Variation in extinction risk among birds: chance or evolutionary predisposition?

Abstract

SUMMARY Collar et al. (1994) estimate that of the 9672 extant species of bird, 1111 are threatened by extinction. Here, we test whether these threatened species are simply a random sample of birds, or whether there is something about their biology that predisposes them to extinction. We ask three specific questions. First, is extinction risk randomly distributed among families ? Second, which families, if any, contain more, or less, threatened species than would be expected by chance ? Third, is variation between taxa in extinction risk associated with variation in either body size or fecundity ? Extinction risk is not randomly distributed among families. The families which contain significantly more threatened species than expected are the parrots (Psittacidae), pheasants and allies (Phasianidae), albatrosses and allies (Procellariidae), rails (Rallidae), cranes (Gruidae), cracids (Cracidae), megapodes (Megapodidae) and pigeons (Columbidae). The only family which contains significantly fewer threatened species than expected is the woodpeckers (Picidae). Extinction risk is also not distributed randomly with respect to fecundity or body size. Once phylogeny has been controlled for, increases in extinction risk are independently associated with increases in body size and decreases in fecundity. We suggest that this is because low rates of fecundity, which evolved many tens of millions of years ago, predisposed certain lineages to extinction. Low-fecundity populations take longer to recover if they are reduced to small sizes and are, therefore, more likely to go extinct if an external force causes an increase in the rate of mortality, thereby perturbing the natural balance between fecundity and mortality.

Reconsidering the Loss of Evolutionary History: How Does Non-random Extinction Prune the Tree-of-Life?

“…There does seem to be a general trend within some clades for threatened species to be overrepresented in speciespoor clades (e.g. in mammals, Purvis et al 2000b andbirds, Bennett andOwens 1997 ). In plants, patterns appear mixed.…”

“…If closely related species also tend to have close geographical proximities, perhaps refl ecting shared habitat preferences or the geographical process of speciation, they will then also be exposed to similar intensity of extinction drivers. There is an increasing weight of evidence suggesting that extinction risk is generally more clustered on a phylogeny than expected by chance (Bennett and Owens 1997 ;Purvis et al 2000a ;Schwartz and Simberloff 2001 ), a pattern also observed within the fossil record. Extinction will thus prune the tree-of-life non-randomly.…”

“…Across studies, high extinction risk is generally associated with large body size, long generation times and small geographic range sizes (Bennett and Owens 1997 ;Russell et al 1998 ;Purvis et al 2000a ;Cardillo 2003 ;Fisher and Owens 2004 ;Cooper et al 2008 ). Conversely, species at low risk of extinction are small, reproduce rapidly, and have a wide niche breadth.…”

“…population density, neonatal mass and litters per year) and extrinsic factors. Such fi ne-scaled analyses can help address whether extinctions are linked to 'bad genes' or 'bad luck' ( Raup 1993 ;Bennett and Owens 1997 ). For mammals, it appears that extinction in small bodied species is more likely a case of bad luck, driven by extrinsic factors.…”

“…Correlates of extinction risk in animal kingdom have been explored extensively using data from the IUCN Red List (Bennett and Owens 1997 ;Russell et al 1998 ;Purvis et al 2000a , b ;Cardillo 2003 ;Cooper et al 2008 ) with particular attention to mammals (Russell et al 1998 ;Cardillo et al 2005Cardillo et al , 2008Isaac et al 2007 ;Huang et al 2012 ), perhaps the best-studied higher taxonomic group. Across studies, high extinction risk is generally associated with large body size, long generation times and small geographic range sizes (Bennett and Owens 1997 ;Russell et al 1998 ;Purvis et al 2000a ;Cardillo 2003 ;Fisher and Owens 2004 ;Cooper et al 2008 ).…”