|
| Aim One prominent explanation for the latitudinal gradient in biodiversity
proposes that its prime cause is the greater age and/or higher origination rates
of tropical clades, and the infrequent or delayed dispersal of their component
species into temperate regions. An alternative is that species’ carrying capacities
vary regionally, which influences rates of time-averaged diversification via ecological
opportunity. We contrast these hypotheses, in order to assess potential
historical influences upon the latitudinal gradient of New World passerine
birds (order Passeriformes), comparing patterns among the two suborders
present (oscines and suboscines), which are known to have had different routes
of dispersal across the region.
Location New World.
Methods We examine diversity patterns, their abiotic and biotic correlates,
and the distributions of phylogenetically old and young species.
Results Strong latitudinal gradients are present within both oscine and suboscine
birds, with maximum diversity towards the equator, but their overall
shapes differ. Among the oscines, older lineages are found towards the north,
with progressively younger lineages present further south. Regional variation in
oscine richness is statistically well explained by a combination of productivity
and elevation (R2 = 0.76). In contrast, few suboscine groups have colonized
the north, so their current diversity is well correlated with temperature seasonality
(R2 = 0.74).
Main conclusions Because the oscines colonized the Americas from the
north, their latitudinal gradient must reflect regional differences in time-averaged
diversification rates, and not the time present within a region. The richness
patterns derived from phylogenetic data and the strong predictive power
of the normalized difference vegetation index (NDVI) suggest that the radiation
of the oscines is consistent with the idea that entry into a new region
stimulates a burst of diversification, which is higher and/or continues for
longer in areas with greater carrying capacity. Conversely, the suboscine distributions
potentially reflect a large historical barrier to dispersal and niche conservatism
of climatic tolerances, possibly coupled with competition from the
oscines. Although contemporary conditions can explain much of the passerine
diversity patterns, history has had an important influence on the taxonomic
composition of this gradient. | |
|