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CRAMP Rapid Assessment. Reference Site LImitations

Results of this investigation show the limitations of using a “reference site” or a “control reef” in determining “reef health” or reef condition. The underlying problem is that selection of a reference site is subjective, even by experts. No two reefs are exactly alike in all respects, so agreement on appropriateness of any “control” or “reference” reef cannot be attained in an absolute sense. Therefore, reference site selection is inevitably subjective and may be biased and inaccurate.

The reference paradigm does not hold up under scrutiny when a large number of sites and measured parameters are available for quantitative comparison. Comparisons between a reference site and a site being evaluated can appear to be a reasonable approach if only a single parameter such as coral cover is being compared. For example, a reef with high coral cover is usually taken as a “reference” for comparison to an “impacted” reef with low coral cover. The comparison begins to break down as more measured parameters are added to the analysis. We begin to see that the two reefs are quite different in many fundamental respects. If we begin to increase the number of sites used in the comparison we note a great deal of heterogeneity and overlap between important parameters both within and between sites. There is high spatial and temporal variability that cannot be encompassed by a single reference site or a small number of reference sites.

The reference site paradigm was not found to be applicable in the Hawaiian marine environment because of the complexity and extreme heterogeneity of coral reef ecosystems. The reference site standard cannot encompass the spatial variability and temporal fluctuations found in the reefs of the Main Hawaiian Islands.

Stratification of marine organisms is principally influenced by depth, spatial complexity, and wave regimes. This pattern is analogous to terrestrial botanical zonation, which is primarily based on elevation, topography and rainfall. These oceanic, geologic, and meteorological differences created diverse habitats, supporting varied biotic distributions and abundances and makes selection of reference sites difficult. Unlike the attributes used to create the index of biotic integrity for freshwater systems, most marine attributes are not comprised of distinct ranges, but instead follow a continuous gradient.

Although considerable overlap exists for the majority of variables, reference sites can be separated from impacted sites based on a few attributes. Severe degradation and effects of sedimentation are detected by strong deviations from reference values. Reference values could not be derived from the majority of environmental variables due to interactions that influence discriminatory power, habitat complexity and extreme variability. This investigation demonstrated the difficulty in the use of reference values as a standard.

  1. The reference sites standard cannot distinguish degree of impairment. The extremes of “severely impaired” and “little or no impact” can be defined, but the high variability in range restricts the ability of reference ranges to discriminate on a finer scale.

  2. Reference site values have limited power in detecting disturbance. High variability among most variables prevents identification of specific causes of disturbance. Natural heterogeneity increases reference ranges and decreases the ability of reference sites to detect impaired reef condition. For example, high wave energy environments naturally have extremely low and variable coral cover values that are not related to anthropogenic factors.

  3. A small sample of reference sites cannot accurately describe the range of biological integrity encountered among reef communities. When attempting to integrate a large number of reference sites, conditions can overlap substantially with non-reference sites. The high heterogeneity of Hawaiian coral reefs impedes the separation of natural from anthropogenic impacts.

  4. Subjective selection of reference sites is flawed. Quantitative analysis showed poor separation and overlap between reference and non-reference sites. Determination of optimal reef conditions is obscured by the lack of knowledge of the anthropogenic history of a site and sliding baselines that change over time. The reference concept is defective largely because it does not embrace the diversity of unimpacted reef communities.

  5. When only reference sites are used in the evaluation of impairment, comparison of a given site with other sites throughout the state is unattainable.

Minimally disturbed sites such as Lehua Island on Ni‘ihau often serve as reference sites for impaired areas.

 

Last Update: 04/21/2008

By: Lea Hollingsworth

Hawai‘i Coral Reef Assessment & Monitoring Program

Hawai‘i Institute of Marine Biology

P.O. Box 1346

Kāne‘ohe, HI 96744

808-236-7440 phone

808-236-7443 fax

email: jokiel@hawaii.edu