II. D. Nearshore Ecologic Models |
As the classification systems have demonstrated, nearshore habitats are defined by a variety of complex interactions between physical, geological, chemical, and biological components. The effects of human-caused changes in physical conditions can cause a change in the structure of habitats, which will ultimately affect the habitat’s function. From this general reasoning, we can derive simple relationships (models) that may help us predict or understand natural and human-caused effects on nearshore ecosystem functions. These models, based on existing knowledge and best professional judgment, are especially useful when there is a pervasive lack of empirical data.
The physical components of an ecosystem are referred to as its “controlling factors” because of the strong dependence of biological entities upon them. For example, the local combination of controlling factors (such as slope, depth, tidal cycle, and wave energy) will define the type of plant species that can exist in that area. Biological communities, which are often spatially constrained by these local controlling factors, serve to further define the structure and functions (e.g., refuge, nutrient cycling) of the nearshore ecosystem (Williams and Thom 2001; Williams et al. 2001) (Table II-1). Once established, biological components may, in turn, influence controlling factors; so biological alterations can impact the ecosystem from a foundational level (for example, temperature regulation and nutrient input from overhanging vegetation).
Table II-1. List of Controlling Factors and Associated Habitat Structural and Functional Attributes (from Williams and Thom 2001).
| Controlling Factors | Habitat Structure |
Habitat Processes |
Ecological Functions |
Depth |
Density |
Production |
Disturbance Regulation |
A conceptual model approach can be used to illustrate the interactions that occur in the nearshore ecosystem as influenced by controlling factors and associated habitat structure and function (for example, the effect of wave energy and light on plant biomass, and resulting links to primary production). Empirical data are often lacking on the impacts of specific activities to a given habitat’s structural and functional attributes. Conceptual models are useful because they allow us to use existing information to identify the linkages between (and among) the controlling factors and biological components of an ecosystem. When changes occur at the controlling factors level, the associated biological and ecological responses can then be inferred and tested. In its most basic form, impact assessment can be approached through the response chain illustrated in Figure II-3 (from Williams and Thom 2001).
Figure II-3. Conceptual model linking shoreline impacts to ecological functions (from Williams and Thom 2001).
This approach provides the necessary framework for assessing complex systems - where data gaps often exist - and will be used throughout this document. The following chapters focus on surveying three key areas within this framework for the Bainbridge Island nearshore environment: physical characteristics and dynamics (Chapter III), habitats (Chapter IV), and biological resources (Chapter V). Based on an understanding of these factors, the potential impacts of nearshore modifications by humans can then be assessed in greater detail (Chapter VI).
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