the ecosystem concept

The Ecosystem Concept
Ecosystem


Ecosystem ecology studies the links between organisms and their physical environment
within an Earth System context. This chapter provides background on the conceptual
framework and history of ecosystem ecology

Introduction

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 management,

as we grapple with the sustainable use

of resources in an era of increasing human

population and consumption and large, rapid

changes in the global environment.

Our growing dependence on ecosystem concepts

can be seen in many areas. The United

Nations Convention on Biodiversity of 1992,

for example, promoted an ecosystem approach,

including humans, to conserving biodiversity

rather than the more species-based approaches

that predominated previously. There is a growing

appreciation of the role that individual

species, or groups of species, play in the functioning

of ecosystems and how these functions

provide services that are vital to human

welfare. An important, and belated, shift in

thinking has occurred about managing ecosystems

on which we depend for food and fiber.

The supply of fish from the sea is now declining

because fisheries management depended on

species-based approaches that did not adequately

consider the resources on which commercial

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 appreciation

that a thorough understanding of ecosystems

is critical to managing the quality and

quantity of our water supplies and in regulating

the composition of the atmosphere that determines

Earth’s climate.

Overview of Ecosystem Ecology

The flow of energy and materials through

organisms and the physical environment provides

a framework for understanding the diversity

of form and functioning of Earth’s physical

and biological processes. Why do tropical

forests have large trees but accumulate only a

thin layer of dead leaves on the soil surface,

whereas tundra supports small plants but an

abundance of soil organic matter? Why does

the concentration of carbon dioxide in the

atmosphere decrease in summer and increase

in winter? What happens to that portion of the

nitrogen that is added to farmers’ fields but is

1

The Ecosystem Concept

Ecosystem ecology studies the links between organisms and their physical environment

within an Earth System context. This chapter provides background on the conceptual

framework and history of ecosystem ecology.

not harvested with the crop? Why has the introduction
of exotic species so strongly affected
the productivity and fire frequency of grasslands
and forests? Why does the number of
people on Earth correlate so strongly with the
concentration of methane in the Antarctic
ice cap or with the quantity of nitrogen entering
Earth’s oceans? These are representative
questions addressed by ecosystem ecology.
Answers to these questions require an understanding
of the interactions between organisms
and their physical environments—both the
response of organisms to environment and
the effects of organisms on their environment.
Addressing these questions also requires
that we think of integrated ecological systems
rather than individual organisms or physical
components.
Ecosystem analysis seeks to understand the
factors that regulate the pools (quantities) and
fluxes (flows) of materials and energy through
ecological systems. These materials include
carbon, water, nitrogen, rock-derived minerals
such as phosphorus, and novel chemicals such
as pesticides or radionuclides that people have
added to the environment. These materials are
found in abiotic (nonbiological) pools such as
soils, rocks, water, and the atmosphere and in
biotic pools such as plants, animals, and soil
microorganisms.
An ecosystem consists of all the organisms
and the abiotic pools with which they interact.
Ecosystem processes are the transfers of energy
and materials from one pool to another. Energy
enters an ecosystem when light energy drives
the reduction of carbon dioxide (CO2) to form
sugars during photosynthesis. Organic matter
and energy are tightly linked as they move
through ecosystems. The energy is lost from
the ecosystem when organic matter is oxidized
back to CO2 by combustion or by the respiration
of plants, animals, and microbes. Materials
move among abiotic components of the system
through a variety of processes, including the
weathering of rocks, the evaporation of water,
and the dissolution of materials in water.
Fluxes involving biotic components include the
absorption of minerals by plants, the death of
plants and animals, the decomposition of dead
organic matter by soil microbes, the consumption
of plants by herbivores, and the consumption
of herbivores by predators. Most of these
fluxes are sensitive to environmental factors,
such as temperature and moisture, and to biological
factors that regulate the population
dynamics and species interactions in communities.
The unique contribution of ecosystem
ecology is its focus on biotic and abiotic factors
as interacting components of a single integrated
system.
Ecosystem processes can be studied at many
spatial scales. How big is an ecosystem? The
appropriate scale of study depends on the question
being asked (Fig. 1.1). The impact of zooplankton
on the algae that they eat might be
studied in the laboratory in small bottles. Other
questions such as the controls over productivity
might be studied in relatively homogeneous
patches of a lake, forest, or agricultural field.
Still other questions are best addressed at the
global scale. The concentration of atmospheric
CO2, for example, depends on global patterns
of biotic exchanges of CO2 and the burning of
fossil fuels, which are spatially variable across
the globe. The rapid mixing of CO2 in the
atmosphere averages across this variability,
facilitating estimates of long-term changes in
the total global flux of carbon between Earth
and the atmosphere.
Some questions require careful measurements
of lateral transfers of materials. A watershed
is a logical unit in which to study the
effects of forests on the quantity and quality of
the water that supplies a town reservoir. A
watershed, or catchment, consists of a stream
and all the terrestrial surfaces that drain into it. By studying a watershed we can compare the
quantities of materials that enter from the
air and rocks with the amounts that leave in
stream water, just as you balance your checkbook.
Studies of input–output budgets of watersheds
have improved our understanding of the
interactions between rock weathering, which
supplies nutrients, and plant and microbial
growth, which retains nutrients in ecosystems
(Vitousek and Reiners 1975, Bormann and
Likens 1979).
The upper and lower boundaries of an
ecosystem also depend on the question being
asked and the scale that is appropriate to the question.The atmosphere, for example, extends
from the gases between soil particles all the way
to outer space.The exchange of CO2 between a
forest and the atmosphere might be measured
a few meters above the top of the canopy
because, above this height, variations in CO2
content of the atmosphere are also strongly
influenced by other upwind ecosystems. The
regional impact of grasslands on the moisture
content of the atmosphere might, however, be
measured at a height of several kilometers
above the ground surface, where the moisture
released by the ecosystem condenses and
returns as precipitation (see Chapter 2). For
questions that address plant effects on water
and nutrient cycling, the bottom of the ecosystem
might be the maximum depth to which
roots extend because soil water or nutrients
below this depth are inaccessible to the vegetation.
Studies of long-term soil development, in
contrast, must also consider rocks deep in the
soil, which constitute the long-term reservoir of
many nutrients that gradually become incorporated
into surface soils (see Chapter 3).
Ecosystem dynamics are a product of many
temporal scales. The rates of ecosystem processes
are constantly changing due to fluctuations
in environment and activities of organisms on time scales ranging from microseconds to
millions of years (see Chapter 13). Light capture
during photosynthesis responds almost instantaneously
to fluctuations in light availability
to a leaf. At the opposite extreme, the evolution
of photosynthesis 2 billion years ago added
oxygen to the atmosphere over millions of
years, causing the prevailing geochemistry
of Earth’s surface to change from chemical
reduction to chemical oxidation (Schlesinger
1997). Microorganisms in the group Archaea
evolved in the early reducing atmosphere of
Earth. These microbes are still the only organisms
that produce methane. They now function
in anaerobic environments such as wetland soils
and the interiors of soil aggregates or animal
intestines. Episodes of mountain building and
erosion strongly influence the availability of
minerals to support plant growth.Vegetation is
still migrating in response to the retreat of Pleistocene
glaciers 10,000 to 20,000 years ago.After
disturbances such as fire or tree fall, there are
gradual changes in plant, animal, and microbial
communities over years to centuries. Rates of
carbon input to an ecosystem through photosynthesis
change over time scales of seconds to
decades due to variations in light, temperature,
and leaf area.
Many early studies in ecosystem ecology
made the simplifying assumption that some
ecosystems are in equilibrium with their environment.
In this perspective, relatively undisturbed
ecosystems were thought to have
properties that reflected (1) largely closed
systems dominated by internal recycling of
elements, (2) self-regulation and deterministic
dynamics, (3) stable end points or cycles, and
(4) absence of disturbance and human influence
(Pickett et al. 1994, Turner et al. 2001).
One of the most important conceptual
advances in ecosystem ecology has been the
increasing recognition of the importance of
past events and external forces in shaping
the functioning of ecosystems. In this nonequilibrium
perspective, we recognize that
most ecosystems exhibit inputs and losses, their
dynamics are influenced by both external and
internal factors, they exhibit no single stable
equilibrium, disturbance is a natural component
of their dynamics, and human activities
have a pervasive influence. The complications
associated with the current nonequilibrium
view require a more dynamic and stochastic
view of controls over ecosystem processes.
Ecosystems are considered to be at steady
state if the balance between inputs and outputs
to the system shows no trend with time
(Johnson 1971, Bormann and Likens 1979).
Steady state assumptions differ from equilibrium
assumptions because they accept temporal
and spatial variation as a normal aspect of
ecosystem dynamics. Even at steady state, for
example, plant growth changes from summer to
winter and between wet and dry years (see
Chapter 6). At a stand scale, some plants may
die from old age or pathogen attack and be
replaced by younger individuals.At a landscape
scale, some patches may be altered by fire or
other disturbances, and other patches will be
in various stages of recovery. These ecosystems
or landscapes are in steady state if there is
no long-term directional trend in their properties
or in the balance between inputs and
outputs.
Not all ecosystems and landscapes are in
steady state. In fact, directional changes in
climate and environment caused by human
activities are quite likely to cause directional
changes in ecosystem properties. Nonetheless,
it is often easier to understand the relationship of ecosystem processes to the current environment
in situations in which they are not also
recovering from large recent perturbations.
Once we understand the behavior of a system
in the absence of recent disturbances, we can
add the complexities associated with time lags
and rates of ecosystem change.
Ecosystem ecology uses concepts developed
at finer levels of resolution to build an understanding
of the mechanisms that govern the
entire Earth System. The biologically mediated
movement of carbon and nitrogen through
ecosystems depends on the physiological
properties of plants, animals, and soil microorganisms.
The traits of these organisms are
the products of their evolutionary histories
and the competitive interactions that sort
species into communities where they successfully
grow, survive, and reproduce (Vrba and
Gould 1986). Ecosystem fluxes also depend on the population processes that govern
plant, animal, and microbial densities and
age structures as well as on community
processes, such as competition and predation,
that determine which species are present and
their rates of resource consumption. Ecosystem
ecology therefore depends on information
and principles developed in physiological, evolutionary,
population, and community ecology
(Fig. 1.2).
The supply of water and minerals from soils
to plants depends not only on the activities of
soil microorganisms but also on physical and
chemical interactions among rocks, soils, and
the atmosphere. The low availability of phosphorus
due to the extensive weathering and
erosional loss of nutrients in the ancient soils of
western Australia, for example, strongly constrains
plant growth and the quantity and types
of plants and animals that can be supported.
Principles of ecosystem ecology must therefore
also incorporate the concepts and understanding
of disciplines such as geochemistry, hydrology,
and climatology that focus on the physical
environment (Fig. 1.2).
Ecosystem ecology provides the mechanistic
basis for understanding processes that occur
at global scales. Study of Earth as a physical
system relies on information provided by
ecosystem ecologists about the rates at which
the land or water surface interacts with the
atmosphere, rocks, and waters of the planet
(Fig. 1.2). Conversely, the global budgets of
materials that cycle between the atmosphere,
land, and oceans provide a context for understanding
the broader significance of processes
studied in a particular ecosystem. Latitudinal
and seasonal patterns of atmospheric CO2 concentration,
for example, help define the locations
where carbon is absorbed or released
from the land and oceans (see Chapter 15).
History of Ecosystem Ecology
Many early discoveries of biology were motivated
by questions about the integrated nature
of ecological systems. In the seventeenth
century, European scientists were still uncertain
about the source of materials found in plants.
Plattes, Hooke, and others advanced the novel
idea that plants derive nourishment from
both air and water (Gorham 1991). Priestley
extended this idea in the eighteenth century by
showing that plants produce a substance that is
essential to support the breathing of animals.At
about the same time MacBride and Priestley
showed that breakdown of organic matter
caused the production of “fixed air” (carbon dioxide), which did not support animal life.
De Saussure, Liebig, and others clarified the
explicit roles of carbon dioxide, oxygen,
and mineral nutrients in these cycles in the
nineteenth century. Much of the biological
research during the nineteenth and twentieth
centuries went on to explore the detailed
mechanisms of biochemistry, physiology,
behavior, and evolution that explain how life
functions. Only in recent decades have we
returned to the question that originally motivated
this research: How are biogeochemical
processes integrated in the functioning of
natural ecosystems?
Many threads of ecological thought have
contributed to the development of ecosystem
ecology (Hagen 1992), including ideas relating
to trophic interactions (the feeding relationships
among organisms) and biogeochemistry
(biological influences on the chemical processes
Earth system
science
Climatology
Hydrology
Soil science
Geochemistry
Physiological
ecology
Ecosystem ecology
Population
ecology
Community
ecology
Context
Mechanism

History of Ecosystem Ecology 7
on the population processes that govern
plant, animal, and microbial densities and
age structures as well as on community
processes, such as competition and predation,
that determine which species are present and
their rates of resource consumption. Ecosystem
ecology therefore depends on information
and principles developed in physiological, evolutionary,
population, and community ecology
(Fig. 1.2).
The supply of water and minerals from soils
to plants depends not only on the activities of
soil microorganisms but also on physical and
chemical interactions among rocks, soils, and
the atmosphere. The low availability of phosphorus
due to the extensive weathering and
erosional loss of nutrients in the ancient soils of
western Australia, for example, strongly constrains
plant growth and the quantity and types
of plants and animals that can be supported.
Principles of ecosystem ecology must therefore
also incorporate the concepts and understanding
of disciplines such as geochemistry, hydrology,
and climatology that focus on the physical
environment (Fig. 1.2).
Ecosystem ecology provides the mechanistic
basis for understanding processes that occur
at global scales. Study of Earth as a physical
system relies on information provided by
ecosystem ecologists about the rates at which
the land or water surface interacts with the
atmosphere, rocks, and waters of the planet
(Fig. 1.2). Conversely, the global budgets of
materials that cycle between the atmosphere,
land, and oceans provide a context for understanding
the broader significance of processes
studied in a particular ecosystem. Latitudinal
and seasonal patterns of atmospheric CO2 concentration,
for example, help define the locations
where carbon is absorbed or released
from the land and oceans (see Chapter 15). History of Ecosystem Ecology
Many early discoveries of biology were motivated
by questions about the integrated nature
of ecological systems. In the seventeenth
century, European scientists were still uncertain
about the source of materials found in plants.
Plattes, Hooke, and others advanced the novel
idea that plants derive nourishment from
both air and water (Gorham 1991). Priestley
extended this idea in the eighteenth century by
showing that plants produce a substance that is
essential to support the breathing of animals.At
about the same time MacBride and Priestley
showed that breakdown of organic matter
caused the production of “fixed air” (carbon
dioxide), which did not support animal life.
De Saussure, Liebig, and others clarified the
explicit roles of carbon dioxide, oxygen,
and mineral nutrients in these cycles in the
nineteenth century. Much of the biological
research during the nineteenth and twentieth
centuries went on to explore the detailed
mechanisms of biochemistry, physiology,
behavior, and evolution that explain how life
functions. Only in recent decades have we
returned to the question that originally motivated
this research: How are biogeochemical
processes integrated in the functioning of
natural ecosystems?
Many threads of ecological thought have
contributed to the development of ecosystem
ecology (Hagen 1992), including ideas relating
to trophic interactions (the feeding relationships