Introduction
Because EO ranks are used to represent the relative conservation value
of an EO as it currently exists, EO ranks are based solely on factors
that reflect the present status, or quality, of that EO. There are three
EO rank factors, each reflecting what is currently known (in an ideal
situation) about an EO: size, condition, and landscape context. These
factors are used as the basis for estimating the viability of an EO,
i.e., its EO rank. Thus:
Size + Condition + Landscape Context
=> Estimated Viability 
EO Rank
For community Elements, the term “viability” is used loosely,
since ecological communities are comprised of many separate species, each with
their own viability. Thus, the viability of an ecological community is considered
to be the sum of the viability or persistence of the component species
and their ecological processes. More directly, EO ranks reflect the
degree of negative anthropogenic impact to a community (i.e., the degree
to which people have directly or indirectly adversely impacted community
composition, structure, and/or function, including alteration of natural
disturbance processes). Occurrences with relatively less impacts would
generally be ranked “A”, “B”, or “C” (at
least “fair” viability), and those with significant degradation
would be ranked “D” (“poor” viability).
It is not necessary to have knowledge of each of the three rank factors
to develop EO rank specifications (especially for species). For some
Elements, information on one factor may be sufficient to rank an occurrence
(e.g., a large population size may require, and thus imply, adequate
condition and landscape context). For other Elements, information may
be scant or incomplete. In such cases, EO ranks will be based on only
one or a combination of the rank factors. The three EO rank factors
are summarized in the table below.
Factor
|
Components
|
Species
|
/Communities
|
| Size |
Area of occupancy |
|
|
| Population abundance |
|
|
| Population density |
|
|
Population fluctuation
(average population and minimum population in worst foreseeable
year)
|
|
|
| Condition |
Reproduction and health
(evidence of regular, successful eproduction; age distribution for
long-lived species; persistence of clones; vigor, evidence of disease
affecting reproduction/survival) |
|
|
| Development/maturity (stability, old-growth) |
|
|
Species composition and biological structure
(richness, evenness of species distribution, presence of exotics) |
|
|
Ecological processes
(degree of disturbance by logging, grazing; changes in hydrology
or natural fire regime) |
|
|
Abiotic physical/chemical factors
(stability of substrate, physical structure, water quality) [excluding
processes]
|
|
|
Landscape
Context |
Landscape structure and extent
(pattern, connectivity, e.g., measure of fragmentation/ patchiness,
measure of genetic connectivity) |
|
|
Condition of the surrounding landscape
(i.e., development/maturity, species composition and biological
structure, ecological processes, abiotic physical/chemical factors) |
|
|
Size
Size is a quantitative measure of the area and/or abundance of an occurrence.
Components of this factor are:
a) area of occupancy;
b) population abundance, (i.e., total count or qualitative estimate)
(for species);
c) population density (for species);
d) population fluctuation (for species).
For communities, size is equal to the area of the occurrence. For
species, the (population) size of an EO can be determined in several
ways. Most commonly, information on both population abundance and the
area of occupancy is used to calculate population size; however when
appropriate (e.g., for territorial and colonial species), population
abundance alone can be used as the EO (population) size. If population
density is to be used in determining the population size of a species
occurrence (e.g., when sampling a population in order to estimate its
size), density must be used in combination with the area of occupancy.
In addition, information on population fluctuations can be considered
when calculating EO population size, particularly for species known
to typically have high fluctuations in population.
EO size varies as a function of both natural and anthropogenic factors.
Larger EOs are generally presumed to be more valuable for conservation
purposes, all other rank factors being equal. Larger occurrences are
typically less influenced by edge effects, and less susceptible to degradation
or extirpation by stochastic events.
Condition
Condition is an integrated measure of the quality of biotic and abiotic
factors, structures, and processes within the occurrence, and the degree
to which they affect the continued existence of the EO. Components of
this factor are:
a) reproduction and health (for species);
b) development/maturity (for communities);
c) ecological processes;
d) species composition and biological structure (see next paragraph);
e) abiotic physical/chemical factors.
Although terrestrial communities are usually classified and assessed
by analyzing their vegetation composition and structure, the zoological
component of a community should be considered in determining EO rank
specifications. For example, an “A”-ranked community should
be large enough to sustain habitat-specific animal species that are
characteristic of that community (e.g., prairie chickens for prairie
communities; edge-sensitive, forest interior birds for eastern forested
communities). Note, however, that wide-ranging animal species (e.g.,
bison) may be dependent on landscape or multi-community factors (i.e.,
above the scale of the individual EO), and the rank specifications need
not consider the ability of the community to support such species. Note
also that the principle of “best EOs that are reasonably and conceivably
achievable” may preclude setting “A”-rank specifications
at levels to support all historically component species. For a particular
community EO, evidence that the fauna currently present no longer represent
the full suite of species to be expected (e.g., due to past aerial spraying
of pesticides, due to destruction of the upper soil horizons and litter
layer) may be cause to significantly lower the condition rating of that
community.
Landscape Context
Landscape context is an integrated measure of the quality of biotic
and abiotic factors, structures, and processes surrounding the occurrence,
and the degree to which they affect the continued existence of the EO.
Components of this factor are:
a) landscape structure and extent, including genetic connectivity;
b) condition of the surrounding landscape (see components of “condition”
listed above, excluding reproduction).
In terms of EO rank, genetic connectivity refers to the degree of
connectivity between different EOs. Although EOs for non-migratory species
are delineated in part by the degree of genetic connectivity between
adjacent populations (see Section 4.3, Separating EOs), there is usually
some small amount of gene flow between adjacent EOs. If, due to an EO’s
isolation, there is virtually no genetic connectivity between a particular
EO and any other EOs, the EO rank might be lowered. For example, the
EO rank for spotted owl is influenced by the degree of isolation of
a population from other populations.
Source: Draft Element Occurrence Data Standard
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