December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
A New Look at Ranking Plant Rarity for Conservation Purposes,
With an Emphasis on the Flora of the American Southwest
John R. Spence,
Glen Canyon National Recreation Area, Page, AZ
Abstract. A new rarity ranking system for prioritizing vascular plants for conservation and research is developed.
This new system, termed the “At-Risk System” (ARS), ranks species using six variables, with each variable scored
from 0-3; rarity type, biology (life-from, breeding system, pollination ecology, and dispersal ecology), population
trend, anthropogenic threats, climate change vulnerability, and number of populations. Scores can range from 0 to
18, with higher numbers indicating greater potential at-risk status. Selected species from the American Southwest are
scored using the new system. Scores range from 2 for Pinus ponderosa to 18 for the critically endangered Arctomecon
humilis. We know little about the biology and status of many rare species in the Southwest. This lack of
knowledge was incorporated by using an automatic 3 score for the variables for which data are not available, which
highlights uncertainty. For many species, the ARS score is not always strongly correlated with other ranking systems
such as its ESA status or NatureServe G rank.
The American Southwest supports one of the richest
floras in North America, with perhaps as many as 6,000
indigenous species distributed among the deserts and
mountains of the region. The area includes six major
arid and semi-arid biomes: the Chihuahuan, Colorado
Plateau, Great Basin, Mohave, and Sonoran Deserts, and
the Madrean region that extends from Mexico into
southern New Mexico and Arizona. A recent compilation
of rare species (Spence unpublished) has put the
number of G1 and G2-ranked species at ca. 700. This
preliminary list does not include the numerous local varieties
of more common and widespread species, nor
those species restricted to the Mexican portions of the
biomes. With scarce resources, relatively few field botanists,
and impending major climate change there is an
urgent need to prioritize these species for conservation
purposes. Unfortunately, very little is known about the
basic ecological and biological characteristics of many
of them. Thus, although there are ca. 260 G1 species in
the American Southwest, we know relatively little about
how to prioritize this list based on the likelihood of
near-term extinction. Any attempt to rank large numbers
of species for conservation funding must thus use a
"triage" approach, based on general biological characteristics
when more detailed quantitative data are not
available. Rarity is also an elusive characteristic that
may not be well reflected in its G rank, as it can change
through time and space, and is not easily defined or
quantified. Thus what defines rarity can vary across
different scales (Harper 1981).
There are three principal ranking systems that exist
for rare plants, two of which have been applied locally.
The California Native Plant Society ranking system has
been developed for the flora of California (California
Native Plant Society 2001). Their system includes four
lists indicating general status inside and outside of California,
combined with a subjective 3-number code indicating
local distribution, rarity and risks. These values
are based on fairly detailed information, which is often
not available for rare plants elsewhere in the Southwest.
The California 1B list of rare endemic species includes
over 1,000 taxa. The Nature Conservancy developed the
G ranking system, which was then applied to state-level
species lists throughout the West. However, the G ranking
is a rather coarse-scale tool given the large number
of species with either a G1 or G2 rank regionally. The
third system, the IUCN threat categorization (Mace
1994; Mace and Lande 1991), has not been applied to
southwestern rare plants. Other more detailed systems
have been developed (e.g., Bond 1994; Kwak and Bekker
2006) but often require detailed information on genetics
and estimates of pollen flow and fecundity. There
remains a need among field biologists, land managers
and conservation planners for a general system that can
quickly rank and prioritize species within the G1-G2-G3
levels.
A significant amount of research has been done in
attempting to correlate species extinction and rarity with
biological traits (see reviews in Brigham and Schwartz
2003; Krupnick and Kress 2005; Kunin and Gaston
1997). In a seminal paper, Rabinowitz (1981) developed
a rarity matrix that was based on geography, habitat specialization,
and local population abundance. This was
further discussed and applied by Kruckeberg and Rabinowitz
(1985). However, this matrix approach to rarity
has been little utilized in rare plant conservation planning.
Kruckeberg and Rabinowitz suggested several
avenues of research to pursue that may provide insight
into rarity, including molecular/genetic, habitat, demographic
and breeding system characteristics. More re-
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