Counter-intuitive results on connectivity

Posted on December 26, 2009

Most models of the effects of habitat patchiness on the dynamics and genetics of populations have assumed that animals migrate freely between patches in two-dimensional space.  This assumption is valid for terrestrial systems, but not for branching streams, where only one route is available for movement between any two places in the network.  The influence of the shape of branching networks on population parameters has been examined by generating a large number of different networks and for each one simulating the population dynamics of a species with a simple life cycle.  The outcomes were very different from those predicted by  “terrestrial” spatial models.  Most significantly, when dispersal rates were low, high connectivity (defined as the number of favourable habitat patches within a given radius) tended to promote local extinction and genetic isolation by distance.   This occurred because in branching networks with only one path between patches,  genes circulate well within groups but not between groups: for example, when a root patch goes extinct, the population as a whole is split in two and each portion has a lower persistence time due to its reduced size. 

Reference:  Labonne, J., Ravigne, V., Parisi, B. & Gaucherel, C.  2008.  Linking dendritic network structures to population demogenetics: the downside of connectivity.  Oikos 117, 1479-1490.