The Biogeography of Community Assembly
Very few studies have been conducted on global-scale patterns of functional trait dispersion, despite well-known latitudinal gradients in species interactions and abiotic filters that assemble communities to create these patterns. Alternatively, the unique histories of particular biogeographic regions may superimpose a significant component of historical contingency over the patterns in community structure we see and override or obscure the underlying latitudinal trends.
Using data from the 2014 Zostera Experimental Network (ZEN) team, we examined the functional trait dispersion of eelgrass-associated peracarid crustacean communities (amphipods, isopods, and allies) in eelgrass beds across 37 degrees of latitude across the coasts of North America and Eurasia, and looked for patterns in dispersion as a result of latitude, biogeographic region, abiotic filters like temperature, and biotic variables like community richness and predation intensity.
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Map of sites where peracarids and other epifauna were collected along with measurements of biotic, and abiotic environmental variables. Colors represent functional trait dispersion; clustered communities (blue) are more functionally similar than expected from a random draw, while overdispersed communities (red) are more functionally distinct than expected.
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Functional trait dispersion showed a significant declining trend with latitude, with low-latitude communities being more functionally overdispersed and high-latitude communities being more functionally clustered. This pattern is consistent with the idea that at high latitudes, abiotic filters like temperature strongly restrict the range of functional traits that allow species to persist in these communities, while at lower latitudes, abiotic filters are weaker and stronger competitive interactions lead community members to avoid niche overlap, favoring more dissimilar traits. We also found that the strength of this relationship differed across ocean basins: in the Pacific, latitude had a stronger effect on dispersion than it did in the Atlantic.
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Interestingly, predation pressure consistently appeared as an important driver among all models we compared across different regional species pools and sets of traits. In nearly all of these cases, as predation intensity increased, peracarid communities became increasingly overdispersed, suggesting stabilizing usage of enemy-free space. In other words, predators of peracarids may drive these crustaceans to use traits associated with different anti-predator strategies to reduce the likelihood of being eaten by a given predator.
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