Which of the following has the largest biodiversity?

Concept: Biodiversity is the variety of plant and animal species that a given ecosystem can support. Across the world's major terrestrial biomes, species richness scales mainly with two factors: the biome's net primary productivity — how much new plant biomass an ecosystem generates each year, which depends on the sunlight, warmth, and rainfall available to it — and the structural complexity of its vegetation, which creates the range of physical niches different species can occupy. A warm, consistently wet climate sustains the highest productivity and the most complex, multi-layered vegetation, and therefore supports far more species than a biome that is cooler, drier, or structurally simpler.

Application: Of the four biomes listed, the tropical evergreen rain forest biome has the highest year-round productivity — warm temperatures and heavy, evenly distributed rainfall all year keep photosynthesis and plant growth at their peak — and its vegetation forms a dense, multi-layered canopy (upper canopy, understorey, and forest floor) that creates an enormous range of niches. This combination of high productivity and high structural complexity is why tropical rain forests — though covering only about 6% of Earth's land surface — are home to more than half of the world's known plant and animal species, making this biome the one with the largest biodiversity among the options given.

Cross-check: Comparing against the other biomes on the same two factors confirms this:

• Temperate deciduous forests have markedly lower annual productivity than tropical rain forests and a simpler, largely single-layered canopy, so they offer fewer niches and support noticeably fewer species overall — not because species become inactive in winter, but because the biome's underlying productivity and structural complexity are lower.

• Temperate grasslands receive far less rainfall, which caps productivity, and their vegetation is structurally simple — mostly grasses with very few trees — so they offer a narrower range of niches and support comparatively few species.

• Savannas have a long dry season that limits productivity for part of the year, and their open, grass-dominated structure with only scattered trees offers fewer niches than a continuously wet, densely layered forest, so they support fewer species overall.

Hence, the tropical evergreen rain forest biome holds the largest biodiversity among the given options.