title: Abundance-area relationships in bird assemblages along an Afrotropical elevational gradient: space limitation in montane forest selects for higher population densities.
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.
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Viničná 7, 12844, Praha 2, Czech Republic.
authors.forename: Michal
authors.initials: M
authors.lastname: Ferenc
authors.affiliation: Center for Macroecology, Evolution, and Climate, Natural History Museum of Denmark, University of Copenhagen, 2100, Copenhagen, Denmark.
authors.forename: Jon
authors.initials: J
authors.lastname: Fjeldså
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Viničná 7, 12844, Praha 2, Czech Republic.
authors.forename: Ondřej
authors.initials: O
authors.lastname: Sedláček
authors.affiliation: Mount Cameroon National Park, Buea, Cameroon.
authors.forename: Francis Njie
authors.initials: FN
authors.lastname: Motombi
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Viničná 7, 12844, Praha 2, Czech Republic.|IRTC-International Research and Training Center, Yaoundé, Cameroon.
authors.forename: Eric
authors.initials: E
authors.lastname: Djomo Nana
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Viničná 7, 12844, Praha 2, Czech Republic.
authors.forename: Karolína
authors.initials: K
authors.lastname: Mudrová
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Viničná 7, 12844, Praha 2, Czech Republic. david.horak@natur.cuni.cz.
authors.forename: David
authors.identifier: http://orcid.org/0000-0002-8073-1617
authors.initials: D
authors.lastname: Hořák
mesh_terms: D000531:Altitude; D063147:Animal Distribution; D000818:Animals; D044822:Biodiversity; D002163:Cameroon; D053476:Extinction, Biological; D065928:Forests; D046109:Passeriformes / Q000502:physiology*; D011156:Population Density
keywords: Density compensation; Extinction filter; Passerines; Specialization; Tropical mountain