the ecosystem concept

Ecosystem ecology studies the links between organisms and their physical environ-

ment within an Earth System context. This chapter provides background on the con-

ceptual framework and history of ecosystem ecology.

Introduction

The supply of fish from the sea is now declin-

ing because fisheries management depended on

Ecosystem ecology addresses the interactions

between organisms and their environment as an

integrated system. The ecosystem approach is

fundamental in managing Earth’s resources

because it addresses the interactions that link

biotic systems, of which humans are an integral

part, with the physical systems on which they

depend. This applies at the scale of Earth as a

whole, a continent, or a farmer’s field. An

ecosystem approach is critical to resource man-

agement, as we grapple with the sustainable use

species-based approaches that did not ade-

quately consider the resources on which com-

mercial fish depend. A more holistic view of

managed systems can account for the complex

interactions that prevail in even the simplest

ecosystems. There is also an increasing appreci-

ation that a thorough understanding of eco-

systems is critical to managing the quality and

quantity of our water supplies and in regulating

the composition of the atmosphere that deter-

mines Earth’s climate.

of resources in an era of increasing human

population and consumption and large, rapid

changes in the global environment.

Overview of Ecosystem Ecology

Our growing dependence on ecosystem con-

cepts can be seen in many areas. The United

The flow of energy and materials through

Nations Convention on Biodiversity of 1992,

organisms and the physical environment pro-

for example, promoted an ecosystem approach,

vides a framework for understanding the diver-

including humans, to conserving biodiversity

sity of form and functioning of Earth’s physical

rather than the more species-based approaches

and biological processes. Why do tropical

that predominated previously. There is a grow-

forests have large trees but accumulate only a

ing appreciation of the role that individual

thin layer of dead leaves on the soil surface,

species, or groups of species, play in the func-

whereas tundra supports small plants but an

tioning of ecosystems and how these functions

abundance of soil organic matter? Why does

provide services that are vital to human

the concentration of carbon dioxide in the

welfare. An important, and belated, shift in

atmosphere decrease in summer and increase

thinking has occurred about managing ecosys-

in winter? What happens to that portion of the

tems on which we depend for food and fiber.

nitrogen that is added to farmers’ fields but is

3

4

1. The Ecosystem Concept

not harvested with the crop? Why has the intro-

tion of plants by herbivores, and the consump-

duction of exotic species so strongly affected

tion of herbivores by predators. Most of these

the productivity and fire frequency of grass-

fluxes are sensitive to environmental factors,

lands and forests? Why does the number of

such as temperature and moisture, and to bio-

people on Earth correlate so strongly with the

logical factors that regulate the population

concentration of methane in the Antarctic

dynamics and species interactions in communi-

ice cap or with the quantity of nitrogen enter-

ties. The unique contribution of ecosystem

ing Earth’s oceans? These are representative

ecology is its focus on biotic and abiotic factors

questions addressed by ecosystem ecology.

as interacting components of a single integrated

Answers to these questions require an under-

system.

standing of the interactions between organisms

Ecosystem processes can be studied at many

and their physical environments—both the

spatial scales. How big is an ecosystem? The

response of organisms to environment and

appropriate scale of study depends on the ques-

the effects of organisms on their environment.

tion being asked (Fig. 1.1). The impact of zoo-

Addressing these questions also requires

plankton on the algae that they eat might be

that we think of integrated ecological systems

studied in the laboratory in small bottles. Other

rather than individual organisms or physical

questions such as the controls over productiv-

components.

ity might be studied in relatively homogeneous

Ecosystem analysis seeks to understand the

patches of a lake, forest, or agricultural field.

factors that regulate the pools (quantities) and

Still other questions are best addressed at the

fluxes (flows) of materials and energy through

global scale. The concentration of atmospheric

ecological systems. These materials include

CO2, for example, depends on global patterns

carbon, water, nitrogen, rock-derived minerals

of biotic exchanges of CO2 and the burning of

such as phosphorus, and novel chemicals such

fossil fuels, which are spatially variable across

as pesticides or radionuclides that people have

the globe. The rapid mixing of CO2 in the

added to the environment. These materials are

atmosphere averages across this variability,

found in abiotic (nonbiological) pools such as

facilitating estimates of long-term changes in

soils, rocks, water, and the atmosphere and in

the total global flux of carbon between Earth

biotic pools such as plants, animals, and soil

and the atmosphere.

microorganisms.

Some questions require careful measure-

An ecosystem consists of all the organisms

ments of lateral transfers of materials. A water-

and the abiotic pools with which they interact.

shed is a logical unit in which to study the

Ecosystem processes are the transfers of energy

effects of forests on the quantity and quality of

and materials from one pool to another. Energy

the water that supplies a town reservoir. A

enters an ecosystem when light energy drives

watershed, or catchment, consists of a stream

the reduction of carbon dioxide (CO2) to form

and all the terrestrial surfaces that drain into

sugars during photosynthesis. Organic matter

it. By studying a watershed we can compare the

and energy are tightly linked as they move

quantities of materials that enter from the

through ecosystems. The energy is lost from

air and rocks with the amounts that leave in

the ecosystem when organic matter is oxidized

stream water, just as you balance your check-

back to CO2 by combustion or by the respira-

book. Studies of input–output budgets of water-

tion of plants, animals, and microbes. Materials

sheds have improved our understanding of the

move among abiotic components of the system

interactions between rock weathering, which

through a variety of processes, including the

supplies nutrients, and plant and microbial

weathering of rocks, the evaporation of water,

growth, which retains nutrients in ecosystems

and the dissolution of materials in water.

(Vitousek and Reiners 1975, Bormann and

Fluxes involving biotic components include the

Likens 1979).

absorption of minerals by plants, the death of

The upper and lower boundaries of an

plants and animals, the decomposition of dead

ecosystem also depend on the question being

organic matter by soil microbes, the consump-

asked and the scale that is appropriate to the