 1982-83 y su subsecuente recuperación,
p.245-258. [11:G. R. Robinsonand E.del Piño (eds.), Galápagos
1982-83: A chronicle of the El Niño. Springer-Verlag, New
York.
Valle, C. A. 1986. Status of the Galápagos penguin and flightless
cormorant populations in 1985. Noticias de Galápagos 43:
16-17.
Vargas, H. 1995a. Informe del censo total de pingüinos y
cormoranes en 1995. Report to Charles Darwin Research
Station and Galápagos National Park Service.
Vargas, H. 1995/7. Comentarios sobre la estimación de las
poblaciones de pingüinos y cormoranes. Appendix 2, Re-
port to Charles Oarwin Research Station.
Kyra 1. Mills, Department of Ecology and Evolutionary
Biology, University of California, Irvine, CA 92717-2525
USA (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, ENDEMIC
TO GALÁPAGO S
By: André Mauchamp
INTRODUCTION
Establishment on islands is a challenge for propagules
arriving with the winds, the marine currents or borne by
animals. Survival capacities of the first individual plants
determine their fate in the short termo Whether they can
form a viable population depends on a complex array of
traits, among which are the characteristics of the breeding
system. It is generally considered that hermaphroditic
self-compatible species have a better chance to establish
on an island, since a single propagule can be sufficient to
build up a population (Baker 1955). On the other hand,
this may lead to low genetic variability, possibly negative
in the long termo The contrasting proportions of different
breeding systems on islands, about twenty percent dioe-
cious in Hawaii and New Zealand (Bawa 1982) to less
than 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. These
observations led to a considerable interest in the evolu-
tion of breeding systems, particularly on islands (Baker
1955,Carlquist1966, Bawa 1980, 1982, Givnish 1982, Baker
and Cox 1984).
Studies of the flora of the Galápagos Islands started
with the very first scientific journeys to the archipelago
(Hooker 1847, Stewart 1902) and culminated with the
publication of the Flora of the Galápagos Is/ands (Wiggins
and Porter 1971). However, relatively few studies focused
on the processes of popula tion d ynamics or the reprod uc-
tive biology of the Galápagos plants. The main studies of
breeding systems show that self-compatibility is highly
represented in the islands (Rick 1966; Grant and Grant
1981; and McMullen 1985, 1986, 1987). The dioecious
species 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 on
three of the Galápagos islands.
THE SPECIES
Croton scoll/eri Hook f. (Euphorbiaceae) is an endemic
species of the Galápagos Archipelago. It is a highly vari-
able taxa usually divided into four varieties, brevifo/ills
Muell Arg.,darwilliiWebster,grandifo/ills Muell Arg. and
scou/eri, according to the size of the lea ves and a few other
morphological traits (Wiggins and Por ter 1971, Lawesson
et al. 1987). It is present on most of the islands of the
archipelago (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 Huttel
collected monoecious specimens in the southern archi-
pelago: on Isla Osborn (specimen Huttel # 2283), Isla
Gardner (Huttel # 2290) and on Isla Española (Huttel #
2440). One monoecious population was found on Isla
Isabela, on the southeastern slope ofV olcán Alcedo, about
850 m elevation, and another on southern Isla Genovesa,
a low, dry island in the north of the archipelago. The
pattern was the same for all three populations. Three
types of plants could be found: female trees with only
female inflorescences of ten to thirty flowers; male trees
that bear male inflorescences with a similar number of
16 NOTICIAS DE GALÁP AGOS No. 58
~ \ \
~~....
Figure 1. Monoecious inflorescence of Croto/! scouleri. The
photograph is fram IsloteOsborn, Bahía Gardner, Isla Española.
The drawing is fram a specimen collected on Volcán Alcedo, Isla
Isabela. On Alcedo the trees were 4 to 6 m high and formed
patches of CrotOIlforest. The fruits are below the male flowers
on the central stem.
flowers; and monoecious trees. The monoecious trees are
mainly male, but about one to five percent of their inflo-
rescences have two to five female flowers, always the most
basal (Figure 1). 1counted 100 plants randomly selected
in both the Volcán Alcedo and Isla Genovesa populations
and the proportions of sexes are shown in Table 1. The
proportion for the Española population could not be de-
termined beca use the plants were never sufficiently
flowered when visited. In all three cases, the female flow-
Table 1. Praportions of sexes among 100 individuals of Croton
scouleri in two Galápagos populations.
Site Male Female
44
47
Monoecious
Alcedo
Genovesa
41
45
15
8
/.~¡f........
/
')
2
cm
2mm
ers of the monoecious inflorescences developed and pro-
duced seeds. On Isla Gardner, seeds produced by a
monoecious plant were collected. Of fourteen seeds, two
germinated and grew; both were male plants. However,
they were young and bore only a few inflorescences each.
Only three percent of the native and endemic plant
species of the Galá pagos Islands are dioecious (Baker and
Cox 1984). Among the thirteen dioecious species, five are
endemic and eight native (Table 2). This low proportion
is similar to the only available figure for the flora of
continental Ecuador, three percent (Gilmartin 1968). Two
dioecious genera from the Galápagos, Baccharis and
Bllrsera, have an endemic and a native species. It is likely
tha t the majority of the dioecious species of the Galá pagos
were already dioecious when they colonized the islands.
Although the information on seed set and seed viability
of the monoecious inflorescences is still very scarce, Croton
scollleri is probably an example of "leaky dioecism," a
Family Genus Species1 Origin2
BURSERACEAE Bursera malacophyla E
COMPOSIT AE Baccharis steetzii E
EUPHORBIACEAE Croton scouleri E
NYCT AGINACEAE Pisonia floribunda E
SIMAROUBACEAE Castela galapageia E
BATIDACEAE Batis maritima N
BURSERACEAE Bursera graveolens N
CHENOPODIACEAE Atriplex peruviana N
COMPOSITAE Baccharis gnidiifolia N
MENISPERMACEAE Cissampelos parelra N
MENISPERMACEAE Cissampelos glaberrima N
RUTACEAE Zanthoxylum fagara N
URTICACEAE Urera caracasana N
1 Nomenclature follows Wiggins and Porter (1971).
2 E =endemic and N = native.
May 1997 NOTICIAS DE GALÁr AGOS 17
Table 2. Dioecious species of the Galápagos islands.
phenomenon that is not rare in island floras (Baker and
Cox 1984). The occasional production of female flowers
and possibly seeds by a mal e plant could favor the
establishment of a new population on an island and species
with such a breeding system might persist on more islands
than those that are strictly dioecious.
LITERA TURE CITED
Baker, H. G. 1955. Self compatibility and establishment after
"long distance dispersal". Evolution 9: 347-348.
Baker, H. G. and P. A. Cox. 1984. Further thoughts on dioecism
and islands. Annals of the Missouri Botanical Garden 71:
244-253.
Bawa,K.S. 1980. Evolutionofdioecyinfloweringplants. Annual
Review of Ecology and Systematics 11: 15-40.
Bawa, K. S. 1982. Outcrossing and the incidence of dioecism in
island floras. American N aturalist 119: 866-871.
Carlquist, S. 1966. The biota of long distance dispersal. IV.
Genetic systems in the floras of oceanic islands. Evolution
20: 433-455.
Gilmartin, A. J. 1968. Baker' s Law in dioecism in the Hawaiian
flora: an apparent contradiction. Pacific Science 22: 285-292.
Givnish, T. J. 1982. Outcrossing versus ecological constraints in
the evolution of dioecy. American Naturalist 119: 848-865.
Grant, B.R. and P. R. Grant. 1981. Exploitation of Opuntia cactus
by birds on the Galápagos. Oecologia (Berlin) 49: 179-187.
Hooker,J. D. 1847. AnenumerationoftheplantsoftheGalápagos
archipelago; with description of those which are new. Trans-
actions of the Linnean Society 20: 163-233.
Lawesson, J. E., H. Adsersen, and P. Bentley. 1987. An updated
and annotated check list of the vascular plants of the
Galápagos Islands. No. 16, Reports from the Botanical Insti-
tute, University of Aarhus, Aarhus, Denmark.
Mabberley, D. J. 1987. The plant book. A portable dictionary of
the higher plants. Cambridge University Press, Great Brit-
ain.
McMullen, C. K. 1985. Observations on insect visitors to flow-
ering plants on Isla Santa Cruz. I. The endemic carpenter
bee. Noticias de Galápagos 42: 24-25.
McMullen, C. K. 1986. Observations on insect visitors to flow-
ering plants on Isla Santa Cruz. n. Butterflies, moths, hover
flies, ants and stilt bugs. Noticias de Galápagos 43: 21-23.
McMullen, C. K. 1987. Breeding systems of selected Galápagos
angiosperms. American Journal of Botany 74: 1694-1705.
Rick, C. M. 1966. Some plant-animal relations on the Galápagos
lslands. In: R. I. Bowman (ed.) TheGalápagos, Universityof
California Press, Berkeley: 215-224.
Snell, H. L., H. M. Snell, P. A. Stone, M. Altamirano, A.
Mauchamp, and I. Aldáz. 1995. La diversidad biológica de
las Islas Galápagos volumen uno: análisis de la flora.
Fundación Charles Darwin, Quito, Ecuador.
Stewart, A. 1911. Expedition of the California Academy of
Sciences to the Galápagos Islands, 1905-1906. n. A botanical
survey of the Galápagos Islands. Proceedings California
Academy of Sciences, Fourth Series 1: 7-288.
Wiggins, I. and D. Portero 1971. Flora of the Galápagos Islands.
Stanford University Press, Stanford, CA.
André Mauchamp, Former Head of the Botany Depart-
ment, CDRS. Station Biologique de la Tour du Valat,
Le Sambuc, 13200 Aries, France.
E-mail: a.mauchamp@sansouire-tourduvalat.fr


Monoecy in the dioecious Croton scouleri, endemic to Galápagos

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Monoecy in the dioecious Croton scouleri, endemic to Galápagos

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