The natural world encompasses every living organism on Earth and every non-living system those organisms depend on, from ocean currents and atmospheric gases to bedrock minerals and the energy pouring in from the sun. It is not a single thing but a web of interacting parts: an estimated 8.7 million species of complex life, an unknown but staggering number of microbes, and the physical and chemical cycles that connect them all. The boundaries of what “counts” as natural are less obvious than they seem, though, especially as human activity reshapes ecosystems faster than at any point in recorded history.
Living Things, from Forests to the Upper Atmosphere
When most people think of the natural world, they picture animals and plants. That picture is real but incomplete. A widely cited modeling study predicted roughly 7.77 million animal species, about 298,000 plant species, around 611,000 fungal species, and smaller numbers of protozoa and chromists, for a total of roughly 8.7 million species of complex (eukaryotic) life on Earth, with about 2.2 million of those in the oceans.1PLoS Biology. How Many Species Are There on Earth and in the Ocean? Most of those species have never been formally described by science. Animals alone dwarf everything else in sheer number of species, but plants and fungi punch far above their weight in biomass, making up the bulk of the living material on Earth’s surface.
Then there are microbes, which upend our sense of scale entirely. Bacteria and archaea are not a footnote to the natural world; they are its foundation. Between 12 and 20 percent of Earth’s total biomass lives in the terrestrial deep subsurface, in rock and sediment hundreds or thousands of meters underground, compared to roughly 1.8 percent in the deep ocean floor.2PubMed Central. A global perspective on bacterial diversity in the terrestrial deep subsurface These organisms survive in conditions that would kill most surface life: extreme heat, crushing pressure, no sunlight, and vanishingly little oxygen.
Life doesn’t stop at the soil surface or the ocean’s edge. Viable bacterial cells make up about 20 percent of the tiny particles floating in the upper troposphere, more than ten times the abundance of fungal cells at that altitude.3PubMed Central. Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications Researchers have detected microbial taxa from every major biological lineage in Earth’s upper atmosphere.4PubMed. Microbes in the upper atmosphere and unique opportunities for astrobiology research Some of those airborne bacteria appear to feed on simple carbon compounds drifting in the air, and the whole floating community may influence cloud formation and the water cycle. The natural world, in other words, is not just where we see green. It’s a thin layer of life that extends from deep rock to the edge of space.
The Non-Living Systems That Hold It All Together
Life gets the attention, but the natural world is just as much about water, rock, air, and energy. Scientists often talk about “Earth’s spheres” as a way to carve up these interconnected systems, and the vocabulary keeps expanding as researchers model how the pieces interact across different scales of space and time.5Progress in Physical Geography: Earth and Environment. Earth’s spheres: Conceptual and definitional debates But you don’t need the jargon to grasp the key point: abiotic systems are not just a backdrop for life. They actively shape it, and life shapes them right back.
Water is the most obvious example. Oceans, rivers, lakes, glaciers, and groundwater cover about 71 percent of Earth’s surface.6Open Access Journal of Agricultural Research. The Impact of Hydrosphere on Climate Change That water doesn’t just sit there. When it evaporates, it cools its surroundings; when it condenses into rain or snow, it releases energy and warms them. Water moves heat from the equator toward the poles, feeds every terrestrial ecosystem, and sets the basic conditions for where different organisms can survive. The cycling of water through atmosphere, land, and ocean is one of the natural world’s most powerful engines.
Chemical cycling matters just as much. The nitrogen and carbon cycles, for instance, are deeply tangled with each other. Human additions of nitrogen to the environment since the 1860s have helped terrestrial ecosystems absorb extra carbon, but nitrogen availability also limits how much additional carbon plants and soils can take up, especially in cooler regions. One model-based estimate found that nitrogen scarcity could reduce future carbon uptake by up to 70 percent in some areas compared to what would happen if nitrogen weren’t limiting.7PubMed Central. Terrestrial nitrogen–carbon cycle interactions at the global scale These nutrient cycles are invisible to the casual observer, but they govern how ecosystems grow, how the atmosphere changes, and ultimately how the climate behaves.
Geology, too, is part of the natural world in a very active sense. Volcanic eruptions recycle minerals and gases from deep in the Earth back to the surface. Tectonic movement creates mountain ranges that steer weather patterns and fragment habitats, driving the evolution of new species. Even the soil beneath your feet is a product of millennia of interactions between rock, water, microbes, and plant roots. There is no clean line between the living and non-living parts of nature; they co-produce each other.
Life Where You Wouldn’t Expect It
One of the more striking lessons of modern biology is that nature shows up in places once thought to be sterile. Deep-sea hydrothermal vents are a dramatic example. These ecosystems, scattered along mid-ocean ridges where tectonic plates are spreading apart, are fueled not by sunlight but by chemical energy from superheated water rich in hydrogen sulfide and metals. Microbes at the base of the food web convert those chemicals into usable energy, and entire communities of tube worms, shrimp, and other animals cluster around the vents in total darkness.8Frontiers in Marine Science. Active hydrothermal vent ecosystems in the Indian Ocean are in need of protection Vent ecosystems are among the most pristine on Earth, and they operate on completely different rules from the sunlit surface world most of us picture when we hear “nature.”
The deep subsurface is another frontier. As mentioned earlier, a substantial share of Earth’s biomass lives underground, and researchers continue to discover new microbial lineages in deep boreholes and mine shafts. These organisms survive on chemical energy from mineral reactions, often metabolizing so slowly that their generation times are measured in centuries rather than hours. Their existence pushes the natural world’s boundary deep into the planet’s crust.
And then there is the atmosphere. The discovery of a viable microbial community in the upper troposphere blurs the line between “nature” and “empty sky.” Some of those bacteria aren’t just passively drifting; they appear to be metabolically active, using atmospheric carbon compounds as food.3PubMed Central. Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications Hurricanes can sweep huge numbers of new cells aloft, reshuffling the airborne community in a single storm. The natural world, it turns out, fills every niche the planet offers.
Where Does “Nature” End and “Human” Begin?
This is the question that makes the concept of the natural world genuinely complicated. For centuries, Western thinking drew a hard line between human civilization and wild nature. That line has always been leaky, and modern ecology has largely abandoned it.
Cities are a good case study. Urban areas contain biological components like plants, animals, and microorganisms interacting with physical components such as soil, water, air, and local climate. In terms of “green infrastructure,” these elements combine natural and constructed materials in ways that support real metabolic processes, biodiversity, and ecosystem services.9IntechOpen. Urban Ecosystem: An Interaction of Biological and Physical Components The peregrine falcon nesting on a skyscraper ledge, the coyote trotting through a suburb, the mycorrhizal network in a park’s soil: all of these are the natural world operating inside a human-built environment. Nature doesn’t respect city limits.
Climate change, species introductions, and land-use shifts are also producing what ecologists call “novel ecosystems,” assemblages of species that have no historical precedent. These ecosystems emerge when conditions change so far from the original state that conventional restoration is no longer practical; they are self-organizing and self-sustaining, not directly managed by humans on an ongoing basis.10Restoration Ecology. Novel and designed ecosystems Ecological novelty is accelerating worldwide, driven by the same global-change pressures that dominate environmental headlines.11Methods in Ecology and Evolution. The making of novel ecosystems: A process‐based framework for measurement, analysis and application A meadow colonized by a mix of native and introduced grasses after farmland abandonment is “natural” in that nobody is tending it, but “unnatural” in that the species mix never existed before humans changed the landscape. Drawing a crisp boundary between nature and artifice becomes an exercise in philosophy rather than science.
Humans as Part of the Natural World
Biologically, humans are unambiguously part of nature. We are primates, shaped by the same evolutionary pressures as every other species. One way to see our ecological position is through trophic level, a measure of where an organism sits in the food chain. A global analysis calculated the average human trophic level at about 2.21, roughly the same as anchoveta (a small schooling fish), because so much of our diet comes from plants and low-level animal products rather than from apex predators.12PubMed Central. Eating up the world’s food web and the human trophic level National averages range from about 2.04 to 2.57, reflecting the enormous diversity of human diets worldwide. That global figure has been rising over time as diets shift toward more meat.
Our trophic position has changed dramatically over evolutionary time, too. Evidence suggests the human lineage started at a low trophic level, climbed to a strongly carnivorous position during the Pleistocene (peaking with Homo erectus), and then reversed with the spread of agriculture in the Neolithic period.13PubMed. The evolution of the human trophic level during the Pleistocene We are, in a real ecological sense, mid-level omnivores whose dietary flexibility has been central to our success.
Many Indigenous worldviews never separated humans from the natural world in the first place. The concept of kincentric ecology, for instance, frames people and the rest of the living world as relatives bound in reciprocal relationships.14PubMed. Mother Earth kinship: Centering Indigenous worldviews to address the Anthropocene and rethink the ethics of human-to-nature connectedness In this view, the question “what is the natural world?” doesn’t produce a list of things separate from people. It produces a web of obligations and connections. Western science is now converging on a similar insight, recognizing through systems modeling and Earth-system science that human activity is so deeply interwoven with planetary processes that it makes little sense to study them in isolation.5Progress in Physical Geography: Earth and Environment. Earth’s spheres: Conceptual and definitional debates
Soundscapes and the Sensory Dimension of Nature
The natural world isn’t only a collection of organisms and chemical cycles. It also has sensory dimensions that matter both ecologically and to human experience. Ecoacoustics, a growing field, treats the soundscape as an integral layer of any landscape. A natural soundscape includes three components: biological sounds produced by animals and insects (biophony), geophysical sounds like wind, rain, and flowing water (geophony), and human-generated noise (anthropophony).15Ecological Indicators. Exploring the relationship between the soundscape and the environment: A systematic review These sounds are not just atmospheric decoration. Animal vocalizations encode information about species identity, territory, and mating readiness. The sound of flowing water signals moisture and food sources. And human noise pollution can mask those signals, disrupting behavior and reducing habitat quality even in places that look untouched.
There is also a psychological angle. The biophilia hypothesis proposes that humans have an innate biological and emotional connection to nature, an evolved tendency to pay attention to and respond positively to natural stimuli.16PubMed Central. A Meta-Analysis of Emotional Evidence for the Biophilia Hypothesis and Implications for Biophilic Design That idea has been influential in architecture and urban design, where “biophilic design” tries to bring natural elements into built spaces. Whether the hypothesis fully holds up to scrutiny is debated; some researchers argue the human attraction to nature is partly learned and culturally shaped, not purely innate.17PubMed. Our not-so-natural connection to nature But the broad pattern is real enough to matter for public health and design. People consistently report feeling calmer and more restored after spending time in environments rich in natural sights and sounds, and this effect shows up across a range of study designs. The natural world is something we experience with all our senses, not just something we catalog in a database.
Planetary Boundaries and the Scale of Human Pressure
Understanding what the natural world includes also means understanding how it is changing. The planetary boundaries framework is one attempt to define “safe operating space” for human civilization within Earth’s systems. A 2023 update found that six of the nine identified boundaries have been transgressed, suggesting that Earth is now well outside that safe space.18PubMed Central. Earth beyond six of nine planetary boundaries The boundaries cover processes like climate change, biodiversity loss, nitrogen and phosphorus flows into ecosystems, freshwater use, land-system change, and the introduction of novel entities such as synthetic chemicals.
The framework is useful because it maps the natural world’s major systems against the pressures humans are placing on them. It reveals that the threats are not isolated. Excess nitrogen from agriculture doesn’t just pollute waterways; it alters how much carbon ecosystems can absorb, which feeds back into climate change.7PubMed Central. Terrestrial nitrogen–carbon cycle interactions at the global scale Biodiversity loss reduces the resilience of ecosystems to handle drought or disease. Land-use change simultaneously drives species extinction, alters local climate, and disrupts water cycles. These interactions are what make the natural world a single, interconnected system rather than a list of separate topics. Crossing one boundary makes it harder to stay within the others.
For anyone wondering what the natural world “includes,” the planetary boundaries offer a practical map. The answer is: the climate system, the ozone layer, the chemical composition of oceans, the nitrogen and phosphorus cycles, freshwater systems, land cover, biodiversity, aerosol pollution, and the chemical pollutants we introduce. All of these interact, and all of them are part of the natural world that sustains human civilization, whether or not we notice them on a daily basis.
What Gets Left Out of Typical Definitions
Popular depictions of the natural world lean heavily on charismatic animals and scenic landscapes, which is understandable but skews perception. A few things routinely get left out:
- Fungi: With an estimated 611,000 species, fungi rival plants in diversity and wildly outstrip them in ecological roles. They decompose dead material, form symbiotic networks with plant roots, and regulate nutrient cycling in soil. Yet they rarely appear in nature documentaries or conservation campaigns.
- Subsurface life: The deep biosphere contains a substantial fraction of all living cells on Earth, yet it is almost invisible in public discussions of nature. These organisms influence mineral weathering, groundwater chemistry, and even the formation of certain oil and gas deposits.
- Atmospheric biology: The airborne microbiome is a functional ecological layer, not just a curiosity. Bacteria and fungal spores drifting at altitude can seed cloud droplets and affect precipitation patterns, linking the living world directly to weather.
- Soil: A single gram of fertile soil can contain billions of microbes, and soil organisms drive the nutrient cycles that feed every terrestrial ecosystem. Soil degradation is one of the least-discussed environmental crises despite being among the most consequential.
Leaving these out isn’t just an aesthetic oversight. It shapes what we choose to protect. Conservation funding and public attention flow disproportionately toward large animals and photogenic habitats, while the microbial, fungal, and geological foundations of those ecosystems receive comparatively little. A fuller picture of what the natural world includes would shift those priorities toward the invisible systems that hold the visible ones together.