Figure 44.6 Resource Partitioning Allows Competitors to Coexist
On the Galápagos Islands there are several finch (Geospiza) species with distinctive beak sizes. Beak depth is an important predictor of the size of seeds that a given species can consume. Schluter and Grant (1984) predicted that partitioning of seed resources occurred on islands and that this partitioning enabled different Geospiza species to coexist on the same island. To test this prediction, the researchers measured seed size and abundance on 15 islands. For each island they also modeled the availability of food for birds with different beak sizes and noted a striking correspondence between the peak availability of food (of a given size) and the beak sizes of the different species. On most islands, the number of peaks in food availability determined the number of species present, suggesting that finch species coexist by using different seed resources. There has been continued study of the finches on these islands, and recently Grant and Grant (2006) documented the importance of competition in determining beak size. Specifically, they studied the finches on the island of Daphne. Initially only G. fortis was present, as shown in the graph in the upper left. Lacking competition, these medium-sized finches consumed a diversity of seed types. In 1982, G. magnirostris first appeared on the island. This large ground finch is almost twice the size of G. fortis and readily eats larger seeds. The two species coexisted on the island until a drought in 2003. The drought resulted in strong competition for seeds in 2004, resulting in considerable bird mortality. Interestingly, the highest G. fortis mortality was for those individuals with larger beaks. In 2005, the Grants documented that the G. fortis population on the island had smaller beaks than before the drought. Smaller beaks facilitate consumption of smaller seeds, allowing them to coexist with the larger seed-eating G. magnirostris. This shift in beak size as a result of interspecific competition is an excellent example of character displacement, which is rarely observed in such an evolutionarily short time period.
Original Paper
Schluter, D., and P. R. Grant. 1984. Determinants of morphological patterns in communities of Darwin’s finches. American Naturalist 123: 175–196.
http://www.jstor.org/stable/2461032
Links
Pennisi, E. 2006. Competition Drives Big Beaks Out of Business. Science 313: 156.
http://dx.doi.org/10.1126/science.313.5784.156
Grant, P. R., and B. R. Grant. 2006. Evolution of Character Displacement in Darwin’s Finches. Science 313: 224–226.
http://dx.doi.org/10.1126/science.1128374
National Geographic News: “Instant” Evolution Seen in Darwin’s Finches, Study Says
http://news.nationalgeographic.com/news/2006/07/060714-evolution.html
Levine, J. M., and HilleRisLambers, J. 2010. The Maintenance of Species Diversity. Nature Education Knowledge 1(10): 67.
http://www.nature.com/scitable/knowledge/library/the-maintenance-of-species-diversity-13240565
Kimball’s Biology Pages: Speciation
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
Wikipedia: Character Displacement
http://en.wikipedia.org/wiki/Character_displacement
University of British Columbia: Dolph Schluter
https://www.zoology.ubc.ca/~schluter/wordpress/