USA Biology Olympiad (USABO) Practice Exam

Which ecosystem typically has the highest number of species per square kilometer?

1. Coral Reef

2. Desert

3. Deciduous Forest

4. Freshwater Lakes

The correct answer is: Coral Reef

Coral reefs are renowned for their incredible biodiversity and are often cited as one of the most vibrant ecosystems on the planet. They typically boast the highest number of species per square kilometer due to a combination of factors that promote rich biodiversity. First, coral reefs provide a complex three-dimensional structure that creates numerous habitats for a wide array of marine organisms. The multitude of niches within the reef structure allows various species to coexist, leading to high species richness. This complexity is enhanced by the interactions between the corals, algae, fish, and invertebrates that form intricate relationships, such as symbiosis and mutualism. Additionally, coral reefs are often located in warm, shallow waters that provide ample sunlight for photosynthetic organisms like zooxanthellae, which are algae living within coral tissues. These algae contribute to the energy needs of corals and also support other life forms that rely on them for nutrition, further increasing the diversity of species present. In contrast, deserts, deciduous forests, and freshwater lakes have their own unique sets of species, but they generally do not reach the same levels of species diversity per square kilometer as coral reefs. Deserts are characterized by extreme conditions that limit the variety of life forms that can thrive. Deciduous forests and