Biotic Exchange and Glacial Cycles

Figures 7.15 and 7.22 demonstrate how downward shifts of high-altitude vegetation (primarily forests) during glacial maxima could have created avenues of dispersal: such a lowering would allow a plant or animal species, previously isolated on individual peaks, to cross ridges and migrate along mountain ranges.

This mechanism is precisely the one invoked by many authors to account for the intracontinental dispersal of vegetation types and associated animals.

When followed by isolation as mesic biomes contracted, it could result in taxonomic disjunctions (Simpson 1975; Vuilleumier and Simberloff 1980).

Glacial cycles may have had similar effects on the distributions of many marine organisms (Figure 7.25).

In the case of cold-water stenothermal species, relatively warm tropical waters serve as an effective physiological barrier to dispersal, limiting their distributions to the middle or high latitudes of the Northern or Southern Hemisphere

During glacial maxima, however, cooling of marine waters could have allowed range expansion into the lower latitudes.

Subsequent rewarming of tropical waters during interglacials could have again caused range contraction and possibly bipolar distributions of these species (Figure 7.25C).

As we noted earlier, eustatic and isostatic changes in sea level greatly altered opportunities for biotic exchange for both terrestrial and marine biotas(see Figure 7.9).

Greatly reduced sea levels during the Wisconsin created extensive landbridges, such as Beringia (connecting Siberia and North America), the Sunda Shelf (connecting Malaysia, and Indonesia), and the Arafura Sea and Bass Straits (conncting New Guinea, Australia, and Tasmania).

While these landbridges eliminated and fragmented marine biotas, they served as important dispersal corridors for terrestrial organisms, In most cases, however, biotic exchange was asymmetrical, withe more species migrating from larger(species-rich) to smaller areas than vice versa (e.g., from Siberia to Alaska; from Southeast Asia to the "islands" of the Sunda Shelf; from Australia to Tasmania). Biotic Exchange of terrestrial organisms across Beringia (see below) contributed significantly to the similarity between Nearctic and Palearctic biotas.

Our own species used this glacial landbridge to colonize North America from Siberia.

Similary, biotic exchange of marine organisms often tended to be asymmetrical, depending on the size and diversity of each species pool and the ocean currents and other factors influencing dispersal.

While Beringia served as a dispersal corridor for terrestrial organisms during glacial maxima, the Bering Strait was an important corridor for the dispersal of marine life during interglacial periods.

Again, more species migrated from the larger, more species-rich region - that is, from the Pacific basin northward.

During the late Cenozoic, 125 species of marine invertebrates invaded the Arctic-Atlantic region from the Pacific, while no more than 16 species colonized in the reverse direction(Durham and Mac Neil 1967; see also Vermeij 1991).

In Chapter 6 we discussed the tectonic events that ultimately formed a Central American landbridge between North and South America (about 3.5 million years B.P.).

The resultant waves of biotic exchange between Nearctic and Neotropical boitas, referred to as the Great American Interchange, were made possible not just by tectonic events, but by eustatic changes and vegetative shifts associated with glacial cycles.

During glacial maxima, the lowering of sea levels increased both the area and the elevation of the Central American landbridge. Perhaps just as important, the relatively dry conditions that prevailed during glacial maxia caused savannas to expand toward the equator and form a continuous habitat corridor for the migration of many species adapted to these open habitats (savannas and shortgrass prairiel see Webb 1991).

We shall return to the profound effects of these waves of biotic interchange in Chapter 16.