Biological diversity, or biodiversity, is the variety of life on Earth, encompassing everything from genes to ecosystems. Determining which organisms are the most diverse depends entirely on the metric used for measurement. One approach focuses on taxonomic diversity, measuring the sheer number of formally described species. The second measure focuses on functional or genetic diversity, examining the breadth of metabolic capabilities and genetic variation within a group.
The Reign of Insects
When diversity is measured by the number of described species, insects are the champions of the animal kingdom. They represent over half of all described species on the planet. Estimates suggest that between 950,000 and 1,000,000 insect species have been formally identified, though the total number is likely much higher, possibly reaching 5.5 million or more.
This species richness is enabled by several evolutionary advantages. Their small body size allows them to exploit a vast array of ecological niches, requiring fewer resources and space than larger organisms. The evolution of flight gave them mobility for dispersal, evading predators, and finding mates and food sources. Short generation times and high reproductive rates also allow them to adapt and evolve more rapidly than long-lived vertebrates.
The most diverse insect orders demonstrate this success, with five groups accounting for approximately 90% of all known insect species. These groups include:
- Coleoptera (beetles), the largest single order
- Lepidoptera (butterflies and moths)
- Diptera (flies)
- Hymenoptera (ants, bees, and wasps)
- Hemiptera (true bugs)
A significant driver of their diversification is the co-evolutionary relationship with flowering plants. Many insects have specialized mouthparts and behaviors to feed on specific plant parts, nectar, or pollen, leading to the rapid proliferation of new insect and plant species.
The Unseen Majority: Prokaryotic Genetic Diversity
While insects dominate species counts, the most diverse forms of life in terms of genetic and metabolic function are the prokaryotes, which include Bacteria and Archaea. These single-celled organisms, lacking a nucleus, exhibit a range of biochemical pathways unmatched in the eukaryotic world. This functional diversity allows them to thrive in environments too extreme for other life forms, such as deep-sea hydrothermal vents or highly acidic hot springs.
The basis for this extensive variation lies in their ability to harness energy and carbon from a wide array of sources. Unlike eukaryotes, which primarily rely on sunlight or organic compounds, prokaryotes include chemoautotrophs that synthesize organic molecules using energy derived from inorganic chemicals. They utilize compounds such as iron, sulfur, ammonia, or methane, supporting entire ecosystems in the absence of light. This metabolic flexibility, categorized by energy source (phototrophs or chemotrophs) and carbon source (autotrophs or heterotrophs), creates nutritional modes that exceed those found in plants or animals.
Genetic variation is further accelerated in prokaryotes through horizontal gene transfer (HGT), a process distinct from the vertical inheritance of genes from parent to offspring. HGT allows bacteria and archaea to rapidly acquire new traits, such as antibiotic resistance or novel metabolic capabilities, by exchanging genetic material directly between cells. This mechanism means that the genetic diversity of the population can shift quickly in response to environmental changes, making them the functionally richest domain of life.
Other Major Contributors to Global Biodiversity
Beyond the numbers of insects and the metabolic diversity of prokaryotes, other groups contribute significantly to global biodiversity. The Kingdom Fungi is a major component of terrestrial ecosystems, with an estimated 2.5 million species globally, though only about 155,000 have been formally described. Fungi play a foundational role as decomposers, recycling nutrients from dead organic matter.
Fungi form symbiotic relationships, such as mycorrhizal associations, which connect with the roots of over 90% of land plants to enhance nutrient and water uptake. The Plant Kingdom provides the basis for nearly all food webs on land, converting solar energy into biomass. Vascular plants show diversity in form and function, from towering trees to small flowering herbs. Plants are also a source of medicinal compounds, with over half of modern medicines derived from natural sources. Both plants and fungi demonstrate high levels of undiscovered diversity, with thousands of new species named annually, highlighting remaining gaps in our understanding of global life.