HGM vs. IBI hydrology
landscape success criteria
floristic quality Abby Rokosch
Jessen Book Siobhan Fennessy
Wetland Restoration Theory:
Before any project begins, Zedler (1996) suggests that those restoring the wetland must have very clear goals for their work. Specific decisions on what aspects of the restoration will be focused on (structure and/or function), and how those goals will be achieved must be made absolutely clear in order to promote success (Grayson et al., 1999). Specifically, restorers have a series of "theoretical" decisions to make:
Two controversial theories in wetland restoration are the theories of self-design and design wetlands. Some postulate that these two restoration theories have evolved from gleasonian and clementsian succession theories:
The controversy of "design" and "self-design" centralizes itself around the question of whether to plant vegetation at a restoration site or to naturally allow the restoration site re-colonize.
In-kind restoration refers to the creation of a new wetland of the type being destroyed (therefore, out-of-kind restoration would be creation of a different wetland type). And on-site refers to a creation of a wetland adjacent to the one being destroyed (Brinson and Rheinhardt, 1996). The differences between each approach can greatly affect the overall outcome, and success, of the restoration project. Ideally, when considering whether to create on-site or off-site, in-kind or out-of-kind wetlands, the engineers should examine which method yields the highest potential for success. Additionally, engineers also need to consider the possibility of creating out-of-kind wetlands in order to create a highly endangered wetland type (Brinson and Rheinhardt, 1996). However, it has traditionally been thought that creating in-kind, on-site restorations are the best fit solution for most situations. Restoring in this manner would seem to be the easiest of all practices because the site has already proven that it can support a wetland of the type to be destroyed; therefore creation of a comparable wetland would seem to have an increased chance of success (Brinson and Rheinhardt, 1996). However, this may not be the most appropriate idea. Kruczynski (1990), suggests that creating on-site, and in-kind may in fact doom the created wetland to failure (in Brinson and Rheinhardt, 1996). Because of the close proximity of on-site wetlands to humans, and human byproducts, the restoration project may be predisposed to failure. Specifically, human introduction of wastes, chemical pollutants, landscape changes to the surrounding area, and introduction of invasive species are all potential human dangers that grow explosively as one approaches human settlement.
By comparing the restored site to an approximate reference site, researchers can determine how well the experimental wetland is mimicking the original (Zedler, 1996; White and Walker, 1997; Grayson et al., 1999; Mitsch and Wilson, 1996). However, White and Walker (1997) and Grayson et al. (1999) contend that the picking of reference sites for comparison is more complicated than just looking at comparable, pristine natural settings. Specifically, Grayson et al. suggest that restored sites must be compared to both non-degraded natural wetlands, and unrestored damaged wetlands. Thus, if the restored project shows signs of success, more knowledgeable conclusions can be drawn as to whether the success has come from the act of the restoration, or whether it is merely a natural response of the ecosystem (which may be evidenced by comparison to the response of the degraded un-restored site). Furthermore, White and Walker (1997) also advocate choosing reference sites that are both temporally and spatially resonant with the restoration site.
However, reference sites must be chosen and used judiciously. (For a list of the limitations of using reference sites, click here.)
As well as using reference sites for comparison, several researchers also suggest the use of more sophisticated, quantifiable, systems of comparison. On such system is the Hydrogeomorphic (HGM) approach to wetland assessment (Brinson, 1993). This model of comparison builds upon the idea of using reference sites (Whigham, 1999). Specifically, HGM classifies wetlands from a functional standpoint in three different, layered, ways. First, HGM classifies wetlands on the basis of their functional differences. Second, HGM defines the functions that each wetland performs. And third, HGM uses reference wetlands to establish a range of "naturally acceptable" differences in function (Burkhardt, 1996). Thus, the HGM model is founded on a landscape perspective and consequently recognizes the impact that geomorphic setting, water source, and hydrodynamics have on wetland structure and function.
Using the principles of the HGM method of classification has led to the development of five major classes of wetlands: riverine, depressional, slope, flats (organic soil and mineral soil), and fringe (estuarine and lacustrine) (Burkhardt, 1996). (For a more detailed breakdown of these classes, click here.) These classes can then be further subdivided regionally. Ultimately, HGM allows wetlands to be classified very specifically based on their "ideal" functional roles in the ecosystem. In turn, this highly specified definition both gives restoration engineers specific goals to uphold, but also provides a simple measurement for restoration success. If the wetland does not approximate the functional characteristics of its HGM classification, then the wetland is a failure.
For a review of recent changes to the HGM classification system, in light of the landscape changes effected by mitigation see "The Changing Face of HGM".
Like the HGM method of classification, IBI attempts to synthesize individual components of the ecosystem. In turn, this synthesis is used to quantify the success/health of the ecosystem. However, unlike the HGM--which is based on physical features of the wetland--IBI focuses its synthesis on biotic factors. Specifically, IBIs prioritize certain, highly susceptible to environmental degradation, species within a wetland. Theoretically, if these "tenuous" species are able to survive in the wetland, then the wetland is free from high levels of disturbance.
The idea of an index of biotic integrity was first developed/applied by Dr. James Karr in 1981. In his examination of stream health, Dr. Karr highlighted 12 different, biotic, factors indicative of stream health (e.g. trophic organization and function, reproductive behavior, fish abundance, fish species richness and composition, and condition-health-of individual fish). With these categories, Dr. Karr ranked the stream in question on a scale from one to five (five representing what would be expected in a stream of high health) for each category. All values were then summed to create a single number representing the overall health of the wetland (Simon and Lyons, 1995). (Format of Dr. Karr's original IBI).
Since the introduction of IBI, modifications have been made. On different scales, and to reflect different base community types, different authors have proposed other, more specific, models of quantification. For an example, see Simon et al., 2000.