SUMMARY

CONCEPT 3.1 Terrestrial biomes are characterized by the growth forms of the dominant vegetation.

3.1.1 Explain why ecologists use plant growth forms to categorize global terrestrial biomes.

Terrestrial biomes are characterized by plant growth forms, particularly of dominant plants. Plants are immobile, so they must be able to cope with the biome's environmental extremes as well as its biological pressures, such as competition for water, nutrients, and light. Plant growth forms are therefore good indicators of the physical environment, reflecting climate zones as well as rates of disturbance.

3.1.2 Describe how global patterns of precipitation and temperature, and their variability, influence the location of terrestrial biomes.

Terrestrial biomes reflect global patterns of precipitation and temperature. Temperature influences the distribution of plant growth forms directly through its effect on the physiological functioning of plants. Precipitation influences the availability of water, which is important in determining the supply of nutrients in the soil, which is also an important control on plant growth form.

3.1.3 Evaluate how human activities impact the actual

distributions of biomes relative to their potential distributions.

The potential and actual distributions of terrestrial biomes differ because of human activities, particularly conversion of land for agriculture, forestry, and grazing.

3.1.4 List the nine major terrestrial biomes.

There are nine major terrestrial biomes: rainforests, seasonal forests and savannas, and hot deserts in tropical and subtropical zones; grasslands, shrublands and woodlands, deciduous forests, and evergreen forests in the temperate zones; and boreal forests and tundra in polar regions.

Biological communities with close analogs to global biomes occur in mountains along elevational bands associated with climate gradients.

CONCEPT 3.2 Biological zones in freshwater ecosystems are associated with the velocity, depth, temperature, clarity, and chemistry of the water.

3.2.1 Summarize how the size of particles on the bottom of streams, as well as water velocity and clarity, change from source streams to large rivers and subsequently influence the organisms that inhabit different zones of moving waters.

Biological communities in streams and rivers vary with stream order and location within the stream channel. Coarse terrestrial detritus is most important near the stream source, while the importance of fine organic matter, algae, and rooted and floating aquatic vascular plants increases downstream. The general feeding styles of organisms change accordingly as the river flows downstream.

3.2.2 Explain how the depth and amount of light penetration in a pond or lake influence the distribution of photosynthetic and nonphotosynthetic organisms.

Biological communities in lakes vary with depth and light penetration. Photosynthetic plankton are limited to the surface layer of water where there is enough light for photosynthesis. Zooplankton—tiny animals and nonphotosynthetic protists— occur throughout the pelagic zone, feeding on detritus as it falls through the water.

CONCEPT 3.3 Marine biological zones are determined by ocean depth, light availability, and the stability of the bottom substrate.

3.3.1 Describe how substrate stability at the bottom of nearshore and shallow ocean zones determines which types of organisms are present, particularly emergent and nonemergent vascular plants and large algae.

Estuaries, salt marshes, and mangrove forests occur in shallow zones at the margins between terrestrial and marine ecosystems. They are influenced by inputs of fresh water and sediments from nearby rivers.

Biological communities at the shoreline reflect the influence of tides and the stability of the substrate (sandy vs. rocky).

Coral reefs and kelp and seagrass beds are productive communities with high diversity associated with the habitat

complexity provided by their photosynthesizers.

3.3.2 Explain how different sources of energy and food affect the type and number of ocean organisms that exist in the water's surface and in the deepest depths.

Biological communities of the open ocean and deep benthic zones contain sparse populations of organisms, whose distributions are determined by light availability and proximity to the bottom.

REVIEW QUESTIONS

1. Why are terrestrial biomes characterized using the growth forms of the dominant plants that occupy them?

2. Describe the close association between the distribution of biomes and the major climate zones described in Chapter 2. In particular, consider how seasonality of both temperature and precipitation influences biome distribution.

3. As streams flow from their sources to the oceans, what physical changes occur that affect the distribution of their biological communities?

4. Why do ocean depth and the stability of the substrate play important roles in determining the composition of marine biological communities?

HONE YOUR PROBLEM-SOLVING SKILLS

Chapter 2 introduced the concept of the rain shadow effect on regional climates. Mountains intercepting air masses enhance precipitation on the windward slopes, with drier and warmer climates occurring on the leeward slopes. The change in temperature and precipitation with elevation therefore differs on west- versus east­facing slopes. The Cascade Range of the Pacific Northwest exemplifies this well. The windward west-facing slopes experience a temperature change with elevation (known as the environmental lapse rate) of 4.5°C per 1,000 m, while the leeward east-facing slopes have an environmental lapse rate of 6.5°C per 1,000 m.

1. Describe the vegetation types you would expect to encounter moving along a line from the base of the Cascade Range on the western side to the summit ridges at 3,000 m, and then descending to the bottom of the mountains on the eastern side. Use the information given in Figure 3.4 to determine the potential vegetation types. The annual average temperature for sites at the base of the west slopes (at sea level) is 12°C, and the average annual precipitation is 120 cm. Annual average precipitation on the summit ridges (3,000 m) increases to about 180 cm (assume the precipitation increase with elevation is linear). Annual average precipitation decreases to 50 cm at the base of the mountains on the leeward slope (elevation is 300 m).

2. Using the same approach you used in Question 1, describe the vegetation types you would expect to find along the same transect with the projected warming of 4°C and a 30% decrease in precipitation associated with climate change over the next 50 years.

LIST OF KEY TERMS

benthic zone Biomes biosphere convergence desertification detritus hyporheic zone intertidal land use change lentic littoral zone lotic macrophytes nekton pelagic zone permafrost photic zone phytoplankton

plankton savannas Tides Zooplankton