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Variation in species richness across environmental gradients results from
a combination of historical non-equilibrium processes (time, speciation,
extinction) and present-day differences in environmental carrying
capacities (i.e., ecological limits, affected by species interactions and
the abundance and diversity of resources). In a study of bird richness
along the sub-tropical east Himalayan elevational gradient, we test the
prediction that species richness patterns are consistent with ecological
limits using data on morphology, phylogeny, elevational distribution, and
arthropod resources. Species richness peaks at mid-elevations. Occupied
morphological volume is roughly constant from low to mid-elevations,
implying more species are packed into the same space at mid-elevations
compared with low elevations. However, variance in beak length, and
differences in beak length between close relatives decline with elevation,
a consequence of the addition of many small insectivores at
mid-elevations. These patterns are predicted from resource distributions:
arthropod size diversity declines from low to mid elevations, largely
because many more small insects are present at mid-elevations. Weak
correlations of species mean morphological traits with elevation also
match predictions based on resources and habitats. Elevational transects
in the tropical Andes, New Guinea and Tanzania similarly show declines in
mean arthropod size and mean beak length, and in these cases likely
contribute to declining numbers of insectivorous bird species richness
along these gradients. The results imply conditions for ecological limits
are met, although historical non-equilibrium processes are likely to also
contribute to the pattern of species richness.
293 views reported since publication in 2019.