Levyns’ Law: explaining the evolution of a remarkable longitudinal gradient in Cape plant diversity

Abstract

It has been known for many decades that the diversity of clades endemic to the Greater Cape Floristic Region (GCFR) declines along a longitudinal (west-east) gradient, in concert with a reduction in the proportion of winter rainfall. In honour of the pioneering work by Margaret Levyns, we recognise this pattern as Levyns’ Law, and illustrate it with distribution data for 23 speciose endemic clades. All patterns were consistent with Levyns’ Law. Here we assess explanations for Levyns’ Law in terms of theories invoked to explain the evolution of diversity patterns along other diversity macro-gradients. Neither the metabolic nor the ecological opportunity hypotheses convincingly explain Levyns’ Law. Surprisingly, almost no research has been done on the evolutionary opportunities associated with the Mid Miocene onset of a winter-rainfall regime in the western GCFR, despite this phenomenon being frequently invoked to explain the diversification of many GCFR clades. In all other respects, niche space both now and historically shows no major differences between the western, winter rainfall, and the eastern, year-round rainfall regions of the GCFR. We suggest that higher Pleistocene climatic and biome stability in the western GCFR – the age and area hypothesis – best explains Levyns’ Law. Pronounced instability in the east reduced diversification rates via increased rates of extinction of lineages. Testing the age and area hypotheses will require, inter alia, proxy data for Pleistocene environmental and biotic dynamics, an analysis of diversification rates in relation to rainfall seasonality, and an assessment of the patterns of phylogenetic diversity in the GCFR.