Answers to Review Questions
1. Plant growth forms are good indicators of the physical environment, particularly climatic and soil conditions. Because plants are immobile as adults (seeds can move), they have evolved morphological features that allow them to cope with their physical environment, including its extremes.
Leaf life span (evergreen vs. deciduous leaves), for example, reflects the fertility of the soil. Some biomes, such as grasslands, can also be indicators of disturbances such as grazing or fire. Animals can be important features of and controls on biome distribution, but their mobility renders them less useful as indicators of biomes.2. Biomes are associated with the major climatic zones described in Chapter 2. Tropical rainforests are associated with a tropical climate characterized by high annual precipitation with only slight seasonal variations in the amount of precipitation. As the seasonality of rainfall becomes more pronounced further north and south from the tropics, regular dry periods occur, giving rise to the seasonal tropical forest biome. High-pressure zones associated with Hadley cells create extremely dry zones that promote the desert biome. Seasonality of both temperature (cool winters, warm summers) and precipitation in the temperate climatic zone gives rise to grassland (wet summers, dry winters) and shrubland (wet winters, dry summers) biomes. Temperate deciduous forests occur where seasonal temperature changes are moderate and both summer and winter are moist. Moving toward the polar climatic zone, winter temperatures and precipitation decrease, and the period of subfreezing winter temperature increases, marking the
transition to the boreal and tundra biomes.
3. According to the river continuum concept, water velocity, stream bed particle size, and input of detritus from riparian vegetation all decrease as rivers move downstream. As a result, the importance of the surrounding terrestrial ecosystems as sources of energy for stream organisms tends to decrease downstream. Stream insects include more shredders near the source of a stream and more collectors in the lower portions. Attached plants and free-floating algae become more abundant downstream.
4. Light penetration varies according to the depth and clarity of the water. Where there is enough light for photosynthesis (the photic zone), photosynthetic organisms provide food for consumers, increasing the abundance of those organisms. The stability of the substrate determines whether organisms can anchor themselves or bury themselves in sand. Nearshore zones with rocky substrata tend to have the most abundant organisms and the most diverse communities. Photosynthetic organisms are more sparse in nearshore zones with sandy bottoms and below the photic zone in the open ocean.