Letters

Rethinking migration

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Science  15 May 2015:
Vol. 348, Issue 6236, pp. 766
DOI: 10.1126/science.348.6236.766-a

We applaud the Report by C. Montes et al. (“Middle Miocene closure of the Central American Seaway,” 10 April, p. 226), whose geochemical analysis pushes back the timeframe of the shoaling of the Isthmus of Panama by 10 to 12 million years. This finding establishes a middle Miocene [13 to 15 million years ago (Ma)] completion of the land bridge between North and South America, marking a substantial shift in our understanding of the merger between these long-isolated landmasses. This has key implications for understanding the mass migration of organisms between these two continents—the so-called Great American Biotic Interchange (GABI).

Titanis Terror bird ILLUSTRATION: JULIUS CSOTONYI

In light of this new hypothesis, previous inferences about the GABI require a fresh reassessment. For example, although the majority of animal migrations appear to have taken place during the previous estimate of when the land bridge was thought to have been completed (∼3.5 Ma), the new date better conforms with evidence of older migrations (10 to 5 Ma) involving flightless animals such as ground sloths, procyonids, gomphotheres, tapirs, peccaries, and flightless terror birds without invoking complicated island-hopping scenarios (13). For plants, however, the route of migration across the Isthmus region appears to have been used much earlier. In the Barbados cherries, for example, numerous independent migration events from South America to Mexico occurred as early as the middle Eocene (46 Ma) based on phylogenetic inference, but increased six-fold beginning in the lower Miocene (23 Ma), just before the newly estimated date (4). Thus, rates of plant migrations between North and South America appear to have been greatly stimulated by the formation of this land bridge, even before its completion.

This reassessment raises a new conundrum: Why does the migration of animals lag so dramatically behind that of plants? It could be that even if the habitat was suitable, there were geological impediments to reaching it (dispersal limitation). However, the migration of flying birds, which conceivably could have overcome such impediments, appears to be as delayed as that of other animals, relative to plants (5). Another possibility is sampling bias in the fossil record. Alternatively, these patterns may hint at ecological, as opposed to geological, barriers underlying biome assembly. Ecological barriers could have included Plio-Pleistocene global climate, as Montes et al. suggest, or the need for vegetative changes to establish suitable habitat before the arrival of the animals. To the extent that this scenario applies, it indicates that geological barriers are not the only factor influencing migration at geological time scales and opens exciting new possibilities for exploring the relative importance of ecological factors in the merger of two continental biotas.

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