Rare Plant Program
Rarity in Vascular Plants
Peggy L. Fiedler (from CNPS Inventory, 6th Edition, 2001)
Vascular plants can be rare for an astonishing variety of reasons. Broadly speaking, however, a species is rare either because it lives in a very limited habitat ("natural" rarity) or because its habitat has been converted by humans to other uses ("anthropogenic" rarity). Natural rarities, therefore, are those species that always have been rare during their evolutionary history, or currently are rare by today's standards. Anthropogenic rarities are those species that may (or may not!) have been widespread in the historic past, but through relatively recent negative interactions with human populations, are greatly fragmented, restricted to a few small, imperiled populations, or both. Each general class of rarity has its own set of consequences that must be considered for a rare species' conservation and management.
Patterns of Rarity
What does it mean to be rare? Essentially, using the word "rare" is a statement about the geographic distribution and population sizes of a particular species. Rarity, in fact, describes at least three different biological possibilities. A rare taxon can be A) broadly distributed, but never abundant where found (e.g., Cypripedium californicum); B) narrowly distributed or clumped, and abundant where found (e.g., Limnanthes bakeri); or, C) narrowly distributed or clumped, and not abundant where found (e.g., Tuctoria mucronata). Geographic distributions of rarity also can include a temporal dimension, such that a rare species is defined by its abundance, distribution, and persistence through evolutionary time (Fiedler and Ahouse 1992). Patterns of distribution and abundance that classify rarity have been described generally for animals (Mayr 1963) as well as for plants (Drury 1974, 1980), but seldom have they been applied to specific problems in biological conservation.
Consideration of such patterns of distribution and abundance, coupled with other inherent biological characteristics, allows rare species to be grouped into distinct classes. Classifications are essential to articulating what differences actually distinguish classes, in this case, of plant rarity, ultimately leading to protection efforts that can be tied explicitly to individual classes of rare species. For example, a particularly persuasive classification of natural rarities was proposed by Rabinowitz (1981), who explicitly tied habitat specificity to geography and abundance. This system identifies seven possible forms of rarity, six of which can be used to classify rarities for floristic regions, as has been done for the British Isles (Rabinowitz, Cairns, and Dillon 1986), but not yet for California. One of the limitations of this scheme, however, is that the causes of rarity (e.g., limited availability of suitable habitats) are not distinguished from the consequences of rarity (e.g., loss of rare alleles). Both cause and consequence have implications for rare plant management. The goal of conserving biological diversity will best be served if politicians, developers, and conservationists recognize that there are different types of rarity among plants, and that each may require a different form of protection and management.
Causes of Rarity
Although rare plants can be classified by different patterns of distribution and abundance, as illustrated above, this does not explain the many biological and anthropogenic factors causing these patterns. Indeed, developing an understanding into the causes of rarity for any particular taxon can be approached one of three ways (Gaston 1994). Most commonly, researchers investigate the biological and ecological parameters of an individual taxon to determine which life history traits limit a species' distribution and abundance. Pavlik's work on Amsinckia grandiflora is seminal in this regard (Pavlik 1994, 1995; Pavlik, Nickrent, and Howald 1993). A second approach is to compare the biology of rare taxa with closely related congeners. Fiedler's (1987) work on Calochortus in California, and Karron's (1987) work on Astragalus in Colorado stand out, and together provided a model empirical framework for understanding rarity in a evolutionary context. Animal ecologists recently have championed this approach (see Kunin and Gaston 1997), although translating insights from rare animal taxa to less mobile organisms, such as plants, can be both limited and inappropriate. Lastly, a third approach, less well developed, is to search for the relationship between a rare species' geographic range and/or population size and any plausible factor that might cause rarity. Gaston (1994) points out that this approach is is strictly correlative at spatial and temporal scales where experimental support cannot be sought.
Nearly a decade ago, Fiedler and Ahouse (1992) identified thirteen general categories of factors that probably contribute to rarity, including ten that were related to the biology of rare species and three that were related to human activities. Within each category, they listed up to ten specific causes of rarity. For example, the general category "Human Uses" includes the specific causes of horticultural trade, aboriginal uses, a species' role in ancient and/or modern medicine, and a species' role in past or present industry; the general category of "Stochasticity" includes both demographic and environmental causes. There are literally dozens of causes that contribute to plant rarity, but the brief discussion below emphasizes only the major genetic explanations.
In California, many vascular plant species are rare because they are either new species (i.e., "neoendemics") or old ones (i.e., "paleoendemics"). Neoendemics are found frequently in geologically youthful habitats, and often their rarity is partly a function of their youth - in some cases these plants have not had time to expand their range from their point of origin to their climatic and geological limits. Such taxa include members of the Limnanthes floccosa complex from vernal pools, and Linanthus arenicola and Oenothera californica ssp. eurekensis from the Mojave Desert. Clarkia lingulata, of the Sierra Nevada foothills, is another well-known neoendemic. In contrast, some of our most famous rare or restricted species, such as Abies bracteata, Carpenteria californica, Lyonothamnus floribundus, Pinus radiata, P. torreyana, and Sequoiadendron gigantea are paleoendemics that were once more broadly distributed, but have retreated to their current ranges in response to dramatic climatic change.
Perhaps the most significant classification of California's rare species is that of Stebbins and Major (1965), who presented an extensive analysis of California's endemic plants, many of them rarities, in the flora. Their classification is based upon taxon age, systematic position, and cytology, and includes the class paleoendemics, as well as the classes schizoendemics, patroendemics, and apoendemics. Most representatives of the last three categories are believed to be new species, although some are of moderate or even relatively old age.
Schizoendemics have more or less simultaneously diverged from a common ancestor, as have many of the rare taxa in Ceonothus sect. Cerastes such as Ceanothus roderickii and C. ophiochilus. The remaining two categories are especially important because they contain most of California's rare species. Patroendemics are diploid species of limited geographic distribution that are related to, and probably ancestral to, a more recent and widespread species. Patroendemics and their probable descendants include, among many others, the very restricted endemic Clarkia imbricata (n = 8) and its derivative polyploid C. purpurea (n = 26); Galium clematis (n = 11) and G. californicum (n = 44, 66); and Tonestus (formerly Haplopappus) eximius (n = 9) and T. peirsonii (n = 45). In contrast to patroendemics, apoendemics are defined as polyploids of limited geographic distribution that are either sympatric or parapatric to more widespread diploid (or lower degree polyploid) species, from which they are likely descended. Apoendemic pairs include the rare Dudleya saxosa ssp. saxosa (n = 68, 85) and its probable parent D. saxosa ssp. aloides (n = 17), which is also a rare plant; Lomatium repostum (n = 22) and L. lucidum (n=11); and Penstemon heterodoxus var. shastensis (n = 22) and the more widespread P. heterodoxus var. heterodoxus (n = 8). Raven and Axelrod (1978) thoroughly summarized the origin of California endemics, and Kruckeberg and Rabinowitz (1985) discuss the biology of endemic plant species in detail.
Many rare species in California and elsewhere are restricted to specific soil types, and are therefore considered "edaphic endemics." The mechanism of plant adaptation and subsequent restriction to unusual soils that generates edaphic endemics may be complex, but typically involves physiological tolerance to mineral imbalances or toxic minerals. California's serpentinite flora is well known worldwide in this regard, and it contains both common and rare species, including 300 taxa in this Inventory. Familiar serpentinite rarities include Calochortus tiburonensis, Streptanthus morrisonii, and Hesperolinon, a genus composed almost exclusively of rare serpentinite endemics. Kruckeberg's (1984) monograph on California serpentinite flora provides an excellent starting point for understanding the evolution, distribution, and management of plant species restricted to these substrates.
Darwin (1859) wrote that rarity is linked inseparably with the extinction process. Indeed, conservationist biologists have established that, all else being equal, rare species are more susceptible to the risk of extinction than widespread taxa. In California, with so many rare species and as many ideas about why any one vascular plant species might be rare, conservation biologists now agree that only infrequently does a single "cause" by itself truly explain why a species is rare. Indeed, many rare and endangered species in California that began as natural rarities have, through one form or another of human-induced detrimental changes in their populations and/or habitat, become anthropogenic rarities needing immediate protection and recovery.
It is still impossible at this time to generalize about why species are rare because rarity is - and not just from a broad perspective - an idiosyncratic biological attribute. Having said this, however, it is still vitally important for California's botanists to seek an understanding of the biology of our rare species, to know the genesis of their rarity, and to understand the current threats to rare species' persistence. Armed with such knowledge, rare plant taxa in California and elsewhere can be more appropriately managed with much better chances for long term survival. Much work lies ahead for botanists in California.
Peggy L. Fiedler is Professor in the Department of Biology, San Francisco State University, San Francisco, CA 91432.
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