O/E: a standardized way to make site-specific assessments of biological condition Chuck Hawkins Western Center for Monitoring and Assessment of Freshwater.

O/E:a standardized way to make site-specific assessments of

biological condition

Chuck Hawkins

Western Center for Monitoring and Assessment of Freshwater Ecosystems

Utah State University

What is O/E? O/E is a measure of the taxonomic

completeness of the biological community observed at a site

E = 8 taxa O = 3 taxa

O/E0.38

Why Site-Specific Assessments are Important:

The world is variable and assessments should lead to biologically meaningful, legally

defensible, and fair assessments that minimize both false positives and false negatives.

Why Standardized Assessments are Important:

They allow uniform criteria and standards to be developed that apply to all waterbodies

(uniform ≠ fixed)

How can we achieve both site-specific and standardized assessments?

• Recognize that natural ecosystems vary considerably in their expected biota.

• Develop ways of describing the expected biota for individual sites (site-specific).

• Assess the degree to which observed biota equal that expected for the site.

• Develop criteria and standards based on the relative degree of departure from the expected biota (standardized).

The Technical Challenge:Accurately and precisely describing the biota expected in different waterbodies in a State.

The O/E Approach1. Select a set of high-quality reference sites that fully

characterize the natural biological variation and potential that occurs within a state.

2. Develop predictive models that relate variation in biota to easily measured predictor variables.

3. Quantify model error.

4. Compare the biota observed at a site to that predicted (expected) to occur for that site ‘type’.

5. Develop biological criteria and standards that incorporate measures of error and are consistent with the designated uses of different waterbodies.

Reference Site Selection Criteria

•Minimal chemical contaminants

•Minimal flow alteration

•Natural riparian vegetation

•Natural mixture of habitat types

Reference Criteria

• Used comparable reference criteria

• Based on:– EMAP reference criteria– Neighboring state reference criteria– Colorado water quality standards

Taxonomic Data

• Resolved ambiguous taxonomic information within and among sites

• Both approaches used comparable taxonomic resolution

Modeling and Assessments1. Classify reference sites into groups based on

their biological composition.2. Estimate the frequencies of occurrence (fo) of

different taxa in each reference site group.3. Develop a statistical model that predicts the

probabilities that an assessed site belongs to each group (pg) .

4. Weight fo by pg to refine estimates of the probabilities of capturing (pc) every taxon in the state at an assessed site.

5. Compute O/E from estimates of pc derived from sample data (O) and predictions (E).

6. Assess site condition in the context of model error.

How O/E is Calculated:

Sum of taxa pc’s estimates the number of taxa (E) that should be observed at the site given standard sampling.

Taxa pc O

1 0.92 *

2 0.86 *

3 0.70

4 0.63

5 0.51 *

6 0.32

7 0.07

8 0.00

E 4.01 3

O/E = 3 / 4.01 = 0.75

O2 O3

*

*

* *

* *

3 3

O/E

Describing Model Error

E

O

1

Assessing Biotic Condition

O/E0 1

Num

ber

of

Ob

serv

ati

on

s

Reference Sites

Criterion defining highbiological integrity

Decreasing biological integrity

Initial Results for Colorado

• No time to go into technical details of modeling.

• Will focus on:– The biological classification of reference

sites.

– The predictors of biotic variability.

– Error of the model.

Low elevation sites

High elevationsites

Plains

Foothills?

10987654321

GROUP

Spatial Distribution of Different Classesof Reference Sites

1 2 3 4 5 6 7 8 910

GROUP

-109-108-107-106-105-104-103-102

GIS

_LO

NG

1 2 3 4 5 6 7 8 910

GROUP

0

1

2

3

4

5

LOG

_WS

A

1 2 3 4 5 6 7 8 910

GROUP

-50

0

50

100

150

TM

EA

N

Temperature, stream size, and geography were the best predictors of what class a site belonged to.

What group does a site with log WSA = 2.9 belong to?

0.0 0.5 1.0 1.5 2.0O/E

12345678910

GROUPThere was no evidence that the model was biased by biotic class or ecoregion. ER O/E

CP 1.04

SR 1.00

WHP 0.96

SWTL 0.97

SD of 0.17 is ~ 2/3 of possible precision.

Summary• The CO model is similar in performance

to models developed for OR, WA, CA, and WY.

• Initial results suggest that site-specific assessments can be made with acceptable error and expressed in standardized units that allow direct comparisons across stream types.

• Assessments are easy to generate and have an intuitive biological meaning interpretable by all stakeholders.