Biodiversity

• 1. Introduction

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  Biodiversity is the variability of life on Earth, and it is not spread evenly across the planet. Tropical forest ecosystems cover less than one-fifth of Earth's terrestrial area, yet they hold about 50% of the world's species. That single imbalance hints at a stranger truth. In 1768, a clergyman named Gilbert White looked at his patch of Selborne in Hampshire and wrote that nature is so full that the most variety is found in the place most examined. Two and a half centuries later, scientists still cannot agree on how many species share the Earth with us. Estimates run from 2 million to 100 million. One 2016 report pushed the figure to a trillion, of which only one-thousandth of one percent has been described. How do you measure the variety of life when most of it remains unknown? Why does life cluster so heavily near the equator? And what does it mean that the variety we can see is now shrinking faster than at almost any moment in the fossil record?

• 2. Defining The Undefinable

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  Biologists most often describe biodiversity as the totality of genes, species and ecosystems of a region. The phrasing is deliberate. It unifies several older ideas of variety: taxonomic diversity measured at the species level, ecological diversity seen through ecosystems, morphological diversity arising from genetic and molecular differences, and functional diversity, which counts how many functionally different species live in a population. A predator and its prey, two creatures with different feeding mechanisms, different ways of moving, all count as functionally distinct. Yet no single fixed definition exists, because the meaning keeps being reimagined. In 2019, the Food and Agriculture Organization of the United Nations called it the variability among living organisms within and between species and the ecosystems they belong to. The World Health Organization settled on variability among living organisms from all sources. The word itself is young. In 1916, J. Arthur Harris used biological diversity in an article titled The Variable Desert in Scientific American. The contracted form biodiversity came later. According to Edward O. Wilson, it was coined by W. G. Rosen for the National Forum on BioDiversity, held in September 1986. Rosen introduced the term, and it spread quickly from there.

• 3. The Uncounted Majority

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  According to estimates by Mora and colleagues in 2011, there are roughly 8.7 million terrestrial species and 2.2 million oceanic species. The authors warned that even these figures probably understate prokaryotic diversity, the world of bacteria and archaea. The numbers below the headline are staggering and uncertain. Insects may number between 10 and 30 million, though only about 0.9 million are known today. Bacteria range from 5 to 10 million, fungi from 1.5 to 3 million, of which just 0.075 million had been documented by 2001. The Global Ocean Sampling Expedition, working from near-surface plankton over the 2004 to 2006 period, identified so many new genes that its findings may eventually change how science defines a species at all. Birds and mammals are the most studied animal groups, while fishes and arthropods are the least studied. Nearly 90% of all arthropods are not yet classified. Because extinction is speeding up, many species may vanish before anyone ever describes them. Less than 1% of all described species have been studied beyond noting that they exist.

• 4. Why Life Crowds The Equator

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  Diversity climbs steadily from the poles toward the tropics, a pattern known as the latitudinal gradient in species diversity. The deepest cause is mean temperature, far higher at the equator than at the poles. Rainforests that have stayed wet for ages, such as Yasuni National Park in Ecuador, are especially rich. Colombia carries this to an extreme. It holds the highest rate of species per unit area in the world and the largest number of endemics of any country, creatures found naturally nowhere else. About 10% of Earth's species live there, including over 1,900 bird species, more than Europe and North America combined. Brazil's Atlantic Forest is a biodiversity hotspot, a term Norman Myers introduced in 1988 for regions rich in endemic species that have suffered great habitat loss. It contains roughly 20,000 plant species, 1,350 vertebrates and millions of insects, about half found nowhere else. The rule is not universal. The latitudinal pattern is contested in marine ecosystems, and soil bacterial diversity peaks in temperate zones rather than the tropics. Madagascar offers a clue to how such concentrations form. Since the island separated from mainland Africa 66 million years ago, its species and ecosystems evolved on their own.

• 5. Three And A Half Billion Years

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  Biodiversity is the result of 3.5 billion years of evolution. The age of the Earth itself is about 4.54 billion years, and the earliest undisputed evidence of life dates from at least 3.7 billion years ago. Until roughly 2.5 billion years ago, all life consisted of microorganisms: archaea, bacteria, and single-celled protozoans and protists. The great acceleration came during the Phanerozoic, the last 540 million years, and above all in the Cambrian explosion, when nearly every phylum of multicellular organisms first appeared. The 400 million years that followed were not a smooth ascent. They were punctuated by massive losses called mass extinction events. When rainforests collapsed in the Carboniferous, limbed vertebrates without amniotic eggs suffered a significant loss. The worst of all came 251 million years ago, the Permian-Triassic extinction event. Vertebrates needed 30 million years to recover. The most recent major event, the Cretaceous-Paleogene extinction 66 million years ago, drew more attention than the rest because it ended the non-avian dinosaurs, along with marine creatures like the ammonites. It is estimated that between 5 and 50 billion species have existed across this span. Greater than 99% of them went extinct before humans ever evolved.

• 6. The Sixth Extinction

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  During the last century, biodiversity loss has been observed with rising frequency. It was estimated in 2007 that up to 30% of all species could be extinct by 2050. Species are vanishing now at a rate 100 to 1,000 times higher than the background rate, and that rate is still climbing. This is the Holocene extinction, the sixth mass extinction, and most biologists agree it is driven primarily by human impact on the environment. The numbers come from many directions. The Living Planet Report 2014 found that mammal, bird, reptile, amphibian and fish populations were on average about half the size they had been 40 years earlier. Latin America took the biggest hit, falling 83 percent. A 2017 study in PLOS One reported that insect biomass in Germany had dropped by three-quarters over 25 years. Dave Goulson of Sussex University warned that humans appear to be making vast tracts of land inhospitable to most forms of life, and are on course for ecological Armageddon. If we lose the insects, he said, then everything is going to collapse. A 2020 World Wildlife Fund report stated that biodiversity is being destroyed at a rate unprecedented in human history, with 68% of the studied populations destroyed between 1970 and 2016. The anthropogenic story may reach back to the end of the last ice age, when overhunting helped drive the megafaunal extinction.

• 7. What Biodiversity Gives Back

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  In 1997, Robert Costanza and his colleagues estimated the global value of ecosystem services not captured in traditional markets at an average of $33 trillion annually. These services fall into three kinds: provisioning ones that produce food, wood and fresh water; regulating ones that lessen environmental change like climate regulation and pest control; and cultural ones that carry human value, from landscape beauty to spiritual meaning. The economic activity of pollination alone represented between $2.1 and 14.6 billion in 2003. Humans cannot easily replace these systems. Insect pollination cannot be mimicked, though some have tried building artificial pollinators using unmanned aerial vehicles. The benefits run deep into human survival. At least 50% of the pharmaceutical compounds on the US market derive from plants, animals and microorganisms, and about 80% of the world's population depends on medicines from nature for primary healthcare. Marine organisms have opened paths to treatments for cancer, viral bacteria, and AIDS. Biodiversity also shapes disease itself. Felicia Keesing of Bard College and Drew Harvell of Cornell University co-authored work showing that the species most likely to disappear are the ones that buffer against infectious disease, while the survivors tend to spread it, including West Nile Virus, Lyme disease and Hantavirus. Roughly 80 percent of humanity's food supply comes from just 20 kinds of plants, yet humans use at least 40,000 species.

• 8. Drawing The Lines Of Protection

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  On the 19th of December 2022, at the United Nations Biodiversity Conference, every country on Earth except the United States and the Holy See signed an agreement to protect 30% of land and oceans by 2030, a target known as 30 by 30. At the moment of adoption, 17% of land and 10% of ocean were protected. The pact also commits to recovering 30% of degraded ecosystems. Protected areas are the front line. There are over 238,563 of them worldwide, covering 14.9 percent of Earth's land surface, and they range widely in size and management. Forest protected areas matter most of all, since forests harbour about 80% of the world's terrestrial biodiversity. More than 700 million hectares of forest fall within legally established protected areas. South America holds the highest share of forests in protected areas at 31 percent. The legal scaffolding stretches across decades and borders. The United Nations Convention on Biological Diversity opened in 1992, and there have since been 16 Conferences of the Parties, most recently COP 16 in Cali, Colombia, in 2024. India passed its Biological Diversity Act in 2002, building in mechanisms to share the benefits of traditional resources fairly. Ordinary people are part of the count too. Platforms like iNaturalist and eBird let volunteers record what they see, and nearly half of all occurrence records shared through the Global Biodiversity Information Facility come from datasets with significant volunteer contributions. The 2019 IPBES Global Assessment concluded that the state of nature has deteriorated at an unprecedented and accelerating rate, and that only transformative change can reverse it.