Biodiversity is high in the islands, with many endemic species, and with many genera shared with Borneo to the south.
A mountain range runs up the middle of the island, with almost half of the slopes averaging over 30 degrees.
The driest month has less than 60 mm of precipitation, and is drier than the average month.
Large areas are limestone forest, featuring cathedral cactus (Euphorbia lacei), Aglaia argentea, and species of Antidesma, Drypetes, and Gomphandra.
There are ultramafic forests around Victoria Peak that support plants that may be associated with soils having heavy metals.