Managers would be better able to prioritize species for conservation intervention if it were possible to predict, on the basis of species traits, those most at risk of extinction. With this in mind, Angermeier (1995) analyzed the traits of the 197 historically native freshwater fish in Virginia, USA, paying particular attention to the characteristics of the 17 species now extinct in Virginia and nine more considered at risk because their ranges have shrunk significantly. Of particular interest was the greater vulnerability of ecological specialists. Thus species whose niche included only one geological type (of several present in Virginia), those restricted to flowing water (as opposed to occurring in both flowing and still water) and those that included only one food category in their diet (i.e. wholly piscivorous, insectivorous, herbivorous or detritivorous as opposed to omnivorous on two or more food categories) had a higher probability of local extinction. It might be supposed that top predators would be at higher risk of extinction than species at lower trophic levels whose food supply is more stable. In a study of beetle species in experimentally fragmented forest habitat (compared to continuous forest) Davies et al. (2000) found that among species whose density declined, carnivores (10 species, reducing on average by 70%) did indeed decline more than species feeding on dead wood or other detritus (five species, reducing on average by 25%).
A pattern that has repeatedly emerged is that extinction risk tends to be highest for species with a large body size. Figure 7.9 illustrates this for Australian marsupials that have gone extinct within the last 200 years or are currently considered endangered. Some geographic regions (e.g. arid compared to mesic zone) and some taxa (e.g. potoroos, bettongs, bandicoots and bilbies) have experienced higher extinction/endangerment rates than others, but the strongest relationship is between body size and risk of extinction (Cardillo & Bromham, 2001). Recall that body size is part of a common life history syndrome (essentially r/K) that associates large size, late maturity and small reproductive allocation (see Section 4.12).
Cortes (2002) has explored the relationship between body size, age at maturity, generation time and the finite rate of population increase X (referred to in Section 4.7 as R), by generating age-structured life tables (see Chapter 4) for 41 populations of 38 species of sharks that have been studied around the world. A three-dimensional plot of X against generation time and age at maturity shows what Cortes (2002) calls a 'fast-slow' continuum, with species characterized by early age at maturity, short generation times and generally high X at the fast end of the spectrum (bottom right hand corner of Figure 7.10a). Species at the slow end of the spectrum displayed the opposite pattern (left of Figure 7.10a) and also tended to be large bodied (Figure 7.10b). Cortes (2002) further assessed the various species' ability to respond to changes in survival (due, for example, to human disturbance such as pollution or harvesting). 'Fast' sharks, such as Sphyrna tiburo, could compensate for a 10% decrease in adult or juvenile survival by increasing the birth rate. On the other hand, particular care should be taken when considering the state of generally large, slow-growing, long-lived species, such as Carcharhinus leucas. Here, even moderate reductions to adult or, especially, juvenile survival require a level of compensation in the form of fecundity or immediately post-birth survival that such species cannot provide.
I] Other species U Extinct and endangered U Extinct
0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8 Logic body weight (g)
... and to set priorities for conservation of endangered species large body size and extinction risk are often correlated
Figure 7.9 Body size frequency distribution of the Australian terrestrial marsupial fauna including 25 species that have gone extinct in the last 200 years (dark orange). Sixteen species currently considered endangered are shown in gray. (After Cardillo & Bonham, 2001.)
Skates and rays (Rajidae) provide a graphic illustration of Cortes' warning. Of the world's 230 species, only four are known to have undergone local extinctions and significant range reduction (Figure 7.11a-d). These are among the largest of their group (Figure 7.11e) and Dulvy and Reynolds (2002) propose that seven further species, each as large or larger than the locally extinct taxa, should be prioritized for careful monitoring.
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