Our Frequent Flier had time zone
confusions, so we were up an hour early for our flight to the Galapagos. We
flew from Quito to the island
of Baltra, where we drove to the ferry
to cross onto Santa Cruz.
Just during the ferry transfer we saw Brown Pelicans, Galapagos Sea Lions,
Sally Lightfeet and Red Billed Tropic Birds. We drove across Santa
Cruz to board our boat, watching marine iguanas and Sally Lightfeet
bask on the rocks, Darwin’s
Finches flitter around chasing flies, Brown Pelicans watching Blue Footed
Boobies dive deep into the water for fish and a Great Blue Heron idly picking
at the seaweed.
We were in the amazing Galapagos
Islands, made famous by the 1835 voyage of the HMS Beagle. Charles
Darwin was invited on the Beagle as gentleman company for Captain Robert
FitzRoy, and was not the official naturalist. When he first saw the islands he
wrote "nothing could be less inviting than the first appearance". The
islands are most arid and barren, volcanic outposts. They are also actually
extremely poor in biodiversity, as only a few species have been able to reach
and thrive on the islands. In fact, over the past 3.5 million years since the
oldest island formed only 400 colonisation events have occurred, one every 12
000 years. The rate may be even slower than this, genetic testing shows that
marine and land iguanas are 15 million years apart, therefore they either
represent separate arrivals or they arrived 15 millions years ago when none of
the current Galapagos Islands had formed and
the sea mounts reached the surface.
Instead of being rich in biodiversity, the
Galapagos are rich in endemics as the long isolation allows each arrival
species to evolve to their new environment. We saw the most famous endemics
that afternoon, when we drove across Santa
Cruz to the moist highlands to see the Galapagos
Tortoises. The tortoises were large and still. Enormous rocks with a
prehistoric face hiding under the shadows or reaching out on a veined and
wrinkled neck. They reminded me of the skeksis from The Dark Crystal, with a
cruel vulture beak and dead grey skin. While they sat like boulders in the mud,
when they started to move they looked like implacable tanks.
There are two types of Galapagos Tortoise,
the Dome-shelled tortoises of the cool moist highlands, and the Saddlebacks of
the low arid lands.
Each has evolved to its environment. The Dome-shelled is
larger (with shells over a metre long the males reach 300kg) and they have
short limbs, to be able to keep their heat better. The Saddlebacks are smaller
(around 50kg on average) and have long limbs and a saddle on their shell so
their neck can reach up higher. This allows them to be able to reach up high
and eat the Opuntia, a type of cactus that has evolved into tree-like
proportions, and it also gives rise to the name “Galapagos”, Spanish for
“saddle”. While the two types of tortoise live separate lives most of the year,
in order to lay eggs the female domed tortoises need to descend to the lowlands
to lay their eggs. Galapagos tortise eggs become male if they are incubated
under 28.5 C and female if they are incubated above 28.5 C, so if the females
stayed in the cool highlands all the eggs would hatch into males.
As well as being shaped by the islands, the
Galapagos Tortoise has altered the islands in return.
The Opuntia has grown so
tall in order to escape the Saddleback, reaching far greater heights on islands
that have the tortoise. The Galapagos tomato seeds now need to spend time in a
tortoise’s digestion system before they can germinate.
There were once 14 different subspecies of
tortoise. Over 100 000 tortoises were harvested by sailors because they were
easy to collect and they could be kept alive on deck without food or water for
months, giving them fresh meat throughout the voyage. This harvesting drove
three subspecies extinct. A fourth might be extinct; there is only one Pinta
tortoise – Lonesome George. He is only 90 years old and he could easily live
for another hundred years, so they are searching for another Pinta tortoise on
other islands, but genetic testing indicates that he may actually be an
Espanola tortoise, making the Pinta extinct.
The other 10 subspecies are now
healthy. The Espanola tortoise dropped down to fourteen individuals, but
captive breeding and release has built the numbers up to over 1000.
As well are Galapagos tortoises we watched Golden
Warblers bathe, and saw some of Darwin’s
Finches, egrets and the introduced Ani. There are fifty one species of land
birds in the Galapagos Islands, with a very high
proportion (43%) being endemic – twenty two endemic species and three endemic
subspecies. The proportion of endemics is even higher when the thirteen vagrant
species (observed once, but not colonised) are counted out, 66%. The observed
vagrant species show that while colonisation events are improbable, they do
occur.
Yet the 22 endemic species represent only seven different colonisation
events – a rail, a hawk, a dove, a martin, a flycatcher, a mockingbird and a
finch. Two endemic owl subspecies, the Barn Owl and Shorteared Owl, came from
two separate colonisation events. The expansion of a well suited immigrant is
reflected by the success of goats on the islands. In 1959 two females and one
male were released on Pinta. By 1970 the population had expanded to 100 000 and
it took until 1986 to eradicate them.
Not all vagrants are so lucky though, it
was only those vagrants that reached the islands in large enough numbers at the
ideal time that were able to flourish. The ones that did diverged from the mainland
populations, as the islands are far enough away that additional individuals
arriving to keep up gene pool flow is extremely unlikely.
Because the small
islands are close enough that most birds will travel in between them, almost
all endemic successes have stayed as a single species (with flightless reptiles
the story is very different, with seven separate endemic species of lava lizard,
six endemic species of gecko and three endemic snakes on different islands).
The mockingbirds (separated into four species) and the finches (separated into
thirteen species) are the exceptions to this rule, for different reasons. The
mockingbirds are very reluctant to fly across water, so the four different
species and two subspecies are all found on separate islands, with travel rare
enough to allow geographical divergence.
The finches were different. Unlike the
other colonisation species they have highly specialised diets, so to take
advantage of an array of unexploited food sources they had to diverge by
adaptive radiation.
So the finches show different species living together on
the same islands, but with beaks adapted to unique food supplies. On every
island with more than one finch species, the finches have at least a 15%
difference in beak size, with small and large forms to specialise in different
food supplies, while the same finch species on islands with only a single finch
tend to have medium-sized beaks to inefficiently take advantage of a variety of
foods. The larger islands that support multiple Finch species show a very high
level of divergence in eating behaviour, with seed eaters, blood drinkers (the
Vampire Finch pecks Boobies to draw blood to drink, and kicks eggs against
rocks to break them), insect eaters and tool users (the Woodpecker Finch breaks
off spines from cacti to delve under the wood to find beetles). Interestingly
there is actually a fourteenth species of Galapagos Finch, the Cocos Island
Finch, which was seeded onto the Cocos Islands
from the Galapagos and not from the mainland.
While these finches are one of the most
striking examples of adaptive evolution, Darwin
didn’t actually include them in “The Origin of Species”. The finches all look
pretty much the same (we certainly couldn’t spot the difference) and he thought
they were just variants of the one species. It was only when he brought his
specimens back that a specialist told him they were separate species, and he
saw how only evolution could adequately explain their existence. Unfortunately
his finch collection was poorly labelled and he wasn’t sure which were from
which islands, so he played it safe and left them out of the book,
concentrating on the mockingbirds.
After watching the tortoises and finches we
walked down a hollow lava tube, the long winding tunnel caused by the surface of
a lava flow solidifying while the centre flowed on, and then had a cocktail in
town before heading back for our first night on the boat.
