Biological diversity or biodiversity refers to the immense variety of life. An ecosystem is a community of living organisms interacting with each other and their non-living environment, such as air, water, and soil. Determining which ecosystem is the “most” diverse is complex, as it depends on the specific metrics used to measure life’s variety. The title of champion often shifts between terrestrial and marine environments based on whether one counts the sheer number of species or the density of life in a small area.
Defining the Metrics of Biological Diversity
Scientists quantify biodiversity using several metrics to compare different regions. Species richness is the most straightforward measure, simply counting the total number of different species present in a community.
Species density refines this by measuring the number of species per unit of area, which helps compare ecosystems of different sizes. A small patch of land may have fewer total species than a large continent, but it has a higher species density if it has a greater concentration of species per square meter. A third metric is endemism, which counts the number of species unique to a specific geographic area and found nowhere else on Earth.
The Terrestrial Champion Tropical Rainforests
Tropical rainforests are recognized as the terrestrial ecosystems with the highest overall species richness. Located near the equator, these environments benefit from a stable climate with warm temperatures and high annual rainfall. This lack of seasonal change allows life to thrive and reproduce year-round, without the need to adapt to harsh conditions.
The physical structure of the rainforest contributes significantly to its diversity by creating highly specialized niches. The layered structure, including the emergent layer, the main canopy, the understory, and the forest floor, provides a variety of habitats and microclimates. The dense, interlocking canopy alone is estimated to host 70 to 90 percent of all rainforest life, offering surfaces for plants like epiphytes to grow on other plants.
This high productivity, fueled by abundant sunlight and moisture, supports complex food webs and intense competition. Nutrients are rapidly recycled and mainly held in the living biomass rather than the poor soil, enabling high rates of primary production. This environmental stability has allowed for high rates of speciation, resulting in many species highly specialized to their corner of the forest. A single hectare may contain over 480 tree species, a level of variety unmatched in temperate forests.
The Marine Champion Coral Reefs
In the marine environment, coral reefs hold the title of the most biologically diverse ecosystem, sometimes exceeding the species density of rainforests. These “rainforests of the sea” are constructed by coral polyps that secrete calcium carbonate, forming massive, three-dimensional limestone structures. This physical complexity allows them to support a vast array of life.
Despite covering less than one percent of the ocean floor, coral reefs are home to at least 25 percent of all known marine species, including over 4,000 fish species. The intricate network of crevices, caves, and branching coral provides shelter, hunting grounds, and nursery areas for countless invertebrates and fish. This structural complexity maximizes the available surface area for organisms to colonize and allows many different species to coexist in a small space.
High productivity is maintained through symbiotic relationships, such as the one between the coral animal and the microscopic algae called zooxanthellae. The algae live within the coral’s tissues and perform photosynthesis, providing the coral with up to 90 percent of its required nutrition. This partnership allows the ecosystem to thrive in nutrient-poor tropical waters, supporting a dense population of herbivores, predators, and specialized organisms.
Why Biodiversity Hotspots Matter
The concept of a Biodiversity Hotspot provides a framework for conservation by identifying areas of immense, yet threatened, diversity. To qualify, a region must meet two criteria: containing at least 1,500 species of endemic vascular plants, and having lost 70 percent or more of its primary native vegetation. This dual focus on richness and threat directs limited conservation funding to the places where it can have the greatest impact.
Globally, 36 areas meet these criteria, representing just 2.5 percent of the Earth’s land surface. These terrestrial regions contain approximately 50 percent of the world’s endemic plant species and 43 percent of all terrestrial vertebrates. While large ecosystems like the Amazon are important, smaller hotspots, such as the Caribbean Islands or the Cape Floristic Region in South Africa, are recognized for their extreme rates of endemism due to long periods of geographical isolation.
These unique regions are considered irreplaceable because the species that live there would vanish from the planet if the habitat is destroyed. Protecting these hotspots helps maintain ecosystem services for human life, including clean water, climate regulation, and pollination. The concentration of unique life in these areas underscores their significance for global biodiversity.