University of Kiel, Ecology Centre, MSc Environmental Science, a seminar paper
Status: completed (2010)
Features of Ecosystem Structures
Regina Schulze
ginaschulze@yahoo.de
Abstract
"A promising goal of ecosystem science is to seek a quantitative description of the interactions of the various components of a community with each other and with the physical domain in which they are embedded". [Ulanowicz, 1995, p.1]
In order to achieve this goal it is crucial to answer the following questions: How is an ecosystem constructed? Where does instability come from and how do systems change in time and space? To visualise and comprehend ecosystem structures is an important step in ecosystem analysis and a precondition of ecosystem modelling and managing. This Wiki will give an overview what ecosystem structures are and how they are applied.
Content
- What is an ecosystem structure?
- What are the features of ecosystem structures?
- 2.1 Boundaries
- 2.2 Hierarchies
- Construction of ecosystem structures
- Examples for other ecosystem structures
- Conclusions and outlook
- References
- 6.1 Literature
- 6.2 Pictures
1. What is an ecosystem structure?
According to Billings (1978) the term ecosystem is "an energy-driven complex of a community of organisms and its controlling environment" [CBD, 2010].
It means also "a dynamic complex of plant, animal and micro-organism communities and their non-living environment interacting as a functional unit" (Article 2 of the Convention)[CBD, 2010]. A structure is the construction or the composition of a thing. In this context the meaning of construction is less important because ecosystems where not constructed by following a (human) plan like it is the case by the construction of a car for example. The natural world is fundamentally different from the world built by humans. In this case the meaning of composition is more crucial: The composition of organisms and natural systems, the visual pattern which we are able to realise (e g. the anatomy of an organism), provides fundamental information as basis for the study functions and a framework where dynamic action takes place [Golley, 2000, p. 21]. The problem is that nature does not provides us with a visual pattern of ecosystems. Ecosystems are rather based on a human definition. Therefore it is necessary to visualize their structures artificially.
Golley [2000, p. 31] merges the terms ecosystem and structure to ecosystem structures:
"An ecosystem structure is a network of interaction between components of the system".
These components are the so called "entities" which are defined by Golley and Keller [1998] as "objects which are bound in space and/or time and which are internally coherent". Fredrick Ferré, a philosopher with affection to metaphysics identified six kinds of ecological entities [Ferré, 1996]:
- Aggregate entities are physical bodies with boundaries, e.g. mountains and lakes; they form a background for the ecological play.
- Systematic entities include ecosystems. These retain structure and function under continually changing environmental conditions.
- Formal entities are based on the knowledge added to them, e.g. a species;
- Organic entities are living organisms. They are creative because they generate unique, new forms of life.
- Compound entities are those which have strong internal relationships but are without apparent internal system dynamics, e.g. water or sodium chloride (chemicals).
- Fundamental entities are of little use for ecologists (Metaphysics needs such a category).
Aggregate, organic and compound entities (1, 4 and 5) are interpretable as structural components which can interact with each other and which are embedded in systematic entities: ecosystems.
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2. What are the features of ecosystem structures?
A structure we observe in the natural world is the consequence of the interactions between the genetic process in the organisms and the selective forces in the environment, thus a structure can be static, fixed, dynamic, flexible and responsive at the same time [Golley 2000, p.21]. The beak of a hummingbird, a structure of a single organism shall give an example for these attributes: The hummingbird (Fig. 1) is living as a high specialized nectarivore, depending on ornithophilous flowers it is feeding upon. Both structures, flower and beak will adapt to each other as response. But adaptation cannot be obtained through change in structure. The hummingbird will never develop a flexible elephant's trunk in exchange to a rigid beak. The genetic conditions in this respect are rather fixed. Hence the flexibility of structure, which is essential for survival, allows the dynamic process of co-evolution between flower and hummingbird.
Figure 1. Hummingbird (Meyer Media LLC, 2006)
These attributes apply to ecosystem structures as well but to understand them on a broader level, boundaries and hierarchies have to be taken into account:
2.1. Boundaries
The boundary of ecological entities is permeable. Flows of energy, matter and information pass into and out of the entity from and to its environment across the boundary. The human organism for example has multiple boundaries. The most apparent one is the surface of the skin. A less obvious boundary, at least for an outsider is the region around the organism which is interpretable by the sense organs and bounded by their capacity. Also less evident is the boundary of the personal community [Golley, 2000, p. 23]. Nowadays, in times of "Facebook" and "Twitter", a friendship network probably has a worldwide extend. In comparison to single organisms, the boundaries of ecosystems are a question of definition like it is with the term "ecosystem" itself. Therefore ecologists rather speak about a transition zone (ecotone) between ecosystems than about a boundary. However, Golley [1974] defines the boundary as a property of an ecosystem as an entity and not as the sum of the boundaries of the individual structural components.
2.2. Hierarchies
To give an idea of the environment of ecosystems, it is helpful to consider the hierarchical theory of Allen and Starr (1982) and O'Neill et al. (1985). They developed this theory as a fundamental concept of ecosystem structure. It makes it possible to arrange systems of different scale and character into coherent patterns of a nested hierarchy where systems of a small scale are components or subsystems of the higher order systems.
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3. Construction of ecosystem structures
By visualising ecosystem models it is possible to emphasize on the components, the interaction between components or both in various levels of abstraction. Three different approaches with different purposes are shown in the following:
One of the first people who operationalised the ecosystem concept (ecosystem approach) was Raymond Lindeman in 1942. He and his wife sampled the biota and environmental factors of the small "Cedar Bog Lake" for about two years. They and classified several feeding groups and arranged them into a trophic pattern, the food cycle. Ooze as sink for organic matter is placed at the centre of the diagram. The conceptual model of the Cedar Bog Lake ecosystem (Fig. 2) contains internal cycles of elements among the system components and an outflow from the system to the larger environment in which it is nested.
Figure 2. Food cycle diagram based on Cedar Bog Lake, Minnesota, [Lindeman 1941]
Lindeman stayed within the biological paradigm where biologically defined components (organisms and species) play a dominant role. In 1957 Lindeman's circular diagram was replaced by the more abstract energy flow diagram developed by Howard T. Odum (Fig. 3). Odum generalised concrete species and organisms to functional taxa. This allows to reduce the biological component to a node and emphasise the connections between the nodes. Hence, the dominant structures are the relationships, not the biological properties.
Figure 3. Energy flow diagram with units in kilocalories per square meter per year in the Silver Springs community, Florida. The letters indicate herbivores (H), carnivores (C), top carnivores (TC) and decomposers (D). [Odum, 1957]
In 1967 Bormann and Likens presented their ecosystem model which consisted of storages of chemical elements and transfer flows (intra system cycle) between compartments, linked to the earth' biosphere by inputs and outputs (Fig. 4).
Figure 4. Relationships of the geological, meteorological and biological components within the ecosystem, connected through inputs and outputs to the biosphere [Borman and Likens 1967]
Golley noted that there is not only one correct or most useful approach to develop an ecosystem structure. It is common to compile a list of species present in the ecosystem (like Lindeman does) and then a list of interactions between species. Depending from the data which can be rather qualitative or more quantitative it is possible to construct quite different maps or diagrams of the interaction network.
In order to find a good compromise between abstraction and reality Golley suggests to focus on local ecotopes, the smallest systems which are homogenius for the properties of interest [Golley 2000, p. 31] .
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4. Examples for other ecosystem structures
Golley defined structure as "construction or the composition of a thing "and ecosystem structure as "a network of interaction between components of the system". Because the following examples show less obvious interaction networks than the trophic structures (food webs) which are described in chapter 3, is a matter of interpretation if spatial and temporal aspects can be a structure as well. Myster [2001, p. 132] speaks about "patterns of organisation at spatial or temporal scale" instead of structure. There is no criterion that a pattern must satisfy before it is also an ecosystem structure. Therefore he suggests amongst others that "structural patterns must be tied to those functions that are critical for the continued operation of the ecosystem". For the sake of simplicity the following examples are treated as patterns and not as structures.
The environment offers various ranges of physical and chemical conditions which can be used to derive horizontal, vertical and distribution patterns of organisms (Fig. 5).
Figure 5. Examples for spatial structures (composed by Schulze 2010)
Environmental conditions are not stable. Fluctuating conditions over time are influencing the structural items and appear in temporal patterns (Fig. 6).
Müller [2010] listed the following types of temporal patterns: circadian rhythms, lunear periodics, tide periodics, seasonal rhythms and long term rhythms. A differentiation should take place between those repetitive rhythms and succession. As ordered process of community development [Müller 2010] succession includes regular changes of the species compositions due to variations of the species interactions and internal processes within the community. Hence, changes in community structures cause changes in physical ecosystem functions.
Figure 6. Examples for temporal patterns (composed by Schulze 2010): the seasonal rhythm of a lake and two examples for succession
It is also possible to join spatial patterns, temporal patterns and trophic structures to a composed ecosystem structure (Fig. 7).
Figure 7. A composed ecosystem structure [NOAA, 2010]
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5. Conclusions and outlook
The features of ecosystem structures are describable with the attributes "static", "fixed", "dynamic", "flexible" and "responsive" (cf. chapter 2).
An ecosystem is relatively static because aggregate entities, which form the background for the ecological play, are more or less fixed (in terms of immobile). Furthermore, compound entities are liable to chemical and physical laws which usually do not change. The ecosystem's attributes "dynamic", "flexible" and "responsive" are based on the behaviour of the organic entities within the system. To specify the emphasis of the features, it is necessary consider the ecosystems environment with respect to their hierarchies and boundaries. Lunear periodics for instance are influencing the earth as a whole as well as various ecosystems since billions of years. Therefore, the tides are a stable phenomenon which not even humans can change. But humans as the "dominating organic entities" on the planet are able to modify trophic structures, destroy horizontal and vertical structures (e.g. from forests) and consequently change ecosystem functions. Homer-Dixon [2006, p. 232] describes the results of human acting within the context of the adaptive cycle (Holling cycle): "This is a moment of great volatility and instability in the world system. We need urgently to do what we can t avoid deep collapse. We also need to figure out how to exploit the opportunity provided by crisis and collapse when they occur, because some kind of systematic breakdown is now almost certain." A breakdown, or collapse would be fatal for humankind but at least the ecosystem structures will recover: "A collapse also liberates the ecosystem's enormous potential for creativity and allows for novel and unpredictable recombination of its elements. It's as if somebody threw the forest's remaining plants, animals, nutrients, energy flows and genetic information into a gigantic mixing bowl and stirred [...]. This is a perfect setting for the forest's plants and animals to experiment with new behaviours and relationships - a pollinator species like a bee or wasp will try gathering nectar from a type of flower it hadn't previously visited" [Homer-Dixon, 2006, p. 228].
The adaptive cycle which embraces two opposites (growth and stability on one hand, change and variety on the other) is hence reflected in the features of ecosystem structures.
From the human point of view it is left to hope that we are responsive and flexible enough to maintain the present ecosystem structures as well as their functions.
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References
6.1. Literature
- Allen, T.F.H., Starr, T. B.: Hierarchy: Perspectives for Ecological Complexity, University of Chicago Press, Chicago 1982
- Bormann, F.H., Likens, G.E.: Nutrient cycling, In: Science 1955 (3761),1967, p. 424−429
- Convention on Biological Diversity: definitions of ecosystem: http://www.cbd.int/ecosystem/ , [accessed on 5.5.2010] Ferré, F.: Being and value: Toward a Constructive Postmodern Metaphysics, State University of New York press, Albany, 1996
- Golley, B.: Ecosystem Structure, In: Jørgensen, S.E; Müller, F. (Ed.):Handbook of ecosystem theories and management, p 21 ff, CRC Press, Boca Raton, 2000
- Golley, F.B.: Structural and functional properties as they influence ecosystem stability, Proceedings first International Congress of Ecology, 1974, pp 97−102
- Golley, F.B., Keller, D.: Science of Synthesis, University of Georgia Press, Athens, 1998
- Homer-Dixon, T.:The Upside of Down − catastrophe, creativity and the renewal of civilization, Island Press, Washington, 2006
- Lindeman, R.: Energy dynamics of a Senescent Lake. Phd dissertation. University of Minnesota, 1941
- Müller, F.: Basics of ecosystem analysis −lecture slides, Kiel, 2010 Myster, R.W.:What is Ecosystem Structure? In: Caribbean Journal of Science, Vol 37, No 1−2, 2001, pp 132−134
- Odum,H.T.:Trophic structure and productivity of silver springs, Florida, In: Ecol.Monogr. 27,1957, pp 55−112
- O'Neill, R.V., Allen, T.F.H. et al.: A hierarchical Concept of Ecosystems, Princeton University Press, Princeton 1985 Ulanowicz, Robert E., Abarca-Arenas, Luis G.: An informational synthesis of ecosystem structure and function, In: Ecological Modelling 95, 1997 pp 1−10
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6.2 Recommended links
Hummingbird: [accessed on 6.5.2010]
Spatial structures: [all accessed on 6.5.2010]
Temporal structures: [all accessed on 6.5.2010]
Composed ecosystem structure: [accessed on 6.5.2010]
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