July 7, 2000.
Land Animals make the Scene
This is Roger Payne speaking to you from the Odyssey as it makes its way through the Galapagos Islands.
The larger land animals such as lizards, iguanas and giant tortoises must have arrived by sea.
They probably rode in on floating rafts of vegetation. Such mats of plant material dead and alive break off from mainland river banks during the rainy season. Sometimes they may be several hectares in extent and some have been known to carry several animal species. As unlikely as such a mechanism may seem it has been postulated that it would have required only half a dozen successful introductions to account for all the reptile species on the Galapagos.
Tiny spiders, minute snails, and other air plankton, as well as many seeds presumably came by air, some of the smaller seeds must have been carried in by birds as well. Darwin did a series of experiments in which he fed seeds to turkeys and then experimented in germinating and growing the seeds after they had made their own voyage through the turkey. Nor did he stop there. He notes that: "The crops of birds do not secrete gastric juice, and do not in the least injure, as I know by trial, the germination of seeds; now after a bird has found and devoured a large supply of food, it is positively asserted that all the grains do not pass into the gizzard for 12 or even 18 hours. A bird in this interval might easily be blown to the distance of 500 miles, and hawks are known to look out for tired birds, and the contents of their torn crops might thus readily get scattered ... Some hawks and owls bolt their prey whole, and after an interval of from twelve to twenty hours, disgorge pellets, which as I know from experiments...include seeds capable of germination." And sure enough, in a paper written in 1975 we find direct observations of owls that killed finches with bellies full of seeds on one island and regurgitated them in pellets on another island. Darwin's work is filled with similar insights all of which demonstrate how well he observed and how thoroughly he considered his observations.
That's all for today, more tomorrow. It turns out that however harrowing the trip might be, getting there is only the beginning of the trials any animal or plant faces when it tries to make a living on a previously lifeless volcanic island.
© 2000 - Roger Payne