What characterizes a tropical rainforest?

The characteristics of a tropical rainforest prominently include high levels of biodiversity and biomass. These ecosystems are renowned for their rich variety of plant and animal species – with estimates suggesting that they house over half of the world's terrestrial species, despite covering just a small percentage of the Earth's surface area. This high biodiversity is a result of the warm and moist climate that provides ideal growth conditions for a multitude of organisms. Additionally, biomass refers to the total mass of living matter in a particular area, which is very high in tropical rainforests due to the dense vegetation and complex layers of flora, including towering trees, understory plants, and a diverse array of epiphytes and vines.

The other options present characteristics that do not accurately describe tropical rainforests. Cold temperatures and low rainfall are contrary to what defines this vibrant ecosystem, as tropical rainforests typically experience warm temperatures year-round with substantial rainfall, leading to lush vegetation. Low levels of biodiversity and resources and sparse vegetation with nutrient-rich soil are also misrepresentations; while rainforests can have nutrient-poor soils due to rapid decomposition and nutrient cycling, the abundant plant life indicates a wealth of biological resources rather than scarcity. Thus, high levels of biodiversity and biomass distinctly characterize tropical rainforests.