Various sci-fi movies often have single-biome planets (such as Tatooine, for example). Obviously a desert planet is possible (witness: Mars), but what about a more complex ecosystem, such as a rainforest? Is the biodiversity inherent in such a biome possible without other biomes to, for example, move heat around the planet or act as carbon dioxide sinks as Earth's oceans do? An answer to this question mentions
Mars is a freezing desert planet, Titan is a hydrocarbon snow "planet", Venus is a crushing, superheated, acidic hellscape planet, Io is a volcano "planet", and there are a number of airless dust 'planets' like Mercury, the Moon, and a number of the larger asteroids and moons.
but, again, all of these are simple 'biomes', and I'm wondering if a planet can support a much more complex single one.