Biogeography and complex traits: dispersal syndromes, in the sea

Abstract

news and update ISSN 1948-6596 commentary Biogeography and complex traits: dispersal syndromes, in the sea How dispersal potential manifests as realized mi- gration (sensu Wright 1931) is a core question that links pattern and process in biogeography, ecology and evolution. It is a question that cannot be an- swered by knowing about dispersal alone; the an- swer requires understanding how traits of organ- isms influence dispersal potential, how abiotic and biotic environments filter dispersers, and how in- teractions between organisms and environments determine dispersal routes (e.g., Johnson and Black 1994, Siegel et al. 2003, Shanks 2009). A new paper by Luiz et al. (2013), investigating de- terminants of range size in reef fishes, emphasizes that understanding the relationship between dis- persal potential and migration also may require understanding the traits of non-dispersive phases. Luiz et al. (2013) report that the characteris- tics of generally non-dispersive adult phases cor- relate strongly with differences in range size, which historically have been considered the con- sequence of differences among dispersive larval phases (e.g., Lester et al. 2007). The significance of this initially counter-intuitive finding is that it greatly enriches recent discussion of dispersal that, after decades focusing on the influence of pelagic larval duration (PLD), has, as the authors put it, “yielded mixed results” (Luiz et al. 2013:16498) By analogy, consider how the state of knowledge about dispersal in terrestrial plants might look if we studied fruits alone and not other diverse aspects of dispersal syndromes, such as the animals involved (e.g., Beaudrot et al. 2013). Luiz et al. (2013:16500) find that the indi- vidual and combined effects of adult body size (independent effect [IE] = 36%), nocturnality (IE = 26%), and schooling behavior (IE = 16%) “are more important in determining the size of geographic ranges than larval dispersal potential” (IE = 13%; Table 1). They suggest that these adult character- istics provide key advantages in terms of in- creased establishment success by decreasing pre- dation risk and reducing Allee effects. The result and inference make sense (Figure 1). Population genetic theory has long recognized the bipartite— dispersal and establishment—nature of migration (e.g., Marshall et al. 2010). Similarly, without dis- persal and establishment, there can be no mean- ingful expansion in range size. The same suite of population theory, how- ever, also highlights a weakness in the inference that correlation with adult traits means the domi- nant effects are in “population establishment af- ter propagule arrival” (Luiz et al. 2013:16499); em- phasis added). For example, adult body size tends to correlate with fecundity and with longevity in many fishes (Beverton 1987, Winemiller and Rose 1992, Hixon et al. 2006:306, Juan-Jorda et al. 2013; see also Figure 1a–d). Thus, although the fecundity advantage of large-bodied species may sometimes be counteracted by greater abundance of small-bodied species, complex relationships between body size and abundance (White et al. 2007) mean that larger fish can be both more fe- cund and equally or more abundant than smaller fish (e.g., Munday and Jones 1998). Thus, the ad- vantage of body size may accrue via post-dispersal processes and/or via increased lifetime output of larvae. The total effect size of dispersal poten- tial—considering lifetime larval production, PLD, and Luiz and colleague’s preferred dataset—is therefore greater than 13% and possibly as large as 49% (i.e., 13.3% + 36.0%; Table 1), which could turn their conclusion upside-down. Moreover, if a substantial effect of body size is mediated via lar- val production, their justification for excluding the high-PLD trans-Pacific fishes is limited and the to- tal effect size of dispersal could be as high as 62% (i.e., 24.8% + 37.6%; Table 1), though this would require that other correlated factors, such as young-of-year growth, contribute little. Conse- quently, despite the advances made by Luiz et al. (2013)—a dataset including ~3-5% of marine fishes, an alternative perspective, and application of mixed models—we remain uncertain about the relative importance of factors influencing migra- tion and range size in this dataset and across frontiers of biogeography 6.1, 2014 — © 2014 the authors; journal compilation © 2014 The International Biogeography Society