The ecosystem is a well-established natural network system that consists of various kinds of organisms and abiotic components. The network is constructed on the basis of solar energy. Solar energy is converted into various materials by plants by utilizing inorganic materials, and the materials produced flow into the grazing and predation network. As a result, the energy is stocked in the ecosystems as various organisms and abiotic components.
The structure of an ecosystem changes continuously for the purpose of utilizing and stocking the solar energy with high efficiency in a given environment, which also dynamically changes. This aspect of changing is very important to understand an ecosystem characteristic; that is why mathematical models which describe the structural dynamics in the ecosystem network are required.
To understand the ecosystem network roughly, the well-known concept called 'food chain' is often quoted. In the food chain, components included in an ecosystem are divided into some trophic levels (i.e., primary producers (plants), primary consumers (herbivorous organisms), secondary consumers (carnivorous organisms), higher-degree consumers and decomposers. For example, in the water ecosystem, the food chain is assumed to be constructed by phytoplankton, zooplankton, planktivorous fish, piscivorous fish, and bacteria. In the food chain, the phytoplankton produces various organic materials by photosynthesis, and is grazed upon by zooplankton. Zooplankton is predated upon by planktivorous fish.
Planktivorous fish is predated upon by piscivorous fish. The detritus produced from organisms is decomposed by bacteria in order to produce inorganic substances. This way of thinking is simple and very clear, and useful to understand the ecosystem state roughly. That is why food-chain structure is described by many ecological models.
On the other hand, many kinds of organisms are included in each trophic level in practice. For example, in water ecosystem, the above-mentioned phytoplankton includes green algae, diatoms, blue-green algae, etc. Zooplankton includes cladocerans, rotifers, copepods, etc. Furthermore, these can be divided into various species and growth stages. The predator-prey relationships among them are thus very complex, making the material flow network very intricate. This network is called as ecological 'food web'. This ecological food-web structure is also described by many ecological models.
As mentioned above, ecosystems are constantly affected by various environmental factors such as nutrient loading, temperature, human activity, etc., as well as solar radiation. If these environmental factors change drastically (e.g., high levels of a nutrient starting to load into an ecosystem abruptly), they have a strong impact on an ecosystem. The ecosystem structure changes extensively, and another ecological structure must be sought consistent with the newly given environment. The structural dynamic model is a tool for predicting or assessing the dynamic changes in such an ecological structure.
In fact, structural dynamic models are among the newly developed ecological models. In the past few decades, a good number of studies have been conducted on the structural dynamic models. Recently, there has been increasing concern for environmental remediation technologies on the basis of ecological methodology, which strongly depends on the ecological structure and also promotes the study on structural dynamic models.
In this article, the mass balance model, which is the basis of the structural dynamic models, is described at first. And the ecological feedbacks, which are important mechanisms to model the structural dynamics, are briefly reviewed. Then, the actual modeling approaches of structural dynamics are introduced, especially the holistic approach using goal functions as well as classical bottom-up-style modeling approach.
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