Thursday, 8 December 2011

How similar are populations on "urban" and "natural" islands?

Although human habitats often differ from natural areas in obvious ways, there are sometimes unexpected similarities. For instance, urban "habitat islands"--or patches of suitable habitat separated by barriers such as buildings and roads--are not unlike geological islands separated by water. Among animals living on natural islands, there are usually fairly low levels of dispersal relative to continuous habitats, since it is difficult for many animals to cross water. This can lead to reductions of genetic diversity, since individuals are surrounded by, and forced to mate with, relatives who share similar genes. It seems logical to hypothesize that anthropogenic habitat islands might have similar genetic patterns as their natural counterparts, and that the the low genetic diversity found in many island-dwelling populations might represent an "end point of the genetic trajectories" of animals in anthropogenic areas. If so, it would be useful to measure and compare the genetic diversities of populations living on both types of islands in order to get an idea of how the genetics of anthropogenic populations might change over time, and over what timescale.

Such an effort was recently undertaken by researchers from the University of Western Ontario and the University of Victoria. They quantified the genetic diversity of 106 song sparrows (Melospiza melodia) breeding across 14 sites, 7 of which were coastal islands near Vancouver, and 7 of which were urban islands in the nearby city of Victoria. Because of similarities in habitat structure, breeding density, and population size at the coastal and urban sites, any genetic differences were likely to have been attributable to island type. Genetic diversity was determined by measuring the heterozygosity--the possession of two different forms of a gene (alleles), as contributed by each of an individual's parents--of each bird at as many as 7 different genetic loci. This allowed the scientists to measure genetic variation within mating pairs, within entire sites, and between sites. The researchers could also use the genetic information to investigate potential mechanisms driving the genetic structure of the different populations of birds--including dispersal between natal and breeding habitats, local population declines, and assortative mating (whereby individuals are more likely to breed with genetically-similar partners).

(Song sparrow, Melospiza melodia)

There were some similarities between the two types of site, but also some surprising differences. On both coastal and urban islands, increasing geographic distance between sites was associated with increasing genetic distance; in other words, nearby birds were more likely to be related than birds separated by large distances. Additionally, dispersal was similarly low at both types of site. Only 8 of 106 sparrows originated from a site other than where they were sampled; 4 of these were sampled on coastal islands, and 4 were sampled on urban islands. At neither site did individuals seem particularly inclined to mate with partners genetically similar to themselves. Despite these commonalities, there were also some marked genetic differences between coastal and urban islanders. Individual birds on coastal islands had significantly higher genetic diversity, and, accordingly, were significantly less inbred, than their urban counterparts. A similar pattern was found when the analysis was conducted at the level of site rather than individual: Birds in coastal areas shared fewer alleles than did birds in urban areas.

Normally, low genetic diversity is explained by the presence of low dispersal rates, differences in population history (for instance, sudden reductions in size that result from natural disasters), and demographic processes (such as assortative mating) that occur within habitats. However, the similarity of these factors between coastal and urban islands suggests that variations in song sparrow heterozygosity were influenced by some other factor. The authors believe that this other factor may be predation, which is more frequent in urban sites than in coastal sites. When conducting nest raids, predators often eat every egg or nestling in a single feeding bout, thus placing an all-or-nothing pressure on particular genetic combinations. This can have significant impacts on genetic variation within sites, especially if parents with certain genotypes are all drawn to more heavily predated territories. Another pressure that can result in chick mortality is brood parasitism by brown-headed cowbirds (Moluthrus ater). Female chicks are less likely to survive nesting with a cowbird "sibling;" this means that males have fewer potential mates to choose from when breeding, and may find themselves stuck with a closely related partner with whom they will create less heterozygous young.

(Satellite image showing the Juan de Fuca Strait, middle of photo, and the coastal islands, right center, on which half of the study birds lived)

It is also possible that urban birds are less heterozygous because this trait is simply not as favorable in urban areas. Heterozygosity is generally thought to be advantageous because the increased level of genetic diversity should equip animals with greater "flexibility" to respond to challenges--especially disease--that they might encounter during their lives. Thus, most animals are programmed to behave in ways that maximize this trait from one generation to the next. However, individuals in urban areas may be more likely to die from causes that cannot be "combated" with heterozygosity and so there may not be as much of a selective pressure on maintaining this trait in urban areas.

On the whole, the differences between coastal and urban islands suggest that anthropogenic islands are not "analogues of their naturally fragmented counterparts," as initially thought. Thus, it doesn't appear that scientists can use information on coastal birds to predict what changes might take place in birds inhabiting anthropogenic islands. However, the comparison was useful in highlighting the unexpected difference in heterozygosity between these two types of site. This will likely be an important characteristic to consider in future conservation efforts, since managers will want not only to preserve individuals, but also to maintain genetic diversity. The marked difference between urban and coastal islands also underlines the fact that animals in human areas face very different selection pressures from their natural counterparts, and therefore may adopt quite different behaviors and life history strategies.

For supplementary photos associated with this post, please visit the Anthrophysis pin board at Pinterest.

MacDougall-Shackleton, E.A., Clinchy, M., Zanette, L., and Neff, B.D. 2011. Songbird genetic diversity is lower in anthropogenically versus naturally fragmented landscapes. Conservation Genetics 12:1195-1203.

Thanks to the following websites for providing the images used in this post:

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