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Into and out of the tropics: the generation of the latitudinal gradient among New World passerine birds

Authors

  • Jonathan D. Kennedy,

    Corresponding author
    1. Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
    2. Department of Ecology and Evolution, University of Chicago, Chicago, IL, USA
    • Correspondence: Jonathan Kennedy, Natural History Museum of Denmark, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.

      E-mail: jonathan.kennedy@bio.ku.dk

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  • Zhiheng Wang,

    1. Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
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  • Jason T. Weir,

    1. Department of Biological Sciences, University of Toronto Scarborough, Toronto, Canada
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  • Carsten Rahbek,

    1. Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
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  • Jon Fjeldså,

    1. Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark
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  • Trevor D. Price

    1. Department of Ecology and Evolution, University of Chicago, Chicago, IL, USA
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Abstract

Aim

One prominent explanation for the latitudinal gradient in biodiversity proposes that its prime cause is the greater age and/or higher origination rates of tropical clades, and the infrequent or delayed dispersal of their component species into temperate regions. An alternative is that species’ carrying capacities vary regionally, which influences rates of time-averaged diversification via ecological opportunity. We contrast these hypotheses, in order to assess potential historical influences upon the latitudinal gradient of New World passerine birds (order Passeriformes), comparing patterns among the two suborders present (oscines and suboscines), which are known to have had different routes of dispersal across the region.

Location

New World.

Methods

We examine diversity patterns, their abiotic and biotic correlates, and the distributions of phylogenetically old and young species.

Results

Strong latitudinal gradients are present within both oscine and suboscine birds, with maximum diversity towards the equator, but their overall shapes differ. Among the oscines, older lineages are found towards the north, with progressively younger lineages present further south. Regional variation in oscine richness is statistically well explained by a combination of productivity and elevation (R2 = 0.76). In contrast, few suboscine groups have colonized the north, so their current diversity is well correlated with temperature seasonality (R2 = 0.74).

Main conclusions

Because the oscines colonized the Americas from the north, their latitudinal gradient must reflect regional differences in time-averaged diversification rates, and not the time present within a region. The richness patterns derived from phylogenetic data and the strong predictive power of the normalized difference vegetation index (NDVI) suggest that the radiation of the oscines is consistent with the idea that entry into a new region stimulates a burst of diversification, which is higher and/or continues for longer in areas with greater carrying capacity. Conversely, the suboscine distributions potentially reflect a large historical barrier to dispersal and niche conservatism of climatic tolerances, possibly coupled with competition from the oscines. Although contemporary conditions can explain much of the passerine diversity patterns, history has had an important influence on the taxonomic composition of this gradient.

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