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Since Baker & Stebbins’s (1965) influential book on the genetics of colonizing species, numerous attempts have been made to generalize traits of good colonists (Sailer, 1978; Parsons, 1983; Simberloff, 1986; Cronk & Fuller, 1995; Williamson, 1996), in particular those on islands (Whittaker, 1998). Successful colonization and establishment on islands for any group of organisms will be influenced by their life histories and reproductive systems. For instance, many invasive plants exhibit self-compatibility, which assures fertilization after long-distance dispersal (e.g. Baker, 1955; Cronk & Fuller, 1995). Some island floras (e.g. Hawaii) have none the less higher than average proportions of dioecy, perhaps a result of successful long-term establishment due to the apparent lack of detrimental inbreeding in immigrant dioecious plants (Whittaker, 1998).
Studies of differential colonization abilities for animals with different reproductive systems have not yet received the same attention. However, introduced insects in the continental United States are clearly over-represented by the insect orders Hymenoptera, Thysanoptera and Homoptera (Simberloff, 1986). The two first groups, and the majority of all introduced Homoptera (scale insects and white-flies), are haplodiploid (Normark et al., 1999) where females are diploid and haploid males result from unfertilized eggs. Many haplodiploid species have strongly skewed offspring sex-ratios and practise extreme inbreeding (Wrensch & Ebbert, 1993), a definite advantage to colonizing organisms. It is generally believed that such organisms do not go through severe inbreeding depression (when close relatives are the only alternative for mating) during colonization because haplodiploidy, and especially regular inbreeding, reduces the potential for inbreeding depression by having effectively eliminated deleterious alleles early in the history of a lineage. That sib-mating species mate before dispersal also makes them independent of mate finding at new sites (Kirkendall, 1993). Population size is thus probably less relevant to the survival of inbreeding colonizing lineages as compared to outbreeding equivalents.
Wood-boring and bark-boring beetles (hereafter referred to as wood-boring for convenience) in the curculionid subfamily Scolytinae are one of only two groups of beetles in which haplodiploidy is known (the other being the bizarre monotypic family Micromalthidae, which also breeds in dead wood) (Mable & Otto, 1998). All haplodiploid scolytines are regular inbreeders (Kirkendall, 1993; Normark et al., 1999) and the haplodiploid genetic system enables females to control their offspring sex-ratios perfectly (Hamilton, 1967). Although other genetic systems are also known from the seven independent origins of regular inbreeding in Scolytinae (e.g. paternal genome elimination [Brun et al., 1995]), the largest clade of inbreeding species is strictly haplodiploid and includes the tribe Xyleborini and the inbreeding genera in the tribe Dryocoetini (Normark et al., 1999). When females of these species have not been fertilized by their brothers, they can produce haploid sons parthenogenetically, mate with the first-born and eat those remaining (Büchner, 1961). Hence, colonizing females of these taxa do not have to be inseminated before dispersal to ensure successful colonization.
Throughout tropical and temperate forests, scolytine beetles dominate the guild of wood-boring beetles that attack recently dead or dying trees. In the tropics this guild also includes beetles of the related family Platypodidae (Browne, 1961; Atkinson & Equihua-Martinez, 1986). Almost all species of Platypodidae and more than half of the tropical species of Scolytinae [including the entire species-rich tribe Xyleborini, and some smaller groups (Beaver, 1989a)] tunnel deep into wood and culture mutualistic yeast-like fungi for larval food (ambrosia beetles). Most of the remaining scolytine species breed in and feed upon phloem (true bark beetles), the pith of twigs, or in seeds. Other wood-boring beetle families contribute much less to the guild, or have very different ecologies, and will not be mentioned further in this paper.
Inbreeding scolytines are significantly more abundant (in terms of species) in tropical latitudes than outside the tropics, and for the Cryphalini and ambrosia beetles the proportions of inbreeding species are significantly higher on small tropical islands than on large islands and mainland regions at similar latitudes (Kirkendall, 1993: his Table 7.2 and Fig. 7.1). Here, we add more data and extend the analyses of tropical incest further, by examining the effects of area, isolation and of island elevation on species numbers. Are there any consistent differences between inbreeders and outbreeders for any of the geographical variables listed? If so, could this be due to different properties associated with different mating systems? To examine the possibility that inbreeding has evolved on the islands themselves, we investigated patterns of endemism for inbreeders and outbreeders. The group with the most widespread species should have the highest colonization potential of the two and perhaps indicate less intra-island speciation. We also examine whether widespread outbreeders on islands are disproportionately associated with plants used as crop cultivars, implying dispersal by human commerce.
Figure 7. Proportions of inbreeding (solid dots) and outbreeding (open dots) species that are endemic, occur on neighbouring islands (less than 1000 km apart), occur in a larger region (e.g. the Pacific) or are widespread (at least Africa + Asia). The eight islands were selected arbitrarily among those having four or more outbreeding species and with a recent review of the wood-boring beetle fauna. Proportions are reported for inbreeding and outbreeding species separately and are connected by lines for illustrative purposes only. See Table 1 for details on sample sizes.
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Different islands will doubtless have different histories of evolution and of human disturbance (and transport). However, by analysing many island faunas together in a macroecological analysis, a unifying picture will probably emerge from the apparent noise resulting from limited sampling of individual islands, and help to explain the current island diversity of wood-boring beetles.