In my last post, I began discussing William J. Sutherland et al.’s article Identification of 100 fundamental ecological questions – specifically, the first question What are the evolutionary consequences of species becoming less connected through fragmentation or more connected through globalization?
This isn’t the first time I’ve given thought to this issue. In the opening chapter of my dissertation I wrote:
The viability of small populations can also be affected by the loss of genetic diversity. Most tropical trees are outbreeders with complex incompatibility systems (Bawa 1974, Zapata and Arroyo 1978, Bawa et al. 1985, Bawa 1990). Since they are less prone to inbreeding in natural conditions, outbreeders are likely to carry a moderately high genetic load of slightly deleterious alleles (Lande 1995). Minimum viable population sizes for tropical forest trees needed to ensure long-term survival have been estimated at effective populations sizes (Ne) of about 5000 (Alvarez-Buylla et al. 1996).
Although the literature was far from cutting edge even when I wrote it (and more than a little long-of-tooth today), I think the basic point is applicable to the fragmentation question – inbreeders should be expected to purge genetic load, outbreeders more likely to tolerate moderate genetic load. So inbreeders and outbreeders are likely to experience fragmentation differently. (There’s are important caveats here, of course: plants may not respond to fragmentation the same way that animals do; tropical plants may differ from extra-tropical plants; trees will probably not react in the same way that shorter-lived species would.)
Thinking about this point reminded me of something other than Preston Aldrich’s work – Gemma White worked on gene flow in fragmented populations of Swietenia humilis, a species of mahogany restricted to the Pacific coast of Mexico. White and colleagues found that increased gene flow between populations in a fragmented landscape counteracted the effects of fragmentation (in this one species in this one context). The problem with this is that Aldrich and Hamrick’s paper was published in 1998, while White et al. is a 2001 vintage. So what has happened since?
This is where Google Scholar becomes both a boon and a trap. Not only does it make it much easier to find literature, it also makes it easy to find who has cited the papers that interest you. Granted, Web of Knowledge does the same thing, but Google Scholar is quicker and has a more intelligent search engine (as you’d expect from Google). White’s PNAS paper, it turns out, has been cited about 226 times, far too often for casual curiosity. And searching those 226 papers can itself be difficult – the top hits may not be the ones that cite White et al. for precisely the reasons you’re interested. At the same time, it does make an exercise in casual curiosity, like this one, feasible.
So what does the literature say? Not surprisingly (given the question posed), it says “we’re not sure”. Caesalpinia echinata in fragmented Brazilian Atlantic Rainforest fragments showed substantial genetic structure in forest fragments – populations in different fragments have become different, presumably due to reduced gene flow. On the other hand, the Australian rainforest tree Elaeocarpus grandis, showed reduced diversity and increased inbreeding in fragmented landscapes, but no increase in genetic structure. But Andrea Kramer and colleagues found that although “theory predicts widespread loss of genetic diversity from drift and inbreeding in trees subjected to habitat fragmentation, yet empirical support of this theory is scarce”, and suggested that ecological degradation, rather than genetic degradation, might be a far more important cause for concern.
These are just a few random excerpt from the literature, but it makes me think that I might been on the right track when I wrote:
However, the studies that led to these conclusions were not done on insular populations. Species native to the Greater and Lesser Antilles should be adapted to much smaller populations than are mainland populations; historically small populations are likely to be more inbred and, as a consequence of this, to carry lower genetic loads (Alvarez-Buylla et al. 1996). This makes them less susceptible to inbreeding depression (reduced viability, seed production and growth rates caused by the segregation of partially recessive lethal alleles).
There’s still a long way to go, but I suspect that a lot of insight from naturally fragmented systems, especially areas like the Caribbean (and obviously, some people have done just that).