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| 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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