Abstract
Aim To assess the geological evolution and biogeographical implications of low mountain passes. In particular, we question the common biogeographical belief that major mountain belts form impervious physical barriers to biological dispersal, and that related taxa found on opposites sides of mountains are necessarily a result of vicariant tectonic processes.
Location The Southern Alps of New Zealand form a long (500 km) narrow mountain belt at the oblique collisional Pacific-Australian tectonic plate boundary. High mountains were uplifted during the Pliocene (2-5 Ma) and uplift has continued to the present day.
Methods We integrate previous work from several disciplines to obtain an overview of inter-relationships between plate tectonic processes, geomorphology and biogeography along the main mountain barrier in New Zealand, and then extend this approach to other major mountain belts.
Results The Southern Alps initially formed a barrier to at least some biological dispersal, including vicariant formation of separate species of freshwater non-migratory galaxiid fish on either side. However, the high mountain barrier was breached in several places when passive transport of topography occurred, from the low-erosion rain shadow on the eastern side towards the high-erosion, high-rainfall western side. This tectonic transport resulted in the capture of eastern rivers by west-draining rivers, leaving low passes at the topographic divide. These low-elevation corridors permitted biological dispersal across the mountains, although continued uplift raises these passes. A new set of passes has formed in the northern part of the mountains where younger faults are cutting across the older mountain topography. These potential dispersal corridors are becoming lower with continued erosion, and more common as the defining structures migrate southwards.
Main conclusions Biological dispersal across the Southern Alps may be facilitated by numerous mountain passes, especially via the new passes formed by cross-cutting faults. More low-lying corridors existed than is readily apparent now, as old river capture-related passes have been blocked by ongoing uplift. The dynamic mountain-building and erosional environment typified by the Southern Alps occurs in all the world's collisional mountain belts, such as the Andes, Himalayas, European Alps and North American Cordillera. Sister taxa occurring across mountain belts are not necessarily a result of vicariance driven by the rise of the mountains, as numerous passes may have permitted intermittent dispersal. The evolution of low passes may have been more prevalent than is currently appreciated, suggesting that topographically complex mountain ranges might be more effectively viewed as dynamic filters within a probability landscape rather than as static and impervious high-altitude barriers to all but the rarest of biological dispersal events. In some cases, the biological disjunctions observed across mountains may more directly reflect habitat differentiation driven by orographic mountain development that has limited the probability of trans-alpine dispersal success.