Continuing our discussion of the meanings and causes of rarity in plants, I’d like to briefly explore some of the historical concepts of rarity.

Especially in the past 80 years, the restriction of plant species to narrow geographic ranges and the discontinuity between plant communities has been the subject of considerable attention. One of the reasons for this interest is the belief that rare species can offer insight into the evolution of species, telling us about their origin, age, and rate of speciation.

There has been a great deal of speculation concerning the relative significance of environmental factors and their effects on the distribution of plant species. On a continental scale, moisture availability, length of the growing season, and the number of frost-free days exert a strong influence over the distribution of broadly defined vegetation types. This is the first filter leading to isolation, which in turn leads to speciation and specialization. While these broad vegetation classes might conform neatly to such generalizations, the more subtle variations in vegetation on a smaller scale are far less clear-cut.

In his “”age and area”” hypothesis, Willis (1922) proposed that species are rare because they have recently evolved and haven’t had time to expand their range. Gleason (1924) pointed out that this hypothesis falsely assumes that all species migrate at the same rate. Fernald (1924) proposed his theory of “”relictual endemism,”” suggesting that narrow endemics are old taxa that had once been more widespread. He held that as a taxon gets older, it becomes senescent and loses its competitive ability.

Griggs (1940) also related rarity in plants to a lack of competitive ability. He described rare species as those that cannot compete successfully with more common, widespread species and find their habitats in the early stages of succession. Stebbins (1942) agreed, but did not support the notion that rare species are restricted to pioneer habitats and are “”dying out.”” Instead, Stebbins suggested that rare species are genetically depauperate, that is, lacking heterogeneity, and that this makes them poor competitors. In their landmark paper on Clarkia franciscana, Lewis and Raven (1958) observed repeated extinction and recolonization events by closely related species. With their theory on “”saltational speciation,”” they proposed that if such events were repeated often enough, the result would be speciation and the formation of highly endemic populations. Lewis (1962) further suggested that sudden, catastrophic alterations in climate could similarly drive selection and lead to the development of endemic species, especially among populations on the margin of a taxon’s range.

As more and more work was done along these lines, it became evident that all these hypotheses offered only partial explanations for rarity. Because natural systems involve complex interactions between highly variable environmental conditions and the genetic history of individual organisms, the usefulness of such generalized concepts is limited. This led to the development of a more holistic view of biological associations and distributional patterns in the 1970s. Stebbins (1980) proposed a “”synthetic approach”” when looking at the history of rare taxa. In his gene pool-niche interaction theory, he attempted to synthesize the effects of a unique discontinuous habitat, the evolutionary history of a population, and the genetic make-up of the population. Fiedler (1985) expanded Stebbins’ line of thought, suggesting that rarity is the product of the age and genotype of a taxon, its evolutionary history, taxonomic position, ecology, populational and reproductive biology, land use history, and recent human uses.

Other factors have had a pronounced effect on our perception of biological rarity. Harper (1981) described rarity as a phenomenon in both time and space, and given the patchiness of all natural environments, suggested that rarity has different meanings to different people. Harper claimed that a major problem concerning rarity results from taxonomic bias, something I alluded to in our last newsletter. Human subjectivity, then, must also be taken into consideration when attempting to account for rarity. In this way, the subject of rarity has become increasingly complex, perhaps more accurately reflecting the dynamic interactions that are the reality of natural systems.

References

Fernald, M.L. 1924. Isolation and endemism in northeastern America and their relation to the age and area hypothesis. Am. J. Bot. 11:558-572.
Fiedler, P.L. 1985. Heavy metal accumulation and the nature of edaphic endemism in the genus Calochorus (Liliaceae). Am. J. Bot. 72:1712-1718.
Gleason, H.A. 1924. Age and area from the viewpoint of phytogeography. Am. J. Bot. 11:541-546.
Griggs, R.F. 1940. The ecology of rare plants. Bull. Tor. Bot. Club 67-575-594.
Harper, J.L. 1981. The meanings of rarity. In H. Synge (ed.). The biological aspects of rare plant conservation. John Wiley and Sons, New York, pp. 189-203.
Lewis, H. 1962. Catastrophic selection as a factor in speciation. Evolution 16:257-271.
Lewis, H. and P.H. Raven. 1958. Rapid evolution in Clarkia. Evolution 12:319-336.
Stebbins, G.L. 1942. The genetic approach to problems of rare and endemic species. Madroño 6:241-258.
Stebbins, G.L. 1980. Rarity and plant species: a synthetic viewpoint. Rhodora 82:77-86.
Willis, J.C. 1922. Age and area. Cambridge Univ. Press, Cambridge, England.