December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
ber of populations is not necessarily equivalent to the
number of collections of a species, especially if these
collections are from the same general area.
3. Abundance. Abundance refers to the number of
individual plants known statewide. If census data are
lacking, abundance can be approximated from the relative
dominance of a species within its area of occupied
habitat.
4. Habitat Specificity. This factor assesses the degree
to which a species is a habitat specialist restricted
to a particular soil or geologic substrate (edaphic endemics)
or vegetation type, or is a generalist found in a
wide variety of substrates or plant communities.
5. Intrinsic Rarity. Analogous to habitat specificity,
intrinsic rarity addresses those attributes of a species’
life history that may limit its establishment or persistence.
Examples include low fecundity, poor dispersal,
low seedling survival, low genetic diversity, or dependence
on specialized pollinators.
6. Magnitude and Imminence of Threats. This criterion
assesses the scope, severity, and immediacy of current
or future negative impacts on a species. Potential
threats include habitat destruction, over-collection, herbivory,
trampling or soil compaction from recreation, or
competition from invasive plants.
7. Population Trend. Trend is the change in population
size, extent, and vigor over time.
Assigning scores to each criterion
Following the model of Rabinowitz (1981), six of the
seven preceding criteria are scored using a binary rating
(high/low or increasing/decreasing). A score of 1 is assigned
to those conditions that make a species highly
vulnerable to extinction or extirpation, while a score of
0 is given for conditions that only moderately impact or
do not adversely affect a species’ persistence in the
state. The only exception is geographic range in which
three scores are possible (0, 1, or 2) to allow greater
weighting of locally endemic species. If there is insufficient
data to rate a specific criterion, or available information
is inconclusive, a value of “U” (unknown) may
be assigned. Scores for individual criteria are shown in
Table 1.
Scoring is based on a review of pertinent literature,
specimen databases, and expert knowledge and should
be supported by corroborating data. Scores can be tabulated
in a spreadsheet or in a simple data form (see Table
2 for an example).
Determining conservation priority
Once the ranking form or table is completed, the individual
scores for each of the seven ranking factors are
summed to derive both a minimum and potential (maximum)
score (Table 2). These scores can range from 0
to 8. The minimum score includes only those factors for
which information is available, with any unknowns
scored as 0. The potential score includes the same values
but with unknowns given a “worst case” score of 1.
The minimum and potential scores are then averaged
(with the sum rounded down) to derive an overall score
(Table 2).
The final score can be used to assign each species to
one of the following six conservation priority categories:
Extremely High (7 or 8 points): species at extreme
risk of extirpation across its range due to all seven of the
following conditions: limited geographic range, small
number of populations, low number of individuals, high
habitat specificity, high intrinsic rarity, high threats, and
downward population trend.
High (6 points): species at high risk of extirpation
rangewide or in the state. High priority species are
scored as vulnerable for at least six of the seven ranking
criteria.
Watch (5 points): species currently secure but vulnerable
to downward changes in status. These taxa are
scored as vulnerable for at least five of the seven ranking
criteria.
Medium (4 points): species secure rangewide but
vulnerable to extirpation in the state. Medium priority
species are scored as vulnerable for at least four of the
seven ranking criteria.
Low (0-3 points): species secure rangewide and in
the state. These species are scored as vulnerable for
three or less of the seven ranking criteria.
Need Data: insufficient data available to score species
for at least three of the seven ranking criteria. If
information were available. these species would likely
be ranked as Extremely High, High, Watch, or Medium
priority rather than Low priority.
An example of ranking a Utah species
The following example demonstrates the application
of the Wyoming protocol. Penstemon gibbensii is a narrow
endemic of extreme NE Utah (Daggett County),
adjacent NW Colorado, and SC Wyoming, earning it 2
points for geographic range. In Utah, it is known from a
single occurrence in the Browns Park area (1 point for
low number of populations) containing approximately
700 plants (1 point for low number of individuals) (Utah
Division of Wildlife Resources 1998). It is restricted to
barren white shales of the Browns Park Formation (1
point for high habitat specificity). Little is known about
the pollination biology or life history of P. gibbensii
(Heidel 2009), suggesting an “unknown” score is appropriate
for intrinsic rarity. Threats from trampling, soil
erosion, and over-collection by gardeners are high
throughout its range (1 point for threats). Trends in
Utah are unknown, although some populations in Wyoming
appear to be declining (Heidel 2009). The mini-
199