July 13, 2000
Colonization, the other half of the battle
It is one thing to heroically survive a trip from the mainland, however, once arrived at the island it pitches up onto, the voyager has to remain alive and reproduce successfully. Even seabirds many of which nest on cliffs do all next on cliffs. Red-footed boobies, pelicans and frigate birds need trees for nesting and would have had to await the arrival of trees and bushes before they could become established. Major problems face each new arrival: In the case of plants that require soil there has to be enough soil already formed for the plant to survive. And the climate has to favor growth, and the right pollinators (or appropriate substitutes) must be already present and spread over the islands, and be making a living by feeding on something else. Without these conditions being met there would be no chance of reproduction and a colonization attempt would fail (which it is probably safe to assume must have happened many times). Besides, there have presumably been thousands of volcanic eruptions in the Galapagos archipelago since its birth, many of which must have caused severe setbacks, even eradication, of recent arrivals, along with their incipient colonies.
Even with birds that could easily have flown to the Galapagos, both sexes would have had to arrive within the reproductive lifetimes of each and the right plants would have had to be established and producing the right nesting sites and material and insect food for their young if the birds were to reproduce and colonize.
And each new arrival would have had to find that the niche for which it was adapted was not filled by some well-established residents against which it could not force a foothold. All arriving animals also had to find enough food to eat and water to drink right away. And once the fruiting season of whatever plant they were eating ended, they would have had to find substitutes. If they were insectivorous, insects had to be present already and in the right phases of their life cycle to be accessible. If they were predators on other vertebrates (the Galapagos hawk is an example) a standing crop of enough prey to support them would have had to be present when they arrived. Requirements such as these must have favored herbivores over carnivores and must have been the chief determinant of success for the earliest arrivals.
One feels at times that there were so many conditions that had to be correct for each arrival that there is no reasonable probability that anything could ever have made it to the islands at the right time and landed in the right place. But it has been calculated that to account for the 522 indigenous species of Galapagos plants only requires about 378 introduction events. As I have said before, the oldest islands have been above sea level for about 5 million years, which means these 378 introductions could have taken place as slowly as one every eight to ten thousand years, and the first European visitors would still have found things in the state of completion that they were when they first arrived.
Modern biologists have watched the colonizing of several volcanic islands, most notably Surtsey about 40 kilometers off the south coast of Iceland. Though much closer to its major source of species, (Iceland) than the Galapagos is to its (the Northern Coast of South America), Surtsey is a good indicator of how the process of colonization of bare lava rock works. Six months after the eruptions ended, bacteria, fungi, seabirds and seeds had all reached the new islands. By 19 months a clump of the beach plant called sea rocket was thriving and eight years after the eruptions ceased, thirteen species of vascular plants and 66 species of mosses had become established. Things may move a lot faster than we think.
That's all from me at present. More next time.
© 2000 - Roger Payne