Element Occurrence Ranks for Species & Ecosystems

An Element Occurrence Rank is used to describe the probability of persistence of the element at a particular location.

An element occurrence (EO) should have conservation value for the element as evidenced by potential continued (or historic) presence and/or regular recurrence at a given location:

• For species – the EO often corresponds with the local population, but may also be a portion of a population (e.g. long distance dispersers), or a group of nearby populations (e.g. a metapopulation)
• For ecological communities – the EO may represent a stand or patch of an ecological community, or a cluster of stands or patches of an ecological community

The rank is based on the probability of persistence or estimated viability or ecological integrity of an EO with respect to other occurrences of the same element. The rank provides an assessment of the likelihood that, if current conditions prevail, the element occurrence will persist for a defined period of time, typically 20-100 years.

EO Ranks may be used in conjunction with Conservation Status Ranks to:

• Prioritize which element occurrences should be recorded and mapped
• Help prioritize EOs for conservation planning or action, both locally and rangewide

Ranks are assigned by considering the following information about each element occurrence:

• Condition
• Size
• Landscape context

For more information about how EO ranks are assigned:

• Review the NatureServe - Element Occurrence Data Standard (PDF)

An uncertainty qualifier (?) or range rank (e.g. CD) can be used to express uncertainty about a particular EO rank. Where not enough current survey information on the EO rank factors is available to estimate viability or ecological integrity of the element occurrence (i.e., assign an A, B, C, or D rank), the E, H, F, or X ranks should be used.

Note that the “?” qualifier is not used with the E (extant) rank; if there is uncertainty whether an EO is extant, then either a U = unrankable or an NR = not ranked is assigned.