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Rarity To determine what species or community is in dire need of conservation, we need to understand what rarity is.
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Rarity To determine what species or community is in dire need of conservation, we need to understand what rarity is. Deborah Rabinowitz (1981, 1986) described a general pattern of species abundances, in which there were 7 different ways that species could be rare. What follows is a reconstruction of the table she developed:
Geographic range LargeSmall Somewhere Common Locally Locally Locally large abundant over abundant in abundant in a large range several specific in a specific habitats, but habitat, but Population habitat type restricted restricted Size geographically geographically Everywhere Constantly Constantly Constantly Constantly small sparse over sparse in a sparse and sparse and a large specific geographically geographically range and in habitat, but restricted in restricted in several over a large several a specific habitats range habitats habitat _______ ________ ________ ________ BroadRestricted BroadRestricted Habitat Specificity
Rabinowitz and her colleagues checked the patterns of distribution and local abundance for the species sufficiently well described in the flora of the British Isles. From maps of individual species, it was possible to determine which species had large geographic ranges and which small. From descriptions of habitats where species had been collected, fellow scientists were asked to decide whether the species were habitat specialists or generalists, as well as whether species were at least somewhere locally abundant.
Descriptions for some species make the categories clear: • Marshes, fens, heaths, woodlands, and waste ground. A common weed of arable land. – this is a description clearly for a habitat generalist, at least some places locally abundant. • Restricted to soil-filled crevices in scree slopes. – this is clearly the description of a habitat specialist. • Scarce where present. Occurs as widely scattered individuals. – this is equally clearly the description of a species that is everywhere scarce.
Rabinowitz worked by consensus. A statistical test indicates the three 'traits' are statistically independent. Think about that one! Common sense ecology would likely suggest that habitat generalists ought to be widespread in distribution, but there are habitat generalists with narrow (small) range. On the next slide is a table of her results.
Geographic range LargeSmall Somewhere large 58 71 6 14 Population Size Everywhere 2 6 0 3 small _____ ______ ______ ______ Broad Restricted Broad Restricted Habitat Specificity Species rarity could occur in three different ways: 1) restricted geographic distribution; 2) narrow habitat distribution; 3) low local population abundance.
Of the British flora analyzed, 39% had no component of rarity (i.e. they were not rare in any way), while among the 61% that were rare in one or more ways: 59% had narrow habitat specificity; 15% had small geographic range; 7% had low population size.
There has been at least one other analysis using this scheme. Pitman et al. (1999) analyzed trees in Peruvian Amazon forest, and set 1 ind./ha as the abundance dividing line, 1 vs. 2 or more forest type occurrences for habitat breadth, and a political boundary (Madre de Dios) for geographic distribution. None of the trees were geographically confined (no small geographic ranges); only 13% were locally rare. The rarest trees occurred at 1 stem per 36 ha; over the forest this 'rare' species would encompass 200,000 stems!
One important consequence of this result is, however, that conservation of truly rare species will require preservation of a lot of land (Ricklefs 2000). Much of the difference between Rabinowitz’s and Pittman’s results may relate to the spatial scale used. Consideration of spatial scale is obligatory when assessing rarity. The lack of apparent habitat specialization by these tropical trees runs in contrast to prevailing views. They also stand in contrast to Rapaport's Rule that high tropical diversity is associated with habitat specialists and small geographic ranges.
Why is the separation of forms of rarity important to conservation biology? Think about how the limited resources available for conservation should be spent. Whole countries identified as centers of diversity cannot be wholly conserved. Within such countries, what should be protected? Clearly, the best approach is to preserve habitats and ecosystems. But which ones? If information about rarity is available (or can be developed) for species of various sorts (not just plants), then the form of rarity can be very useful.
Species rare due to limited geographic distribution, but with fairly generalized habitat requirements and locally abundant are good candidates for introduction into areas outside their current distribution. Species with wide distributions and high local abundance, but narrow habitat requirements, probably cannot be successfully moved. Species that are locally sparse, but widely distributed and generalized in habitat requirements probably need no protection. In sum, species restricted to narrow niche characteristics in two or more of the three traits are those most likely to need help.
Kunin and Gaston (1993), in a review of rare and common species, found that rare species (i.e. locally rare and geographically restricted) differ from more common species. Rare species (plants) have lower levels of self-incompatability, a greater tendency to asexual reproduction, lower overall reproductive effort and poorer dispersal abilities. In a sense, they tended to make the best of a bad situation.
Groups such as the World Conservation Union (IUCN) use indicators to determine which species are rare and at risk of extinction. Typical indicators used for this purpose include: 1) rarity; 2) rate of decline (high rates being bad); 3) degree of population fragmentation. In the IUCN model, species are considered critically endangered if the extent of occurrence is <10km2, endangered if they occupy <500km2, and vulnerable if they occupy <20,000km2 .
Rate of decline is measured by assessing population size over at least 2 points in time. Four possible measures are possible using only range size and population number (declines in range but not population number, declines in population number but not range, both, neither). Scale has been shown to make a difference in these results. The IUCN method considers population structure with respect to dispersal potential (i.e. fragmentation extent). However, severe fragmentation (many small, isolated populations) and lack of fragmentation (a single or few subpopulations) can be considered as indicative of increased risk of extinction.
References (Rarity) Harte, J. and E. Hoffman. 1989. Possible effects of acidic deposition on a Rocky Mountain population of the tiger salamander Ambystoma tigrinum. Conservation Biology 3:149-158. Hartley, S. and W. Kunin. 2003. Scale dependency of rarity, extinction risk, and conservation priority. Conservation Biology 17: 1559-1570. Kunin, W.E. and K.J. Gaston. 1993. The biology of rarity: patterns, causes and consequences. TREE 8:298-301. Pitman, N.C.A. et al. 1999. Tree species distributions in an upper Amazonian forest. Ecology 80:2651-2661. Rabinowitz, D. 1981. Seven forms of rarity. pp. 205-217 in The Biological aspects of rare plant conservation. Ed. by H. Synge. Wiley. Rabinowitz, D., S. Cairns and T. Dillon. 1986. Seven forms of rarity and their frequency in the flora of the British Isles. In M.E. Soule; (ed.) Conservation Biology: The Science of Scarcity and Diversity. pp. 182-204. Sinauer. Ricklefs, R.E. 2000. Rarity and diversity in Amazonian forest trees. TREE 15:83-84.
