Fragmentation alters evolutionary processes

Throughout this book we've emphasized the interactions between ecological and evolutionary processes, particularly through adaptation and impacts on population dynamics. Human alteration of landscapes adds a twist to these interactions, which we emphasized in our consideration of conservation biology in the previous chapter.

Chapters 10 and 23 considered the genetic and demographic problems of small, isolated populations. The evolutionary consequences of fragmentation were studied by Marcel Goverde and his colleagues by watching bumblebee behavior in the Jura Mountains of Switzerland (Goverde et al. 2002). Their experimental plots included meadow fragments of different sizes (created by mowing) and control plots in unfragmented meadow habitat. The researchers studied the foraging behavior of bumblebees as they visited the flowers of wood betony (Stachys officinalis), which were common in both experimental fragments and control plots. The bees visited fragments less frequently than they visited control plots, and once there, they tended to stay longer in the fragments. Ultimately, these two changes in bumblebee behavior resulted in a lower probability of pollination and an increased likelihood of inbreeding for the wood betony in the fragments, resulting in an altered evolutionary trajectory for those plants.

In many other cases, habitat fragmentation has been shown to increase rates of inbreeding and genetic drift for those species confined to fragments. For example, Keller and Largiader (2003) found significant genetic divergence between populations of the flightless ground beetle Carabus violaceus that had been isolated by roads. Habitat fragmentation can also alter selection pressures on organisms. Where plant populations become small and isolated, their chances of encountering their pollinators, their pathogens, their herbivores, their dispersers, and their competitors may all be reduced, with subsequent ecological and evolutionary consequences. Similar effects have been observed in animals, whose breeding systems and survival patterns can be altered in small fragments (Barbour and Litvaitis 1993).

The evolutionary implications of habitat fragmentation continue to be important topics for study. As we'll see in the next section, however, the evolutionary dynamic associated with fragmentation is only one part of what must be considered in designing nature reserves that will work well to maintain biodiversity in landscapes increasingly modified by humans.