Unprecedented Low Levels of Genetic Variation and Inbreeding Depression in an Island Population of the Black‐Footed Rock‐Wallaby

Abstract

It has been argued that demographic and environmental factors will cause small, isolated populations to become extinct before genetic factors have a significant negative impact. Islands provide an ideal opportunity to test this hypothesis because they often support small, isolated populations that are highly vulnerable to extinction. To assess the potential negative impact of isolation and small population size, we compared levels of genetic variation and fitness in island and mainland populations of the black‐footed rock‐wallaby (Petrogale lateralis[Marsupialia: Macropodidae]). Our results indicate that the Barrow Island population of P. lateralis has unprecedented low levels of genetic variation ( He= 0.053, from 10 microsatellite loci) and suffers from inbreeding depression (reduced female fecundity, skewed sex ratio, increased levels of fluctuating asymmetry). Despite a long period of isolation (∼1600 generations) and small effective population size ( Ne∼15), demographic and environmental factors have not yet driven this population to extinction. Nevertheless, it has been affected significantly by genetic factors. It has lost most of its genetic variation and become highly inbred ( Fe= 0.91), and it exhibits reduced fitness. Because several other island populations of P. lateralis also exhibit exceptionally low levels of genetic variation, this phenomenon may be widespread. Inbreeding in these populations is at a level associated with high rates of extinction in populations of domestic and laboratory species. Genetic factors cannot then be excluded as contributing to the extinction proneness of small, isolated populations.

History and fate of a small isolated population of Weddell seals at White Island, Antarctica

“…The question of whether to prioritize genetic or demographic factors when investigating the risks facing small populations has been debated at length (Caro and Laurenson 1994;Caughley 1994;May 1995;Hedrick et al 1996;Eldridge et al 1999). However, it seems apparent that these processes do not act independently (Gilpin and Soulé 1986;Nunney and Campbell 1993;Hedrick 1996;Frankham 1997).…”

A taxonomic overview and key to the ants of Barrow Island, Western Australia

“…By way of contrast, only 52 ant species from Barrow Island (i.e., 44.4% of the total recognized) are included within the 243 species of ants identified by Gunawardene and Majer (2004) from the southern Carnarvon Basin, the latter reflecting a much more temperate fauna. This is an interesting observation in view of the fact that Barrow Island was part of the Carnarvon Basin until 8000 years ago, when it was separated by rising sea levels (Eldridge et al 1999). Five of the ants collected from Barrow Island are definitely not represented elsewhere in the Curtin Ant Collection and, of these, four are almost certainly unnamed (i.e., Rhytidoponera ?micans complex sp.…”

The identity of the Depuch Island rock-wallaby revealed through ancient DNA

“…Having undergone severe population retractions, the remaining animals are highly vulnerable to introduced predators (Kinnear et al 1988(Kinnear et al , 1998, inbreeding depression and a multitude of interacting anthropogenic factors that further threaten population viability. Offshore islands have provided a refuge for many species, and currently harbor 14 species of Australian mammals (Eldridge et al 1999) that are now extinct on the mainland. Such isolated islands can offer valuable insights into speciation and adaptive radiation and provide a refuge and conservation site for terrestrial mammal species (Maxwell et al 1996;Abbott 2000).…”

“…Among those species most severely affected have been the medium-sized species, more specifically marsupials that are in the 'critical weight range' (between 35 and 5500 g) (Johnson and Isaac 2009). As a result, combinations of environmental pressures such as introduced feral predators, and demographic and genetic stochasticity have contributed to putting isolated populations of critical-weight marsupials, such as those that exist on islands (Frankham 1997;Eldridge et al 1999), under extreme pressure.…”