Environmental causes for plant biodiversity gradients.

One of the most pervasive patterns observed in biodiversity studies is the tendency for species richness to decline towards the poles. One possible explanation is that high levels of environmental energy promote higher species richness nearer the equator. Energy input may set a limit to the number of species that can coexist in an area or alternatively may influence evolutionary rates. Within flowering plants (angiosperms), families exposed to a high energy load tend to be both more species rich and possess faster evolutionary rates, although there is no evidence that one drives the other. Specific environmental effects are likely to vary among lineages, reflecting the interaction between biological traits and environmental conditions in which they are found. One example of this is demonstrated by the high species richness of the iris family (Iridaceae) in the Cape of South Africa, a likely product of biological traits associated with reproductive isolation and the steep ecological and climatic gradients of the region. Within any set of conditions some lineages will tend to be favoured over others; however, the identity of these lineages will fluctuate with a changing environment, explaining the highly labile nature of diversification rates observed among major lineages of flowering plants.