Patterns of Nestedness in Remote Polynesian Ant Faunas (Hymenoptera: Formicidae)

The entire ant faunas of remote Polynesian islands consist of introduced species. An important question concerning the assembly of Pacific island ant faunas is whether these species are a random assortment of the available species pool, or whether they exhibit highly ordered occurrence patterns (i.e., nested subsets of species). I evaluated nestedness for the ant faunas of two island groups in remote Polynesia: (1) the Hawaiian Islands, and (2) French Polynesia and the Cook Islands. Wilcoxon two-sample tests were used to analyze nestedness patterns for individual species and islands; the degree of nestedness for species assemblages and archipelagos was determined by combining tail probabilities of individual species and islands. Both island groups revealed highly significant nestedness at the level of the assemblage (a per-species approach) as well as the archipelago (a per-island approach). Considered individually, most species (73–95%) and most islands (89–100%) demonstrated significant nestedness. Instances of nonsignificant nestedness were frequently associated with low statistical power. These results reveal a strong deterministic element in the assemblage of remote Polynesian ant faunas. Dispersal opportunities along with presence of appropriate habitat type are likely the most important mechanisms underlying the observed patterns. The entire ant faunas of the more remote Polynesian islands (east of Rotuma, Samoa, and Tonga) consist of introduced species. Although most introductions have occurred within the last 200 yr, some introductions may date to the ancient Polynesians (Wilson and Taylor 1967a). These islands represent rare opportunities to study patterns of community organization among species that share no common coevolutionary history in the region. A collection of early survey records suggested that the ant faunas of these islands were depauperate, and that strong interspecific aggression resulted in complementary distribution patterns (Wilson and Taylor 1967b); this remained the conventional wisdom for many years (Hölldobler and Wilson 1990). A more comprehensive reevaluation based on more thorough collections revealed more species were present than previously thought and found no evidence of mutually exclusive distribution patterns among islands (Morrison 1996a). Ant communities examined on the island of Moorea revealed patterns of structure and function similar to those found in more speciose, coevolved mainland communities (Morrison 1996b). Here I examine whether the ant species occurring on remote Polynesian islands are a random assortment of the available species pool, or whether they exhibit highly ordered occurrence patterns (i.e., nestedness or nested subsets of species). Nestedness occurs when the species composition of each island represents a subset of the species assemblage on more speciose islands. A high degree of nestedness implies that the geographic distribution of species is a deterministic (and predictable) function of physical, biological, or anthropogenic processes, and this has Pacific Science (2008), vol. 62, no. 1:117–127 : 2008 by University of Hawai‘i Press All rights reserved 1 Manuscript accepted 20 February 2007. 2 Department of Biology, Missouri State University, 901 South National Avenue, Springfield, Missouri 65897 (phone: 417-836-3119; fax: 417-836-8886; e-mail: [email protected]). important implications for biogeography, evolutionary ecology, and conservation (e.g., Patterson and Atmar 1986, Patterson 1987, Simberloff and Martin 1991, Cook 1995, Cook and Quinn 1995, Kadmon 1995, Hecnar and M’Closkey 1997, Wright et al. 1998, Hecnar et al. 2002). The following questions are posed: (1) Are the introduced ant faunas of remote Polynesian islands significantly nested at the level of species assemblages and archipelagos? (2) Which individual species or islands contribute to this nestedness, and which do not? and (3) What are the potential mechanisms of the observed nestedness? materials and methods