Grant, B. Rosemary

Grant, Peter R.

English

Aquatic Commons

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December 1999No. 60
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Noticias de Galapágos
EFFECTS OF THE 1998 EL NIÑO ON DARWIN'S FINCHES ON DAPHNE
Peter R. Grant and B. Rosemary Grant
El Niño events are amplifications of normal seasonal
processes in the eastern tropical Pacific region
(Philander 1990), and they bring an abundance of rain
to the Galápagos. Since 1973 we have been studying
how populations of Darwin' s finches on the small
island of Daphne Major respond to conditions that vary
from draughts of scarcity to El Niño years of plenty.
For us this variation is like a gigantic watering experi-
ment that decreases or increases the food supply of the
finches (seeds, caterpilJars, spiders, pollen, nectar, etc.)
at the normal time of breeding, while leaving other
potentially influential factors either unaffected
(photoperiod) or affected in a minor way (temperature).
The experiment is not under our control, nevertheless
we can learn a lot by comparing the responses of finches
in different years. The finches in question are four
species of Geospiza ground finches: large (G. mag-
nirostris), medium (G. fortis), and small (G. fuliginosa)
graund finches, and the cactus finch (G. scandens).
We have studied finches on the island thraughout the
entire breeding season in four El Niño years (1982-83,
1987, 1991, and 1998), as well as ten other years of
breeding (Grant et al., ms.) and three other years of
drought and no breeding (1985, 1988, 1989). Three
features of breeding vary among years. They are the
number of eggs a female lays in a c1utch, the success in
raising the young to the point of fledging; and the number
of times a female breeds. The sum total of these com-
ponents is the annual praduction. Praduction is on
average four times higher in El Niño years than in non-
El Niño years. The main reason for the high praduction
is that finches breed four to eight times in El Niño
years, instead of the usual one (or none) to four times in
non-El Niño years. Clutch sizes are also larger in El
Niño years than in non-El Niño years, and fledging suc-
cess is slightly higher in El Niño years, although hatch-
ing success tends to be a little lower in part because the
foraging of the parents is interrupted by rain.
Remarkably, in the longest breeding seasons of all
(1982-83 and 1987), young finches hatched at the begin-
ning of the season were breeding three months later
and contributing to the total praduction; some finches
became grandparents within one season!
How did the responses of finches in 1998 compare
with responses in other El Niño years? Rainfall on
Daphne in 1998 amounted to two-thirds of the quantity
that fel! in the mega-El Niño of 1982-83. This should have
resulted in pralonged and pralific breeding of the finches
to an extent exceeded only in 1982-83, but it didn't. A
few birds bred in January following the heavy and
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repeated rains that began in the last few days of 1997.
By February breeding was in full swing and continued
in March, April, and May, but by the second week of
June, breeding had ceased. Thus the start to the breeding
season was sluggish, and the end was precipitous, and
neither was expected fram the pattern of rainfall. For
example, rain continued into early June, but finches did
not continue to breed. This pattern was seen in all four
species.
In two respects finch breeding in 1998 was typical
of breeding in a Niño year. First, c1utch sizes were
distinctly higher than in non-El Niño years; c1utches of
five eggs were moderately common in 1998, whereas
they are exceedingly rare, if they occur at all, in non-El
Niño years. The larger number of eggs in a c1utch
suggests that females find it easier to get enough food
to make eggs in Niño years, and 1998 was no exception
to the rule. Second, hatching and fledging success were
also about the same as in other El Niño years. The
anomalously low praduction in 1998 was c1early
brought about by an unexpectedly short breeding
season. Why?
More specifically, why did breeding start slowly
and end rapidly? Normally, the length of the breeding
season appears to be governed by a food supply that is
replenished for as long as the rain continues. In 1998,
the length of the breeding season was not so c1early
determined by the pattern of rainfall, but perhaps the
populations of caterpillars took a long time to increase,
and then decreased quickly at the end of May. Our
previous work on both Daphne and Genovesa (Gibbs
and Grant 1987, Grant and Grant 1989) makes this a
plausible explanation. However, at best it is only partly
correct; although their numbers were dec1ining, cater-
pillars were still fairly plentiful in our samples in June,
when finch breeding had all but ceased.
The explanation may lie instead in a combination of
unusual conditions; high initial densities of finches and
exceptionally high temperatures. Breeding may have
been delayed as a result of much interference. An
unknown factor that might have contributed to the
delay is a carry-over effect fram the extensive breeding in
the previous (El Niño) year; finches may not have been
physiologically ready to breed when the rains returned
at the end of 1997. Breeding ceased early in 1998, per-
haps because the energetic benefits gained fram a
declining food supply was offset by rising energetic
costs of nest and territory defense as overall density,
adults and fledglings combined, increased. High air
temperatures may ha ve contributed to the increase in
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December 1999Noticias de GalápagosNo. 60
costs. They were higher than in all previous El Niño
years.
Whatever the reasons, patterns of breeding on
Daphne are not unique, but are likely to occur in par-
allel fashion elsewhere on other islands when effects
of El Niño conditions are experienced throughout the
archipelago. In previous El Niño years, Darwin's
finches on two widely separated islands, Genovesa
(Grant and Grant 1989) and Daphne Major (Gibbs and
Grant 1987), responded in the same way to heavy rain
and a prolonged wet season (Grant and Grant 1996).
Clutch sizes were elevated and finches bred repeatedly.
In 1998, short-term observations of ground finches
on other islands indicated repeated breeding, but
an early cessation, perhaps even earlier than on
Daphne (D. Day, M. Hau, and M. Wikelski,
pers. comm.).
One of the things we have learned from this long-
term study is that no two El Niño years are alike. An
obvious reason for this is the amount of rain and the
number of months in which it falls varies among El
Niño years. However, this is not the only reason.
Responses to El Niño conditions are determined in part
by preceding conditions, be they dry or wet, food-rich
or food-poor. Those preceding conditions in turn are
determined by whether drought or normal conditions
precede the Niño perturbation and on the interval since
the previous Niño event. For example, finch population
densities were much higher at the beginning of 1998
than at the beginning of 1983, and finches probably
interfered with each other' s attempt to breed at the
beginning of 1998. This, we believe, is one reason why
finch breeding took so long to get underway in 1998.
Thus, effects of El Niño can be fully understood only by
placing them in their temporal contexto We would
never have learned this lesson if the Niño watering
experiment had been under our control.
ACKNOWLEDGMENTS
Fieldwork was conducted with permission and
support from the Galápagos National Park Service and
the Charles Darwin Research Station, Galápagos.
Funding has been received from NSERC (Canada) and
NSF, most recently BSR-9615383. We thank Solanda
Rea at the Charles Darwin Research Station for supplying
temperature data, the numerous assistants who have
given help in the field, L.E Keller, K. Petren, E. Monson,
and C. Edwards for help in the 1998 season, and G.
Estes for reading the rain gauge when we were not on
the island.
LITERATURE CITED
Gibbs, H.L. and PR Grant. 1987. Ecological consequences of
an exceptionally strong El Niño event on Darwin's finches.
Ecology 68: 1735-1746.
Peter R. Grant, Princeton Üniversity, Princeton, New
Jersey (e-mail: prgrant@princeton.edu). B. Rosemary
Grant, Princeton University, Princeton, New Jersey
(e-mail: prgrant@princeton.edu).
EL NIÑO AND INTRODUCED INSECTS IN THE GALÁPAGO S ISLANDS:
DIFFERENT DISPERSAL STRATEGIES, SIMILAR EFFECTS
Lázaro Roque-Albelo and Charlotte Causton
INTRODUCTION
Oceanic islands are considered fragile ecosystems.
The introduction of one or more alien species can often
lead to a series of important ecological alterations.
Alien species are often able to rapidly colonize oceanic
islands in the absence of natural enemies and other
biotic factor s that, in their native range, maintain popu-
lation numbers at a natural level. These species often
possess characteristics that enable them to occupy a
wide variety of niches that have yet to be filled by
native species and, in some cases, can displace them
(Vitousek 1998).
A 960-km oceanic barrier has isolated the
Galapagos Islands from continental America, but recent
colonization by humans has increased the introduction
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of alíen species to the islands. Peck el al. (1998) have
identified 292 introduced insects from the Galápagos
Archipelago and it is estimated that there are many
more to be found. Accidental transport by humans is
the principal cause for these introductions, while a
variety of methods is responsible for the distribution of
these insects within the Archipelago, including disper-
sal by air currents. It has long been suspected that the
periodic El Niño events contribute to insect dispersal
to and within the Archipelago. However, to our
knowledge, published information about the impact of
these irregular cJimatic conditions on the behavior and
dispersal strategies of alíen insects is non-existant.
Six alíen insect species are highly aggressive and a
threat to the flora and fauna of the Galápagos Islands.
In 1997, the Entomology Program of the Charles
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Effects of the 1998 El Niño on Darwin's finches on Daphne

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Effects of the 1998 El Niño on Darwin's finches on Daphne

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