The principles that underpinned Gillespie et al.'s study are actually fairly straightforward. First, organisms tend to stay closest to areas that they are comfortable in (taken here to be the area immediately around Tora Bora). Second, within those area, organisms tend to concentrate on larger 'islands' where propitious conditions are found - in this case, it meant cities where individuals can be more inconspicuous, on top of having access to the amenities needed for both hiding and coordinating further action. Lastly, within such islands, organisms will tend to map onto localities that can support their life history characteristics, in this case the size of bin Laden's entourage and the physical space and specific resources they would need to be able to thrive.
A lot of the comments on the Science page and other places are critical of the study, saying it wasn't, in fact, very precise, and saying its findings have been misrepresented. Fair enough. But what's really interesting is that the study was still correct enough to identify the region, the kind of place (i.e., a city) and the kind of structure (a large-ish compound, not a cave!) where bin Laden was found. To focus on the fact that they didn't identify the exact city is to miss the broader point here: biogeography is useful to predict the kinds of areas that given organisms will be found in, and usually not pinpoint exact locations, though informed suggestions as to these can certainly be made. The authors' suggestion that bin Laden would be found hiding in a city within one of the provinces or FATAs (federally administered tribal areas) closest to Tora Bora (which was bin Laden's last reported position according to outside sources) basically proved correct. Detractors will say that this is so general as to be useless, but if you think about how you would structure the use of resources to track an individual in such a large region, you could do a lot worse than that as a starting point to develop a plan of action.
Now, you may well be asking why the hell am I talking about bin Laden and biogeography on this blog which usually focuses on archaeology and paleoanthropology. Well, that's because biogeography is becoming increasingly used in paleoanthropological research to map out areas that would have been best suited for the needs of various kinds of past hominin communities. Specifically, ecological niche modeling has been used to reconstruct the preferred habitats of given species or industries (Banks et al. 2006). This is done specifically by extrapolating from the ecological contexts in which given sites have been recovered what would have been the preferred ecological conditions for the hominins responsible for accumulating them. An example of this is a paper in which the potential extent of various Upper Paleolithic bifacial tools was inferred based on where they have been found over the past 150 years or so of research (Banks et al. 2009). Some members of the same team have also used this approach to reconstruct what the preferred habitats of Neanderthals and early European modern humans (Banks et al. 2010). In this case, they were able to suggest that the ecological niches most prized by both groups of hominins would have overlapped to a very large degree, implying that the two would have likely come into direct contact, leading to a situation whereby modern humans probably drove Neanderthals to extinction through some form of competitive exclusion.
Ecological niche modeling yields very interesting and though-provoking results, but it remains a means to en end, a tool rather than a be-all-end-all kind of approach. As in most models dependent on inputted information, the quality of the patterns it generates is constrained by the quality of the empirical data used in the first place. This was best demonstrated by a recent paper by Lozier et al. (2009) in which they used georeferenced sightings of sasquatch drawn from the Bigfoot Field Researchers Organization website to extrapolate the eoclogical niche and most likely range of that cryptozoid in western North America (as an aside, do check out the BFRO website, for a good look into how organized modern cryptozoology can be - and while you're there, since this week's my birthday, consider ordering a BFRO polo shirt for me!). The paper yields a convincing map of where sasquatch are most likely to be found... a distribution, which incidentally corresponds almost exactly to the modeled ecological niche of brown bears, providing one explanation for what many (if not all) sasquatch sightings may actually have really been of.
The point here is that you can model the ecological niche of any georeferenced dataset, so you need to amke sure your data are really appropriate for the question you're trying to answer. Just because you get an output doesn't mean you necessarily had reliable data on which to base it in the first place. This is a good example of the issues related to the GIGO problem (reconceptualized for non-computer scientists by M. Wolpoff here). In the case of modern humans and Neanderthals, this means specifically that you're limited to dated sites with good provenience information, which captures only part of the relevant sites. It also means that you're basing a species-level attribution on lithic assemblages, which is notoriously problematic (e.g., Clark and Riel-Salvatore 2006).
This is also true for biogeographic approaches in general, and especially as they are applied to people, who have broader sets of motivations than most other animals. Returning to 'where in the world
Edit 05/05/2011: Whoa - this has been the most popular post ever on this little blog! It was even picked as one of this week's ResearchBlogging 'Editor's Selection'. The moral of all this: obviously, I need to write about terrorism and Sasquatch a lot more frequently!
Banks, William E., Francesco d'Errico, Harold L. Dibble, Leonard Krishtalka, Dixie West, Deborah I. Olszewski, A. Townsend Peterson, David G. Anderson, J. Christopher Gilliam, Anta Montet-White, Michel Crucifix, Curtis W. Marean, María-Fernanda Sánchez-Goñi, Barbara Wohlfarth, and Marian Vanhaeren. 2006. Eco-Cultural Niche Modeling: New Tools for Reconstructing the Geography and Ecology of Past Human Populations. PaleoAnthropology 2006:68-83
Banks, W., Zilhão, J., d'Errico, F., Kageyama, M., Sima, A., & Ronchitelli, A. (2009). Investigating links between ecology and bifacial tool types in Western Europe during the Last Glacial Maximum Journal of Archaeological Science, 36 (12), 2853-2867 DOI: 10.1016/j.jas.2009.09.014
Banks, W., d'Errico, F., Peterson, A., Kageyama, M., Sima, A., & Sánchez-Goñi, M. (2008). Neanderthal Extinction by Competitive Exclusion PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003972
Clark, G.A., and J. Riel-Salvatore. 2005. Observations on Systematics in Paleolithic Archaeology. In Transitions before the Transition (E. Hovers and S.L. Kuhn, eds.), pp. 29-56. Springer, New York.
Gillespie, T.W., J.A. Agnew, E. Mariano, S. Mossler, N. Jones, M. Braughton, and J. gonzalez. 2009. Finding Osama bin Laden: An application of biogeographic theories and satellite imagery. MIT International Review: http://web.mit.edu/mitir/2009/online/finding-bin-laden.pdf
Haider, M. 2009. A Response to Professors Gillespie and Agnew on "Finding Osama bin Laden". MIT International Review: http://web.mit.edu/mitir/2009/online/finding.html
Lozier, J., Aniello, P., & Hickerson, M. (2009). Predicting the distribution of Sasquatch in western North America: anything goes with ecological niche modelling Journal of Biogeography, 36 (9), 1623-1627 DOI: 10.1111/j.1365-2699.2009.02152.x