Population/Occurrence Delineation and Viability Criteria
Assembling and using information on the location, extent, and condition of species populations
and ecological community stands is a principal focus of natural heritage programs. This detailed
and spatially explicit information serves as the basis for many conservation planning and natural
resource management decisions. Species and community occurrences are identified and mapped according
to standard criteria, and they are assigned condition ranks that reflect estimated viability or
probability of persistence. Consistent application of such criteria is especially important for
conservation planning that spans geopolitical boundaries.
Criteria for delineating occurrences and for assessing viability frequently are developed for
groups of taxonomically or ecologically related species or communities (e.g., a genus or family,
or a group of species that occupies a similar habitat and exhibits similar mobility). This assures
that the criteria are consistent within groups of similar species and make sense in comparison to
criteria for other groups of species with different demographic characteristics. As an example,
criteria should be similar for most darter species, but sturgeons and prairie skippers would each
have different criteria.
Delineation of populations or occurrences refers to the process of distinguishing discrete occurrences
and representing them in a mapped form. Criteria for such delineations are developed from a
rangewide perspective and used to promote standardization in how these occurrences or populations
are defined and mapped at local levels. The primary intent of these standardized criteria is to
ensure that occurrences are delineated such that: 1) for species they reflect populations or
metapopulations whenever possible so that population viability can be meaningfully assessed;
and (2) they are consistently defined and mapped.
Occurrence criteria describe the evidence required to establish a valid occurrence
(e.g., the minimum size, quality, or persistence required) and the barriers and distances or
factors that separate one occurrence from another. In the absence of a barrier, locations not
farther apart than the separation distance(s) are assumed to represent the same occurrence.
Separation distances attempt to delineate population units between which gene flow is minimal.
For most species, gene flow data do not exist; thus, decisions on separation distances are made
on the basis of best information available and by consideration of factors related to gene flow,
such as dispersal distance and spatial and temporal patterns of occurrence. In addition, consideration
of gene flow may not be practical for certain species that disperse widely (e.g., many birds), that
have very long generation times (e.g., plants characterized by long-term seed banking or
dormancy persisting clones) or that are dependent on rare but recurrent phenomena for dispersal
(e.g., floods, major storms). For these, separation distances are necessarily somewhat arbitrary
but should be based on practical considerations. See
Animal Separation and IE Distances and
Separation Distance and Mapping Guidelines.
For many plants, occurrence delineations may be based on consideration of the intervening habitat
separating observations (see
Habitat-based Plant Element Occurrence Delimitation Guidance).
For all species and communities, minimum values for separation distances have been established to
ensure that occurrences are not separated by unreasonably small distances, which would lead to the
identification of inappropriately fragmented populations or stands as potential targets for
conservation planning or action.
Population/Occurrence Viability Criteria
Population/occurrence viability criteria provide guidelines for assessing the probability of
persistence of an occurrence based on its size, condition, and landscape context. For species,
probability of persistence is referred to as viability, whereas for ecological communities it is
referred to as ecological integrity. Application of these criteria to an occurrence provides an
assessment of the likelihood that, if current conditions prevail, the occurrence will persist
for a period of time, at least 20-30 years, and sometimes longer, depending on the species or
community. Occurrence viability ranks are designed to be used in conjunction with species or
community-level conservation status assessments to help prioritize occurrences for purposes of
conservation planning or action, both locally and range-wide. The viability or ecological integrity
of occurrences is also a factor in NatureServe's assessment of the overall conservation status for
a particular species or community (see conservation status assessments).
Conservation status and occurrence viability ranks are important in setting conservation priorities,
but they are not the sole determining factors. Additional factors that may influence conservation
priorities include: the evolutionary uniqueness of the species; the co-occurrence of the species
or community with other species or communities of conservation concern at a site; the likelihood
that conservation action will be successful; and economic, political, and logistical considerations.
The number of reproductive individuals (adult population size) can be a meaningful indicator of a
population's probability of long-term persistence (exceptions include species that require currently
rare circumstances for successful reproduction). Thus adult population size often plays a role in
assessing occurrence viability. However, defining precise threshold values for population size is
problematic because of temporal variation in population size, as well as variation among survey
techniques and observers. Therefore, population size, if known, should be considered in concert
with other factors, such as quantity and quality of occupied habitat, known history of population
persistence, condition (e.g., evidence of successful reproduction for long-lived species), current
threats, and landscape context, in estimating a population's probability of persistence. For example,
population sizes of certain amphibians can be expected to decline with ongoing loss or degradation of
habitat near breeding sites (e.g., due to expanding residential development and increasingly heavy
road traffic), even if breeding sites are "protected." In these and similar cases of unavoidable
habitat loss or degradation it may be appropriate to reduce an occurrence viability rank below the
level indicated by current population data. Suggested population sizes for various occurrence
ranks are somewhat arbitrary but are based on the generalized results of population viability
analyses that suggest that occurrences with a minimum population size of at least several hundreds or
thousands of adults should exhibit relatively good to excellent long-term (e.g., 100+ years)
population persistence whereas the viability of most occurrences with fewer individuals is likely to
be fair or poor. Of course, this generalization is not without exceptions; large populations sometimes
do plummet to extirpation, whereas some small populations may persist for long periods.
Note that because occurrences of most birds, large carnivores, and other highly mobile species are
particularly hard to circumscribe, and often must be arbitrarily delimited, occurrence viability
criteria for these occurrences do not necessarily reflect estimated viability in the same way that
viability criteria do for species that occur in discrete populations.
For a more detailed description of factors considered in estimates of probability of persistence,
see Generic Element Occurrence Ranking Guidelines.