Environmental Engineering Reference
In-Depth Information
establishing, evaluating, and communicating biological integrity include the BCG and the IBI,
which will be described in the following sections.
7.8.1 bcG
The BCG is a conceptual model that describes how the ecological integrity of aquatic systems
changes in response to increasing levels of human disturbance. The BCG is consistent with eco-
logical theory and has been veriied by aquatic biologists throughout the United States. The BCG
model is based on irst identifying the critical attributes of an aquatic community that can be used
to evaluate its biological integrity and then evaluate each attribute on a scale indicating the level
of disturbance (USEPA 2005; Davies et al. 1993; Davies 2003; Davies and Jackson 2006).
Ten attributes are used to assess the changes in an aquatic ecosystem's response to increasing
levels of stressors (USEPA 2005):
1. Historically documented, sensitive, long-lived, or regionally endemic taxa
2. Sensitive rare taxa
3. Sensitive ubiquitous taxa
4. Taxa of intermediate tolerance
5. Tolerant taxa
6. Nonnative or intentionally introduced taxa
7. Organism condition
8. Ecosystem function
9. Spatial and temporal extent of stressor effects
10. Ecosystem connectance
The 10 attributes are based on the spatial or temporal scales at which data are collected for an
analysis. Attributes 1-6 are ecological attributes based on taxonomic composition and structure.
Attributes 7 and 8 deal with organism condition and system performance. Attributes 9 and 10 are
scale-dependent for physical-biotic interactions of importance in evaluating long-term impacts,
restoration potential, and recoveries.
The BCG is further divided into six tiers of biological conditions along the stressor-response
curve, ranging from observable biological conditions found at no or low levels of stress to those
found at high levels of stress (Davies and Jackson 2006; Figure 7.8):
1. Native structural, functional, and taxonomic integrity is preserved; the ecosystem function
is preserved within the range of natural variability.
2. Virtually all native taxa are maintained with some changes in biomass, abundance, or
both; ecosystem functions are fully maintained within the range of natural variability.
3. Some changes in structure due to loss of some rare native taxa; shifts in the relative abun-
dance of taxa but sensitive ubiquitous taxa are common and abundant; ecosystem functions
are fully maintained through redundant attributes of the system.
4. Moderate changes in structure due to the replacement of some sensitive ubiquitous taxa
by more tolerant taxa, but reproducing populations of some sensitive taxa are maintained;
overall a balanced distribution of all the expected major groups; ecosystem functions are
largely maintained through redundant attributes.
5. Sensitive taxa are markedly diminished; a conspicuously unbalanced distribution of major
groups from that expected; organism condition shows signs of physiological stress; system func-
tion shows reduced complexity and redundancy; increased buildup or export of unused materials.
6. Extreme changes in structure; wholesale changes in taxonomic composition; extreme altera-
tions from normal densities and distributions; organism conditioning is often poor; ecosystem
functions are severely altered.
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