Overview – Models
Once the input data layers are created, analysis in CAPS requires a model to be defined for each natural community or broad ecological system. Each community's model entails selecting a number of metrics and weighting them by importance for that community. This model parameterization was originally done by a number of expert teams with expertise in particular ecological systems (i.e., forests, grasslands & shrublands, wetland & aquatic communities, coastal ecosystems, and transportation impacts on landscape connectivity). Additional parameterization and some necessary modifications were done for this project by Brad Compton, Scott Jackson and Kevin McGarigal.
Results from the landscape metrics are rescaled, weighted, and then combined into an overall index of ecological integrity. First, the results of each metric are rescaled by percentiles for each community so that, for instance, the best 10% of marshes have values ≥ 0.90, and the best 25% have values ≥ 0.75. This is done to adjust for differences in units of measurement among metrics and to account for differences in the range of metric values among communities. The rescaling by community is done to facilitate identification of the "best" of each community, as opposed to the best overall – which is strongly biased towards the dominant, matrix-forming communities (i.e., forest).
CAPS Landscape Metrics lists the landscape metrics used in CAPS.
Next, the rescaled values are weighted (weights are assigned by expert teams), to reflect the relative importance of each metric for each community, and then added together to compute an overall index of ecological integrity. Thus, the final index of ecological integrity for each cell is a weighted combination of the metric outputs for that cell, based on the community within which the cell falls.
CAPS Community Model Parameters lists the metrics selected for each of the ecological communities and their relative weights used in the models.
Note that metrics do not apply to developed land—all cells corresponding to developed land cover types are given an index of ecological integrity (IEI) score of zero, even though we recognize that some developed land may contribute to the conservation of biodiversity.
The default CAPS analysis rescales the IEI values to the full extent of the assessment area (e.g. statewide). Separate analyses allow IEI to be rescaled by percentiles within each watershed or ecoregion. For example, if the IEI is rescaled by watershed, a marsh with a value of 0.85 would be interpreted as being in the 85th percentile of marshes for its watershed.
In our CAPS assessment of Massachusetts (November 2011) we rescaled results at three extents (full extent, rescaled by major watershed, and rescaled by ecoregion), plus a final integrated rescaling. The integrated rescaling uses the maximum score from statewide and watershed analyses for each cell in wetland and aquatic communities, and the maximum score from statewide and ecoregion analyses for cells in upland communities. The resulting IEI is then rescaled again by community to preserve the interpretation (i.e., the top 10% of IEI values represent 10% of the landscape).
Table 1. Summary of different scalings of the Index of Ecological Integrity.
|Each community is scaled across the full extent (statewide)
|Each community is scaled separately within each ecoregion
|Each community is scaled separately within each major watershed
|IEI result for each community are integrated using combinations of statewide, watershed and ecoregion results
The following figures from our Massachusetts assessment show statewide IEI, IEI scores rescaled by watershed and by ecoregion, and integrated IEI, with darker colors indicating higher-valued cells. Note that at the statewide extent most of the high-value falls in forests in the western half of the state. In the other figures the ecoregion and watershed scaling has reallocated the high IEIs across the state.
CAPS treats the results for each community separately, thus IEI should be compared only within communities. IEI is a relative measure, thus a powerline shrubland may have a high IEI, meaning that it has high integrity compared to other powerlines—this does not imply that it is pristine, or that it has more integrity than a medium-IEI wetland.
The following figure depicts the integrated IEI using a five-color scheme that makes it easier to differentiate among various groups of ecological communities. The five colors represent five broad groups of ecological communities: forest (green), shrubland (orange), freshwater wetland and aquatic (blue), coastal wetland (cyan) and coastal upland (yellow to brown). By using different colors to represent these five broad community types it is easier to recognize high-quality stream segments and patches of shrubland, wetlands and coastal communities that might otherwise go unnoticed among the large patches of forest throughout much of the state.