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The effects of the rainstorm El Niño from 1982-1983 had a significant effect on the ecology of the islands Daphne Major and Genovesa. Plant growth hastened across the islands and produced more seeds than in previous years. The population of Geospiza finches increased concurrently. The sudden surge in the populations of Geospiza fortis, G. scandens and G. conirostris led to food shortages on the islands, and the finch were forced to adapt. On Daphne Major, G. fortis developed smaller beaks to aid in the ingestion of small and soft seeds. On Genovesa, G. conirostris males developed deep bills to aid in tearing the pads of Oputnia for food and to provide a form of bird song that could help it overtake another male's territory. G. scandens did not go through any major adaption. Through research it has been determined that due to an extreme environmental event, these species of Geospiza on two geographically separate islands have gone through natural selection in a short amount of time to better adapt to their environment.
Through detailed studies of organisms on the Galapagos Islands, a chain of volcanic islands found west of South America on the equator, Charles Darwin was able to formulate the theory of natural selection (Riley, 2011). Natural selection is the process by which individuals with certain beneficial traits leave more offspring than others with less beneficial traits (Campbell and Reece, 2009). To further prove his theory, Darwin came up with four affirmations that he used to define natural selection. The first is that each individual member of a species is different. Second, Darwin stated that some variations are passed on to offspring. Third, in a certain population, there will always be more individuals born than can survive. Lastly, the ability to survive and reproduce is not random, but instead based on the traits the individual is born with (Riley, 2011).
Extreme environmental conditions have affected the Galapagos Islands a multitude of different times. In particular, the ecology of the islands was significantly affected by the climatic disturbance of the El Niño Southern Oscillation. El Niño was a period of extensive rainfall lasting eight months; starting in November 1982 and finally ending in July 1983. To put this into perspective, most of the major storms of the Galapagos have only lasted four to five months (Grant and Grant, 1993). I will be exploring the effects of the El Niño on the native Geospiza finch population on two geographically separated islands, Daphne Major and Genovesa. Both Daphne Major and Genovesa received significant amounts of rainfall from 1982-1983, altering their respective ecologies (Grant and Grant, 1987; Grant and Grant, 1993). It is estimated that 1359 mm was the amount of rain to fall on Daphne Major, and 2408 mm was the amount of rain to fall on Genovesa (Grant et al. 2000; Grant and Grant 1987). Most of this paper will be concerned with the species G. fortis and G. scandens on Daphne Major, and G. conirostris on Genovesa, although the species G. magnirostris and G. fuliginosa will be referenced as well. It is my hope to detail how these species adapted to changes in the ecology of each island and provide specific observations that support Darwin's idea of natural selection. Similar observations of natural selection in the Galapagos Islands are present in a companion paper (Grant and Grant, 1989).
Daphne Major is a low island, only reaching 120 meter at its highest point. It has two seasons, a wet season the first four months of the year and a dry season the rest of the year. Due to El Niño, the wet season lasted much longer in 1983 (Boag and Grant, 1984; Grant et al. 2000). This had significant effects on the vegetation and allowed plant life to flourish. Plant growth excelled across the island, with as much as 5% more land coverage than in the previous year. As plants grew, so did the overall seed biomass on the island. The overall seed biomass in 1983 was 11 times greater than in 1982. The vast majority of the seeds that caused the increased biomass were small and soft. This is associated to the genera Heliotropium, Cacabus and Chamaesyce, each containing plants that thrive in wet conditions and produce small and soft seeds. The plants that produce large and hard seeds were not well suited for the new wet environments and did not produce as many seeds as they normally do (Gibbs and Grant, 1987; Grant and Grant 1993).
The high seed biomass increased the carrying capacity of Daphne Major and allowed the Geospiza finches to flourish. Coupled with the high food levels, the population of finches rose because mating season occurs during the wet season, and El Niño extended the wet season in 1983 (Gibbs and Grant, 1987). On Daphne Major, the most common types of finch are G. fortis and G. scandens (Grant et al. 2000) These finch tend to be on to be medium/small, however G. scandens is slightly larger. There are also small numbers of G. magnirostris and G. fuliginosa. G. magnirostris is a large with a large beak and G. fuliginosa is small with a small beak. G. fortis produced 3.5 times as many young in 1983 than they did in 1981, and G. scandens produced 4 times as many young in 1983 than 1981. However, even though a large amount of young was produced, a vast majority of them did not survive. Between December 1st, 1983 and August 31st, 1983 about 38% of G. fortis lived until 1984, while 40% of G. scandens lived until 1984. This highlights one of Darwin's affirmations about natural selection, not all individuals that are born can survive (Gibbs and Grant, 1987).
The finches that lived until 1984 soon faced another challenge. The explosion of the population of finches on Daphne Major put a strain on food availability, even after the increase in overall seed biomass (Gibbs and Grant, 1987). Environmental pressures began to select for certain individuals and there became an interspecies competition among all the species of finch on the island. The most immediate effect of the food strain was the disappearance of G. magnirostris. With their large beak, G. magnirostris are located in a niche where they eat large and hard seeds that none of the other finches can eat. After El Niño, very little large and hard seeds developed and soon there were none remaining come dry season. At this point G. magnirostris began to starve and the entire population on the island either died or immigrated to a distant island. Finches with large beaks were not selected for after El Niño and as a result their population died off (Gibbs and Grant, 1987; Grant and Grant, 1993).
Although it is not yet detailed, a similar event is expected to have occurred for G. fuliginosa. G. fuliginosa are small and can eat the small and soft seed, however they are significantly smaller than the other three finch species, and thus have trouble competing for resources with them. During the food shortage they were not able to compete with the bigger G. fortis and G. scandens finches for the small seeds, so they either died or immigrated to another island (Boag and Grant, 1984).
The change is seed composition to small and soft caused intraspecies competition in populations of G. fortis, the species most capable of eating smaller seeds. G. fortis come in many different shapes and sizes, as stated by Darwin's first affirmation. The ability to eat smaller seeds became based on size after the rain from El Niño. It was found that a small body size G. fortis, with a smaller beak depth and width, was more capable of eating the small and soft seeds than the larger body sizes. The result of this natural selection was a shift in the population towards G. fortis with smaller bodies, beak depth and beak width because these individuals were more able to reproduce and offspring. This supports both Darwin's second and fourth affirmations. The only species of finch on the island to not be selected for by El Niño was G. scandens, who ate Oputnia seeds which none of the other species could eat (Grant and Grant, 1993).
Similar to the effects of El Niño on Daphne Major, the island Genovesa was affected by the significant rainfall. Plants flourished on the island and vegetation became thick and plentiful. G. conirostris benefitted from an increase in small seeds and Oputnia seeds, its major food source. Like on Daphne Major, with an increase in the food the carrying capacity increased. Coupled with a longer wet season, breeding among G. conirostris skyrocketed. The breeding season for G. conirostris lasted 7-8 months, double what it normally is (Grant and Grant, 1987).
As the population peaked, food began to become scarcer. By the end of the dry season, almost all small seeds and Oputnia seeds had been eaten and the population was beginning to starve. By the start of 1984, only 12% of the G. conirostris born in 1983 survived. In order to survive to continue to survive, intraspecies competition began to occur. Unlike on Daphne Major, intraspecies competition occurred in two ways on Genovesa: phenotypically and reproductively (Grant and Grant, 1987; Grant 1985).
With no small seeds or Oputnia seeds left in 1984, G. conirostris had to find another source of food. The next best food source was Bursera seeds, but these too were soon eaten. G. conirostris thus was forced to eat beetle and fly larvae, found inside rotting Oputnia pads. The pads required ripping and tearing by the beak before the larvae could be extracted. Studies show that larger G. conirostris with deeper bills were more able to tear open the pads of Oputnia and reach the food inside. The population shifted towards larger birds with deeper bills as smaller birds began to die from lack of food. Deeper bills also were better at eating the minute number of Bursera seeds remaining on the island (Grant and Grant, 1989).
In order to reproduce, G. conirostris males must obtain a territory before the female will consider him. Males compete with each other for territories using song. It has been found that males with long and deep bills create a 'winning' song and can take the territory easier. This means that males who are able to create the better song will better attract the female and be able to pass his variation to his offspring. G. conirostris with smaller, shallower bills will not attract the females as well and not be able to pass on their traits. The outcome will be a shift in the population towards larger, deeper bills (Grant, 1985)
On two geographically separated islands, natural selection was observed on distinct species of Geospiza from the same extreme environmental event. Each adhering to Darwin's four affirmations, G. fortis and G. conirostris provide ample amounts of data to help researchers better confirm the theory of natural selection. On Daphne Major it was observed that the population of G. fortis shifted to having smaller beaks, a result of the vast amount of small and soft seeds (Grant and Grant, 1993). On Genovesa, G. conirostris developed deeper bills to help reproduce and eat (Grant and Grant, 1989; Grant, 1985). Due to El Niño, each population shifted dramatically in a short amount of time. The theory of natural selection is further supported through the disappearance of G. magnirostris and G. fuliginosa from Daphne Major. These species could not compete with the more biological fit G. fortis and were forced to leave the island or die. Competition for shared resources put stresses on individuals of a species carrying certain traits, causing them to die or emigrate (Hendry et al. 2008). In cases where species are not able to leave an island because they are physiologically trapped, researchers can observe how events such as extinction occur. It is not a coincidence that on two separate islands, two different species were naturally selected for based on the stresses of the same event. It is instead evidence that natural selection is not a theory but a fact of life that must be embraced by all.