University of Kiel, Ecology Centre, MSc Environmental Science, a seminar paper
1st version completed 1/18/2010 by Martin Hitzemann (firstname.lastname@example.org)
2nd version (new) 2010 by Veronika Grünwald-Schwark (email@example.com)
What is an Ecosystem?
This paper gives a general introduction into ecosystems, describes their general properties and shows the importance of ecosystem biodiversity and of ecosystem research as a prerequisite for environmental management. An overview of the historical development of different ecosystem concepts is given, and several ecosystem definitions are illuminated.
Furthermore, the most important characteristics of ecosystems and ecosystem models are pointed out, and a possible classification scheme of natural and semi-natural terrestrial and aquatic ecosystems is presented. It is shown how to deal with the complexity of ecosystems and which methods can be applied for simplifying ecosystem processes, which helps to tackle current environmental problems like global change and biodiversity loss.
In view of rapid changes of ecosystems it is indispensable to analyze the complex interactions from a holistic and interdisciplinary point of view. Structural and functional attributes should be taken into account as well as water, matter and energy budgets and inputs, outputs, internal flows and efficiencies (Müller et al. 2010). Environmental managers should involve as many interactive biotic and abiotic factors as possible in their management, including the human environment, and should use indicators for ecosystem assessment.
Key words: ecosystem analysis, ecosystem definitions, ecosystem biodiversity, ecosystem history, ecosystem characteristics, ecosystem models, ecosystem types
- History of ecosystem concepts and definitions
- Characteristics of ecosystems and ecosystem models
- Types and examples of ecosystems
- Useful links
Human activities affect the global environment in many respects and have a myriad of effects on ecosystems. To understand and to manage these complex systems it is essential to analyze their innumerable chemical and physical interactions as well as the consequences of human-caused changes. Recent global changes have made scientists aware of alterations in ecosystem processes, which interact significantly and cannot be understood in isolation.
2. History of ecosystem concepts and definitions
To comprehend the development of the ecosystem approach, it is useful to have a look at its history and evolution. In the 19th and 20th century, many naturalists concerned themselves with research on the biochemical processes that are integrated in the functioning of ecosystems.
In 1866 Ernst Haeckel (Fig. 1) was the first to use the term "ecology" in relation to the "household of nature" and the science of the relationship of the organism to the environment.
Figure 1. Ernst Haeckel (Wikipedia 2010a)
1927, Charles Sutherland Elton, an English zoologist, researched on concepts of trophic structures (food chains), which act as a basic principle for the comprehension of the material flow through ecosystems. The trophic structures of Summerhayes & Elton (1923) shown in Fig. 2 are the first illustration of a complex food web. It involves interactions between animals, plants and bacteria on Bear Island in Norway.
Figure 2. Illustration of a complex food web (Summerhayes & Elton 1923)
In 1935 Arthur Tansley coined the term "ecosystem" as a basis for the fundamental understanding of ecosystem dynamics and pointed out the relevance of material interchanges between organisms and their abiotic environment: "But the more fundamental conception is, as it seems to me, the whole system (in the sense of physics), including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment of the biome – the habitat factors in the widest sense. It is the systems so formed which, from the point of view of the ecologist, are the basic units of nature on the face of the earth. These ecosystems, as we may call them, are of the most various kinds and sizes. They form one category of the multitudinous physical systems of the universe, which range from the universe as a whole down to the atom" (Tansley 1935). As Tansley very clearly emphasized, the whole system of biotic and abiotic factors has to be seen as an ecological entity. Tansley’s ecosystem definition is still valid today and can be considered a framework for the current ecosystem research and ecosystem management.
1954, Herbert George Andrewartha and Louis Charles Birch defined ecology as "the scientific study of the distribution and abundance of organisms and the interactions that determine distribution and abundance" (Andrewartha & Birch 1954). This definition, based on the biotic effects of the animate nature on organisms, is also universal until today.
1969, Eugene Pleasants Odum (Fig. 3) and his brother Howard Thomas Odum further developed the system approach to suggest generalizations of ecosystem functions without exhibiting all the interrelating mechanisms and interactions by using radioactive tracers to quantify energy flows through a coral reef (Odum 1969).
Figure 3. Eugene Pleasants Odum (NGE 2010)
In 1986 Crawford Stanley Holling (Fig. 4) developed the adaptive four-phase figure eight model of ecosystem dynamics (Fig. 5) and postulated that catastrophic events and the following system renewals, which indicate the resilience of ecosystems, are at least as important as the exploitation and conservation stages of such systems (Holling 1986).
Figure 4. Eugene Pleasants Odum (NGE 2010)
Holling’s adaptive cycle shown in Fig. 5 represents ecosystem dynamics as a progression of four stages: the pioneer stage or growth (exploitation) [r], the conservation stage [K], the release or creative destruction (collapse) [Ω] and the reorganization stage [α]. The arrows designate the cycle flow, short arrows indicating a slowly changing and long arrows a rapidly changing situation. The abscissa determines the connected processes, the ordinate the potential range options.
Figure 5. Four-phase figure-eight model of ecosystem dynamics (Holling & Gunderson 2002)
Holling’s cycle model includes disturbances as a normal procedure of ecosystem development and focuses on destruction and reorganization processes which are often ignored in aid of growth and conservation. Including these two additional functions – release and reorganization – corresponds to our modern understanding of ecosystem dynamics, ecosystem organization and ecosystem resilience. In comparison to the traditional concept of succession the model of the adaptive cycle emphasizes also, beside the first foreloop phase from r to K, the significant reorganization backloop phase from Omega to Alpha, which leads to renewal (Resilience Alliance 2010).
Müller (i.p.) defined the term ecosystem as follows (translated from German): Ecosystems represent dissipative structure models of interactive biotic and abiotic subsystems, in which compositional and energetic gradients are generated, maintained and degraded by cycling processes on functionally connected, hierarchical levels.
According to Ellenberg et al. (1986), an ecosystem is an interacting system of different organisms, which have become attuned to one another and to the abiotic conditions in their habitat to such a degree that they form an integrated whole.
In 1992 the Convention on Biological Diversity (CBD) formulated that an "Ecosystem means a dynamic complex of plant, animal and microorganism communities and their non-living environment interacting as a functional unit" (CBD 1992). With the CBD and the Millennium Ecosystem Assessment, the term "ecosystem" gained political relevance. The Millennium Ecosystem Assessment (2005) evaluated the consequences of ecosystem changes for human well-being and defined humans as an integral part of ecosystems. Today, the importance of ecosystems receives increasing attention all over the world, the more so as 2010 is the "International Year of Biodiversity".
According to Müller et al. (1997), the openness of systems is to be considered as the basic requirement of their development and self-organization ability. Ecosystems are only functional as open systems if they exchange regulated imports and exports with their environment (Breckling & Müller 2006)
Roweck (2010) coined the following definition: "Ecosystem = biotope + biocoenosis; a relation complex of organisms and the conditions of the biotope they live in; used when looking at processes and interactions. Built up by
- structures : horizontal and vertical, nutrients and matter, trophic levels (from producer to highest consumer level), life forms, food chains (feeding relationship in an ecosystem), and food webs (totality of food chains in an ecosystem); and
- functions: cycling of nutrients and material, energy flow, and biotic and abiotic interactions."
3. Characteristics of ecosystems and ecosystem models
The term "ecosystem" can be divided into the terms "ecology" and "system".
What is ecology? Ecology is "the study of the interrelation of living organisms and their environment" (Environmental Dictionary 2009).
What is a system? "Systems are complexes which are interrelated, purposeful abstractions, units in space and time with interacting sub units, irreducible, hierarchical, self-regulated and self organized" (Müller 2010).
What are ecosystems? They are…
- very, very complex hierarchical networks of organisms and their abiotic environment
- interacting energy machines and matter processors
- real parts of the environment acting as multifunctional models of colluding structural units (Müller 2010, Campbell & Reece 2002).
As shown in Fig. 6, an ecosystem can be seen both as a defined section of the biosphere and a deduced model conception. The model, a theoretical mathematical image of an ecosystem, is verified or falsified, respectively, on the basis of measurements (Breckling & Müller 2006).
Figure 6. Illustration based on the graphic "Ökosystem" (Breckling & Müller 2006, p. 7; modified by Grünwald-Schwark (2010, translated from German)).
The complexity of ecosystems illustrated in Fig. 7 makes environmental management and future prognoses difficult. There are, however, methods to deal with these complexities (Müller 2010). For simplifying the management, spatial boundaries have to be imposed around them (Emris 2010).
Figure 7. Illustration of ecosystem complexity (Müller 2010)
An ecosystem model describes the major pools and fluxes in an ecosystem and the factors that regulate these fluxes (Chapin et al. 2002). Fig. 8 represents a more detailed Ecosystem model of the organization of an ecosystem.
Figure 8. Structure of an ecosystem (Heinrich & Hergt 1990, modified by Grünwald-Schwark, translated from German)
Heinrich & Hergt (1990) describe the organization of an ecosystem as followed: The ecosystem model emphasizes mainly the trophic correlations, the effects of abiotic and biotic factors. It clarifies the absolute dependence on the radiation and the classification in more or less definable compartments. Those are to a large extent homogeneous, have specific functions in the ecosystem and can be characterized through different input and output parameters. The productivity of an ecosystem can be assigned as the output of biomass (g, kg or t) per unit area (m² or ha) and time (d or y).
Plants, as producers, remove water and nutrients from the soil with their roots and build up glucose (by means of radiation and CO2), which is the basis for other organic associations (e.g. photosynthesis). These associations are used for metabolism maintenance and constitution of higher substances (phytomass). During this process energy is lost via respiration and heat dissipation. Only a little part of the radiation, about 1%, is converted into chemical energy. Only the producers are able to build up organic matter from inorganic substances (autotrophic).
The consumers are animal organisms. They are dependent on other biota, because they need energy-rich organic matter for their nutrition (heterotrophic) and to build up higher substances (zoomass). A part of the energy is lost through respiration.
The primary consumers are herbivores, e.g. herbivorous insects. They can serve as nutrition for secondary consumers, e.g. lizards, which, in turn, are nutrition for the tertiary consumers, e.g. buzzards.
The heterotrophic decomposers, mainly bacteria and fungi, decompose organic into inorganic matter (mineralization). Their activity is supported by many invertebrates (e.g. wood lice, mites, insect larvae). The saprophagous animals feed on dead organic material; the coprophagous animals feed on animal excrement (in this process, the colonizing bacteria and fungi form an important part of the food web); the necrophagous animals are scavengers. The destruents’ activities close the cycle of matter with the generation of CO2, NH3, H2S, CH4, H2 and ions like PO43–, Cl–, Na+, K+, Ca2+ and others.
A short cycle is formed by only producers and decomposers; a longer one also involves consumers. From more than 100 elements just 40 are indispensable for life. In contrast to the energy transport, which occurs non-circular in an ecosystem, the material transport is carried out in cycles, across the biota are correlated to their environment.
The mass transport in an ecosystem is rarely balanced. Scarcities are overcome via connections to other ecosystems. The negative calcium balance of terrestrial ecosystems, for example, is compensated by aquatic ecosystems through the water cycle.
Characteristics of ecosystems
Life conditions’ variability in space and time is very high. The biota have to be able to adapt to new conditions, to develop new life strategies. An ecosystem can, as shown in the Holling cycle (Fig. 5) in section 2, still be disturbed by catastrophic events, when it has adapted to prevailing environmental conditions (Jørgensen et al. 2007).
Due to their properties, some of which are listed below, ecosystems have the flexibility and adaptability to deal with disturbances and changed conditions "and still maintain the system far from thermodynamic equilibrium" (Jørgensen et al. 2007):
- Living (biotic) and non-living (abiotic) components
- Self-organization and self-regulation
- Structural and functional units
- Buffer capacity
- Direct effects and indirect effects
- Short term and long term effects
- Local and delocalized effects
- Scale-dependence (space and time)
- Elements and subsystems
- Open systems
According to Jørgensen et al. (2007), the Ecosystem Theory presents the seven basic principles of ecosystems listed below. (Jørgensen (2008) describes the ecosystem theory as "an integration of several contributions from a number of system ecologists".)
- "Ecosystems have thermodynamic openness" (within the existing temperature range of 250–350 K there is a good balance between the ordering and disordering processes of building up biochemical important compounds and decomposition of organic material),
- "Ecosystems have ontic openness" (mass exchange with the surroundings) ,
- "Ecosystems have directed development",
- "Ecosystems have connectivity" (ecological entities do not exist in isolation, and they are connected to others, whereas networks have synergistic effects on the different components),
- "Ecosystems have hierarchic organization" (e.g. levels from atoms to molecules, cells, organs, organisms, populations, communities, ecosystems and the ecosphere),
- "Ecosystems have complex dynamics: growth and development" (possible by a biomass and network cycle increase and an increase of the system’s information, i.e., the system moves away from thermodynamic equilibrium, whereas the growth forms are associated with an increase of stored eco-exergy as well as the energy flow in the system), and
- "Ecosystems have complex dynamics: disturbance and decay" (due to disturbances, which can be advantageous in the long term, ecosystems are able to create new solutions for life).
These general principles help to predict and to understand ecosystem changes caused by perturbations, but they need indicators to be measured. The assessment of ecosystem indicators is useful in environmental management (Jørgensen et al. 2007).
4. Types and examples of ecosystems
Ecosystems can be classified (Fig. 9) according to the level of human impact into:
Natural ecosystems: Ecosystems with no noticeable interference; according to Ellenberg (1973) the mass balance is primarily dependent of the (prevailing) radiation;
Semi-natural ecosystems: Ecosystems that are like natural ecosystems, but changed by humans; the mass balance is, according to Ellenberg (1973), also primarily dependent of the (prevailing) radiation;
Artificial ecosystems: Ecosystems that are created and maintained by humans.
Figure 9. Ecosystem classification scheme (TutorVista 2010)
There are hardly any on earth. The entire planet Earth can be considered a natural ecosystem as well as e.g. a pristine rain forest (Figs. 10 and 11).
Figure 10. Natural ecosystem planet earth (Thompson 2010)
Figure 11. Natural ecosystem rain forest (Desktopnature 2010)
Ellenberg (1973) considers the biosphere of the earth the most extensive and most diverse ecosystem. The biosphere comprises the complete geosphere that is colonized by biota, including the oceans up to their maximal depth.
Cities, reservoirs and croplands can be looked upon as artificial ecosystems, because they are built by humans (Figs. 12 and 13).
Figure 12. City of Denver, Colorado, USA (Photo: Grünwald-Schwark 2009)
Figure 13. Fort Peck Reservoir, North Dakota, USA (Photo: Grünwald-Schwark 2009)
According to Ellenberg (1973), who refers to the artificial ecosystems as "urban-industrial" ecosystems, they are primarily dependent of energy sources made accessible by man, e.g. coal, oil, other fossil energy sources and nuclear energy.
Semi-natural terrestrial ecosystems (on landmasses of the continents or islands):
Desert, prairie, glacier and (coastal) forest ecosystems (Figs. 14–17) as well as grasslands can be regarded as semi-natural terrestrial ecosystems.
Figure 14. Desert ecosystem, Grand Sand Dunes NP, Colorado, USA (Photo: Grünwald-Schwark 2009)
Figure 15. Prairie ecosystem, Wind Cave NP, South Dakota, USA (Photo: Grünwald-Schwark 2009)
Figure 16. Glacier ecosystem, Bugaboo PP, BC, Canada (Photo: Grünwald-Schwark 2009)
Figure 17. Coastal forest ecosystem, Vancouver Island, BC, Canada (Photo: Grünwald-Schwark 2008)
Semi-natural aquatic ecosystems (water bodies): Freshwater ecosystems
Within the semi-natural aquatic ecosystems, freshwater (Figs. 18 and 19) and marine (salt water, Figs. 20 and 21) ecosystems can be distinguished. The former may further be classified into lentic (standing water, Fig. 18) and lotic (flowing water, Fig. 19) ecosystems.
Figure 18. Lentic (standing water) lake ecosystem, Kananaskis Lake, AB, Canada (Photo: Grünwald-Schwark 2008)
Figure 19. Lotic (flowing water) river ecosystem, Bow River, AB, Canada (Photo: Grünwald-Schwark 2008)
Figure 20. Ocean ecosystem: Pacific Ocean, Vancouver Island, BC, Canada (Photo: Grünwald-Schwark 2008)
Figure 21. Sea ecosystem: Baltic Sea near Kiel, Germany (Photo: Grünwald-Schwark 2009)
Semi-natural aquatic ecosystems (water bodies): Marine ecosystems
Marine ecosystems can be divided into oceans (Fig. 20) and seas (Fig. 21), the latter being saltwater areas located on the margins of oceans, smaller than oceans and at least partially enclosed by land.
Ellenberg (1973) classifies the above-mentioned types of ecosystems (like artificial, semi-natural terrestrial, semi-natural freshwater and marine ecosystems) as mega-ecosystems, subordinated ecosystem types like forests and grasslands as macro-ecosystems and smaller subordinated ecosystem entities like e.g. a boreal deciduous forest or a tropical rain forest as meso-ecosystems.
For sustainable environmental management, ecosystem research is one of the most significant principles. Ecosystem research tackles many environmental and social problems like climate change, biodiversity loss, poverty and population growth. As stated in the Millennium Ecosystem Assessment (2005), humans are an integral part of ecosystems. Ecosystem theory is an important scientific concept to describe the multitude of complex interrelations between organisms (including humans) and their environment, which can be very useful in solving problems of human interest. Since ecosystem theory can be applied to explain ecological findings, it is a particularly helpful tool in environmental management, which would otherwise solely depend on time-consuming and hence expensive observations (Jørgensen 2008).
According to Jørgensen (2008), the theoretical basis of ecology will be substantiated by "any step toward a wider application of an ecosystem theoretical explanation of ecological rules and observations". The wider the theoretical explanations of ecology are used, the better becomes the theory, because each application will either support or improve the theory or, if the theory fails, the theory needs to be modified (Jørgensen 2008). Ecosystem theory also provides a stringent guide for resource conservation and environmental management (Jørgensen et al 2007). With increasing knowledge and experience of ecosystem research it can be expected that ecosystem theory will become an equally recognized tool between decision-makers and environmental managers, which supports thinking in a more holistic and interdisciplinary way.
This thinking requires the ecosystem to be understood as an interacting structure of biotic and abiotic components, in which the properties of individual organisms, the populations they build and all their habitat factors can be described together, i.e. in a holistic manner (Breckling & Müller 2006). "A well-defined ecosystem has strong interactions among its components and weak interactions across its boundaries" (Jørgensen et al. 2007). The detailed knowledge about what an ecosystem is and e.g. what properties and cycling processes it implements, is a prerequisite for sustainable decisions in environmental management.
Ecosystems must be non-isolated (like the earth) because otherwise they could not get the energy to maintain the system far from thermodynamic equilibrium. Ecosystems are ontic open systems because they have to exchange mass, energy and information (e.g. water through precipitation and evaporation) with the surroundings (Jørgensen et al. 2007). Due to the limited energy flow from the sun and the limited energy capture possibility the ecosystem must be able to "develop better utilization of the exergy that it is able to capture" (Jørgensen et al 2007) to move further away from thermodynamic equilibrium. The development of networks whose components are in tune with each other enhances the exergy efficiency.
Life conditions’ variability is rather high in time and space. Although an ecosystem has adapted to certain conditions, it can be suddenly disturbed by catastrophic events. Due to their specific properties, ecosystems have the flexibility and adaptability to meet changed conditions and keep the system far from thermodynamic equilibrium. Regarding long-term ecosystem processes, disturbances may be advantageous and beneficial for creative survival strategies (Jørgensen et al 2007).
Due to current and rapid ecosystem changes and rising environmental problems, further ecosystem research on the basis of an interdisciplinary, holistic communication and integration in natural and social sciences is indispensable. To reach this aim, further development of methods, systems, tools and technologies are required.
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