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CADDIS Volume 4: Data Analysis

Advanced Analyses

Analyzing Trait Data


Using traits for causal analysis

Author: A.I. Pollard

Analyzing Trait Data

Ecological traits are attributes of an organism that provide morphological, behavioral, and functional characteristics that describe how an organism interacts with the environment. Ecological traits are inherent characteristics of individuals, e.g., the presence of suckers or hooks to maintain position in a stream (Figure 1). A trait-based approach analyzes information about ecological traits to better understand why organisms occur in particular environmental conditions.

What are the advantages of using this approach?

There are two primary advantages of using a trait-based approach in biological assessment. First, trait-based descriptions of biological communities are applicable at various spatial and temporal scales. Environmental managers are asked to assess the condition of communities over a variety of spatial scales, for example by comparing condition in a local stream to general condition across a state. Descriptors based on taxonomic identity, such as the number of Ephemeroptera taxa, may be suitable for local analyses, but, unlike the taxonomic identity of individual invertebrates, responses of trait-based descriptors to changes in environmental conditions are thought to be uniform across large spatial scales. A traits-based approach, therefore, would be applicable across biogeographic boundaries. Second, a trait-based approach may provide mechanistic insight into the relationship between changes in environmental conditions and the occurrence of organisms. Better mechanistic understanding may improve the ability to predict community composition based on environmental characteristics.

How do I calculate trait-based metrics?

Figure 1. Beetle larvae (family Gyrinidae) with a close up of the terminal hooks.
Courtesy of U.S. EPA’s Region 3 Environmental Science Center.

A tool is available in CADStat that uses existing trait data from the USGS Exit EPA Disclaimer to compute trait-based metrics from observations of benthic macroinvertebrates.

How can traits be used in causal analysis?

The mechanistic relationships between trait and environmental characteristics may contribute evidence towards building a case to identify the cause of the observed impairment. An analysis of traits supports the verified prediction line of evidence, where the knowledge of a cause's mode of action permits prediction and subsequent confirmation of previously unobserved effects. The power of verified prediction resides in making specific predictions rather than explaining the reasons for observations post hoc. Therefore, traits can be used in the following way: A resource assessor may suspect that increased sedimentation is an issue in a stream. She predicts that the increased sedimentation would lead to a lower proportion of clinging invertebrates in the sample from the degraded site compared to the community from the reference site. She tests this prediction by comparing community composition at the two sites.

More information

  1. More background information is available from the National Institute of Water and Atmospheric Research Exit EPA Disclaimer.
  2. Cleaned traits data and tools to calculate trait-based metrics are available in CADStat.
  3. The original trait database Exit EPA Disclaimer is publically available through USGS.
  4. Pollard AI, Yuan LL (2010) Assessing the consistency of response metrics of the invertebrate benthos: a comparison of trait- and identity-based measures. Freshwater Biology 55:1420-1429.

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