Biodiversity

Biodiversity refers to the variability of life on Earth, encompassing the range of genes, species, ecosystems, and evolutionary lineages that exist within the biosphere. It provides the structural and functional basis for ecological processes, ecosystem services, and the long-term resilience of living systems. Biodiversity varies across regions and ecosystems, with distribution patterns shaped by climate, productivity, geological history, and evolutionary processes. Although the concept may be defined in multiple ways, it is commonly used as a comprehensive term that captures the complexity and interdependence of life at all biological levels.

Levels and Definitions of Biodiversity

Biodiversity can be understood across several hierarchical levels, each contributing to ecological stability:

• Genetic diversity involves the variation in genetic composition within and between populations of a species. This diversity enables adaptation to changing environmental conditions.
• Species diversity refers to the variety and abundance of species within ecological communities. It is frequently measured using indices of species richness and evenness.
• Ecosystem diversity encompasses the variety of habitats, biotic communities, and ecological processes within a given region.
• Phylogenetic diversity considers the evolutionary relationships among species, representing the breadth of their shared ancestry.

Definitions continue to evolve as scientific understanding expands. Organisations such as the Food and Agriculture Organization and the World Health Organization describe biodiversity as the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems. These broad descriptions reflect efforts to capture the full extent of biological variation, from microscopic life to complex ecosystems.

Global Patterns and Distribution

Biodiversity is unevenly distributed across the planet. The most pronounced pattern is the latitudinal gradient, in which species richness increases towards the equator. Tropical ecosystems, characterised by warm temperatures and high primary productivity, contain particularly high concentrations of species. Although tropical forests occupy less than one-fifth of Earth’s land area, they are estimated to harbour approximately half of the world’s species. Similar gradients appear in marine environments, where tropical and subtropical waters support a greater diversity of organisms than temperate zones.
Environmental factors shaping these patterns include energy availability, climatic stability, and evolutionary time. Regions with long periods of stable climate have provided greater opportunities for speciation, niche diversification, and ecological complexity.

Biodiversity Through Geological Time

The history of life on Earth has been marked by dramatic fluctuations in biodiversity. Since the emergence of life through abiogenesis, there have been six major mass extinctions alongside several smaller events. The Phanerozoic aeon, covering the last 540 million years, witnessed rapid diversification, beginning with the Cambrian explosion during which most major animal phyla first appeared.
Over the subsequent hundreds of millions of years, repeated biodiversity crises occurred. Significant examples include:

• Carboniferous rainforest collapse, which caused widespread loss of plant and animal species.
• Permian–Triassic extinction, approximately 251 million years ago, the most severe mass extinction on record. Recovery of vertebrate diversity took tens of millions of years.

These events demonstrate that Earth’s biological systems have undergone substantial restructuring, with long-term consequences for evolutionary pathways.

Human Impacts and the Sixth Mass Extinction

Human activities have accelerated biodiversity loss to rates far exceeding natural background extinction levels. This trend is often referred to as the Holocene extinction or the sixth mass extinction. Habitat destruction, driven by agricultural expansion, urbanisation, and deforestation, remains one of the dominant causes of species decline. Climate change contributes additional pressures by altering temperature regimes, shifting biomes, and increasing the frequency of extreme events.
Overhunting and overexploitation have also played significant roles. Some research suggests that the extinction of megafauna near the end of the Pleistocene may have been influenced by human hunting practices in combination with climatic changes.
Current estimates suggest that up to 30% of species may face extinction by the mid-twenty-first century. Species are currently being lost at rates calculated to be 100 to 10,000 times higher than the expected background rate, indicating a widespread erosion of genetic and ecological diversity.

Estimating Species Numbers

Efforts to quantify biodiversity reveal immense complexity. A widely cited estimate proposes approximately 8.7 million terrestrial species and 2.2 million marine species, though the true number may be significantly higher due to the difficulty of detecting and describing microscopic life. Other estimates include:

• Around 220,000 known vascular plant species.
• Between 0.7 and 1 million marine species.
• Approximately 10 to 30 million insect species, of which fewer than one million have been formally described.
• Roughly 510 million bacterial species, representing a vast, poorly understood component of global biodiversity.
• Around 1.53 million fungal species inferred from molecular and ecological studies, although only a fraction has been documented.

The Global Ocean Sampling Expedition further expanded awareness of microbial diversity by identifying numerous previously unknown genetic sequences from marine plankton.
Because many taxa remain undiscovered, particularly in tropical regions and deep-sea environments, ongoing extinction may eliminate species before they are scientifically recorded.

Contemporary Biodiversity Loss

Evidence from global monitoring programmes shows steep declines in biodiversity. Reports from international conservation organisations indicate that nearly half of vertebrate populations have decreased significantly since the 1970s. Freshwater ecosystems have been particularly affected, experiencing declines exceeding 70% in some assessments.
Key findings from recent studies include:

• Approximately one-eighth of known plant species are threatened with extinction.
• Mammal populations show declining trends, with some projections suggesting that a quarter of species could face extinction within decades.
• Insect biomass in some regions has decreased by as much as 75% over recent decades, raising concerns about the stability of ecosystems reliant on pollination and nutrient cycling.

Differences in regional trajectories reflect disparities in land use, conservation capacity, and resource consumption. High-income countries have reported modest increases in monitored biodiversity metrics, though these gains are often offset by externalised environmental pressures on lower-income nations.

Consequences of Biodiversity Loss

Declining biodiversity threatens the resilience of ecosystems and the services they provide. Ecosystem services, including pollination, soil formation, water purification, climate regulation, and cultural value, depend upon diverse biological communities. Losses in biodiversity diminish natural capital and compromise the adaptive capacity of ecological systems.
The Anthropocene, characterised by significant human modification of Earth’s systems since the mid-twentieth century, has seen unprecedented environmental change. Reduced biodiversity undermines the stability of ecosystems, increases vulnerability to disturbances, and poses substantial risks to human well-being.
Scientific assessments highlight that species are being lost at 100 to 1,000 times the background rate, with extinction pressures continuing to escalate. Conservation frameworks attempt to address these challenges by identifying threatened species, protecting habitats, and promoting sustainable resource use.

Monitoring and Conservation Assessment

Formal classification systems, such as those used to assess rare, threatened, and endangered species, provide essential tools for tracking biodiversity trends. Despite substantial progress, many species remain unassessed, and millions more may be at risk without formal recognition. Advances in molecular biology, remote sensing, and ecological modelling continue to improve the accuracy of biodiversity assessments, yet the pace of extinction often outstrips scientific capacity to document and respond to losses.