Page 658 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

622 part IV Soils, ecosystems, and Biomes
Ice and snow
Tundra
Needleleaf forest
Temperate deciduous forest
rain forest
Tropical rain forest
Ice and snow
Tundra
Latitudinal zonation Increasing latitude from the equator
(a) Progression of plant community life zones with changing elevation or latitude.
▲Figure 19.15 Vertical and latitudinal zonation of plant communities. [robert Christopherson.]
plant associations in relationship to climatic conditions are discussed further.
As discussed in this chapter’s Geosystems Now, recent scientific studies show that climate change is causing plants and animals to move their ranges to higher elevations with more suitable climates as established life zones shift. Evidence exists that some species have run out of space on mountains, as environmental conditions are pushing them to elevations beyond their mountains’ reach, essentially taking them “out of bounds,” forcing them to either move elsewhere or into extinction.
limiting Factors
The term limiting factor refers to physical, chemical, or biological characteristics of the environment that deter- mine species distributions and population size. For ex- ample, in some ecosystems, precipitation is a limiting factor on plant growth, through either its lack or its ex- cess. Temperature, light levels, and soil nutrients all af- fect vegetation patterns and abundance:
• Low temperatures limit plant growth at high elevations. • Lack of water limits growth in a desert; excess water
limits growth in a bog.
with a small habitat in the Florida Everglades, is a specialist that feeds on only one specific type of snail. In contrast, the Mallard Duck (Anas platy- rhynchos) is a generalist, feeds from a variety of widely diverse sources, is easily domesticated, and is found
throughout most of North America (Figure 19.16a).
For some species, one critical limiting factor deter- mines survival and growth; for other species, a combina- tion of factors is at play, with no one single factor being dominant. When taken together, limiting factors deter- mine the environmental resistance, which eventually
stabilizes populations in an ecosystem.
Each organism possesses a range of tolerance for
physical and chemical environmental characteristics. Within that range, species abundance is high; at the edges of the range, the species is found infrequently; and beyond the range limits, the species is absent. For exam- ple, the coast redwood (Sequoia sempervirens) is abun- dant within a narrow range along the California and Oregon coast where foggy conditions provide conden- sation to meet the tree’s water needs. Redwoods at the limit of their range—for example, at higher elevations above the fog layer—are shorter, smaller, and less abun- dant. The red maple (Acer rubrum) has a wide tolerance range and is distributed over a large area with varying moisture and temperature conditions (Figure 19.16b).
Needleleaf forest
(b) Treeline for a needleleaf forest in the Canadian Rockies marks the point above which trees cannot grow.
• Changes in salinity levels affect aquatic ecosystems.
• Low phosphorus content of soils limits plant growth.
• The general lack of active chlo- rophyll above 6100 m limits primary productivity.
For animal populations, limit- ing factors may be the number of predators, availability of suitable food and habitat, availability of breeding sites, and prevalence of disease. The Snail Kite (Rostrha- mus sociabilis), a tropical raptor
Temperate deciduous
forest
Tropical
Treeline
Vertical zonation Increasing elevation from sea level