The extent of dispersal by pelagic larvae in marine environments, including coral reefs, is central for understanding local population dynamics and designing sustainable marine reserves. We present here the first example of a clear stepping-stone genetic structure throughout the Caribbean basin for a common coral reef species, the French grunt (Haemulon flavolineatum). Analysis of microsatellite DNA markers indicated that French grunt population structure may be characterized by overlapping populations throughout the Caribbean, influenced by independent population dynamics but with no fixed geographical boundaries. In addition, different spatial genetic patterns were found in different oceanographic regions. A second species, the bluehead wrasse (Thalassoma bifasciatum), has a much longer pelagic larval duration than French grunts and showed no explicit spatial pattern of genetic variation. This finding is concordant with the hypothesis of a positive relationship between larval dispersal and duration in the plankton. While the magnitude of the genetic signal of population structure in French grunts was very low (F(ST) approximately 0.003), the pattern of isolation-by-distance throughout the Caribbean indicated considerable population structure with important ecological and conservation significance.

1 Dispersal in many species is sex-biased. Greenwood (1980, Animal Behaviour, 28, 1140–1162) hypothesized that the direction of sex bias was dependent on the mating system, and suggested that female-biased dispersal in birds is associated with monogamy and resource defence by males, while mammalian male-biased dispersal is associated with polygyny and male defence of mates. 2 Cooperatively breeding brown jays Cyanocorax morio live in large territorial groups and are one of the few bird species known to exhibit male-biased dispersal. Survivorship of adults is high, and both males and females delay breeding for many years. 3 In this study, dispersers almost always moved to neighbouring social groups, rarely in response to a known breeding vacancy, and both sexes dispersed into groups with fewer members of the same sex than their origin group. 4 Dispersal and extra-group forays were strongly male biased. Forays were more common during nest building and egg laying, suggesting that males were monitoring mating opportunities, and many males later dispersed into the groups they visited. Genetic parentage analyses indicated that males mainly obtain paternity outside their natal groups through extra-group forays and dispersal. 5 The major routes to breeding status for females are inheritance of breeding positions on their natal territories and/or breeding there concurrently with an established breeder. Females usually dispersed on their own into groups where reproductive success had previously been poor. 6 Kin facilitation appears to affect male dispersal options: related males formed coalitions and tended to disperse to groups where male relatives were already established. 7 Male dispersal, female philopatry, and a matrilineal society correspond to three of Greenwood's seven predictions for a ‘mate-defence’ system, but overt mate defence itself is absent, territorial defence is communal, and genetic polyandry is common. Social behaviour appears to affect a reversal in sex-specific costs and benefits of dispersal, so that females benefit from philopatry and matrilineal inheritance, while males benefit from expanded dispersal options in part through kin facilitation, but without effective mate defence or clear cut sex-specific resource defence.

Invasive species are believed to spread through a process of stratified dispersal consisting of short-distance diffusive spread around established foci and human mediated long-distance jumps. Brazilian peppertree (Schinus terebinthifolius), native to South America, was introduced twice as an ornamental plant into Florida, USA, just over 100 years ago. A previous study indicated that these two introductions were from genetically differentiated source populations in the native range. In this study, we took advantage of these contrasting genetic signatures to study the spatial spread of Brazilian peppertree across its entire range in Florida. A combination of spatial genetic and geostatistical analyses using chloroplast and nuclear microsatellite markers revealed evidence for both diffusive dispersal and long-distance jumps. Chloroplast DNA haplotype distributions and extensive bands of intra-specific hybridization revealed extensive dispersal by both introduced populations across the state. The strong genetic signature around the original introduction points, the presence of a general southeast to northwest genetic cline, and evidence for short-distance genetic spatial autocorrelation provided evidence of diffusive dispersal from an advancing front, probably by birds and small mammals. In the northernmost part of the range, there were patches having a high degree of ancestry from each introduction, suggesting long-distance jump dispersal, probably by the movement of humans. The evidence for extensive movement throughout the state suggests that Brazilian peppertree will be capable of rapidly recolonizing areas from which it has been eradicated. Concerted eradication efforts over large areas or the successful establishment of effective biocontrol agents over a wide area will be needed to suppress this species.