Biodiversity - Week 5

These pages are being generated as part of a senior seminar course directed by Neal Williams at Bryn Mawr College during fall semester, 2007. This week's topic is "Conserving BioDiversity: Climate Change and the Significance of Genetic Diversity at Species Range Boundaries."

Reading to be discussed:

  • Parmesan, C. and Yohe, G. (2003) A globally coherent fingerprint of climate change impacts across natural systems. Nature 421: 37-42.
  • Hampe, A. and Petit, R.J. (2005) Conserving biodiversity under climate change: the rear edge matters. Ecology Letters 8: 461-467.
  • Webb, S.L., Glenn, M.G., Cook, E.R., Wagner, W.S, and Thetford, R.D. (1993) Range edge red spruce in New Jersey, U.S.A.: bog versus upland population structure and climate responses. J. Biogegraphy 20: 63-78.
Background notes (Word document)
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Comments

Paul Grobstein's picture

Diversity and age

I'm intrigued by several general issues this conversation helped me to think more about. For both human genetic and language diversity there is evidence suggesting that older, founder populations exhibit greater diversity than the distributed populations to which they give rise. My presumption, without thinking much about it, was that this was because the daughter populations had made use of the initial diversity to adapt to new circumstances and so displayed less diversity. An alternative is that the older/founder populations weren't in fact more diverse originally; that the enhanced diversity simply reflects their age and hence time for random variation to occur. I'd like to know more about which is in fact the case. It goes back to the earlier discussion of whether diversity is random accumulation or in some sense represents "meaningful" experiences (ie is a product of selection).

Along these same lines, there was a potentially interesting distinction made between "evolutionary" potential and "biological" potential, with the former being understand to relate specifically to possibilities of future change in genes and their frequencies, and the latter to all possible ways in which organisms might develop and explore in new ways. And this in turn takes us back to earlier conversations. Does a "generalist" have greater potential to radiate into new species? Does a generalist have greater genetic diversity? Or is a generalist simply an organism in which a given genetic potential is less "canalized" than in specialists (birds have the genetic potential to create teeth but it isn't normally expressed).

Yes, I think all this has relevance to "conservation", at the very least by encouraging us to think more clearly about what it is we might want to "conserve" and why.

nwilliams's picture

evolutionary potential?

I had always seen evolutionary potential as being the result of a population having greater within-population genetic variation. By chance such a population is more likely to contain alleles that code of traits that might give their holders greater fitness under a new set of conditions. Such a population would then evolve in response to selection. A population with fewer alleles would not have the opportunity. The same could occur through drift rather than selection?? This evolutionary potential is of course possible among populations, so that a species with greater evolutionary potential is one with overall greater diversity. In this case selection would be more about sorting among populations. Paul's comment reminded me of the importance of time scale. PERSISTENCE might allow for rise in variation through mutation. However, this will depend on the fact that new alleles are not lost through drift. In small populations accumulation of such variation might me hard.

 

Neal Williams

rkumazaw's picture

Climate change will surely

Climate change will surely have effects on biomes, as well as species. Simulations have told us that we should expect loss of tundra, boreal dieback, and forest expansion. With climate change, we would also expect species range shifts towards the poles or higher altitudes. Analyses conducted by Parmesan and Yohe using previous studies showed that out of the 434 species studied, 80% shifted in the expected direction. Analyses on phenologies were conducted for 677 species, of which 87% shifted in the direction expected. Parmesan and Yohe concludes concludes that there is high confidence that climate change is not going to affect living systems in the future, but that it is already affecting living systems.
So how should conservation reserve designs be formulated? It is suggested that the areas being preserved should be bigger rather than smaller, intact rather than fragmented, close to each other rather than isolated from each other, clumped rather than linear, round rather than flat, and there should be corridors connecting the areas. What Hampe and Petit suggests in their rear edge article is to the contrary, however.
Hampe and Petit suggests that because the rear edge is actually more stable and has the highest level of both gamma and beta diversity, it would be beneficial, in terms of preserving biodiversity, to preserve these "pockets" of stable populations. What they are suggesting is that preserving fragmented areas is better than intact with stable rear edge populations because of the high level of between population diversity. In this special case, losing one population is not as critical because within-population diversity is low, therefore the risk of losing biodiversity is spread over a wide range of populations, thus adding stability. What is troubling about what this reading suggests is that it implies a strong criteria to which species would have to meet in order to be this special type and a dependency on the geography. What is also questionable is whether this theory still holds with the change in climate in the opposite direction as the past. Will this be the pattern observed regardless to whether the change is in the warming trend or cooling trend.
While Hampe and Petite's article argued against the center-periphery hypothesis, which predicts that marginal populations are genetically less diverse and therefore more prone to extinction than those from the center, Webb et al.'s findings with upland vs bog red spruce population supports the hypothesis. They discovered that upland populations were of more recent colonization, as opposed to the wetlands which spanned a broader range of ages, suggesting a higher level of diversity among the trees in the wetland.

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