Page 679 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 20 Terrestrial Biomes 643
▲Figure 20.2 The unique Australian biogeographic realm. Australia’s flora and fauna evolved in isolation; here a western gray kangaroo stands near a dry river with eucalyptus trees. [A. Held/www.agefotostock.com.]
slightly more detailed divisions in use today, shown in Figure 20.1b.
Species interactions occurred as continents collided and remained attached; species became separated when continents drifted apart. Consequently, the organisms within each realm are a product of plate tectonics and evolutionary processes. For example, the Australian realm is unique for its approximately 450 species of Eucalyptus among its plants and for its 125 species of marsupials— animals, such as kangaroos, that carry their young in pouches, where gestation is completed (Figure 20.2). The presence of monotremes, egg-laying mammals such as the platypus, adds further distinctiveness to this realm.
The boundaries of biogeographic regions are usually determined by climatic and topographic barriers, such as deserts, rivers, mountain ranges, and oceans. Austra- lia’s unique native flora and fauna are the result of its early isolation from the other continents. During criti- cal evolutionary times, Australia drifted away from Pan- gaea (see Chapter 12, Figure 12.14) and never again was reconnected by a land bridge, even when sea level low- ered during repeated glacial ages. New Zealand, although relatively close in location, is isolated from Australia, ex- plaining why it has no native marsupials. However, other factors resulted in the grouping of New Zealand within the Australian realm in the most recent classification of biogeographic realms.
Wallace noted the stark contrast in animal species between several of the islands of present-day Indonesia— Borneo and Sulawesi, in particular—and those of Australia. This led him to delineate a dividing line between the Orien- tal and Australian realms over which he believed species did not cross. This deep water barrier existed even during the lower sea levels of the last glacial maximum, when land connections existed between the two realms. His boundary is today known as “Wallace’s line.” Modern biogeographers have modified this line so that it now encircles the island region between Java and Papua New Guinea, an area that never had a land connection to the mainland and is now sometimes referred to as Wallacea.
Biomes
A biome is defined as a large, stable, terrestrial or aquatic ecosystem classified according to the predominant veg- etation type and the adaptations of particular organisms to that environment. Although scientists identify and de- scribe aquatic biomes (the largest of which are separated into freshwater and marine), biogeographers have applied the biome concept much more extensively to terrestrial ecosystems, associations of land-based plants and ani- mals and their abiotic environment.
Vegetation Types Scientists determine biomes based on easily identifiable vegetation characteristics (vegeta- tion includes the entire flora in a region). Earth’s vegeta- tion types can be grouped according to the growth form (sometimes called life form) of the dominant plants. Ex- amples of growth forms include:
• Winter-deciduous trees—large, woody, perennial plants that lose leaves during the cold season in re- sponse to temperature.
• Drought-deciduous shrubs—smaller woody plants with stems branching at the ground that lose leaves during the dry season.
• Annual herbs—small seed-producing plants without woody stems that live for one growing season.
• Bryophytes—nonflowering, spore-producing plants, such as mosses (see GeoReport 20.2).
• Lianas—woody vines.
• Epiphytes—plants growing above the ground on other
plants and using them for support (see Figure 19.14).
These are just a few examples among many growth forms that are based on size, woodiness, life span, leaf traits, and general plant morphology. Vegetation can
Georeport 20.1 A New Look at Wallace’s Zoogeographic Regions
In 2013, a group of scientists published a new map of zoogeographic realms based on the present distributions of amphibians, birds, and nonmarine mammals, as well as their phylogeny (the evolutionary relationships between organisms
based on their ancestors and descendants). The research group identified 11 zoogeographic realms, which are roughly similar to
but more detailed than Wallace’s original six realms. This updated map provides a baseline for a variety of biogeographical and conservation-oriented studies; see the map, published in the journal Science, at macroecology.ku.dk/resources/wallace/credit_journal_ science_aaas.jpg/.