Fragmentation is a problem only if there is insufficient habitat to sustain an stable population for the long term. Keep in mind that most species are not uniformily located across thier range map, they tend to be found in fragmented populations within that range... often with population sinks surrounding the range (this is particularly true of amphibians).

Gene flow doesn't always have to be constant. I'd have to look for the references but I think (stressing think here) that in some box turtle populations the addition of one male to a population was sufficient gene flow for the population.

Ed

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