Nestedness of Southern Ocean island biotas: ecological perspectives on a biogeographical conundrum

Abstract

Aim To use patterns of nestedness in the indigenous and non-indigenous biotas of the Southern Ocean islands to determine the influence of dispersal ability on biogeographical patterns, and the importance of accounting for variation in dispersal ability in their subsequent interpretation, especially in the context of the Insulantarctic and multi-regional hypotheses proposed to explain the biogeography of these islands.Location Southern Ocean islands.Methods Nestedness was determined using a new metric, d1 (a modification of discrepancy), for the indigenous and introduced seabirds, land birds, insects and vascular plants of 26 Southern Ocean islands. To assess the possible confounding effects of spatial autocorrelation on the results, islands were assigned to 11 major island groups and each group was treated as a single island in a following analysis. In addition, nestedness of the six Southern Ocean islands comprising the South Pacific Province (New Zealand islands) was analysed. All analyses were conducted for species and genera, for each of the taxa on its own, and for the complete data sets.Results Statistically significant nestedness was found in all of the taxa examined, with nestedness declining in the order seabirds > land birds > vascular plants > insects for the indigenous species. Vagility had a marked influence on nestedness and the biogeographical patterns shown by the indigenous species. This influence was borne out by additional analyses of marine taxa and smallsized terrestrial species, both of which were more nested than the most nested group examined here, the seabirds. Assemblages of non-indigenous species also showed nestedness, and nestedness was generally more pronounced than in the indigenous species. Surprisingly, vagility had a significant effect on nestedness in these assemblages too.Main conclusions Nestedness analyses provide a quantitative means of comparing biogeographical patterns for groups differing in vagility. These comparisons revealed that vagility has a considerable influence on biogeographical patterns and should be taken into account in analyses. Here, investigations of more vagile taxa support hypotheses for a single origin of the Southern Ocean island biota (the Insulantarctica scenario), whilst those of less mobile taxa support the more commonly held, multi-regional hypothesis. All biogeographical analyses across the Southern Ocean (and elsewhere) will be influenced by the effects of dispersal ability, with composite analyses dominated by sedentary groups likely to favour multi-regional scenarios, and those dominated by mobile groups favouring single origins. Mechanisms underlying nestedness in the region range from nested physiological tolerances in more mobile groups to colonization ability and patterns of speciation in less vagile taxa. Journal of Biogeography (J. Biogeogr.) (2005) 32, 155-168 ª 2005 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 155

Spatial and temporal variability across life's hierarchies in the terrestrial Antarctic

“…Moreover, patterns in species richness and the mechanisms underlying these patterns have been comprehensively investigated Greve et al 2005). Across the islands, species richness of indigenous and alien vascular plants covaries significantly with available energy and the same is true of indigenous and alien insect species richness.…”

“…For isolated islands such as those of the Southern Ocean, and in indigenous taxa such as vascular plants and insects, immigration has probably been low owing to the considerable distance of the islands from source areas , but see also Greenslade et al 1999). Moreover, high endemicity (Chown 1990a;Greve et al 2005) indicates substantial in situ speciation. Little data exist for emigration and extinction (but see Convey et al (2000b) for examples of proposed short-term colonization and then extinction associated with ephemeral geothermally active locations on the maritime Antarctic South Sandwich Islands).…”

“…The islands also differ markedly in their glaciological and geological histories, including the extent of current glaciation (LeMasurier & Thomson 1990;Hall 2002). Again, this multi-scale spatial variation in the environment has profoundly affected the distribution of diversity on the islands Greve et al 2005).…”

“…In islands close to continental or other landmasses genera and sometimes species are shared with what appears to be the source areas (Kuschel 1971;Vogel 1985;Marris 2000), while in more distant archipelagos, endemism is high and relationships are enigmatic. Indeed, the origin and closest relatives of many of the insect taxa from the South Indian Ocean Province Islands (Prince Edward Islands, Crozet Islands, Kerguelen Islands, Heard and McDonald Islands) remain a subject of vigorous contention, as they have been since the discovery of these species (see Jeannel 1964 for early discussion and Greve et al 2005 for recent partial review). Although regional species pools and biogeographic history might determine what species can potentially occur at a given site, actual occurrence is determined by site suitability.…”

Nestedness analysis as a tool to identify ecological gradients

“…That means that smaller fragments selectively lose species that are habitat specialists with low abundance; these same species have a better chance of persistence in larger and/or less isolated fragments. Blake (1991), Greve et al (2005) and Driscoll (2008) pointed to differences in environmental tolerance as a cause of nestedness. Environmental tolerance implies a gradient Box 1.…”