, Volume 18, Issue 6, pp 971–984 | Cite as

On Bird Functional Diversity: Species Richness and Functional Differentiation Show Contrasting Responses to Rainfall and Vegetation Structure in an Arid Landscape

  • Colleen L. SeymourEmail author
  • Robert E. Simmons
  • Grant S. Joseph
  • Jasper A. Slingsby


Biodiversity affects ecosystem function through species’ functional traits. Although it is possible to predict species richness (SR) patterns along environmental gradients, whether functional diversity (FD) changes in predictable ways is not known. In arid environments, SR typically increases with rainfall. Aridity may limit functional differentiation by allowing only certain traits, but could also be associated with diverse traits associated with various strategies for coping with spatial and temporal variation in resources. Rare species may have unique traits, making them particularly important to continued ecosystem function. We investigated SR, FD, and functional differentiation in bird assemblages along an aridity gradient, with attention to functional uniqueness of rare species. We surveyed bird communities in open savanna, bush-thickened, and riparian habitats at five sites of increasing aridity (~150–400 mm rainfall year−1) in wet and dry seasons for 3 years in Namibia. We calculated the standardized effect size of FD (sesFD) as a measure of functional differentiation and used mixed models to ascertain how SR, FD, and sesFD relate to rainfall, season, and habitat type. SR and FD increased with increasing rainfall. Conversely, sesFD declined with increasing rainfall and was lower in woody habitats, suggesting habitat filtering and greater niche overlap. Rare species were more functionally unique than common species, in all three habitats, so loss of rare species could degrade ecosystem function. Our results are consistent with a linear diversity–productivity relationship maintained by regular disturbance (drought) preventing strong competitors from excluding weaker competitors in higher productivity environments.


aridity gradient bush encroachment bush thickening environmental gradients environmental filtering functional uniqueness null models rare species shrub encroachment standardized effect size 



Landowners and the Desert Research Foundation (Gobabeb) are thanked for site access and rainfall records; S. and L. Camp for accommodation; BIOTA Southern Africa, Namibian Nature Foundation and NRF (Grant 91039) for funding.

Supplementary material

10021_2015_9875_MOESM1_ESM.xlsx (59 kb)
Supplementary material 1 (XLSX 58 kb)
10021_2015_9875_MOESM2_ESM.docx (41 kb)
Supplementary material 2 (DOCX 41 kb)


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Copyright information

© Springer Science+Business Media New York 2015

Authors and Affiliations

  • Colleen L. Seymour
    • 1
    • 2
    Email author
  • Robert E. Simmons
    • 2
  • Grant S. Joseph
    • 2
  • Jasper A. Slingsby
    • 3
    • 4
  1. 1.Kirstenbosch Research CentreSouth African National Biodiversity InstituteClaremontSouth Africa
  2. 2.DST-NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, Department of Biological SciencesUniversity of Cape TownRondeboschSouth Africa
  3. 3.South African Environmental Observation Network (SAEON), Fynbos NodeNewlandsSouth Africa
  4. 4.Department of Biological Sciences, Centre for Statistics in Ecology, Environment and ConservationUniversity of Cape TownRondeboschSouth Africa

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