Grant, B. R.

Grant, P. R.

English

Aquatic Commons

December 1989 NOTICIAS DE GALAPAGOS 13 
claw tree had grown into an impenetrable thicket of 
small trees, shrubs, and ground level plants. The 
number of species had also considerably increased. 
Only photographs of its former state could show us 
how startling the regrowth of vegetation had actually 
been (Figs. 1 and 2). One new quadrat was established 
in the bracken fern (Pteridium) zone at the very 
summit where it adjoined the Zanthoxylum zone, to 
permit determination in the future whether this 
bracken zone was a permanent feature or whether it 
would be encroached upon or colonized by elements 
from the Zanthoxylum forest zone. 
The final day's work consisted in rechecking the 
growth, death, and replacement of trees in another 
vegetation plot in the palo santo (Bursera) zone (125 
m above sea level) on the lower southeastern slopes. 
Again as at other plots, it was obvious that rapid 
regrowth of all species was taking place in the absence 
of goats. Here the study was extended to include 
measurements of Scalesia, Bursera, and Opuntia 
cactus outside the vegetation plot so as to be able to 
monitor the changes in the population structure in the 
years ahead. Similar measurements were taken of 
these species at a lower elevation (45 m above sea 
level). It was obvious that the plant populations 
consisted of either very old, large trees or lots of very 
young trees (which had grown in the absence of goat 
grazing). Obviously, the trees that should have been 
intermediate in size had all been eliminated earlier by 
goats. While doing the work, the team was closely 
watched by a family of Galapagos hawks, which sat 
in the low trees above us-almost at arm's reach. 
The goat extermination campaign by the Park and 
Station, though expensive, has been a spectacular 
success with rapid vegetation regeneration and 
reestablishment on the west and southern slopes of 
Pinta (the other parts of the Island are mainly lava 
fields without vegetation). The team saw no visible 
signs of goats, but a few goats still remain (a crew 
member from our boat may have heard one near the 
western anchorage). Eradication of these few 
remaining individuals will be difficult and expensive, 
but it must be done to prevent the population from 
recovering the high population levels that were so 
deleterious for the Island. At middle elevations, we 
encountered several characteristic areas, where dense, 
forestlike vegetation was interspersed with open, 
grassy meadows. We considered these areas to be the 
prime tortoise habitat on Pinta, but sadly we did not 
find traces of the Pinta tortoise. It appears that 
Lonesome George will still be lonesome; no mate is 
available for this sole survivor of the Pinta tortoise 
population. 
P.M. Whelan, Charles Darwin Research Station, 
Isla Santa Cruz, Galapagos, Ecuador. O. Hamann, 
Botanic Gardens, University of Copenhagen, 0ster 
Farimagsgade 2 B, DK-1353 Copenhagen K, 
Denmark. 
THE SLOW RECOVERY OF OPUNTIA MEGASPERMA ON 
ESPANOLA 
By: P.R. Grant and B.R. Grant 
The mature Opuntia cacti of Espanola have sturdy 
trunks that support numerous pads from lateral 
branches. Conspicuous as individuals, they are also 
conspicuously rare. The best guess as to why they are 
so scarce is that goats have damaged many old trees 
and eaten, trampled, or pushed over many of the 
younger ones. This was the view of Stewart (1911, 
1915) who wamed that the cactus was in danger of 
becoming extinct. For example, the density of mature 
cactus is much higher on the neighboring Island of 
Gardner which has never had goats. There is no 
obvious climatic or edaphic difference between the 
Islands that would account for the difference in 
cactus density. A parallel situation exists on Floreana 
(goats common, cactus very rare) and its two satellite 
Islands, Champion and Gardner (no goats, cactus 
dense). 
Goats were introduced to Espanola some time in 
the last century. By the time the California Academy 
of Sciences expedition visited the Island in 1905-06 
the goats were already well established and Opuntia 
were scarce (Stewart 1911,1915; Slevin 1931). Goats 
14 NOTICIAS DE GALAPAGOS No. 48 
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NUMBER OF STEM UNITS 
Figure 1. The number of plants observed in each size class as 
measured by counting stem units on Espanola in 1979. El 
numero de plantas en catagorias de tamano determinado por el 
numero de segmentos en sus truncos. Isla Espanola. 1979. 
were removed by hunting by 1978 (Hoeck 1984). 
This should have resulted in good survival of young 
plants if goats had previously been destroying them. 
We visited the Island in August 1979, and our 
observations suggested that indeed Opuntia 
recruitment was well underway. 
We attempted to count all young individuals on 
the top of Red Hill, in an area of approximately 
10,000 m2• The hill is between 0.5 and 1.0 km inland 
behind Gardner Bay. To characterize the size of the 
young plants, we counted the number of units in the 
stem which are demarcated by constrictions. 
Altogether we found 85 unbranched saplings, 
ranging from 1 to 9 stem units, with a mean of 4.6 ± 
1.24 (SO) and a mode of 5 units (Fig. 1). If 1 stem is 
added each year, and growth starts in the year after 
the seeds are formed, the years of greatest production 
would have been 1973 and 1974. Heavy rains fell in 
1972 and 1973, but not in 1974. According to our 
observations on Genovesa, Daphne Major, and 
Champion, seed germination is promoted by wet 
conditions, so we are inclined to think that the modal 
class of 5 stem units was 7 years old in 1979. 
Regardless of the exact ages, a strong production of 
young Opuntia saplings coincided with both heavy 
rains in the early 1970s and the removal of the last 
goats. 
In addition to the 85 unbranched saplings, one was 
found with three lateral pads on the top of a 9-unit 
stem. Seven more branched ones were found further 
inland. They ranged from 6 to 10 stem units high, and 
had from 2 to 10 lateral pads. The most complex form 
was an 8-stem unit trunk supporting two lateral 
branches offourpads each, with two more pads at the 
end of one of them. Unless they had matured more 
rapidly, these were older by a few years than the 
unbranched ones on Red Hill. Nevertheless, young 
saplings made up the vast majority of the recruits, on 
Red Hill and elsewhere. 
We returned to Espanola in early February 1988, 
and made the same survey on Red Hill. If goats had 
been the sole agent inhibiting recruitment prior to the 
1970s, the large number of young plants seen in 1979 
should have survived well and should have been 
augmented by many new ones produced in the next 
few years. Far from being augmented, however, the 
population of young Opuntia was drastically reduced. 
Despite intensive searching we could find only eight 
unbranched individuals; 1 had 4 stem units, 1 had 5, 
2 had 7, and 4 had 9. In 1979 there had been only one 
branched sapling, but in 1988 there were 10. The 
number of stem units ranged from 6 to 9 (mean 8.0 ± 
1.33 SO), and the most complex in growth form was 
about as complex as the most elaborate one found on 
the Island in 1979. 
Clearly recruitment had been poor in the 
intervening years, and mortality among the saplings 
had been high. Given the age estimation in 1979 of 
7 years for an unbranched plant with 5 stem units, the 
10 branched plants in 1988 would have been the 
remnants of the 85 unbranched ones in 1979. We do 
not know if this is a normal rate of attrition for 
Opuntia plants elsewhere, but the absence of 
replacement cohorts was not expected. 
The key to understanding the poor performance of 
Opuntia may lie in the extraordinary EI Nino event of 
1982-83. Probably more rain fell on Espanola then, 
as it did on other Islands, than at any other time this 
century (Grant 1984). Prolific rains did not inhibit 
flowering on the other Islands we visited in 1983 
(Genovesa, Daphne Major, and Champion), and they 
facilitated germination. But they did contribute to 
the death of several cactus bushes. Roots rotted in 
waterlogged soil and the bushes, made heavier by 
extensive uptake of water, were blown over by strong 
winds (Grant and Grant 1989). The heavy rains also 
promoted extensive growth of vines which smothered 
December 1989 NOTICIAS DE GALAPAGOS 15 
whole Opuntia bushes on Genovesa and Daphne 
Major, as a result of which several died (Grant 1986). 
A second El Nino event in 1987, while not as severe 
or as long, had similar effects on vines and Opuntia 
bushes (Grant and Grant 1989). On Espanola, we 
suspect, the rampant growth of vines and other plants 
in these 2 years (together with the direct effects of 
rain) caused the demise of all but the tallest and 
sturdiest of the saplings that germinated prior to 
1979, and smothered almost all those that germinated 
afterwards. In February 1988 the ground cover of 
Red Hill was a rich, dense tangle of dead herbs. 
These observations show that the goat removal 
"experiment" has still not produced the expected 
effect on the cactus population. Either the two recent 
EI Nino events, especially the first, have delayed the 
expected effect, or other factors not yet identified 
have been responsible for the weak recruitment and 
low adult density of Opuntia megasperma on 
Espanola. Time, and further study, will tell. The 
observations also highlight our ignorance of 
population processes in Opuntia in general. This 
could be rectified by a well-designed, long-term 
investigation at the Charles Darwin Research Station, 
as well as by continued monitoring on Espanola and 
Islands similarly affected by goats such as Santa Fe 
and Pinta (Hamann 1975). 
LITERATURE CITED 
Grant, B.R., and P.R. Grant. 1989. Evolutionary 
dynamics of a natural population: the Large Cactus 
Finch of the Galapagos. University of Chicago 
Press, Chicago. 
Grant, P.R. 1984. Extraordinary rainfall during the 
EI Nino event of 1982-83. Noticias de Galapagos 
39:10-12. 
Grant, P.R. 1986. Ecology and evolution of Darwin 's 
Finches. Princeton University Press, Princeton, 
New Jersey. 
Hamann, O. 1975. Vegetational changes in the 
Galapagos Islands during the period 1966-73. 
Biological Conservation 7:37-59. 
Hoeck, H.N. 1984. Introduced fauna. pp.223-245 
In R. Perry (ed.) Galapagos. Pergamon Press, 
Oxford. 
Slevin, J.R. 1931. Log of the schooner "Academy" 
on a voyage of scientific research to the Galapagos 
Islands, 1905-1906. Occasional Papers of the 
California Academy of Sciences 17: 1-162. 
Stewart, A. 1911. Expedition of the California 
Academy of Sciences to the Galapagos Islands, 
1905-1906. II. A botanical survey of the Galapagos 
Islands. Proceedings of the California Academy 
of Sciences, Fourth Series 1:7-288. 
Stewart, A. 1915. Some observations concerning 
the botanical conditions on the Galapagos Islands. 
Transactions of the Wisconsin Academy of 
Sciences, Arts, and Letters 18, Part 1:272-340. 
P.R. Grant and B.R. Grant, Department of 
Biology, Princeton University, Princeton, New 
Jersey 08544, USA. 


The slow recovery of Opuntia megasperma on Española

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The slow recovery of Opuntia megasperma on Española

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