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INDICES OF TAXON-BY-TAXON DISAGREEMENT BETWEEN OBSERVED AND EXPECTED ASSEMBLAGES
Citation:
VAN SICKLE, J. INDICES OF TAXON-BY-TAXON DISAGREEMENT BETWEEN OBSERVED AND EXPECTED ASSEMBLAGES. Presented at 2007 Annual Meeting of the North American Benthological Society, Columbia, SC, June 02 - 07, 2007.
Description:
RIVPACS-type models infer impairment from the level of disagreement between observed and expected assemblages. The popular O/E index measures the disagreement between the observed and expected number of taxa. Thus, O/E ignores the additional information offered by a taxon-by-taxon contrast between assemblages. I compare O/E to a new index, the mean absolute difference (MAD) and also to the log-likelihood (LL) index. MAD and LL both summarize, across taxa, the disagreement between each taxon's observed occurrence and its model-predicted occurrence probability. However, neither MAD nor LL appear superior to O/E in practical terms. First, the site-specific rescalings required for MAD and O/E obscure interpretations of their numeric scores. In contrast, O/E can be directly interpreted in terms of taxa loss. Second, in a macroinvertebrate predictive model applied to 499 Mid-Atlantic Highlands stream sites, O/E differentiated between 5 independently-assigned site disturbance classes, and also between reference and nonreference condition, as strongly as did LL or MAD. Correlations between O/E and either MAD or LL, across Mid-Atlantic sites, were strong (>0.85) when indices included only locally-common taxa (Pc > 0.5), but were weak (<0.25) when all taxa were included, probably because MAD and LL have greater sensitivity to occasional rare-taxon occurrences.