December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society cently, Bevill and Louda (1999) examined the literature and documented 71 plant variables that had been used in the study of rarity and extinction. Many of these require detailed demographic or lab-based genetics research, and are thus impractical at a scale of the American Southwest in the near-term. Also, many reviews and studies have failed to find a strong link between specific variables and rarity (e.g., Aizen et al. 2002; Lavergne et al. 2004), although other studies have found such links (e.g., Sjöström and Gross. 2006). The general consensus is that demographic and breeding system data and molecular investigations overlaid with climate envelope analyses provide the best research approaches to understanding rarity (Brigham and Schwartz 2003; Gitzendanner and Soltis1999; Kunin and Schmida 1997). Beyond these more detailed techniques, some regional surveys have found traits that are correlated with rarity, including life-form and longevity (Harper 1979; Sjöström and Gross 2006;), breeding system (Sakai et al. 2002; Sjöström and Gross 2006; Sodhi et al. 2008;), pollination ecology (Robertson et al. 1999; Sakai et al. 2002; Sodhi et al. 2008) and dispersal ecology (Bond 1994). Any ranking system should be general enough to be widely applicable, but also provide the ability to rank species from potentially low to potentially high "at-risk" status. It should also be based on what is known about the biology of rarity. I have developed a preliminary ranking system, with an emphasis on the arid and semiarid flora of the American Southwest, based on six elements. This system should be viewed as a draft attempt to provide an early warning ranking for those species that potentially may be most at-risk in the Southwest. The time-frame is the next 50 years, when anthropogenic threats and climate change are likely to push many rare plants towards extinction. The elements are rarity type, biology, population trends, number of populations (TNC element occurrences), direct anthropogenic threats, and climate change vulnerability. Each of these is discussed below, with examples and ranking criteria in tabular form. ELEMENTS OF THE RANKING SYSTEM Each element is scored from 0 (low risk) to 3 (high risk). The six elements are rarity, biology, population trends, anthropogenic threats, climate change vulnerability, and G rank (here called the N rank, see below in Table 3). The final score is called the at-risk score (ARS) for each species. Each of the elements is described below. 1. Rarity Type Table 1 lists the seven forms of rarity as defined by Rabinowitz (1981), scored either 0 or 1 for each level of the three categories. Each of the eight types is also coded from RI-RVIII. Since the three variables used in the system are subjective, they need to be defined in the local context of the American Southwest. Widespread species are defined as those that are typically found across one or more provinces (more than one subprovince) as defined by McLaughlin (2007). Thus a widespread species could occur in the Southwestern Region, Sonoran Province, and both the Sonoran and Mohavian subprovinces. Geographically local species are defined as those endemic to a single subprovince, such as the Colorado Plateau or Great Basin. Habitat specialists are defined as species that are generally restricted to one or a very few similar soils or substrates, while habitat generalists occur across a variety of substrates and soil types. Wetland species are also typically characterized as habitat specialists. Abundance at local sites (population or element occurrences) is more difficult to define, and remains subjective in my system. Many endemic species can be quite abundant locally (cf. Lesica et al. 2006; Spence unpublished data), while other species always seem to be uncommon or sparse where they are found. A species can have a final score of 0 for widespread common habitat generalists, to 3 for local sparse habitat specialists. 2. Biology Detailed studies have shown that traits most closely related to rarity include low population size, demo- Table 1. The Rabinowitz rarity matrix (Rabinowitz 1981) scored from 0-3 for the ranking system. Abundance at Sites Geographic Range Widespread (0) Local (1) Habitat Specificity Generalist (0) Specialist (1) Generalist (0) Specialist (1) Common (0) RI 0 RII 1 RIII 1 RIV 2 Sparse (1) RV 1 RVI 2 RVII 2 RVIII 3 26
Calochortiana December 2012 Number 1 graphic factors, competitive abilities, inbreeding depression, low pollen/ovule ratios and pollen limitation, or highly specialized breeding, pollination and dispersal systems (Brigham and Schwartz 2003; Byers and Meagher 1997; Cole 2003; Gitzendanner and Soltis 2000; Purdy et al. 1994; Schemske et al. 1994; Walck et al. 1999). In some cases rarity may also occur due to extreme and rapid habitat loss in formerly common species (eg., Ge et al. 1999). In the absence of data on these variables, proxies are needed that can be linked to the biological characteristics of rare species. Although the literature is somewhat ambivalent (e.g., Aizen et al. 2002; Bevill and Louda 1999; Sakai et al 2002), some studies have suggested that a variety of basic plant characteristics may be used to provide preliminary indications of possible vulnerability and general extinction probabilities. Thus self-incompatible breeders, species with highly specialized and rare pollinators, and species that depend on highly specialized dispersers are often considered to be more at risk than species with more generalized biology (Aizen et al. 2002; Bond 1994; Buchmann and Nabhan 1996; Kwak and Bekker 2006). I have developed a set of four proxy variables based on the literature, with the caveat that these are only general indicators of potential at-risk status. Detailed studies are clearly needed for most species in the Southwest in order to determine the exact causes of rarity, which are likely to be taxon (genus, species) specific (Bevill and Louda 1999). The four proxy variables (traits) are life-form, breeding system, pollination ecology, and dispersal ecology. Each of these is discussed with examples in Table 2. This element is scored for a species by selecting the single highest score among the four biological traits, rather than averaging them. Table 2. The biology element scored using four principal biological traits to characterize the vulnerability of a species to extinction. Biological Traits Ranked Explanation Score Examples Long-lived woody species Short-lived woody species or long-lived herbaceous species Short-lived perennial herbaceous species Annual or biennial Life-form and longevity Long life buffers against environmental change (>100 yrs) Some vulnerability, but generation times tend to buffer against short-term changes (>25 yrs) Vulnerable, short generation time may not be able to track environmental changes (3-25 yrs) Extremely vulnerable, with seed bank longevity a critical factor in persistence of populations Breeding System 0 Conifers, Coleogyne 1 Atriplex, Ericameria, Pediocactus 2 Astragalus, Eriogonum Penstemon 3 Cryptantha, Ipomopsis, Phacelia Autogamous Mixed mating Facultative outcrossing; some autogamy Obligate outcrossing; xenogamy, dioecy, loss of sexual reproduction, or breeding system unknown Can set seed despite small numbers of individuals through selfing Flexible system that allows for reproduction with or without pollinators, selfing occurs Generally requires pollen transfer between individuals, with low selfing rates often associated with reduced fitness Requires more than one individual, specialized pollen transfer 27 0 Many annuals 1 Many generalized insect pollinated taxa 2 Many insect pollinated taxa 3 Orchidaceae, dioecious species
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December 2012 Number 1 - Utah Native Plant Society

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