Aldáz, Iván

Tye, Alan

English

Aquatic Commons

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Deeember 1999No. 60 Noticias de Galápagos
EFFECTS OF THE 1997-98 EL NIÑO EVENT ON THE VEGETATION OF
ALCEDO VOLCANO, ISABELA ISLAND
Iván Aldaz and Alan Tye
INTRODUCTION
On the majority of the islands, islets, and even
rocks that form the Galápagos Archipelago, El Niño
caused changes in the native plant communities of
Galápagos (see also article by A. Tye and 1.Aldaz in this
issue). In this article, we discuss the effects produced
by the intense and constant rainfall on the vegetation of
Alcedo Volcano, Isabela Island, by means of studies
carried out before and after El Niño.
METHODS
Alcedo is one of the five active volcanoes comprising
Isabela Island. At present it is a site with enormous
ecological problems caused primarily by goats (Capra
hircus), whose numbers were estimated before El Niño
to be between 75,000 and 100,000 (Cayot 1997). In
November 1995, a vegetation monitoring system was
begun on Alcedo, whose objective is to evaluate the
recovery of the vegetation during campaigns to eradicate
introduced animals. Fourteen quadrats of 900 m2 were
established in the area of greatest impact, that is, on the
CowleyIsIet
o
Skm
Figure 1. Map of Aleedo Voleano, lsabela lsland, showing vegetation
monitoring quadrats. Cire/es indieate the areas of the 14 permanent
quadrats for vegetation monitoring; squares indieate the quadrats for
the study ofOarwiniothamnus tenuifolius; and triangles indieate
the quadrats for the study ofScalesia microcephala.
-----------------------------------
southern portion of the volcano. In addition, one year
later, four quadrats of 400 m2 were set up to monitor the
status of Oarwiniothamnus tenuifolius and Scalesia micro-
cephala (Figure 1). Monitoring was carried every six
months, alternating between the garúa season (light
mists, from May to October) and the season with
intense rainfall, from November to April. Basically, the
monitoring consists of making a floristic inventory,
measuring the plant cover, and measuring the diameter
at breast-height (DBH) of trees and bushes. The last
variable is usually measured every 12 months. More
details on methodology of vegetation monitoring can be
found in the monitoring manual of Mauchamp (1996).
RESULTS
The results from the monitoring program allow an
evaluation of the effects of El Niño on the vegetation of
Alcedo Volcano, comparing the data obtained before
the phenomenon (monitoring between November 1995
and June 1997), during El Niño (monitoring in
N ovember 1997 and April1998), and after it (monitoring
in October 1998). In this article we present a prelimi-
nary analysis that gives a general idea of the changes in
the floristic composition on Alcedo during El Niño.
The precipitation data are only indicative, since the
measurements are from the meteorological station at
the Charles Oarwin Research Station on Santa Cruz
Island. It should be mentioned that the quantity of
millimeters of rain is that which accumulated between
each date. A fuller analysis will be published following
the acquisition of additional data from the vegetation
monitoring on Alcedo Volcano.
Floristic inventory
Figure 2 indicates the changes in the tota] number
of species per monitoring trip in the fourteen quadrats
of 900 m2, re]ated to the precipitation at the Darwin
Station. Ouring the first monitoring, undertaken in
November 1995 (garúa season), 32 species were recorded;
the rainfall was minimal. In the second monitoring sur-
vey in May 1996 (rainy season), annua] species
appeared, giving a total of 57 species recorded. During
the third monitoring, undertaken in a new period of
garúa, the number of species dropped to 50, possibly
because it had stopped raining. In June 1997, there was
heavy rainfall, which diminished in the months of
September and November. Nevertheless, the number
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December 1999Noticias de GalápagosNo. 60
of species recorded remained high in November. The
highest peak in rainfall was in April 1998, when the
greatest number of species, 102, was recorded. Six
months later, this declined to 86 species, probably due
to the dry season then prevailing in the islands.
In other sites, uncommon species were found, such
as Phytolacca octandra (native), Jaltomata werfti (native),
and Pleuropetalum darwinii (endemic).
Variations in the native vegetation
due to rainfall
The majority of plants on Alcedo are abundant
during the rainy season. A case in point is Ipomoea alba,
which was not found in any of the regular monitor-
ing quadrats until June 1997, but, during the winter and
with the appearance of El Niño, it recovered remark-
ably, forming very dense patches. In the last monitor-
ing, carried out in October 1998, the vegetation in gener-
al had suffered serious modifications, particularly 1.
alba, due to the drought and the continuing destructive
action of the goats. In addition, the percentage cover of
Paspalum conjugatum increased enormously during the
rainy season, probably because it is a grass resistant to
competition from other herbs.
In general terms, the vegetative cover was high
during the 1997-98 El Niño. Apart from 1.alba, the dom-
inant and common species that covered the soil in the
humid zone (south) of the volcano during El Niño were
the following: Borreria laevis, Cyperus brevifolius,
Dichondra repens, Pteridium aquilinum, Cuphea carthage-
nensis, Plantago major, Hyptis rhomboidea, Physalis pubes-
cens, Elaterium carthagenense, and Sida rhombifolia.
Figure 3 shows the variability in vegetative cover and
includes the species that are considered most frequent
120
100
80
11ü
!.
.
'O 60j
E
"
z
40
Figure 2. Changes in the total
number of plant species in relation
to clima tic changes during monitor-
ing of quadrats on Alcedo Volcano.
Data on precipitation provided by
the meteorological station, CDRS.
Accumulated precipitation during
each period of the study.
20
o
Nov-95 May-96
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An 1
and, at the same time, the most abundant, during the
monitoring on Alcedo.
Appearance and dispersion of introduced species
El Niño may have favored the appearance of some
introduced species, such as Ricinus communis. About
ten adult plants of 3 m height were found, along with a
good number of subadults and seedlings. The plant had
previously been recorded at the site and it is likely that
dormant seed had remained in the soil until helped to
germinate by Niño rainfall. The area of dispersion of this
introduced plant was just on the slopes of the south-
eastern side of the volcano (200 m north of vegetation
monitoring quadrat No. 12). The individuals of this
species were eliminated immediately and the seeds
collected and burned. Other introduced species that
appeared, in order of dominance, are Sida rhombifolia,
Hyptis rhomboidea, Plantago major, Synedrella nodiflora,
Eleusine indica, Cuphea carthagenensis, Sonchus oleraceus,
Stachytarpheta cayennensis, Canna lutea, and Datura stramo-
nium. The most aggressive species, given their dispersal
and invasion of large areas, their displacement of native
herb species, and the possible impediment to the natural
regeneration of Scalesia microcephala,Darwiniothamnus
tenuifolius, Tournefortia rufo-sericea, Zantlwxylum fagara,
and Cordialeucophlyctis,are for the most part S. rhombifolia
and, to a lesser extent, H. rlwmboidea.
Growth in diameter of marked trees and bushes
The measurement of diameter at breast height
(DBH) of trees and bushes within the vegetation moni-
toring quadrats began in May 1996, with the marking of
438 individuals of the following species: Scalesia micro-
2000
1800
1800
1400
1200 Eg
c:
o
1000 J
i!.Ü
800 l.
600
400
200
o
Nov-96 Jun-97 Nov-97 Apr-98 Oct-98
Monitoring dal88
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_.-9No. species +-Precipitation (mm).-
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26
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80
70
60
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No. 60
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December 1999Noticias de Galápagos
I-+--p. aquilinum---S.laevis ~P. conjugatum
-*-c. brevifolius---1. arbaf
Figure 3. Variatiolls ill emicr by predominallt species durillg the
vegetatioll mOllitorillg 011 Alcedo.
cephala, Tounzefortia rufo-sericea, Zanthoxylum fagara,
Croton scouleri, Psidium galapageium, Cordia leucophlyctis,
and lochroma ellipticum. The largest diameters were
those of Zanthoxylum, Scalesia, and Tournefortia, which
varied between 20 and 25 cm. The growth of DBH during
the first year, May 1996 to June 1997, is slightly less than
that which occurred in the second year, June 1997 to
October 1998 (Figure 4).
Mortality of marked trees and bushes
During the first year of the monitoring project, the
greatest decrease in the number of individuals was in P.
galapageium, C. leucophlyctis, T. rufo-sericea, and S. micro-
cephala. During the second year, there was a greater
percentage of trees and bushes that died, with the
species affected being the same as during the first year,
as well as C. scouleri on this occasion (Figure 5).
Evidently, the high rainfall, combined with the steep
slopes, caused many of the trees to fall and conse-
quently die. The population of Darwiniotl1amnus
tenuifolius suffered a great decrease; in 1996 about 150
plants of this species were recorded in the quadrats
established for its study, while in 1997 only 21 were
found, and in 1998 the number dropped to 12. Possible
causes of death of the plants were the excessive rainfaU,
consumption by goats, and the invasion of S. rllOmbifolia,
an introduced plant that currently seems to be impeding
the regeneration of Darwiniothamnus. The regeneration of
tree and bush species in the humid forest zone
increased after winter and El Niño, particularly T. rufo-
sericea, S. microcephala, C. scouleri, Solanum erianthum,
and Psycl10tria rufipes.
DISCUSSION
According to the monitoring that has been
conducted to date on Alcedo Volcano, we can state that
the number of plant species varies markedly in relation to
the intensity of rainfall, with the number of species being
greater during El Niño. According to the reference
rainfall data, the rainfall decreased progressively from
June until November 1997, with light rains, which
allowed the number of species to continue increasing.
However, from April to October 1998, there was an
abrupt change in the weather; heavy rains lasted until
May, after which there was a period of drought. This is
the most probable reason that fewer species were found
in the monitoring of October 1998. On the other hand, it
is possible that this variation is related to the campaigns
to eradicate introduced mammals. Hamann (1985) and
Luong (1985) indicate that there was an increase in
species, particularly herbaceous ones, during El Niño,
followed by a decline in the number of species during the
subsequent "La Niña" drought. This pattern seems to be
occuring the current El Niño-La Niña cycle on Alcedo.
One of the species on Alcedo whose increase
depended on the rainy or very rainy season is 1. alba.
Van der Werff (1978, 1985) and Hamann (1981) indicate
the presence of this species, forming a dense mat over
trees and bushes. During the 1982-83 El Niño, this
climbing species covered extensive areas in the south-
east, east, and northeast inside and outside the crater of
Alcedo, forming impenetrable thickets of vegetation (c.
Márquez, pers. comm.). Subsequently, the population
of l. alba decreased due to the lack of rainfall and, above
all, consumption by goats (Freire 1992). In Aprill998,
this climbing plant was dominant and we observed
extensive patches on the rim of the volcano, the upper
slopes, and the lower slopes, but in October of the same
year, it was uncommon to find these dense patches.
E
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.a>'b- ~'b- ~0<:- .§ &~ ,;)~
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Species measured
Figure 4. Average al1l1ualgrowth of DBH of marked trees and
bushes alive until October 1998.
27
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Noticias de Galápagos December 1999No. 60
J9 200
c:
cuQ. 150
CI)
~
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...
~ 50
E
~
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1996 1997
Measurement periods
1998
-+- S. microcephala --- D. tenuifolius
P. galapageium -*- T. rufo-sericea
--- C. scouleri -+- C. leucophylctis
Figure 5. Annual decrease in t}¡e number of plants for the
measurement of DBH and Darwiniothamnus tenuifolius, until
October 1998.
Nevertheless, it may reeuperate with the next rains. On
the other hand, figure 3 shows the deerease in the per-
eentage eover of P. aquilinum, perhaps due to the eom-
petition that oeeurred just when 1.alba was reeorded for
the first time, in the monitoring of June 1997. l. alba
totally covered P. aquilinum, impeding its regeneration
and survival. In the last months of 1998, it was
observed that the soi] had been eroded by the rains and
the presenee of introdueed mammals, whieh has given
rise to areas devoid of vegetation.
Ricínus communís is eonsidered invasive in the
Galápagos Arehipelago. It disperses rapidly in the rainy
season. Van del' Werff found some plants on Alcedo in
1978; subsequently they were eliminated by the
Galápagos National Park Serviee, but possibly some seeds
remained that germinated in 1998. Onee again this speeies
has been eliminated and the seeds bumed. Nevertheless
it is advisab]e to frequent1y visit the area where this p]ant
was most reeently found. Other introdueed species that
have expanded rapidly throughout Alcedo are S. rhombí-
folía and H. rJlOmboidea.These species were reeorded by
van del' Werff (1978), Hamann (1981), Lawesson (1987),
and Freire (1992). Aeeording to the vegetation monitor-
ing, S. rhombifoJía was first found in May 1996,while H.
rhomboídea was found in June 1997. During the last moni-
toring, in Oetober 1998, we observed that S. rhombifolía
had beeome a potentially damaging species, with individ-
ua]s up to 2.5 m high. Apparent1y this species deereases
during periods of drought, but it has a great number of
seeds that remain dormant in the soil, awaiting a period of
rain to reappear with vigor.
Ihe forest benefitted from the quantity of rainfa11and
showed an exeel1ent natural regeneration of bushy and
tree speeies during El Niño. The opposite oeeurred with
adult trees and bushes that grow on the fragile soil and the
middle slope of the volcano. These were at greater risk of
dying due to the erosion eaused by the formation of
rivu]ets and gullies. With respeet to the slight inerease in
n
-
DBH in trees and bushes a]ive during 1998, it seems that
there exists a direet inf]uenee from the rains; this shou]d
beeome c1earer following further monitoring periods.
ACKNOWLEDGMENTS
We would like to express a speeial thanks to the fol-
]owing peop]e of the CDRS, who have eontributed
information and eomments prior to the pub]ieation of
this artic1e: Mark Gardener, Cruz Márquez, Solanda
Rea, Mareo Altimirano, Eduardo Espinoza, Paola
Buitrón, Waeho LJerena, Heidi Snell, and Kristin
Bergeson. We wou]d ]ike to thank Howard Snell for
he]p with the design of the maps. We would a]so like
to thank Sol Rojas for eheeking the Spanish version of
this artic1e and to Gayle Davis-Merlen for help with
translating it to Eng]ish.
LITERATURE CITED
Cayot, L. J. 1996. Alcedo update. Noticias de Galápagos
57: 3-4.
Freire, M. 1992. Ecología de los chivos ferales (Capra hircus
L.) en volcán Alcedo, isla Isabela, Galápagos, Ecuador.
Thesis for Licenciatura degree, Universidad Técnica de
Ambato, Ambato, Ecuador.
Hamann, O. 1981. Plant communities of the Galápagos
Islands. Dansk BotaniskArkiv 34 (2): 1-163.
Hamann, O. 1985. The El Niño influence on the Galápagos
vegetation. In G. Robinson and E. del Pino, eds., El Niño
in the Galápagos Islands: The 1982-1983 Event, pp. 299-
330, Charles Darwin Foundation, Quito.
Lawesson, J. E., H. Adsersen, and P. Bentley. 1987. An updat-
ed and annotated check Jist of the vascular plants of the
Galápagos Islands. Reports from the Botanicallnstitute,
University of Aarhus 16: 1-74.
Luong, T. T. and B. Toro. 1985. Cambios en la vegetación de
las islas Galápagos durante "El Niño" 1982-1983. In G.
Robinson and E. del Pino, eds., El Niño in the Galápagos
Islands: The 1982-1983 Event, pp. 331-342, Charles
Darwin Foundation, Quito.
Luong, T. T. and B. Toro. 1986. Cambios en la vegetación de
las islas Galápagos durante El Nii'io 1982-83. In G. Davis-
Merlen, ed., Informe Anual ECChO 1983, pp. 10-11,
Charles Oarwin Foundation, Quito.
Mauchamp, A. 1996. Manual de germinación. Unpublished
manuscript, CORS, 19 pp.
van der Werff, H. H. 1978. The vegetation of the Galápagos
lslands. Ph.D. thesis, Rijksuniversiteit, Utrecht.
van del' Werff, H. H. 1980. The vegetation types of Santa
Cruz and Alcedo. Noticias de Galápagos 31: 11-13.
Iván Aldaz, Charles Darwin Research Station,
Galápagos, Ecuador (e-mail: ivan@fcdarwin.org.ec).
Alan Tye, Charles Darwin Research Station,
Galápagos, Ecuador (e-mail: atye@fcdarwin.org.ed.
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Effects of the 1997-98 El Niño event on the vegetation of Alcedo Volcano, Isabela Island

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Effects of the 1997-98 El Niño event on the vegetation of Alcedo Volcano, Isabela Island

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