Our data suggest that dispersal potential, described as mean PLD, is not a universal driver of range size in reef fishes, but does appear to be important under certain circumstances (e.g. in the Indo-Pacific). A few explanations may account for this result, most notably ocean basin size and the spatial distribution of habitat and dispersal barriers. The Pacific is considerably larger than the Atlantic; the maximum potential range is approximately 28
km in the Indo-Pacific, compared to 12
km in the Atlantic. Thus, the relationship between PLD and range size may only be apparent over large scales if dispersal potential does not limit range size in smaller oceans, where range size is more constrained. Since there is no comparably large tropical ocean, it is difficult to determine whether basin size per se
, or other unique aspects of the Pacific, drive our results.
Variations in the spatial arrangement of habitat among ocean basins may also account for our results. The Pacific contains vast expanses of ocean between suitable reef fish habitat and has few islands that could function as dispersal ‘stepping stones’ to its most remote areas. The Hawaiian Islands and Easter Island are isolated from the nearest tropical reef area by approximately 2000
km, and transpacific species must cross almost 5000
km in a single dispersal event to reach the tropical eastern Pacific. Furthermore, the few islands nearest to the eastern Pacific tend to be small, and hence may be unable to sustain large populations. Smaller populations will produce fewer offspring, thereby limiting the number of potential long-distance dispersers. In contrast, while transatlantic species must cross a large oceanic stretch (the minimum distance is 2800
km from Brazil to Senegal), the oceanic Atlantic as a dispersal barrier is bounded on either side by extensive continental habitat, increasing population sizes and the subsequent pool of dispersing larvae, as well as the size of potential targets (i.e. continental coasts) where larvae may settle after long-distance dispersal. However, due to the number of factors that may influence species' range sizes, such as environmental tolerance, historical factors (e.g. rise of the Panamá Isthmus) and biotic interactions, the absence of a relationship between PLD and range size in the Atlantic may not be surprising.
Given the spatial arrangement of habitat in the Pacific, species whose ranges extend to more isolated, peripheral areas like Hawaii and the East Pacific tend to have the largest ranges. These species may therefore be disproportionately important in strengthening the relationship between PLD and range size if they require a long PLD to reach such areas. We conducted two tests of this hypothesis. First, we compared the relationship among Indo-Pacific species (e
) to that among species restricted to the West Pacific (f
), excluding the 51 species whose ranges extend to Hawaii, Easter Island and/or the East Pacific. The R2
value drops by more than half (from 0.257 to 0.117) when widespread species are excluded. For comparison, a simulation randomly excluding 51 of the 257 Indo-Pacific species iterated 100 times produced a mean R2
value of 0.253. This suggests the strength of the PLD and range size relationship in the Indo-Pacific is reinforced by the inclusion of these widespread species. Second, we compared the PLDs of the 51 widespread species to the PLDs of the West Pacific species; widespread species have significantly longer larval durations (mean=48 and 27 days, respectively; t
=257). This result is consistent with that of some previous work (Brothers & Thresher 1985
; Thresher et al. 1989
). In contrast, transatlantic species do not have longer PLDs than species restricted to the western Atlantic (t
=45). These results support the idea that certain configurations of habitat strengthen the relationship between range size and PLD.
While a long PLD may be necessary for species to expand their ranges to the peripheries of ocean basins, rare colonization events of these distant areas may also result in endemics with long PLD and small range (i.e. the points in the bottom right of the ‘U’ in b
). Hawaiian endemics have significantly longer PLD than their more widely distributed congeners (paired t
=11). Other studies confirm that island endemics, despite their small ranges, are not limited in their dispersal potential. In a recent review of the biological characteristics of tropical reef fishes endemic to small, isolated islands, Robertson (2001)
concluded that endemics do not tend to have a shorter PLD than related species with more widespread ranges. Victor & Wellington (2000)
found that island endemics from two fish families in the East Pacific have a longer PLD than their widespread congeners. That species colonizing these islands are subsequently able to speciate despite their great larval dispersal potential highlights the importance of local retention mechanisms (Swearer et al. 2002
) and/or limited larval supply.
The positive relationships between dispersal potential and range size that we found are stronger at higher taxonomic levels. There is a considerable increase in explanatory power from the species to the genus to the family level in the Indo-Pacific (R2: 0.26, 0.54 and 0.85, respectively; and ). An unmeasured factor that is phylogenetically constrained and is correlated with both PLD and range size may be causing the stronger relationships at higher taxonomic levels. Potential factors include body size, habitat specificity, reproductive output, generation time and speciation rate. Body size is relatively uncorrelated with larval duration and range size for the Indo-Pacific species in our dataset (r=0.29 and 0.30, respectively; body size estimates from distributional references listed in the Electronic Appendix). Habitat specialization could be correlated to dispersal potential, assuming habitat specialists experience greater selective pressure for limited dispersal than habitat generalists; but habitat specificity is unlikely to be constrained by family, at least for most fish families. High reproductive output or short generation times could be related to effective dispersal by increasing the absolute number of individuals that comprise the tail of the dispersal kernel, thereby enhancing the number of long-distance dispersal events. However, reproductive output has not been quantified for most reef fishes and our data lends little support for generation time, as relatively long lived families are found on both extremes of the range of data plotted in .
Speciation rate is a more plausible mediating mechanism, assuming taxa with a shorter PLD are more likely to speciate (due to genetic isolation and local adaptation) and speciation results in smaller range sizes (younger species have less evolutionary time for range expansion). We predict that families with short mean PLD should be more speciose if they have a higher speciation rate. To test this prediction, we determined the approximate number of species within the Indo-Pacific for each family (Lieske & Myers 2002
); there is a significant negative relationship between family-level species richness and the mean larval duration for that family (n
). While these data support the idea that speciation rate could help drive the relationship between PLD and range size, additional evidence regarding species' evolutionary ages and the relative importance of extinction rates, both of which may be influenced by dispersal potential (see Jablonski 1986
and references therein), is necessary to further evaluate this hypothesis.
In summary, by incorporating data from multiple oceans and families, we have addressed the conflicting evidence regarding the relationship between dispersal potential and range size in tropical reef fishes. We demonstrate that PLD appears to influence range size only over large scales when significant barriers to dispersal are present. The tails of dispersal distributions may therefore potentially be critical in allowing species to colonize the most isolated areas. This highlights the need for more complete data describing dispersal kernels, particularly better estimates of maximum PLD. Our mechanistic investigation of this relationship has implications for other taxa and makes predictions which could be tested in other systems.