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Ecological Gradient Model

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Ecological Gradient Model



This Ecological Gradient Model (EGM) serves as a tool for managers and scientists to identify the condition of a reef by comparing factors at a site of interest to nearly 200 sites throughout the Main Hawaiian Islands (MHI).   `

It utilizes physical factors of wave energy and depth as a first approximation, to separate natural from anthropogenic impacts.  There is a further refinement based on biological and environmental factors. 

Although this model has limited capability to predict future reef condition, it has the ability to detect degraded conditions.  The EGM has the ability to distinguish levels of impairment for numerous variables and make comparisons between sites.  It provides metrics that can be ranked to form an index of biotic integrity (IBI) (Karr and Chu 1999).  It can be used to assist managers in classifying areas of concern or identifying high-quality candidates for marine protection.  The EGM is designed to describe reef condition in an objective and quantitative manner along a continuum.  Further, the model increases in power as more sites are evaluated and added to the data base.  The EGM allows comparisons across a wide range of sites throughout the MHI.  The model describes reef condition by ranking a single or a group of factors within a habitat class based on wave energy and depth.  A link to specific types of disturbance may be highlighted in the rankings of these variables (eg. a low comparative ranking among sites for fine grain fraction may indicate sedimentation impacts, low IBI rank among sites for fishes in the largest size class may be indicative of overfishing).  Since many biological organisms are stratified by depth and wave energy, sites are grouped by these influential variables based on the Hydrogeomorphic Model (HGM) (Smith et al. 1995) concept of using major natural hydrogeomorphic forcing functions to facilitate separation of natural from anthropogenic variability.

To identify which environmental factors were most important in structuring coral and fish assemblage characteristics and to narrow the field of variables, multiple regression, correspondence analysis, and a non-metric multi-dimensional scaling techniques were used.  Multivariate procedures were used to link biological data to environmental data to find patterns in coral communities and to determine the contribution of each species to site similarities.  These results were used in the development of the final model to determine the CRAMP weights for each factor.  The CRAMP IBI weighs each factor based on an objective analysis of the primary factors defining reef condition.  In addition there is an unweighted IBI and a weighted IBI.  The weighted IBI option is provided that allows the operator to change the weights to suit a particular management or ecological question.  For example, one may chose to place heavy importance on macroalgae and less importance on coral or fish parameters.  The final available unweighted option places equal importance on all factors. Graphs of all three options are available for viewing.

Download the model - Very high and high levels of security will not allow the macros in this model to run.  To allow macros in Excel go to: Tools/Macros/Security.  Selecting medium security will allow you to choose whether or not to run potentially unsafe macros. Any lower level is not recommended.

Ecological Gradient Model Instructions

Modeling Hawaiian Coral Reef Conditions

Power Point audio EGM presentation




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Last Updated: 05/02/2008

By: Erin Naughton

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