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2.2.1 Mechanisms monitoring design: Overview

Overview    Choosing a spatial and temporal design    Response design    Inference design

Mechanism by Dennis Milligan
Image by Dennis Milligan, used with permission.

This section is designed to assist you with choosing which large-scale spatial and temporal designs are most appropriate and feasible for conducting monitoring programs that are aimed at learning about causal mechanisms behind observed changes in salmon indicators.   As discussed briefly on the previous page, "Monitoring Program Design: Introduction", the emphasis here is on the design of large-scale monitoring programs.  Specifically, learning about causal mechanisms, especially those arising from climatic effects, usually requires coordinated effort for designing monitoring programs across contrasting levels of variables that reflect such mechanisms. Such contrasts in climatic effects cannot usually be found in small, local study areas, and will therefore require instead that sampling be done across environmental gradients. However, there could be local situations in which monitoring could provide evidence for causal mechanisms; they are likely to be relatively unusual though.

Also, as we describe in Step 1 under "Mechanisms, Basic Questions", the best that such designs can hope to do is to estimate the relative importance of different causal mechanisms of changes in salmon. We have limited ability to interpret results regarding causal mechanisms from even carefully designed monitoring programs because potential driving mechanisms that are not under human control often inadvertently change simultaneously with those that are under our control. This situation creates confounding, which precludes separating the effects of the different mechanisms. Nevertheless, the results from such monitoring programs will still help management agencies identify priorities for taking actions to improve the status of salmon populations.

Considerable literature exists that reviews the numerous issues related to monitoring designs -- the variety of designs, costs and constraints, and relative merits and disadvantages of different designs. In this web site, we describe some of these key issues, but we cannot reflect all detailed points and recommendations from those studies. We therefore encourage users of this web site to carefully examine some of the most important review documents. These include Stewart-Oaten et al. (1986), Walters et al. (1988), Hicks and Brydges (1994), Underwood (1994), Schmitt and Osenberg (1996), Stewart-Oaten (1996a,b), Griffith (1997), Schwarz (1998), Downes (2002), Roni (2005, especially including Chapter 2 by Roni et al. 2005), and Peterman et al. (2005).

We also recommend that you look at the United States Environmental Protection Agency's web site on "Causal Analysis/Diagnosis Decision Information System (CADDIS)" at CADDIS is an online application that helps scientists and engineers to find, access, organize, use, and share information to conduct causal evaluations in aquatic systems. Many of the topics covered on that web site address issues relevant to our focus on this Salmon Monitoring Advisor web site, that is, understanding habitat-based causal mechanisms that affect salmon.


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