Galapagos: A Traveler's Introduction(Firefly, 2018), by Wayne Lynch, takes readers with him through his travels of the Galapagos Islands. Lynch shares his travel experiences through vivid story-telling and photography. Readers will gain a better understanding of the life that habitats the islands and how the plants and animals may have landed on the islands. Find this excerpt in Chapter 3, Tiny Targets in a Giant Sea.
Tiny Targets in a Giant Sea
Before I started to study the Galapagos Islands I never thought much about how islands are colonized by plants and animals, and it turns out to be an unexpectedly interesting process. The Galapagos are oceanic islands that arose from the repeated eruptions of underwater volcanoes. As with all volcanic islands, once the lava cooled, life had to settle on a landscape of barren rock. Today the islands boast a varied assortment of reptiles and seabirds but not a single amphibian or freshwater fish, and very few mammals. Biologists describe life on the Galapagos Islands as “unbalanced” — vastly different from Central and South America, where the majority of the colonists originated. How, then, did the Galapagos Islands acquire the unique mix of flora and fauna that they have today?
Think of the islands as a tiny target adrift in the vast eastern Pacific, a thousand kilometers or more from the nearest land. To get an idea of the challenge facing any animal or plant that might settle on these islands, imagine yourself sitting in the top row of a large football stadium. There is a grape lying on the ground in the middle of the field. It is too far away for you to see, but you must hit the grape with a dart. If you fail, you will die. Good luck!
For most successful colonists to the islands, it was indeed sheer luck that determined their fate. Many failed and perished. None of those successful animal pioneers were actively searching for the islands; all arrived by accident. Even so, for some colonists the odds were stacked in their favor — they were good long-distance travelers.
All the plants and animals that settled on the islands arrived in one of three ways: by sea, by air or carried by wildlife. Plants that came by sea either had to float on their own or were carried on a raft of vegetation. Mangrove seeds are famous for their long-distance journeys by sea, and most tropical coastlines worldwide are rimmed by forests of mangroves. Four species grow in the Galapagos: red, black, white and button mangroves. All are tolerant of salt water and their fruit and seeds float, even if only for a limited time, allowing dispersal by ocean currents.
Charles Darwin was very interested in how plants disperse. After his visit to the Galapagos Islands in 1835, he spent several decades making experiments to learn how they do this. Once he started thinking about it, he couldn’t stop; he had become obsessed with knowing. In one experiment he investigated how long seeds could stay alive inside the crop of a bird that had died at sea and then floated with the ocean currents. He discovered that they could survive for a month inside the bird, yet those same seeds would die after three or four days if they were dropped directly into seawater, unprotected.
Many plants have used wind power to colonize distant islands. Today the three most common plant groups in the Galapagos are lichens, grasses and ferns. All have very small, lightweight seeds or spores that can easily be carried by the wind. As well, all have seeds or spores that are resistant to the dry, freezing conditions in the upper atmosphere, where the jet stream is strongest and most capable of distant dispersal. Composites, a group of daisy-like flowers, produce seeds with parachute-like devices that are easily blown by the wind. The giant daisy trees belonging to the genus Scalesia that grow in the moist uplands are a good example of a Galapagos composite.
What about plants that might have colonized the islands with the help of wildlife? Darwin, ever curious, devised experiments to look into that as well. In one case he collected three tablespoonfuls of sticky mud from a pond near his home. Eventually nearly 500 seedlings of multiple species sprouted from the muddy goo. The inquisitive naturalist reasoned that the muddy feet of birds might have accidentally carried the seeds of grasses and sedges to the distant Galapagos. Today, three birds with incriminating muddy feet that regularly migrate to the islands are Arctic-breeding semipalmated plovers; Wilson’s phalaropes, which breed on the Canadian prairies; and yellow-crowned night herons, which stalk the mudflats of coastal Central America. Any one of them could be guilty of transporting seeds to the islands.
Many seeds are covered with barbs and hooks that can stick to a bird’s plumage and then be carried to distant islands. Migrating birds can also unwittingly transport undigested seeds in their intestinal tracts, depositing them in distant lands in their droppings. When scientists analyzed how all the native plants had arrived in the Galapagos, they concluded that only a few of the original colonists arrived by sea, roughly a third were carried by the wind, and a little less than two-thirds were transported by wildlife. Of those carried by wildlife, around 65 percent were carried internally as undigested seeds, while the remainder arrived in roughly equal amounts either stuck to feathers or glued to muddy toes.