Reasons for rarity

Rarity is most commonly defined as some combination of geographic range (small) and abundance (low) that might make a species vulnerable to extinction (see Gaston and Kunin 1997). Just because a species is rare, however, does not mean it is declining. Species may be rare at three stages in their evolution: first, they may just have speciated and be in the process of expansion. Second, they may have expanded their geographic range as far as possible, but still remain rare. Third, they may be rare because their range has declined from its maximum extent (Figure 13.1).

The danger of extinction while a species is still young is illustrated by the fates of hybrid plant species that have formed in historical times (Chapter 12). The Malheur Wire Lettuce, Stephanomeria malheurensis, is a recent derivative of Stephanomeria exigua known only from a single hilltop in Oregon (Figure 13.2). The species is probably now extinct, largely due to changes in the habitat and invasion of an introduced grass, Bromus tectorum.

A contrasting story is that of the cordgrass Spartina anglica, a tetraploid formed through hybridization of Spartina maritima and Spartina alternifo-lia, and first recorded in Lymington, UK, in 1892. Both the hybrid and its parents grow in coastal mudflats. S. anglica spread rapidly following its discovery, and in a few decades had spread along much of the southern English coast and onto the French coast. It has since been introduced to other European countries as well as China and Australia to stabilize low lying coastal zones. The species cannot be considered at risk of extinction, and one factor favouring its long-term survival has been its rapid exit from the vulnerable early stage of its formation.

Fig. 13.1 Changes in geographic range during a species' lifespan, giving rise to rarity. Species are generally rare at their origination (1) and at the end of their lifespan (5), but in between may be either widespread (3) or local (2), (4).

Fig. 13.1 Changes in geographic range during a species' lifespan, giving rise to rarity. Species are generally rare at their origination (1) and at the end of their lifespan (5), but in between may be either widespread (3) or local (2), (4).

Fig. 13.2 Malheur Wire Lettuce, S. malheurensis, at its only known site in Oregon. Photo courtesy of Leslie Gottlieb.

Therefore, species that survive their initial rarity, like S. anglica, may enter a second stage in their evolution, during which they expand their range to some maximum because adaptations allow them to sustain positive rates of increase beyond the area of their birth. Dispersal is obviously required (Chapter 6) as are broad ecological tolerances (Chapter 8). Some species, so-called endemics, will,because of their site of origin or specialization, never attain a large geographic range. At least 44% of vascular plant species and 35% of vertebrates are endemic to 25 biodiversity 'hot spots' which represent only 1.4% of the land area of the globe (Myers et al. 2000). Many of these hot spots include island archipelagos, such as the Pacific islands, the Carribean, New Zealand, the East Indies, and the Indian Ocean islands. Evolutionary changes may occur among rare species that make range expansion less likely. For example, island species may evolve reductions in dispersal ability (Roff 1994; Grant 1998), such as flightlessness in birds (see Figure 13.3) and insects, and reduction in seed dispersal in plants (Chapter 6), that reduce their chances of increasing their geographic range. Rare species may also develop adaptations that favour persistence. Kunin and Schmida (1997) have identified changes to plant breeding systems and flower architecture as possible adaptations to rarity. They tested the breeding systems of 52 Israeli crucifers against their abundance and found that species with sparse populations tended to be more self-fertile. In addition, rare self-incompatible plants had unusually large flowers, which should enhance the chances of cross-pollination.

After expanding their range to some maximum, species might also become rare later in their evolutionary history through declining abundance or geographic range. The agents of decline can be either biotic or abiotic. Biotic agents of decline are extremely well illustrated by the recent effects of introduced species. The Lord Howe woodhen, Tricholimnas sylvestris (Figure 13.3), for example, is a flightless rail that lives on Lord Howe Island, about 600 km East of the Australian mainland. The island was first discovered in 1788 and settled in 1834, with the consequent introduction of

Fig. 13.3 Two Lord Howe Woodhens, T. sylvestris, a species of flightless rail restricted to Lord Howe Island in the South Pacific. Photo courtesy of Ian Hutton.

pigs, dogs, cats, goats, and black rats. By 1853, the woodhen was restricted to mountainous parts of the islands, and by 1920 the population was almost entirely restricted to one single mountain top of 25 ha, containing no more than 10 breeding territories. Miller found that the range of feral pigs did not overlap with the range of the woodhen and could have been the cause of decline through predation on nesting adults and eggs (Miller and Mullette 1985). After removal of the pigs, and a subsequent release of captive bred birds into other areas of the island soon filled the entire available habitat on the island. Declines due to introduced species illustrate the importance of co-evolutionary forces (Chapter 11) on species abundance. Many island birds, for example, are vulnerable to introduced ground predators because they have evolved adaptations in the absence of predators that become detrimental in presence of predators. These adaptations include flightless-ness, ground-nesting, and lack of escape responses (Grant 1998).

Abiotic changes will typically involve changes in climate. Many species have experienced well-documented contractions or expansions in their range during the last several thousand years as a result of changes in global temperature associated with the glacial and interglacial periods. The redwood trees Sequioadendron giganteum and Sequoia sempervirens were widespread in North America before the Pleistocene glaciations, but the former is now restricted to a few valleys in the Sierra Nevada mountains in California, and the latter to the fog belt of the coastal ranges. Aphodius holderi was the most abundant dung beetle in Britain and other parts of Eurasia during the colder parts of the last glaciation, but today is restricted to a very small area in the high plateau of Tibet (Coope 1973).

Thus, at any stage of their evolutionary lifespan species may become rare. Most will be rare at their origins, but if they can expand rapidly their long-term prospects are good. Many species, however, may not ever achieve large geographic ranges or population sizes, a contributory factor to which will be their evolved characteristics. Finally, some species will experience declines after reaching their range or abundance maximum, and one contributory factor may be the evolutionary interactions with other species. When populations are rare, what might deliver the final coup de grace?

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