The Equilibrium Theory of Island Biogeography (ETIB) is a widely applied dynamic theory proposed to explain why islands have coherent differences in species richness. The development of the ETIB was temporarily challenged by the alternative static Theory of Ecological Impoverishment (TEI), which suggests that the number of species on an island is determined by its number of niches or habitats. With no clear evidence relating species richness to the number of niches, the TEI was almost abandoned in favour of the ETIB. Here, we show that the number of climatic niches on islands is an important predictor of the species richness of plants, herpetofauna and land birds, and we propose a new model called the General Theory of Island Biogeography (GTIB) that merges the ETIB and TEI theories. When we test our GTIB for resident land birds in Krakatau Islands it reveals a good correspondence with observed species richness, immigration and extinction rates.