Sarah Marcinko
Classic Paper
710871992
Pattern, process, and consequences of rarity
What causes rarity? The answer to that question, or at least the endeavor to answer it, has
been analyzed and debated since Darwin (1859) who suggested that rarity was a compulsory
precursor to extinction. Contrary to popular belief, rarity is the norm rather than the exception
(Preston 1948), yet identifying why some species are common and others are not has been an
ongoing challenge. While most agree that rarity in vascular plants may be for a variety of
reasons, a universal consensus regarding their evolution and persistence has not been reached.
This has prompted scientists to search for generalities in explaining both the patterns and
processes behind rare species distributions. Some of these earlier efforts to establish general
principles have been fundamental to shaping our present understanding of rarity.
Griggs (1940) was perhaps the first to place plant rarity into an ecological context,
acknowledging that plants do not statically interact with their surrounding environment. He also
observed a geographic component to rare species distributions: some occur only in very
restricted sites while others are more widespread, but sparsely populated. Like others before him
(Fernald 1918), Griggs speculated the former were relicts that had previously covered a larger
area, but had become “slowly dying vestiges of races once widespread” (p 575), or were recent
arrivals on the evolutionary scene and had not yet expanded their ranges (Willis 1922). In
addition, he proposed that “habitat choice” or specificity was an ecological determinant of rare
species occurrence and that certain types of habitats (e.g. rock outcrops, oceanic islands, and
river margins) consistently harbored rare plant assemblages.
The most resounding theory produced from this seminal paper was the notion that many
plants are rare because they are poor competitors, predominantly existing in early successional or
“ecologically young” environments where competitive pressures are low. Griggs, however,
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could not account for the irregularities in habitat preferences. Why, for example, does a typically
acid loving plant such as Pinus resinosa occur on limestone barrens in Canada (Griggs 1940)?
These idiosyncrasies have been reported for many rare taxa and continue to be a stumbling block
for more contemporary generalizations on rarity. Nevertheless, Griggs’ first attempt to explain
both the patterns and causes of rarity initiated a groundswell of theoretical considerations.
Drawing on genetic theory, Stebbins (1942) purported that rare species had depauperate
gene pools and categorized plants as once widespread and genetically diverse or never common,
having diverged from a small group of individuals. The notion that rarity is correlated with
genetic impoverishment has gained momentum (Drury 1974, Lande 1988, Hamrick & Godt.
1989, Falk and Holsinger 1991), but remains a controversial topic (Bowen 1999). Trying to
advance beyond single-factor explanations, Stebbins (1980) later put forward a synthetic
approach. According to his gene pool-niche interaction theory, the conundrum of why certain
areas harbor narrowly endemic taxa as well as their widespread congeners can be elucidated by a
combination of genetic and ecological attributes. For example, temperature, climate, and
edaphic factors can influence distribution patterns, but may also interact with genetic structure to
determine a species’ reproductive potential (e.g. dispersal ability).
Whereas earlier attempts primarily focused on reasons for rarity, Drury (1974) pursued
its meaning. What exactly does it mean to be rare? He reasoned that if rarity is measured solely
in terms of numbers, then most species of plants (and animals) are rare. Instead, he deliberated
on the biological attributes of rare taxa, suggesting that there are several ‘kinds’ of rarity each of
which is a function of individual evolutionary histories, selective pressures, and habitat
characteristics. Drury distinguished species based on adaptive strategies: more common taxa
that can compete in biologically favorable habitats and widely separated species that have
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evolved to exploit extreme habitats at the cost of being a good competitor. Furthermore, he
indicated that rare taxa might have developed reproductive strategies (e.g. variation in breeding
systems) in order to survive in a stressed and isolated environment. Consequently, Drury
proposed that an operational definition of rarity should include the requirement that species
occur in small, disjunct populations to prevent interbreeding between sub-populations, or are
constrained to a single population.
Our current definition of rarity, however, has largely been attributed to Rabinowitz
(1981) who identified seven forms of rarity based on dichotomies of distribution, habitat
specificity, and abundance (Figure 1). This categorization illustrates the divergence from a rigid
and “monolithic” state of rarity to a more flexible and informative schema. Kruckeberg and
Rabinowitz later (1985) limit their examination to the ecological and evolutionary attributes of a
single form of rarity—narrow endemics, or constantly sparse and geographically restricted taxa.
Like Griggs (1940), Rabinowitz was puzzled by sparse species, particularly those that do not
seem to have a favored habitat and occur over a large range, but in low abundances. However, in
contrast to earlier suppositions made by Griggs (1940), Rabinowitz and Rapp (1981) found rare
prairie grasses to be competitively superior to common grasses. Therefore, if sparse species are
competitively superior, then their competitive abilities cannot be the reason for their low
abundance. This ambiguity between causes and consequences continues to hinder more precise
definitions of rarity (Fiedler & Ahouse 1992).
Figure 1. A typology of rare species based on three characteristics: geographic range, habitat selection,
and local population size. From Rabinowitz (1981).
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Harper (1981) conceded the concept of rarity is embedded as much in time as it is in
space and its definition depends largely on our scale of observation. A species may, for
example, seem imperiled on human time scales, but relatively stable on longer ecological ones.
Furthermore, plant populations and the availability of their irrespective habitats fluctuate for a
variety of reasons including random events, species interactions, dispersal capabilities, as well as
limiting resources, all of which can obscure any generalizable patterns.
More recent attempts to explain rarity, however, have typically followed the
classification scheme first proposed by Rabinowitz (1981), but have attempted to break from the
limits imposed by a dichotomous framework. Schoener (1987) characterized species along a
rarity continuum as either diffuse or suffuse, depending on whether a species is sometimes rare
in certain parts of its range and common in others or rare throughout. While the latter is
exercised more commonly in describing rarity, Schoener argues that the former, a “pseudo” or
marginal type of rarity, occurs with greater frequency. Interestingly, the difference between the
two has implications for reserve design, suggesting that a marginal species is best protected by
conserving its center of distribution whereas multiple sites are required for a species that never
abundant where found.
Fiedler and Ahouse (1992) also found the framework proposed by Rabinowitz to be
limiting and called attention to a three dimensional space of abundance, distribution, and time, or
taxon persistence (Figure 2a & 2b). Under this view, species are distinguished along a
continuous gradient of short to long persistence time and narrow to wide geographic distribution.
It is important to note that both parameters are sensitive to the biology of the species in question.
There is little utility, for example, in comparing long-lived, dispersal limited species to shortlived ones that disperse more quickly when categorizing taxa.
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(b)
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(a)
Figure 2: (a) Rarity is described by a combination of abundance and distribution values, from very narrow distributions
with high abundance to very broad distributions composed of only a few individuals. (b) Three primary dimensions are
used to categorize taxa. From Fiedler & Ahouse (1992).
Despite the large amount of research and conceptual models produced, our understanding
is incomplete and novel explanations for rarity are still emerging. Several studies have
elucidated the association between rarity and reproductive biology (Weller 1994; Kunin &
Schmida 1997) while Kilronomos (2002) empirically demonstrated that rare plant densities may
be affected by the accumulation of species-specific soil pathogens, suggesting that plants differ
in their ability to manipulate their abundance by altering soil community structure. The most
complete synthesis on rarity is credited to Gaston (1994), who compiled over a century’s worth
of research and discourse.
Even so, causes of rarity are generally divided into those that are intrinsically rare and
those that are rare as a result of negative interactions with humans (Weller 1994). The recent
arrival of the global extinction crisis and widespread habitat fragmentation has prompted the
conservation of biodiversity in general and rare species in particular. Thus, it seems the need to
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understand the factors underlying rarity is more urgent than ever. Yet, given that species follow
different routes to rarity, how do we best conserve rare taxa in the face of uncertainty?
Rabinowitz (1981) argued “a great amount of fascinating heterogeneity among rare
species is unfortunately obscured by generalities” (p 206). Many maintain the conservation of
rare and endangered plants requires individualized field studies for a given taxa and a large
number of empirical analyses have followed. Numerous studies have examined the ecological
and demographic factors affecting the population dynamics of rare taxa (Gawler et al. 1987;
Menges 1990) while others have looked at comparisons between rare plants and their more
widespread congeners (McIntyre 1997; Bevill & Louda 1999). Finally, Lozano & Schwartz
(2005) address the need for prioritization in rare plant conservation and determined that rarity is
concentrated in species rich taxonomic groups. This suggests that rare species distributions can
be predicted based on phylogenetic and geographic parameters.
Without a doubt, our knowledge of the patterns and processes underlying rarity has
progressed considerably since Griggs (1940) and others, but considering the uncertainty and lack
of focus, we must ascertain if we are any closer today to understanding rarity than we were
nearly 100 years ago. Do general principles exist that predict rarity? Or, is our understanding of
rarity reduced to a series of individual case histories and biological attributes? Clearly, the
debate is far from over and future research will be critical to the management of rare and
endangered taxa during a time of unprecedented rates of species loss.
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