Convergence, divergence, and parallelism in marine biodiversity trends: Integrating present-day and fossil data

Abstract

Paleontological data provide essential insights into the processes shaping the spatial distribution of present-day biodiversity. Here, we combine biogeographic data with the fossil record to investigate the roles of parallelism (similar diversities reached via changes from similar starting points), convergence (similar diversities reached from different starting points), and divergence in shaping the present-day latitudinal diversity gradients of marine bivalves along the two North American coasts. Although both faunas show the expected overall poleward decline in species richness, the trends differ between the coasts, and the discrepancies are not explained simply by present-day temperature differences. Instead, the fossil record indicates that both coasts have declined in overall diversity over the past 3 My, but the western Atlantic fauna suffered more severe Pliocene-Pleistocene extinction than did the eastern Pacific. Tropical western Atlantic diversity remains lower than the eastern Pacific, but warm temperate western Atlantic diversity recovered to exceed that of the temperate eastern Pacific, either through immigration or in situ origination. At the clade level, bivalve families shared by the two coasts followed a variety of paths toward today's diversities. The drivers of these lineage-level differences remain unclear, but species with broad geographic ranges during the Pliocene were more likely than geographically restricted species to persist in the temperate zone, suggesting that past differences in geographic range sizes among clades may underlie between-coast contrasts. More detailed comparative work on regional extinction intensities and selectivities, and subsequent recoveries (by in situ speciation or immigration), is needed to better understand present-day diversity patterns and model future changes.

Keywords: biogeography; diversification; extinction; latitudinal diversity gradient; marine biodiversity.

Fig. 1.

Present-day diversity of marine bivalves on Northern Hemisphere coastlines. (A) Bivalve diversity patterns along the focal coasts (E Pacific and W Atlantic), with W Pacific included for scale. (B) Proportional diversity differences between the two coastlines do not correspond to SST differences for the same latitudinal bins (solid line connects successive latitudes). Thus, diversity is roughly 30% greater in the E Pacific (E.P.) than the W Atlantic (W.A.) across a range of conditions: when the E Pacific SST differs from the W Atlantic by −3 °C, 0 °C, and +5 °C; locations having similar temperatures can exhibit a range of diversities, e.g., where the E Pacific is ∼3 °C cooler than the W Atlantic, its biota can have 50% less, 10% less, and 30% more diversity than the W Atlantic.

Fig. 2.

Pliocene−Recent extinction patterns. (A) Combined regional and global extinction within each bivalve family is generally more severe in the Virginian region than in California. The dashed lines represent y = x and y = 0.5x, triangles represent families having more than five species on both coasts, and darker symbols indicate multiple families showing the same patterns. (B) In the Virginian region, species that became extinct (gray bars) were significantly less widespread than those that persisted in the area (open bars; a Kolmogorov−Smirnov test shows the two distributions are significantly different, P = 0.02). Geographic range is quantified as the number of sampled regions in which a species occurred (following ref. 53); results are insensitive to alternative binning schemes.

Fig. 3.

Convergent, divergent, and parallel pathways in diversity of the 48 families shared by the two coasts, from the Pliocene to the present day. Numbers by arrows are the number of families that follow each path. Blue numbers are similarities at that step; red numbers are differences at that step. Among the two sets of nine families that follow the paths crossing in the center of the diagram, two families that started with similar diversities, and eight families that started with different differences, experienced different extinction intensities since the Pliocene. See Results, Regional History for details.