Ecology in Further Detail ❮ 231
frozen throughout the year. This keeps plants from forming deep roots in this soil and
dictates what type of plant life can survive. The plant life that tends to predominate is
short shrubs or grasses that are able to withstand difficult conditions.
- Water biomes.Both freshwater and marine water biomesoccupy the majority of the
surface of the earth.
The general distribution of biomes on the earth’s surface is shown in Figure 18.8.
Biogeochemical Cycles
One last topic to briefly cover before we wave goodbye to ecology is that of biogeochemical
cycles.These cycles represent the movement of elements, such as nitrogen and carbon,
from organisms to the environment and back in a continuous cycle. Do not attempt to
become a master of these cycles, but you should understand the basics.
Carbon cycle.Carbon is the building block of organic life. The carbon cyclebegins
when carbon is released to the atmosphere from volcanoes, aerobic respiration (CO 2 ), and
the burning of fossil fuels (coal). Most of the carbon in the atmosphere is present in the
form of CO 2. Plants contribute to the carbon cycle by taking in carbon and using it to per-
form photosynthetic reactions, and then incorporating it into their sugars. The carbon is
ingested by animals, who send the carbon back to the atmosphere when they die.
Nitrogen cycle. Nitrogen is an element vital to plant growth. In the nitrogen cycle
(Figure 18.9), plants have nitrogen to consume thanks to the existence of organisms that per-
form the thankless task of nitrogen fixation—the conversion of N 2 to NH 3 (ammonia). The
only source of nitrogen for animals is the plants they consume. When these organisms die, their
remains become a source of nitrogen for the remaining members of the environment. Bacteria
and fungi (decomposers) chomp at these organisms and break down any nitrogen remains. The
NH 3 in the environment is converted by bacteria into NO 3 (nitrate), and this NO 3 is taken up
Tundra
Taiga
Temperate
deciduous
forest
Equator
Tundra
Figure 18.8 General distribution of biomes on the earth’s surface. (The other
land biomes such as grassland and desert are interspersed in temperate and tropical
regions with water as the limiting factor.)
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