Scale effects on the body size frequency distributions of African birds: patterns and potential mechanisms

Authors

  • Bernard W. T. Coetzee,

    Corresponding author
    • Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa
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  • Peter C. le Roux,

    1. Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa
    Current affiliation:
    1. Department of Geosciences and Geography, University of Helsinki, Helsinki, Finland
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  • Steven L. Chown

    1. Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa
    Current affiliation:
    1. School of Biological Sciences, Monash University, Victoria, Australia
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  • Editor: Shai Meiri

Correspondence: Bernard W. T. Coetzee, Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa.

E-mail: bwtcoetzee@gmail.com

Abstract

Aim

To describe and analyse the body size frequency distributions (BSFDs) of avian assemblages at several spatial scales in the Afrotropics. We also tested if the variation in median body size across assemblages at different spatial scales was related to environmental variables and whether purely stochastic processes could explain BSFDs.

Location

The Afrotropical biogeographic realm.

Methods

Avian body masses for 1960 species where analysed at continental, biome, ecoregion and local spatial scales with standard metrics. Variation in median assemblage body size was modelled as a function of environmental and spatial explanatory variables to assess non-random assemblage structure. We tested if BSFDs of smaller spatial scale distributions are random subsets of the larger spatial scale assemblages in which they are embedded, and used three different null model randomizations to investigate the influence of stochastic processes on BSFDs.

Results

The African avifauna's continental BSFD is unimodal and right-skewed. BSFDs generally become less skewed and less modal with decreasing spatial scale. The best-fit model explained 71% of median body size values at the ecoregion scale as a function of latitude, latitude2, longitude, species richness and species range size. BSFDs at smaller scales show non-random assembly from larger scale BSFDs distributions.

Main conclusion

African avifaunal BSFDs are quantitatively dissimilar to African mammal BSFDs, which are bimodal at all spatial scales. Much of the change in median body size with spatial scale can be captured by a range-weighted null model, suggesting that differential turnover between smaller- and larger-bodied species might explain the shift in the central tendency of the BSFD. At the local scale, energy may well contribute to structuring BSFDs, but this pattern is less pronounced at larger spatial scales.

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