Grant, B. Rosemary

Grant, Peter R.

English

Aquatic Commons

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14 NOTICIAS DE GALAPAGOS No. 51
GLOBAL WARMING AND THE GALAPAGOS
By: Peter R. Grant and B. Rosemary Grant
The idea that organisms evolve was transformed
during the last century from conjecture to fact. As the
present century draws to a close, we are experiencing
another transformation. The conjecture that the
world's temperature is gradually rising has become
widely accepted as a demonstrated fact. A warming
trend took place from 1880 to 1940. There followed
a slightreversal, butfrom 1975 onwards the warming
has resumed, and the 1980s were undoubtedly the
warmest decade of the century (Fig. 1).
There has been much discussion about the exact
role of human activity in producing this effect,
through, for example, the large-scale cultivation of
rice and domestication of cattle and sheep, combus-
tion of fossil fuels, destruction of forests, the alteration
of gases in the atmosphere such as carbon dioxide
and methane, and by all this the creation of a green-
house effect. Itis still tooearly to be sure ifthe human
influence is substantial and increasing, in which case
global warming will continue well inlO the next cen-
tury; or if it is minimal, in which case the trend may
once again be reversed. Effects of global warming
are likely to be more severe in parts of the temperate
zone than in tropical Galápagos. Neverthe1ess, we
should be thinking about the implications of global
warming for Galápagos.
We should not wait until we can be sure that Galápa-
gos is affected. Change is already underway. A global
rise in sealevel over the last 100years from the melting
of polar ice has been estimated at 10-30 cm. A mea-
sured 6% decrease in the extent of arctic ice occurred
over the comparatively short time of 15 years, from
0.6
-0.6
1860 1880 1900 1920 1940 1980 20001960
YEAR
Figure 1. Global warming over the last 100 years. Adapted from the National ScienccFoundation (USA) publicationM osaic (1989,
volume 20, number 4).
July 1992 NOTICIAS DE GALAPAGOS 15
1973 to 1988. Projecting these trends into the future
is hazardous of course, but by one calculation a glo-
bal warming of 4°C to 5°C will occur by the year
2050, and much more arctic ice will have melted as
a result. As reported in the April1991 issue of Ecol-
ogy (72:373-412), predicted temperaturesforthe next
century will be higher than any experienced by the
earth's biotaduring the last several million years, and
the projected rates of change may be more than an
order of magnitude faster than any global change in
the past 2 million years!
The extraordinary El Niño eventof 1982-83 (Rob-
inson and del Pino 1985, Glynn 1988) gives us some
basis for estimating the effects of a general warming
on life in Galápagos. Glynn (1990) has speculated
that effects of El Niño might mimic on a very short
time scale what could happen over a much longer
time period through global warming. Given the wide-
spread decimation of corals during 1982-83, Glynn
suggested that they are in danger ofbeing eliminated.
If sea temperatures increase markedly over a century,
it is doubtful if natural selection could act fast enough
to be effective in molding an adaptive response in
long-lived species like these to stressful thermal con-
ditions. A rise in the sea level as a result of polar ice
melting, decreased salinity, and increased turbidity
could exacerbate the stress. The combination of all
these factors and other physicochemical changes
would kill coral s and cause extinctions, as happened
off the Pacific coast ofPanamá during the 1982-83 El
Niño event (Glynn and de Weerdt 1991). Thepropor-
tions and numbers of organisms associated with them,
like damselfish and sea urchins, would change, per-
haps drastically, as a consequence.
A possible key to predicting the future lies at the
interface of the ocean and the atmosphere, for it is
here that interactions determine the occurrence ofEI
Niño-Southern Oscillation events, their severity,
duration, and frequency (Philander 1990). The atmo-
sphere is more thermally labile than the oceans, hence
the climate will undergo change more rapidly than
will the sea.
The most likely effect of global warming on
Galápagos is higher average temperatures, with per-
haps greater contrasts between the warm and wet El
Niño years and the cool and dry intervening years.
After the 1982-83 El Niño event, several populations
of animals, land-based entirely or in pan, suffered
crashes. These inc1uded various species of sea birds
(Rosenberg et al. 1990), land and marine iguanas
(Laurie 1990), and sea lions and fur seals (Limberger
1990). Most recovered fairly quickly, possibly more
quickly than much ofthe vegetation (Grant and Grant
1990). Their recovery was helped by the more mod-
erate effects of the next event in 1987. Nevertheless,
it is not difficult to envisage major and long-Iasting
effects of severe events like the 1982-83 one if they
were repeated every 4 years. Populations of plants
and animals will have altered distributions in the
Galápagos. Some will be driven extinct by a combi-
nation of direct and indirect effects: by the direct
effects of high temperatures, high rainfall, and high
annual variation in both, and the unpredictable indi-
rect effects arising from predators, parasites,
competitors, and various interactions in the commu-
nity food web.
The prediction ofhigh temperatures and rainfall is
not certain however. This is underlined by the fact
that global greenhouse warrning could intensify along-
shore windstress on theeasternPacific Ocean surface,
leading to greater coastal upwelling (Bakun 1990).
This, in tum, would cause a cooling of the ocean
surface and an amelioration of dailing heating of the
ocean offshore, with the possible result of reduced
convective cooling and precipitation on Galápagos.
However, alongshore windstress has been increasing
during the last 40 years, and so have trade winds
(Bakun 1990), without a noticeable drop in precipi-
tation recently.
In the issue of Ecology referred to above, a com-
mittee of the Ecological Society of America
confronted the task of planning globally for an uncer-
tain future. Several key points were made about the
possible effects of global warming that are as rele-
vant to Galápagos as elsewhere. For example, it was
pointed out that a challenge for ecologists is to under-
stand processes which link species and ecosystems
with climate and to predict ecological responses un-
der climates that do not presently existo Evolutionary
responses are important too (Grant 1991). It is known
that natural selection favoring tolerance to heat or
dessication can lead to the rapid evolution of general
stress-tolerant genotypes which are resistant to a
variety of environmental stressors and which have



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