Page 682 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

646 part IV Soils, Ecosystems, and Biomes
Russian Olive
Tamarisk
Cottonwoods
Kudzu
▲Figure 20.4 Needleleaf forest landscape modified by human activity, Norway. Edge species often occupy the varied habitats where natural habitat borders disturbed land. [Bobbé Christopherson.]
After arriving from a different ecosystem, probably 90% of introduced non-native species fail to move into established niches in their new community or habitat. However, some species are able to do so, taking over niches already occupied by native species and thus be- coming invasive species. The 10% that become invasive can alter community dynamics and lead to declines in native species. Prominent examples are Africanized
(a) Zebra mussels cover most hard surfaces in the Great Lakes; they rapidly colonize on any surface, even sand, in freshwater environments.
“killer bees” in North and South America; brown tree snakes in Guam; zebra and quagga mussels in the Great Lakes (Figure 20.5a); Russian olive and tamarisk trees along streams in the U.S. Southwest (Figure 20.5b); and kudzu in the U.S. Southeast (Figure 20.5c). For informa- tion on invasive species prevention and management, see www.ec.gc.ca/eee-ias/ or invasions.bio.utk.edu/ or www .invasivespecies.gov/.
Consider the example of Purple loosestrife (Lythrum salicaria), which was introduced from Europe in the 1800s as a desirable ornamental plant with some medic- inal applications. The plant’s seeds also arrived in ships that used soil for ballast. This hardy perennial, meaning a plant that lives for more than 2 years, escaped cultiva- tion and invaded wetlands across the eastern portions of Canada and the United States, through the upper Midwest, and as far west as Vancouver Island, British Columbia, replacing native plants on which wildlife de- pend. The plant’s invasive characteristics are its ability to produce vast quantities of seed during an extended flowering season and to spread vegetatively through un- derground stems, as well as its tendency to form dense, homogenous stands once established (Figure 20.6).
On a large spatial scale, invasions can alter the dynam- ics of entire biomes. For example, in the Mediterranean shrubland of southern California, the native vegetation is adapted for wildfire. Non-native species often are able to colonize burned areas more efficiently than do native spe- cies; thus the presence of exotic plants can change the suc- cessional processes in this biome. The establishment of non-natives leads to thick undergrowth, providing more fuel for fires that are increasing in frequency. Native veg- etation is adapted for fires that occur at intervals of 30 to 150 years; the increase in fire frequency with climate change puts these species at a disadvantage. More frequent fires in the region combine with the increasing numbers of non-native species to cause the conversion of southern California’s shrubland to grassland.
◀Figure 20.5 Exotic species. [(a) Purestock/ Alamy. (b) lindsay reynolds, Colorado State University/USgS. (c) Bobbé Christopherson.]
(b) Invasive Russian olive (green-gray colour) and tamarisk
(dark green colour in the shade) along Chinle Wash, New Mexico, in riparian habitat formerly occupied by native cottonwoods.
(c) Kudzu, originally imported for cattle feed, spread from Texas to Pennsylvania; here it overruns pasture and forest in western Georgia.