Abstract
Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size play in determining spatial patterns of diversity remains unclear. Here we analyse spatial community structure - β-diversity - for several planktonic and nektonic organisms from prokaryotes to small mesopelagic fishes collected during the Malaspina 2010 Expedition. β-diversity was compared to surface ocean transit times derived from a global circulation model, revealing a significant negative relationship that is stronger than environmental differences. Estimated dispersal scales for different groups show a negative correlation with body size, where less abundant large-bodied communities have significantly shorter dispersal scales and larger species spatial turnover rates than more abundant small-bodied plankton. Our results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversity.
Highlights
Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents
The scale-dependence of β-diversity can be described as a ‘distance-decay’ rate, measured as the slope of a linear relationship between the logarithm of community similarity and the logarithm of geographic distance among pairs of sites[14]. In both oceanic and terrestrial ecological communities, distance-decay patterns are set by three major mechanisms[15,16]: (1) local niche-based processes, which are summarized by the statement that, below 1-mm body size, “everything is everywhere, but the environment selects”[7,17,18]; (2) the effects of dispersal limitation, as hypothesized by the neutral theory of biodiversity[19]; these effects lead to a negative relationship between community similarity and geographic distance, even in a completely homogeneous environment; and (3) the spatial configuration of the seascape, which can dictate the rate at which organisms disperse among sites[1]
In our analysis, the spatial arrangements of the sampled assemblages reveal that surface ocean transit time explains a larger fraction of the variability in planktonic and micro-nektonic community similarity than do environmental factors. This indicates that passive dispersal with surface ocean currents—arguably an ecologically neutral process affecting all planktonic and micro-nektonic organisms—is a stronger determinant of community structure than niche-filtering factors[19]
Summary
Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. We have quantified empirically derived distance-decay slopes and measured dispersal scales for a number of planktonic and micro-nektonic organisms, spanning a wide range of body sizes and abundances, from prokaryotes to small mesopelagic fishes With these analyses, we have tested the hypothesized sizedependence of community dispersal scales and resulting spatial patterns of regional connectivity. Our results show that the species composition of plankton and micronekton communities in tropical and subtropical open ocean is in large part determined by oceanic currents Given this finding, we explored the dispersal scale of each biological group and found a negative relationship between dispersal scale and body size: less abundant large-bodied plankton and micro-nekton communities in near-surface epipelagic waters show significantly shorter dispersal scales and larger spatial species-turnover rates compared to more abundant small-bodied plankton
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