THE ECOSYSTEM CONCEPT

There are many definitions for ecosystem. The definition of Christopherson

(1997) seems quite workable, but so are quite a few others. Ecosystem definition: An

ecosystem is a natural system consisting of all plants, animals and microorganisms

(biotic factors) in an area functioning together with all the non-living physical

(abiotic) factors of the environment (Christopherson 1997). 

The term ecosystem was coined in 1930 by Roy Clapham, to denote the

physical and biological components of an environment considered in relation to each

other as a unit. British ecologist Arthur Tansley later refined the term, describing it as

the interactive system established between biocoenosis (a group of living creatures)

and their biotope (the environment in which they live). Central to the ecosystem

concept is the idea that living organisms are continually engaged in a set of

relationships with every other element constituting the environment in which they

exist. Ecosystems can be bounded and discussed with tremendous variety of scope,

and describe any situation where there is relationship between organisms and their

environment.

The term ecosystem (a contraction of ecological system) is generally

understood as to the entire assemblage of organisms (plant, animal and other living

beings—also referred to as a biotic community or biocoenosis) living together in a

certain space with their environment (or biotope), functioning as a loose unit.

Together, these components and their interactions with and relationships to each other

form a dynamic and complex new whole, functioning as an "ecological unit", with

additional characteristics that can't be found in the individual components. Nor could

any organism live completely on its own without involving any other species of

organism.  

There are no conceptual restrictions on how large or small a space or an area

must be to host an ecosystem, nor on the minimum numbers species or individual

organisms to be present.

Early conceptions of an ecosystem were as a structured functional unit in

equilibrium of energy and matter flows among constituent elements. Some considered

this vision limited, and preferred to view an ecosystem in terms of cybernetics, which,

like any other type of system, is governed by the rules of systems science and

cybernetics, as applied specifically to collections of organisms and relevant abiotic

components. The branch of ecology that gave rise to this view has become known as

systems ecology. 

Politically, the concept has become important, since the Convention on

Biological Diversity, (CBD), signed by almost 200 nations. The CBD formulates the

concept in the following definition: "Ecosystem" means a dynamic complex of plant,

animal and micro-organism communities and their non-living environment interacting

as a functional unit" (Convention on the Biological Diversity, 1992).

With the need of protecting ecosystems, the political need arose to describe

and identify them within a reasonable time and cost-effectively. The IUCN task

force on Protected Areas System Composition and Monitoring (Vreugdenhil et al

2003) argued that this could most effectively be achieved using a physiognomic-

ecological classification systems, as they are easily recognizable in the field as well as

on satellite images. They argued that the structural characteristics - such as forest,

savannah, bush-like, prairie vegetations, seasonality of the vegetation and leaf-

morphology - complemented with geophysical data - such as elevation, humidity,

drainage, salinity of water, characteristics of water bodies - each are determining

modifiers that separate partially distinct sets of species. They argued that this is true

not only for plant species, but also for species of animals, fungi and bacteria. The

degree of ecosystem distinction is subject to the physiognomic classifiers or modifiers

that can be recognized on a satellite image and/or in the field. Based on that principle,

they developed a methodology for ecosystem mapping, which you can find here.

The principle is that physiognomic ecological vegetation classes also represent

ecosystem classes, as those vegetation classes represent ecological conditions with

partially distinct assemblages of both plants and animal species. 

Several physiognomic-ecological are available: the UNESCO system:

Physiognomic-Ecological Classification of Plant Formations of the Earth, Mueller

Dombois and Ellenberg, 1974, and its derivatives, developed by the United States

Vegetation Committee USVCS and the FAO developed Land Cover Classification

System, LCCS. Several systems aquatic systems are available and an effort is being

made by the USGS to design yet another ecosystem classification system that covers

both terrestrial and aquatic ecosystems, but it is not clear how that has been

progressing. The UNESCO system related classification variants all allow fairly to

rather detailed (depending on the use of floristic elements) distinction of biounits with

a reasonable degree of geographical consistency. From the previous analysis of

modifiers, it may be clear that these classification systems not only provide

information that leads to definitions of the vegetation types, but about conditions that

determine the suitability of that location to representatives of any taxon - including

fauna - particularly when complemented with additional faunal characteristics when

appropriate. 

From the previous consideration, it may be deducted that different

recombinations of modifiers most likely lead to partial different assemblages of

species. Particularly by incorporating an aquatic “formation”, sets or assemblages of

ecosystems and species can be be added that were not considered in the original

design of the UNESCO classification system. The different ecosystem classification

systems and definitions are reviewed here.

CHAPTER 9: Introduction to the Biosphere  

(j). Introduction to the Ecosystem Concept

Introduction

In topic 9d, an ecosystem was defined as a dynamic entity composed of a

biological community and its associated abiotic environment. Often the dynamic

interactions that occur within an ecosystem are numerous and complex. Ecosystems

are also always undergoing alterations to their biotic and abiotic components. Some of

these alterations begin first with a change in the state of one component of the

ecosystem which then cascades and sometimes amplifies into other components

because of relationships.

In recent years, the impact of humans has caused a number of dramatic

changes to a variety of ecosystems found on the Earth. Humans use and modify

natural ecosystems through agriculture, forestry, recreation, urbanization, and

industry. The most obvious impact of humans on ecosystems is the loss of

biodiversity. The number of extinctions caused by human domination of ecosystems

has been steadily increasing since the start of the Industrial Revolution. The

frequency of species extinctions is correlated to the size of human population on the

Earth which is directly related to resource consumption, land-use change, and

environmental degradation. Other human impacts to ecosystems include species

invasions to new habitats, changes to the abundance and dominance of species in

communities, modification of biogeochemical cycles, modification of hydrologic

cycling, pollution, and climatic change.

 

Major Components of Ecosystems

Ecosystems are composed of a variety of abiotic and biotic components that

function in an interrelated fashion. Some of the more important components are: soil,

atmosphere, radiation from the Sun, water, and living organisms.

Soils are much more complex than simple sediments. They contain a mixture of

weathered rock fragments, highly altered soil mineral particles, organic matter, and

living organisms. Soils provide nutrients, water, a home, and a structural growing

medium for organisms. The vegetation found growing on top of a soil is closely

linked to this component of an ecosystem through nutrient cycling.

The atmosphere provides organisms found within ecosystems with carbon

dioxide for photosynthesis and oxygen for respiration. The processes of

evaporation, transpiration, and precipitation cycle water between the atmosphere

and the Earth's surface.

Solar radiation is used in ecosystems to heat the atmosphere and to evaporate

and transpire water into the atmosphere. Sunlight is also necessary for

photosynthesis. Photosynthesis provides the energy for plant growth and metabolism,

and the organic food for other forms of life.

Most living tissue is composed of a very high percentage of water, up to and

even exceeding 90%. The protoplasm of a very few cells can survive if their water

content drops below 10%, and most are killed if it is less than 30-50%. Water is the

medium by which mineral nutrients enter and are translocated in plants. It is also

necessary for the maintenance of leaf turgidity and is required for photosynthetic

chemical reactions. Plants and animals receive their water from the Earth's surface

and soil. The original source of this water is precipitation from the atmosphere.

Ecosystems are composed of a variety of living organisms that can be

classified as producers, consumers, or decomposers. Producers or autotrophs, are

organisms that can manufacture the organic compounds they use as sources of energy

and nutrients. Most producers are green plants that can manufacture their food

through the process of photosynthesis. Consumers or heterotrophs get their energy

and nutrients by feeding directly or indirectly on producers. We can distinguish two

main types of consumers. Herbivores are consumers that eat plants for their energy

and nutrients. Organisms that feed on herbivores are called carnivores. Carnivores

can also consume other carnivores. Plants and animals supply organic matter to the

soil system through shed tissues and death. Consumer organisms that feed on this

organic matter, or detritus, are known as detritivores or decomposers. The organic

matter that is consumed by the detritivores is eventually converted back into

inorganic nutrients in the soil. These nutrients can then be used by plants for the

production of organic compounds.

The following graphical model describes the major ecosystem components and their

interrelationships (Figure 9j-1).

 

Figure 9j-1: Relationships within an ecosystem.

 

Energy and Matter Flow in Ecosystems

Many of the most important relationships between living organisms and the

environment are controlled ultimately by the amount of available incoming energy

received at the Earth's surface from the Sun. It is this energy which helps to drive

biotic systems. The Sun's energy allows plants to convert inorganic chemicals into

organic compounds.

Only a very small proportion of the sunlight received at the Earth's surface is

transformed into biochemical form. Several studies have been carried out to determine

this amount. A study of an Illinois cornfield reported that 1.6% of the available solar

radiation was photosythetically utilized by the corn. Other data suggests that even the

most efficient ecosystems seldom incorporate more than 3% of the available solar

insolation. Most ecosystems fix less than 1% of the sunlight available for

photosynthesis.

Living organisms can use energy in basically two forms: radiant or fixed.

Radiant energy exists in the form of electromagnetic energy, such as light. Fixed

energy is the potential chemical energy found in organic substances. This energy

can be released through respiration. Organisms that can take energy from inorganic

sources and fix it into energy rich organic molecules are called autotrophs. If this

energy comes from light then these organisms are called photosynthetic autotrophs.

In most ecosystems plants are the dominant photosynthetic autotroph.

Organisms that require fixed energy found in organic molecules for their

survival are called heterotrophs. Heterotrophs who obtain their energy from living

organisms are called consumers. Consumers can be of two basic types: Consumer

and decomposers. Consumers that consume plants are know as herbivores.

Carnivores are consumers who eat herbivores or other carnivores. Decomposers or

detritivores are heterotrophs that obtain their energy either from dead organisms or

from organic compounds dispersed in the environment.

Once fixed by plants, organic energy can move within the ecosystem through

the consumption of living or dead organic matter. Upon decomposition the chemicals

that were once organized into organic compounds are returned to their inorganic form

and can be taken up by plants once again. Organic energy can also move from one

ecosystem to another by a variety of processes. These processes include: animal

migration, animal harvesting, plant harvesting, plant dispersal of seeds, leaching,

and erosion. The following diagram models the various inputs and outputs of energy

and matter in a typical ecosystem (Figure 9j-2).

Figure 9j-2: Inputs and outputs of energy and matter in a typical ecosystem.

 

 

Study Guide

 Source: The IUCN task force on Protected Areas System Composition and

Monitoring (Vreugdenhil et al 2003)

Key words: ecosystem, concept, definition

This page  is part of our web-book on Biodiversity Conservation. For organized

reading go to our on-line Table of Content, or download our book in pdf format.

Additional Readings

  Internet Weblinks   Citation: Pidwirny, M. (2006). " Introduction to the Ecosystem

Concept". Fundamentals of Physical Geography, 2nd Edition. Date Viewed.

http://www.physicalgeography.net/fundamentals/9j.html