There is no single “most important species on earth,” because importance depends on what you’re measuring: oxygen production, soil fertility, food supply, or the sheer number of other species that would collapse without it. But several organisms have such outsized effects on the planet that removing any one of them would be catastrophic. The strongest candidates aren’t the ones most people would guess.
Why Ecologists Avoid Picking One Winner
Ecology uses two key concepts to describe species that punch above their weight. A keystone species is one whose presence holds an entire community together; remove it, and biodiversity collapses. A foundation species physically shapes the environment other organisms depend on, usually as a primary producer bringing energy into the system. These categories overlap, and they measure different things. A tiny bacterium can be globally essential for the atmosphere while a massive tree is locally essential for a forest canopy. Asking which is “most important” is a bit like asking whether the heart or the lungs matter more.
That said, some species affect so many ecosystems simultaneously that they deserve special attention. Here are the top contenders, ranked by the scale and irreplaceability of what they do.
Phytoplankton: The Planet’s Lungs
If you had to pick one group, ocean microorganisms have the strongest case. Oceanic plankton, including drifting plants, algae, and photosynthetic bacteria, produce the majority of the oxygen in Earth’s atmosphere. One bacterium in particular stands out: Prochlorococcus, the smallest photosynthetic organism known, generates up to 20 percent of all the oxygen in the biosphere. That single microbe, invisible to the naked eye, contributes roughly as much breathable air as all the tropical rainforests combined.
Phytoplankton also form the base of nearly every marine food chain. They pull carbon dioxide from the atmosphere and, when they die, sink it to the ocean floor. Without them, atmospheric oxygen would decline, marine life would starve, and the global carbon cycle would destabilize. No other group of organisms simultaneously supports both the atmosphere and the ocean food web at this scale.
Nitrogen-Fixing Bacteria: Feeding the Soil
The atmosphere is 78 percent nitrogen, but plants cannot use it in gas form. The only organisms capable of converting atmospheric nitrogen into a biologically usable form are certain bacteria and other prokaryotes. Among them, Rhizobium bacteria form partnerships with legumes (beans, peas, clover, lentils) and represent the primary source of fixed nitrogen in land-based ecosystems.
This matters because nitrogen is the nutrient most often limiting plant growth. Without biological nitrogen fixation, soils would deplete rapidly, crop protein levels would drop, and agriculture as we know it would require even more synthetic fertilizer than it already does. These bacteria quietly provide well over half of all biologically fixed nitrogen on land. Every terrestrial food chain, from grasslands to forests to farms, depends on the nitrogen they make available.
Mycorrhizal Fungi: The Underground Network
Roughly 90 percent of land plants form symbiotic relationships with fungi that colonize their roots. The fungus threads itself through the soil, dramatically extending a plant’s reach, and delivers nitrogen and phosphorus the plant couldn’t access on its own. In return, the plant feeds the fungus carbon from photosynthesis. In some species, nutrient uptake through this fungal partnership is 10 to 60 times more efficient than uptake through roots alone.
These fungi don’t just help individual plants. They decompose complex organic matter, recycling nutrients back into the soil. They connect trees in a forest through underground networks, allowing resources to flow between individuals. Without mycorrhizal fungi, most forests, grasslands, and croplands would struggle to sustain themselves. Soil fertility as we understand it is largely a product of this ancient partnership.
Pollinators: Sustaining the Food Supply
Bees, butterflies, moths, bats, and other pollinators are responsible for the reproduction of roughly 75 percent of flowering plant species. Their economic value to global agriculture exceeds $235 billion per year, according to the United Nations Food and Agriculture Organization. Without pollinators, crops like almonds, apples, blueberries, cocoa, and coffee would either vanish or require expensive and inefficient hand-pollination.
Bees are the most important group within this category, but they’re far from the only ones. Flies, beetles, and even some birds contribute. The decline in pollinator populations over the past two decades is one of the reasons this group frequently appears in “most important species” discussions: their loss is already measurable, and its consequences are already being felt in reduced crop yields and rising food prices in some regions.
Ants: Tiny Engineers With Massive Reach
Ants are sometimes overlooked, but their collective impact on terrestrial ecosystems is enormous. Their estimated global population is in the quadrillions, and their combined biomass rivals that of all wild birds and mammals put together. Through nest construction and maintenance, ants physically restructure soil, improving aeration and drainage. Studies on harvester ants in Mediterranean grasslands found that soil near ant colonies contained 51 percent more available phosphorus and 39 percent more ammonium than surrounding areas.
Ants also disperse seeds, decompose organic material, and serve as a food source for countless other animals. Their engineering activities alter soil texture, nutrient availability, and even which plants can grow in a given area. They occupy nearly every terrestrial habitat on Earth, making their cumulative influence on soil health and plant communities difficult to overstate.
Humans: The Hyperkeystone Problem
Ecologists have proposed classifying humans as a “hyperkeystone” species, meaning a species that doesn’t just influence its own ecosystem but affects multiple keystone species across entirely different habitats. The median rate at which humans exploit marine and terrestrial species is up to 15 times higher than that of any other predator. No other organism simultaneously alters ocean chemistry, forest composition, grassland ecology, and atmospheric conditions.
This influence cuts both ways. Humans have driven hundreds of species to extinction, disrupted nitrogen and carbon cycles, and fragmented habitats worldwide. But humans also protect endangered species, restore degraded land, and manage ecosystems in ways no other organism can. The designation of “most important” here is less about ecological value and more about sheer impact, both constructive and destructive.
So Which One Is It?
If the question is which single species’ disappearance would cause the most widespread damage, the answer is probably Prochlorococcus or, more broadly, phytoplankton as a group. They produce most of the planet’s oxygen, anchor marine food webs, and regulate atmospheric carbon. Losing them would make Earth uninhabitable far faster than losing any other single organism.
But “importance” really describes a web of interdependence. Phytoplankton need the nutrients that bacteria and fungi cycle through soil and ocean. Plants need pollinators to reproduce and fungi to absorb nutrients. Soil needs ants and microbes to stay fertile. Each of these groups is irreplaceable in its own domain, and the collapse of any one would cascade through ecosystems worldwide. The honest answer is that Earth’s most important species is whichever one you remove first.