A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet how and why this gradient arose remains unresolved. Here we review two major hypotheses for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that tropical climates are older and historically larger, allowing more opportunity for diversification. This hypothesis is supported by observations that temperate taxa are often younger than, and nested within, tropical taxa, and that diversity is positively correlated with the age and area of geographical regions. The diversification rate hypothesis holds that tropical regions diversify faster due to higher rates of speciation (caused by increased opportunities for the evolution of reproductive isolation, or faster molecular evolution, or the increased importance of biotic interactions), or due to lower extinction rates. There is phylogenetic evidence for higher rates of diversification in tropical clades, and palaeontological data demonstrate higher rates of origination for tropical taxa, but mixed evidence for latitudinal differences in extinction rates. Studies of latitudinal variation in incipient speciation also suggest faster speciation in the tropics. Distinguishing the roles of history, speciation and extinction in the origin of the latitudinal gradient represents a major challenge to future research.

The diversity of life is ultimately generated by evolution, and much attention has focused on the rapid evolution of ecological traits. Yet, the tendency for many ecological traits to instead remain similar over time [niche conservatism (NC)] has many consequences for the fundamental patterns and processes studied in ecology and conservation biology. Here, we describe the mounting evidence for the importance of NC to major topics in ecology (e.g. species richness, ecosystem function) and conservation (e.g. climate change, invasive species). We also review other areas where it may be important but has generally been overlooked, in both ecology (e.g. food webs, disease ecology, mutualistic interactions) and conservation (e.g. habitat modification). We summarize methods for testing for NC, and suggest that a commonly used and advocated method (involving a test for phylogenetic signal) is potentially problematic, and describe alternative approaches. We suggest that considering NC: (1) focuses attention on the withinspecies processes that cause traits to be conserved over time, (2) emphasizes connections between questions and research areas that are not obviously related (e.g. invasives, global warming, tropical richness), and (3) suggests new areas for research (e.g. why are some clades largely nocturnal? why do related species share diseases?).

Aim Elevational gradients distributed across the globe are a powerful test system for understanding biodiversity. Here I use a comprehensive set of bird elevational gradients to test the main drivers of diversity, including sampling, area, mid-domain effect, temperature, temperature and water availability, and hypotheses of evolutionary history.Location Seventy-eight elevational gradients of bird diversity from mountains in both hemispheres spanning 24.5 ° S to 48.2 ° N, including gradients from various climates, biogeographical regions and habitat types.Methods Data on bird elevational diversity were taken from the literature. Of the 150 datasets found or compiled, only those with a high, unbiased sampling effort were used in analyses. Datasets sampled all birds, all breeding birds or all forest birds; a few studies detailed seasonal, elevational shifts. Eighteen predictions of diversity theory were tested, including three sets of interactions.Results Birds display four distinct diversity patterns in nearly equal frequency on mountains: decreasing diversity, low-elevation plateaus, low-elevation plateaus with mid-peaks, and unimodal mid-elevational peaks. Bird elevational diversity strongly supports current climate as the main driver of diversity, particularly combined trends in temperature and water availability. Bird diversity on humid mountains is either decreasing or shows a low-elevation plateau in diversity, while on dry mountains it is unimodal or a broad, low-elevation plateau usually with a mid-elevation maximum. The predictions of sampling, area and mid-domain effect were not consistently supported globally. The only evolutionary hypothesis with preliminary support was niche conservatism.Main conclusions Both water and temperature variables are needed to comprehensively predict elevational diversity patterns for birds. This result is consistent for breeding and forest birds, for both hemispheres, and for local-or regional-scale montane gradients. More analyses are needed to discern whether the mechanism underlying these relationships is ecological, based on direct physiological limitations or indirect food resource limitations, or historical, based on phylogenetic niche conservation or other evolutionary trends related to climate. The species-area and middomain effects are not supported as primary drivers of elevational diversity in birds.