A causal examination of the effects of confounding factors on multimetric indices

Ecological Indicators
By: , and 



The development of multimetric indices (MMIs) as a means of providing integrative measures of ecosystem condition is becoming widespread. An increasingly recognized problem for the interpretability of MMIs is controlling for the potentially confounding influences of environmental covariates. Most common approaches to handling covariates are based on simple notions of statistical control, leaving the causal implications of covariates and their adjustment unstated. In this paper, we use graphical models to examine some of the potential impacts of environmental covariates on the observed signals between human disturbance and potential response metrics. Using simulations based on various causal networks, we show how environmental covariates can both obscure and exaggerate the effects of human disturbance on individual metrics. We then examine from a causal interpretation standpoint the common practice of adjusting ecological metrics for environmental influences using only the set of sites deemed to be in reference condition. We present and examine the performance of an alternative approach to metric adjustment that uses the whole set of sites and models both environmental and human disturbance effects simultaneously. The findings from our analyses indicate that failing to model and adjust metrics can result in a systematic bias towards those metrics in which environmental covariates function to artificially strengthen the metric–disturbance relationship resulting in MMIs that do not accurately measure impacts of human disturbance. We also find that a “whole-set modeling approach” requires fewer assumptions and is more efficient with the given information than the more commonly applied “reference-set” approach.
Publication type Article
Publication Subtype Journal Article
Title A causal examination of the effects of confounding factors on multimetric indices
Series title Ecological Indicators
DOI 10.1016/j.ecolind.2013.01.015
Volume 29
Year Published 2013
Language English
Publisher Elsevier
Publisher location Amsterdam, Netherlands
Contributing office(s) National Wetlands Research Center
Description 9 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Ecological Indicators
First page 411
Last page 419
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