Ecosystem distribution is related to precipitation; regions that receive large amounts of precipitation are wet and therefore have increased vegetation.
Explanation:
Precipitation in the form of rain and climate determines the vegetation of an ecosystem. Ecosystems with abundant rainfall will have dense vegetation and one with scarce rainfall will have limited or no vegetation.
Tropical rainforest ecosystems receive large amounts of rainfall distributed year round and have thick vegetation.
Water is the prime growth factor for plants and rainfall is the main source of water for plants. Any ecosystem, maybe a desert, grassland, fertile plain, bushlands, evergreen forest etc depend upon rainfall.
The rainfall determines the density of vegetation, the type of vegetation, greenness, structure of plants or trees like foliage, height, branching etc