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Wisconsin Plant of the Week

Wetland Indicator Status

As part of the National Wetland Inventory undertaken by the U.S. Fish and Wildlife Service, a national wetland plant list was developed in cooperation with a federal interagency review panel (Reed, 1988). This “National List” ranks individual vascular plant species according to their probability of occurrence in wetlands, or Indicator Categories, as described below.

A plant species may be a better indicator of wetlands in some regions than it is in others.  To reflect this the National List contains regional indicators for thirteen different regions. The indicator category used for the Wisconsin Plant of the Week is that for Region 3, which includes Illinois, Indiana, Iowa, Michigan, Missouri, Minnesota, Ohio and Wisconsin.

Each Region still covers a very large geographic area and, in most cases, includes several floristic provinces.  As a result intraregional differences exist for some species, but such assignment was beyond the scope of the original National List.  Where species in Wisconsin are known to diverge from the category used in the National List we include this information in the text for that species.

The National List was revised in 1996 and a further revision, the 1998 List, is in preparation as of March 2000. Since both revisions are still drafts the indicator status used at this site corresponds to that on the 1988 List.  Electronic copies of the 1988 List and the draft 1996 List are available at the USFWS Branch of Habitat Assessment.

Aside from the caveat of intraregional variations, the application of indicator status for any species still requires caution. The wetland indicator categories should not be equated to degrees  of wetness. Many obligate wetland species occur in permanently or semi-permanently flooded wetlands, but a number of obligates also occur in and  some are restricted to wetlands which are only temporarily or seasonally flooded.

Plant species are not precise indicators of certain levels of environmental factors. A species presence at a site depends on a complex of climatic, edaphic and biotic factors and the effect of a single factor, e.g. wetness, is impossible to isolate. Also, while an indicator status is assigned to a species as a whole, the plants we actually observe of that species may be ecotypes adapted to relatively wetter or drier conditions than “average”.  These ecotypes may or may not be distinguished morphologically. Tiner (1991) gives a detailed discussion of the indicator categories and guidelines for their interpretation.

  Indicator Categories:

  • Obligate Wetland (OBL). Almost always occurs in wetlands (estimated probability > 99%) under natural conditions
  • Facultative Wetland (FACW).  Usually occurs in wetlands (estimated probability 67% - 99%), but occasionally found in non-wetlands.
  • Facultative (FAC).  Equally likely to occur in wetlands (estimated probability 34% - 66%) or non-wetlands.
  • Facultative Upland (FACU).  Usually occur in non-wetlands (estimated probability 67% - 99%), but occasionally found in wetlands (estimated probability 1% - 33%).
  • Obligate Upland (UPL). Occur almost always (estimated probability > 99% in non-wetlands under natural conditions.

A positive (+) and negative (-) sign is used for the facultative categories. The (+) sign indicates a frequency towards the wetter end of the category (more frequently found in wetlands) and the (-) sign indicates a frequency towards the drier end of the category (less frequently found in wetlands).

A wetland indicator status in brackets [] is for a species either not included or varying from that on the 1988 List, and reflects the opinion of the author as to the probability of occurrence in wetlands of that particular species.

 

References:

Reed, Porter. 1988. National List of Plant Species That Occur in Wetlands -- North Central (Region 3).  National Wetland Inventory, U.S. Department of the Interior, Fish and Wildlife Service, St. Petersburg, FL. 90 pp. 

Tiner, Ralph. 1991. The Concept of a Hydrophyte for Wetland Indentification. Bioscience 41 (4) 236-246.

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