Abstract
The usual positive inter-specific relationship between range size and abundance of local populations can have notable exceptions in Afrotropical montane areas, where range-restricted bird species are unusually abundant. We tested how the area occupied locally by passerines and their geographic range size relate to local abundances along a tropical elevational gradient of Mt Cameroon, West-Central Africa. Data on bird assemblages were collected at six forested elevations (350, 650, 1100, 1500, 1850 m, 2200 m a.s.l.) using a standardised point count at 16 locations per elevation. Elevational ranges of birds were compiled from published sources and their geographic range sizes were determined as the occupancy of 1° x 1° grid cells. The observed relationship between local abundance and geographic range size within the entire passerine assemblage on Mt Cameroon disagrees with the most frequently reported positive pattern. However, the patterns differ among elevations, with positive trends of the abundance-range size relationship in lowland changing to negative trends towards higher elevations. Interestingly, the total assemblage abundances do not differ much among elevations and population size estimates of species occupying different parts of the gradient remain relatively constant. These patterns are caused by relatively high abundances of montane species, which might be a result of long-term ecological specialization and/or competitive release in species-poor montane locations and possibly facilitated by an extinction filter. Our data suggest that montane species’ abilities to maintain dense populations might compensate for less area available near mountain tops and help these populations to circumvent extinction.
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Acknowledgments
We wish to thank the communities of Bokwango and Bakingili villages for their support during the fieldwork. We are grateful to Francis Luma for fieldwork management. We thank the Department of Environmental Sciences of the University of Buea, Cameroon and in particular Eric Fokam, Philip Tem Dia, Philip Ngafor and Bertilla Sirri for their assistance during the fieldwork, and Jana Vokurková and Andrew Reeve for stimulating comments on earlier drafts of this manuscript. Tomáš Albrecht and David Storch helped us during point counts. We also thank Kevin Njabo and the IRTC of the University of California, Los Angeles in Yaoundé for their technical support. This study was performed with the authorisation no. 0104/PRBS/MINFOF/SG/DFAP/SDVEF/SC of the Ministry of Forestry and Wildlife of the Republic of Cameroon and with the support of Mt Cameroon National Park authorities. The research was funded by the Czech Science Foundation (project no. P505/11/1617), and J. F. acknowledges the Danish National Research Foundation for support to the Centre for Macroecology, Evolution and Climate.
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D. H., M. F., J. F. and O. S. conceived the ideas; D. H., O. S., F. N. M., E. D. N., M. F. and K. M. collected the data; M. F. performed the statistical analysis; M. F., D. H. and J. F. wrote the manuscript; O. S. and E. D. N. contributed to the writing.
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Communicated by Ola Olsson.
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Ferenc, M., Fjeldså, J., Sedláček, O. et al. Abundance-area relationships in bird assemblages along an Afrotropical elevational gradient: space limitation in montane forest selects for higher population densities. Oecologia 181, 225–233 (2016). https://doi.org/10.1007/s00442-016-3554-0
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DOI: https://doi.org/10.1007/s00442-016-3554-0