Process and Pattern in Communities and Ecosystems

We begin our discussion of processes with solar energy input and precipitation because nearly all ecological processes depend, either directly or indirectly, on the amount and seasonal pattern of solar energy input and supply of water. Next we'll discuss what ecologists have learned by studying interactions among species and how the influences of various factors on community patterns change over space and time. As you read, be aware that the structure of human language forces us to discuss these factors one at a time, but several of them typically operate simultaneously.

Solar energy and precipitation drive ecosystem processes

All organisms depend on inputs of energy (in the form of sunlight or high-energy molecules), water, and nutrients for their metabolism and growth. With the exception of a few ecosystems (some caves, deep-sea hydrothermal vent systems) in which solar energy is not the main energy source, all energy utilized by organisms comes (or once came) from the sun. Even the fossil fuels—coal, oil, and natural gas—upon which the economy of modern human civilization is based are reserves of captured solar energy locked up in the remains of organisms that lived millions of years ago.

Solar energy enters ecosystems by way of plants and other photosynthetic organisms. Only about 5 percent of the solar energy that arrives on Earth is captured by photosynthesis. The remaining energy is either radiated back into the atmosphere as heat or consumed by the evaporation of water from plants and other surfaces. Gross primary productivity is the rate at which energy is incorporated into the bodies of pho-

55.3 Energy Flow through an Ecosystem In this diagram, the width of each channel is roughly proportional to the amount of energy flowing through it. The arrows indicate directions of energy flow.

Sun tosynthetic organisms. The accumulated energy is called primary production; that is, productivity is a rate, and production is a product. Plants use some of this energy for their own metabolism; the rest is stored in their bodies or used for their growth and reproduction. The energy available to organisms that eat plants, called net primary production, is gross primary production minus the energy expended by the plants on their respiration. Only the energy content of an organism's net production—its growth plus reproduction—is available to other organisms that consume it (Figure 55.3).

The distribution of the total amount of energy that plants assimilate by means of photosynthesis reflects the distribution of land masses, temperature, and moisture on Earth (Figure 55.4). Close to the equator at sea level, temperatures are high throughout the year, and the water supply typically is adequate for plant growth much of the time. In these climates, productive forests thrive. In lower-latitude and mid-


| | Photosynthesis |—| Digestion, assimilation, '—' and growth | | Excretion and death | | Respiration

The greatest amount of energy is lost to respiration and is unavailable to organisms.

Detritivores 'jP^^(decomposers)

The greatest amount of energy is lost to respiration and is unavailable to organisms.


| | Photosynthesis |—| Digestion, assimilation, '—' and growth | | Excretion and death | | Respiration

Desert Decomposers

55.4 Primary Production in Different Ecosystems The primary production of Earth's ecosystems can be measured (a) by the geographic extent of the different ecosystems; (b) by net annual primary production; and (c) by the percentage of Earth's total primary production contributed by each ecosystem.


Open ocean

Continental shelf

Extreme desert, rock, sand, ice

Desert and semidesert

Tropical rainforest


Cultivated land

Boreal forest (taiga) Temperate grassland Woodland and shrubland Tundra

Tropical seasonal forest Temperate deciduous forest Temperate evergreen forest Swamp and stream Lake and stream Estuary

Algal beds and reefs Upwelling zones

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Essentials of Human Physiology

Essentials of Human Physiology

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