Page 129 - Environment: The Science Behind the Stories

the ground. The robin might then fly through the air (the recognized that biological entities are tightly intertwined
atmosphere) to a tree (an organism), in the process respiring with chemical and physical entities. Tansley felt that there
(combining oxygen from the atmosphere with glucose from was so much interaction between organisms and their abiotic
the organism, and adding water to the hydrosphere and carbon environments that it made the most sense to view living and
dioxide and heat to the atmosphere). Finally, the bird might nonliving elements together. For instance, in the Chesapeake
defecate, adding nutrients from the organism to the lithosphere Bay estuary—a water body where rivers flow into the ocean,
below. The study of such interactions among living and non- mixing fresh water with saltwater (p. 448)—aquatic organ-
living things is a key part of ecology at the ecosystem level. isms are affected by the flow of water, sediment, and nutri-
ents from the rivers that feed the bay and from the land that
feeds those rivers. In turn, the photosynthesis, respiration,
Ecosystems and decomposition that these organisms conduct influence
the chemical and physical conditions of the Chesapeake’s
An ecosystem consists of all organisms and nonliving entities waters.
that occur and interact in a particular area at the same time. Ecologists soon began analyzing ecosystems as an engi-
The ecosystem concept builds on the idea of the biological neer might analyze the operation of a machine. In this view,
community (Chapter 4), but ecosystems include abiotic com- ecosystems are systems that receive inputs of energy, process
ponents as well as biotic ones. In ecosystems, energy flows and transform that energy while cycling matter internally, and
and matter cycles among these components. produce outputs (such as heat, water flow, and animal waste
products) that enter other ecosystems.
Ecosystems are systems of interacting Energy flows in one direction through ecosystems. Most
living and nonliving entities arrives as radiation from the sun, powers the system, and exits
in the form of heat (Figure 5.6a). Matter, in contrast, is gen-
The ecosystem concept originated early last century with erally recycled within ecosystems (Figure 5.6b). Energy and
scientists such as British ecologist Arthur Tansley, who matter pass among producers, consumers, and decomposers

Consumers Energy is released as heat Consumers
in one-way flow through Matter is conserved and
system cycles within system
Sun
Hawk Hawk
Earthworm Earthworm

Rodent Rodent
Soil bacteria Soil bacteria
Detritivores and Detritivores and
decomposers decomposers

Grasshopper Grasshopper
Heat
Twigs Leaves Twigs Leaves
Detritus Detritus
(non-living organic (non-living organic
matter) matter)

Grass Grass
Producers Producers
Chemical energy Nutrients

(a) Energy flowing through an ecosystem (b) Matter cycling within an ecosystem
Figure 5.6 In systems, energy flows in one direction, whereas matter is recycled. In (a), light energy
from the sun (yellow arrow) drives photosynthesis in producers, which begins the transfer of chemical energy
(green arrows) among trophic levels (pp. 98–100) and detritus. Energy exits the system through respiration in
the form of heat (red arrows). In (b), nutrients (blue arrows) move among trophic levels and detritus. In both
diagrams, box sizes conceptually represent quantities of energy or matter content, and arrow widths represent
relative magnitudes of energy or matter transfer. Such values may vary greatly among ecosystems. For simplic-
ity, various abiotic components (such as water, air, and inorganic soil content) of ecosystems have been omitted.
Based on the figure, which transfer of chemical energy is the largest in ecosystems? Which transfer is
128 the largest for nutrient cycling in ecosystems?

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