Element Occurrences

The species data that underlie NatureServe Surveyor are called element occurrences. An element occurrence is NatureServe’s basic unit of record for documenting and delimiting the presence and extent of a species on the landscape. Element occurrences, or “EOs,” are defined as an area of land and/or water where a species is, or was, present, and which has practical conservation value. EOs for species commonly reflect populations or subpopulations.

What Gets an EO?

Our member programs generally create element occurrences for native species that are considered at-risk within their jurisdictions, with an emphasis on the most imperiled species. These include: those assessed as critically imperiled (S1) or imperiled (S2) using NatureServe’s conservation status assessments, and, depending on resources available to the program, those assessed as vulnerable (S3); those that have state/province/territory protection status; and those listed under the U.S. Endangered Species Act or the Canadian Species at Risk Act. Species that are assessed by NatureServe as critically imperiled or imperiled globally (G1 or G2) generally have EOs throughout their range because, by definition, these species are imperiled in every jurisdiction in which they occur.

Ours is the most comprehensive databas of its kind, but, even so, the data do have some taxonomic gaps. In general, the data are more complete for:

  • Vertebrates
  • Selected species of invertebrates from groups including freshwater mussels, freshwater and terrestrial snails, crayfishes, butterflies, underwing and Papaipema moths, and dragonflies and damselflies
  • Vascular plants

Member programs vary greatly in their capacity to maintain information on other invertebrate groups, non-vascular plants, fungi, and other lifeforms.

What’s in an EO?

Every element occurrence has two components: the mapped location and associated tabular information. The mapped location—that is, the polygon that delineates the boundaries of the occurrence—usually has a recorded level of mapping accuracy associated it. This can range from highly precise, where the locality is very well understood and documented, to very low precision, which is often used for recording older records that may have been quite vague as to the exact locale. Surveyor is conservative in that any overlap with your survey area—even a very small overlap with a very low-precision EO polygon—will return a result.

There are two important aspects of the tabular data for an EO:

  • The date the species was last observed at that location by natural heritage scientists or as reported to them by a credible observer
  • The viability rank, an assessment of the likelihood that that species will persist at that location based on the size and condition of the occurrence, and its landscape context. In brief, viability ranks are defined as:

A: Excellent Viability
B: Good Viability
C: Fair Viability
D: Poor Viability
E: Verified Extant
H: Historical
F: Failed to Find
X: Extirpated
U: Unrankable
NR: Not Ranked

Details about what the ranks mean and how we delineate and rank element occurrences are available from NatureServe Explorer, our free online encyclopedia of species and ecosystems data.