The biogeography of community assembly: latitude and predation drive variation in community trait distribution in a guild of epifaunal crustaceans

Abstract

While considerable evidence exists of biogeographic patterns in the intensity of species interactions, the influence of these patterns on variation in community structure is less clear. Studying how the distributions of traits in communities vary along global gradients can inform how variation in interactions and other factors contribute to the process of community assembly. Using a model selection approach on measures of trait dispersion in crustaceans associated with eelgrass (Zostera marina) spanning 30° of latitude in two oceans, we found that dispersion strongly increased with increasing predation and decreasing latitude. Ocean and epiphyte load appeared as secondary predictors; Pacific communities were more overdispersed while Atlantic communities were more clustered, and increasing epiphytes were associated with increased clustering. By examining how species interactions and environmental filters influence community structure across biogeographic regions, we demonstrate how both latitudinal variation in species interactions and historical contingency shape these responses. Community trait distributions have implications for ecosystem stability and functioning, and integrating large-scale observations of environmental filters, species interactions and traits can help us predict how communities may respond to environmental change.

Keywords: community assembly; eelgrass epifauna; functional traits; historical contingency; latitudinal gradient; predation.

Figure 1.

Zostera experimental network (ZEN) sites used in our analyses. Sites spanned 30° of latitude on the Pacific and Atlantic coasts of North America and Eurasia, including the Baltic and Mediterranean seas, covering most of the range of Zostera marina (eelgrass). Colours indicate trait dispersion (standard effect size, mean nearest taxon distance (SES_MNTD) calculated using the tip shuffle algorithm); positive values of SES_MNTD indicate greater dispersion in traits than expected from a random draw from the global species pool, whereas negative values of SES_MNTD indicate clustering in traits relative to a random draw. See the electronic supplementary material, figure S1 for more detailed information about site locations. (Online version in colour.)

Figure 2.

Trait dispersion (SES_MNTD) in eelgrass-associated peracarid crustacean communities across trait sets. In general, communities at sites in the Pacific Ocean were more overdispersed, while communities at Atlantic sites were less dispersed than expected. The dashed horizontal line represents an SES_MNTD value of 0, indicating random assembly. Asterisks indicate means significantly different from zero (two-tailed one-sample t-tests; see the electronic supplementary material, table S2); error bars represent standard errors. The figure shows SES_MNTD calculated according to the tip shuffle permutation algorithm; results were comparable across permutation algorithms and SES values. (Online version in colour.)

Figure 3.

The effects of predation (a), latitude (b), epiphyte load (c) and in situ temperature (d) on trait dispersion (SES_MNTD using the tip shuffle algorithm) in univariate analyses. In all of the best models of dispersion, sites with higher predation intensity had more overdispersed communities, while those with lower predation intensity had more clustered communities (a; R² = 0.15, p = 0.012). In the best models that had a non-zero latitude effect, sites at lower latitudes had more overdispersed communities, while those at higher latitudes had more clustered communities. This effect was stronger in the Pacific than the Atlantic species pool (b; R² = 0.36, interaction p = 0.0076). In the best models with a non-zero epiphyte effect, sites where eelgrass had lower epiphyte density had more overdispersed communities, while sites with more heavily fouled blades had clustered communities (c; plot shows SES_MNTD for microhabitat traits in the Atlantic species pool; R² = 0.15, p = 0.046). In situ temperature appeared only sporadically across permutations and dispersion metrics, and was not significant for total trait dispersion (R² = 0.0094, p = 0.54). The dashed horizontal line represents an SES value of 0, indicating random assembly; sites in bold italics are those for which SES is significantly different from 0 at α = 0.05. See the electronic supplementary material, figure S1 for an explanation of site codes. (Online version in colour.)