Saturday, March 20, 2010
Island Adaptation
We read in Chapter 4, The Geography of Life, when introduced to volcanic or foreign islands, mainland species often do amazingly well. According to Mr. Coyne, what are the methods by which these species can arrive at these islands? How does the way the organism arrives at the island affect the success of the organism (see p. 104-109)? Is it possible that the organisms on those islands evolve the exact same way as their mainland ancestors, or that they eventually deviate from the original species? If so, for either staying the same or changing over time, how?
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Mr. Coyne notes on page 87 that one of the methods by which new species arrive on an island is through introduction by humans. For example, on Mas a Tierra, the goats, rats, and cats that inhabited the island with Selkird were all "introduced by earlier sailors" (87). Furthermore, because the islands may be "continental islands" meaning that they were once connected to the mainland, native species may have simply traveled over to the island by land. Other than human intervention, animals themselves, have specific adaptations that allow them to arrive at these volcanic or foreign islands. For example, birds can fly over the sea, carrying eggs and seeds from plants. Angiosperm seeds can float across the ocean or hitchhike on "rafts". The spores of ferns, fungi, and mosses can be carried by the wind, along with insects. In the Ecology unit earlier this year, we learned about how the distance the island is from the mainland affects the biodiversity of the island. Thus, if the island were farther from the mainland, it would be more difficult for native species to swim, fly, or be carried by the wind, so there would be fewer mainland species on the island (Campbell 1216).
ReplyDeleteOrganisms that can fly or float across to the island have greater success rates than other animals such as land mammals and reptiles that can't swim or fly. Because organisms such as insects and weeds are such good colonizers, by the theory of natural selection, they have adopted ways to successfully inhabit islands. Consequently, terrestrial animals that are not successful colonizers, therefore oceanic islands lack them. Adaptations such as protective coating in seeds that allow prolonged exposure to seawater allow for greater success of the organism.
Mr. Coyen notes on pg.106 that "the animals and plants on oceanic islands are most similar to species found on the nearest mainland." However, the species on the island will not evolve completely the same way as their mainland ancestors. This is due to accidental colonization and the fact that these islands usually lack the competition and predation levels that the mainland has. Therefore, natural selction "goes wild" on islands, which in turn produces "adaptive radiations". This relates to the theme of interdependence in nature, which describes how organisms depend upon natural conditions for survival. A change in environment or any natural change has impacts beyond a single organism or species. Also, species rarely exist alone in nature. Thus, interaction between organisms or processes in relation to natural occurences is also a part of interdependence in nature. (student.britannica.com). Because predation and competition, key components of interdependence in nature are different between the mainland and the islands, the organisms will evolve differently. Although they descend from the same ancestor, due to dependence on environmental factors, it is inevitable that they will diverge from each other.
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ReplyDeleteConnie mentioned several correct ways that organisms come to colonize islands. To expand, I’d like to point out how some methods of colonization may be more successful than others. Species that arrive via methods in which multiple seeds or individuals arrive together are undoubtedly destined to be more successful. Methods such as seed floating are most likely less successful because fewer organisms can be carried in this manner. If only one or two angiosperm float across the ocean independently or attached to a raft, only those two seeds have the potential of germination and then surviving to be mature enough to reproduce. When time comes for the two angiosperms to reproduce, it is less likely fertilization will occur because there are simply so few gametes on the whole island. In contrast, if a flock of birds eat many angiosperm seeds on the mainland, then fly together to an island, it is more likely that more seeds will be germinated on the island. With a larger starting population, the species has a greater chance of successfully colonizing the island. Ways that bring more individuals to the island initial will yield a greater success rate of colonization. The fact that islands nearer to a mainland have a higher migration rates than further off islands shows that birds (who can fly between the island and mainland) play a significant role in facilitating the colonization of islands (Campbell 1216). This shows the theme interdependence in nature. Species are depended on other species and natural events (wind, tides ect.) to travel to colonize new places such as islands.
ReplyDeleteConnie briefly mentions that islands, in particular oceanic islands, have fewer land animals. This is certainly true: Coyne states that land mammals, reptiles, and amphibians are all missing from islands (104). In addition to providing evidence that the organisms on islands were not placed there by an intelligent designer, this piece of information can account for the evolutionary success of many island species. For example, the population of birds on islands in not kept in check by predators. Because there are not mammals or reptiles to eat the birds or their eggs, the island population of birds can grow to be very high. With a larger population, more mutations will occur and as a result, more speciation of birds can occur as birds evolve to fill different niches. The lack of predators is a key factor that allows the extensive speciation to occur on islands. Coyne points out many times that islands are home to myriad species not found anywhere else, more evidence that the lack of predators on islands contribute to the astounding biodiversity that exists there.