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Population decline in Darwin's finches due to invasive species 

Fessl, B., G. H. Young, R. P. Young, J. Rodriguez-Matamoros, M. Dvorak, S. Tebbich, and J. E. Fa. "How to save the Rarest Darwin's Finch from Extinction: The Mangrove Finch on Isabela Island." Philosophical Transactions of the Royal Society B: Biological Sciences 365.1543 (2010): 1019-030. Web. 14 Sept. 2014. . Different invasive species are threatening the mangrove finch on Isabela Island to the brink of extinction. The high mortality rate of the finch is in part due to rats and an invasive parasitic fly. With rat control and other population control methods, the mangrove finch should experience a rise in the population numbers.

O'connor, Jody A., Rachael Y. Dudaniec, and Sonia Kleindorfer. "Parasite Infestation and Predation in Darwin's Small Ground Finch: Contrasting Two Elevational Habitats between Islands." Journal of Tropical Ecology 26.03 (2010): 285. Web. 14 Sept. 2014. . This research paper examines the parasitic fly larvae in nests of Darwin’s finches on Floreana Island at different elevations. The study conducted in this research suggests that the introduced parasite is limited by its niche requirements and availability of resources.

O'connor, Jody A., Jeremy Robertson, and Sonia Kleindorfer. "Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests." Ed. M. Herberstein. Ethology 120.3 (2014): 228-37. Web. 14 Sept. 2014. . The parasitic fly was experimentally manipulated in nests of Darwin’s finches to test its effects on begging intensity of the chicks and observe the feeding behavior of the parents.

Huber, Sarah K., Jeb P. Owen, Jennifer A. H. Koop, Marisa O. King, Peter R. Grant, B. Rosemary Grant, and Dale H. Clayton. "Ecoimmunity in Darwin's Finches: Invasive Parasites Trigger Acquired Immunity in the Medium Ground Finch (Geospiza Fortis)." Ed. Laurent Rénia. PLoS ONE 5.1 (2010): E8605. Web. 14 Sept. 2014. . Parasite-specific antibody responses to the parasitic fly and the pox virus in finches show acquired immune response. This immune response may help combat the devastating effects of parasitism.

Sulloway, Frank J., and Sonia Kleindorfer. "Adaptive Divergence in Darwin's Small Ground Finch: Divergent Selection along a Cline." Biological Journal of the Linnean Society 110.1 (2013): 45-59. Web. 14 Sept. 2014. . In a single year, phenotypic divergence is observed in a 560 meter elevation gradient in Darwin’s finches on the Galapagos Islands.

https://en.wikipedia.org/wiki/Talk:Philornis_downsi

I believe this stub can be improved with some additional information:


 * 1) The parasitic fly Philornis downsi has been accidentally introduced to the Galapagos Islands in the 1960s. Because of its introduction, the mortality rate of the Mangrove finch (Camarhyncus heliobates) increased by 14% when fly larvae are present in the nest sites.[1]
 * 2) It has been observed that adult flies lay eggs inside of the finch's nests when the adult finches are not present. When the larvae hatch, they migrate up into the nares of chicks and feed on inner tissues of the baby birds.[2]
 * 3) Fly larvae feed for 4 to 7 days and then pupate. They then emerge as adult flies in 7 to 18 days and fly away. Finch chicks have been observed as having a healthy behavior right up until their death. Most chicks die of internal organ damage and open body cavities suffered from fly larvae feedings. [3]

https://en.wikipedia.org/wiki/Small_ground_finch

There have been observable phenotypic differences between finches that live in lowlands and ones that live in highlands, and this change is most likely attributed to adaptation.

Population decline in Darwin’s finches due to predation by invasive species

The Galapagos are an archipelago located off the coast of Ecuador, and possess extraordinary biodiversity with species that are diverse yet similar to one another. Darwin was the first to discover this in 1830s, and collected finches from different islands for examination (Herbers 2014). These finches, which consist of about 15 different species and afterwards dubbed Darwin’s finches, have been one of the most studied passerine birds for a long time. However, the mangrove finch (Camarhyncus heliobates), small ground finch (Geospiza fuliginosa), and medium ground finch (Geospiza fortis) are on the decline due to predation of invasive species, loss of habitat, disease, increased tourism, and other factors. Island species tend to be exceptionally vulnerable to invasive species because they evolved from isolated environments (O’Connor et al. 2010). Recently, increased inter-island connectivity helps the spread of disease and invasive species. One of the main invasive species that is affecting population decline is a nest fly (Philornis downsi) that was first introduced to the island in the 1960s (Fessl et al. 2010). This fly is vegetarian in its adult stage, but the larvae hatched in finch nests tend to crawl into chicks’ nares and consume the inner tissues. The newly-hatched finch offspring have a high mortality rate if afflicted with the larvae due to severe facial and beak deformations and blood bloss (O’Connor et al. 2010). The mangrove finch (Camarhyncus heliobates) is considered to be the rarest of the Darwin’s finches, and their numbers are rapidly declining due to recent predation of invasive species. These birds, unsurprisingly, live in a very limiting mangrove habitat and their population has been reduced to about 100 individuals (Fessl et al. 2010). Due to their small population size, genetic drift is active on mangrove finches, though it is unknown if it is “overpowering” natural selection. Genetic drift is described as random fluctuations in allele frequencies over time due to sampling error (Bergstrom et al. 2012). Regrettably, Kimura’s rule of thumb (if selection is greater than 1/(2Ne), where Ne are the breeding individuals, then selection predominates over drift) could not be used here to determine the answer, because the strength of selection is unknown (Herbers 2014). The main predators of the mangrove finch are cats, fire ants, paper wasps, and especially destructive black rats and parasitic flies. The black rats (Rattus rattus) are predators that account for 54% mortality rate of the mangrove finch during egg incubation, while the larvae of the parasitic fly (Philornis downsi) add an additional 14% mortality rate of newly-hatched chicks (Fessl et al. 2010). If the mortality rates continue to be this high, then the rapid population decline will lead to certain extinction. Due to high predation rates, in 2007 and 2008, rat poison was spread throughout different mangrove sites where the finches lived, which decreased rat predation to 30% mortality of the finch eggs. A year before the rat poison was dispersed (2006), predation was observed in 70% of nests and the average success of nesting was 18%. Although the effective population size (the number of breeding individuals in an idealized population, Ne) is not known, it is estimated that the mangrove finch numbers will rebound if measures of rat control and culling of the parasitic flies are put into place (Bergstrom et al. 2012). Darwin’s small ground finch (Geospiza fuliginosa) has been a species of interest, as it is helping shed light on speciation via adaptive divergence. Adaptive divergence is a phenomenon in which different populations that share a common ancestor inhabit dissimilar habitats, and over generations they develop traits separate from one another. Because natural selection only works on phenotypes, these traits are always phenotypic. The finches seen in highlands had larger, more pointed beaks and smaller feet and claws compared to the lowland variety (Sulloway et al. 2013). Interestingly enough, they are on a cline (series of biocommunities on a continuous gradient), and individuals in the hybrid zone have intermediate traits. This is an example of parapatric speciation, where (in this case) the elevation gradient of 560 meters is causing differentiation in traits, but hybrids are well adapted in their “hybrid zone” (Sabree 2014). Scientists have observed adaptive divergence with gene flow in the small ground finch, and believe that this population might be in the beginning of speciation. This speciation event is most likely caused by different ecological habitats of the populations, consisting of arid and humid zones. Although the distance between the zones is short enough to permit gene flow (2 km), there is evidence of disruptive selection during low rainfall periods (Galligan et al. 2014). Disruptive selection is a type of natural selection that favors extreme values over intermediate values for a specific trait, which add to genetic variance and can lead to speciation (Bergstrom et al. 2012). Therefore, the disruptive selection that the finches are undergoing during low rainfall periods helps maintain their adaptive divergence (Galligan et al. 2014). The small ground finch, like the mangrove finch, has also suffered high mortality rates from the parasitic fly, ranging from 16% to 95% over a four year period (2002-2006). In fact, scientists believe that the small ground finch is at the highest risk for predation from invasive species in the Galapagos Islands (O’Connor et al. 2010). Thirteen islands were examined for infestation of the parasitic larvae in the finch nests, and it was found that infestation was four times less likely to occur in the nests of islands on lower elevation (<200 m) than higher elevation (>400 m). The lowlands seem to be a less favorable environment for the parasitic fly as the climate is drier and the nest sites are fewer and more spread out than in highlands. Overall, these finches should be closely monitored because they are sensitive to parasitism and their populations can decline from high rates of predation and infestation. In addition to studying parasitism, behavioral observations were also recorded in the small ground finch. Scientists hypothesized that nests infested with the parasitic larvae would have chicks that beg for food at different intensities, and that parents would respond more to higher intensities (O’Connor et al. 2014). This ties back to the concept of parent-offspring conflict, where the offspring wants a higher level of parental care than a parent is willing to incur, because the cost of that parental care outweighs the benefits for the parent (Herbers 2014). Also, when there are plentiful resources, parents tend to feed their offspring more during parasitism in order to help combat its destructive effects. Since small ground finches experience a great fitness cost due to severe parasitism of P. downsi, they make for a great model of studying behavioral begging intensity. Indeed, it was observed that chicks who begged louder and for a longer time were parasitized, and in return their parents fed them first and more often than the other chicks (O’Connor et al. 2014). The medium ground finch (Geospiza fortis), found on different islands of Galapagos, is another one of Darwin’s finches and has fascinating morphological traits. Evolution has been seen in real time in this species. In 1977, there was a drought on the island and the supply of seeds these finches would eat diminished. Due to this event, the birds had to turn to harder seeds they found less appetizing. As a result, over generations, beak length increased to 10%. The wing shape, on average, seems to change with ecological shifts. Different selective pressures act on the wing shape of the finches, such as natural and sexual selection (Vanhooydonck et al. 2009). Sexual selection is a form of natural selection that acts on traits that confer a mating advantage (Herbers 2014). In this case, the males seem to have shorter, rounder wings, which help with maneuvering around a female during sexual displays (Vanhooydonck et al. 2009). Since females are the ones that choose the males, this makes it a classic intersexual selection case. Intersexual selection is also at play when the female finches receive direct benefits from the males, who construct the nests (Herbers 2014). In addition, sexual dimorphism is also seen between the sexes, because the males are larger than the females. Sexual dimorphism is also observed in their plumage, as the males have black feathers and the females have streaky-brown feathers. It is advantageous for the females to be smaller due to earlier breeding periods, and females tend to choose bigger males when the sex ratio starts to be off-balance in favor of the males (Vanhooydonck et al. 2009). This is an excellent example of frequency-dependent selection. Frequency-dependent selection occurs when the fitness of a trait is dependent upon the frequency of that trait in a population (Bergstrom et al. 2012). The medium ground finch has also been under parasitism of P. downsi, as well as the avian pox virus (Poxvirus avium). Although outbreaks of the virus have been historically rare, in 2008 there was an outbreak that showed to be present in 50% of the finches tested (Huber et al. 2010). As a result, the finches have developed antibodies to fight specific invasive parasites. The finches with the highest amount of antibodies tend to have the highest fitness, and therefore produce more viable offspring. Of course, this is the backbone of natural selection, so these results are not surprising, but further establish confidence in the “survival of the fittest” hypothesis. In addition to battling predation, the populations of small ground finches have been experiencing inbreeding depression due to small population numbers (Markert et al. 2004). Inbreeding depression occurs when there is a decrease in fitness due to individuals mating with genetic relatives (Bergstrom et al. 2012). Typically, this leads to a loss of genetic diversity and a reduction in heterozygosity (Herbers 2014). Heterozygous individuals have two different alleles on the same locus (with A1A2 being a classic marker). Although the mangrove finch, small ground finch, and medium ground finch are in a constant struggle for existence, they are not completely helpless. Evolution is taking place and making them more adapted to invasive species and disease. However, the rate of evolution by itself may be too slow in preserving the different species. Preserving Darwin’s finches and their phylogenetic diversity (amount of diversity in evolutionary history of a taxon) is important to scientists that are studying the Galapagos (Bergstrom et al. 2012). Without continued human intervention, the finches could keep declining in numbers until extinction.

References

Bergstrom, C, & Dugatkin, L (2012). Parasite Infestation and Predation in Darwin's Small 	Ground Finch: Contrasting Two Elevational Habitats between Islands. Journal of 	Tropical Ecology, 26, 285.

Fessl, B, & Young, G (2010). How to save the Rarest Darwin's Finch from Extinction: The 	Mangrove Finch on Isabela Island. Philosophical Transactions of the Royal Society B: 	Biological Sciences, 365, 104.

Fessl, B, & Loaiza, A (2014). Feeding and Nesting Requirements of the Critically Endangered 	Mangrove Finch Camarhynchus Heliobates. Journal of Ornithology, 2, 152.

Galligan, T, & Donnellan, S (2014). Panmixia Supports Divergence with Gene Flow in Darwin’s 	Small Ground Finch, on Santa Cruz, Galápagos Islands. Molecular Ecology, 2106, 105.

Herbers, J (2014). Evolution lectures. The Ohio State University.

Huber, S, & Owen, J (2010). Ecoimmunity in Darwin's Finches: Invasive Parasites Trigger 	Acquired Immunity in the Medium Ground Finch (Geospiza fortis). Public Library of 	Science, 5, 1-6.

Markert, J, & Grant, P (2004). Neutral Locus Heterozygosity, Inbreeding, and Survival in 	Darwin’s Ground Finches (Geospiza fortis and G. scandens). Heredity, 306, 15.

O'connor, J, & Robertson, J (2014). Darwin's Finch Begging Intensity Does Not Honestly Signal 	Need in Parasitised Nests. Ethology, 228, 37.

Sabree, Z (2014). Evolution lectures. The Ohio State University.

Sulloway, F, & Kleindorfer, S (2013). Adaptive Divergence in Darwin's Small Ground Finch 	(Geospiza fuliginosa): Divergent Selection along a Cline. Biological Journal of the 	Linnean Society, 110, 45-59.

Vanhooydonck, B, & Herrel, A (2009). Wing Shape Variation in the Medium Ground Finch 	(Geospiza fortis): An Ecomorphological Approach. Biological Journal of the Linnean 	Society, 129, 38.

Added my research paper to https://en.wikipedia.org/wiki/Mangrove_finch https://en.wikipedia.org/wiki/Small_ground_finch and https://en.wikipedia.org/wiki/Medium_ground_finch