Connectivity is more than corridors, and corridors are more than linear strips of habitat. Rather, connectivity involves linkages of habitats, species, communities, and ecological processes at spatial scales ranging from fencerows to biomes, and at temporal scales ranging from daily movements of animals to migrations of floras and faunas as climate changes over centuries and millennia. Any piece of land or water may be either a corridor or a barrier to dispersal, depending on the life histories of the organisms concerned. Biogeographers use the term 'filters' to describe areas through which organisms disperse, because species that are poor at dispersing through the habitats involved will be selectively removed. Human activity may either increase connectivity (favoring biological invasions) or decrease connectivity (favoring extinction of isolated populations). Roads are particularly troublesome as barriers, conduits for exotic invasions, and mortality sinks. A general recommendation for conservation of biological diversity is to maintain landscapes with high connectivity of natural habitats, but with low connectivity of artificial habitats such as roadsides, clearcuts, and agricultural fields. Special attention should be given to the needs of species that require broad corridors of habitat with little disturbance by humans, to minimizing opportunities for spread of exotic species, and to maintaining corridors at regional and continental scales to accomodate migration of all native species as climate changes.