An ecosystem is every living organism in an area plus every non-living condition those organisms depend on, all functioning as a connected system. Strip away either half and the whole thing collapses: plants cannot grow without soil, water, and sunlight, and bare rock with no life on it is just geology, not ecology. What makes ecosystems fascinating is that the living and non-living parts do not simply coexist; they constantly reshape each other in ways that can surprise even researchers who study them for a living.
The Non-Living Foundation
The non-living components of an ecosystem, often called abiotic factors, set the physical and chemical stage on which life plays out. Temperature is the most studied of these factors, partly because decades of thermal biology research have made it the most accessible abiotic variable to measure and model. But temperature is far from the only thing that matters. Global gradients in oxygen availability, light intensity, atmospheric and water pressure, pH, and water availability all shift with latitude, elevation, and ocean depth, and species show strong physiological adaptations to these variables within their native ranges.1Ecography. The challenge of novel abiotic conditions for species undergoing climate‐induced range shifts
Soil pH is a good example of an abiotic factor with outsized influence. In soil ecosystems, pH shapes which microbes thrive and which struggle, and it does so both directly and indirectly. Research on tundra habitats found that soil pH acts as what the authors called an “ultimate environmental driver,” triggering cascading differences in plant communities, soil organic matter quality, and microbial populations between acidic and non-acidic sites.2PubMed. Links between plant community composition, soil organic matter quality and microbial communities in contrasting tundra habitats Incubation experiments have confirmed that strongly acidic soils (pH around 4) produce less microbial biomass in response to added organic material than soils closer to neutral, showing a direct pH effect on how quickly dead matter gets broken down.3Soil Biology and Biochemistry. Substrate inputs and pH as factors controlling microbial biomass, activity and community structure in an arable soil The bacterial community that assembles around decomposing material depends on the interaction between pH and the quality of the organic matter itself, while fungal communities respond primarily to pH alone.4European Journal of Soil Biology. Interactive effects of soil pH and substrate quality on microbial utilization
Water is another abiotic factor that does more than just keep organisms hydrated. In Alaskan tundra, channels of subsurface water flow boost plant productivity by roughly 2.4 times compared with adjacent areas that lack that flow. The moving water carries dissolved nitrogen and phosphorus to plant roots faster than those nutrients could diffuse on their own, which largely explains why a single sedge species produced ten times more aboveground growth in the water tracks than outside them.5Ecology. Productivity and Nutrient Cycling of Alaskan Tundra: Enhancement by Flowing Soil Water The water, nitrogen, and carbon cycles in terrestrial ecosystems are tightly coupled: as the water flux through a landscape increases, the potential for photosynthesis and the inputs and losses of nitrogen all rise together.6PubMed. Equilibration of the terrestrial water, nitrogen, and carbon cycles Change one and you inevitably change the others.
The Living Cast
The biotic side of an ecosystem includes every organism present, from bacteria in the soil to apex predators. These are often grouped by what they eat: producers (plants, algae, and certain bacteria that make their own food from sunlight or chemical energy), consumers (animals that eat other organisms), and decomposers (fungi, bacteria, and invertebrates that break down dead material). But the boundaries between these roles are blurrier than textbook diagrams suggest.
Dead organic matter, or detritus, deserves special attention because it tends to get treated as a footnote when it is actually a major player. Detritus serves as both a resource and a habitat for many species. Traditional food-web diagrams emphasize the path from living plants to herbivores to predators, but the detrital pathway often carries more energy, especially on land. Through its influence on food-web composition and dynamics, detritus frequently increases system stability and persistence, with substantial effects on trophic structure and biodiversity.7Ecology Letters. Detritus, trophic dynamics and biodiversity In forests, how quickly carbon, nitrogen, phosphorus, and sulfur are released from soil organic matter depends on the nutrient ratios in that material, not just its total mass.8Frontiers. Soil Organic Matter Mineralization as Driven by Nutrient Stoichiometry in Soils Under Differently Managed Forest Stands Nutrient-poor forest soils can actually release proportionally more nutrients per unit of organic carbon than nutrient-rich ones, which is not what you would expect if you assumed decomposition simply scales with nutrient content.
How Energy Flows Through the System
Energy is the currency that links the living and non-living parts of an ecosystem. In most ecosystems, the chain starts with sunlight captured by photosynthesis. That energy then passes from producers to herbivores to predators, losing a large chunk at each step. For decades, ecologists used a rule of thumb: about 10% of the energy at one level makes it to the next. A global synthesis of over 2,000 estimates from 122 studies found the real average is closer to 6% for energy transfer, well below that familiar figure. Marine ecosystems came closest to the old rule at about 8%, freshwater systems averaged around 5.5%, and terrestrial ecosystems lagged far behind at roughly 1.5%.9Science Advances. Global synthesis reveals systematic variation in trophic transfer efficiency across and within ecosystems
That inefficiency matters because it means terrestrial food webs can support fewer trophic levels before running out of energy entirely. The difference between aquatic and terrestrial systems comes down largely to the producers themselves. Phytoplankton are tiny, lack structural tissues like wood, grow fast, and are more nutritious to the animals that eat them than most land plants. Size and trophic position are also more tightly correlated in plankton food webs, while on land, a small beetle can occupy a higher trophic level than a large deer.10PubMed Central. All wet or dried up? Real differences between aquatic and terrestrial food webs
Warming appears to make this energy transfer even less efficient. A seven-year warming experiment on freshwater plankton found that 4°C of warming decreased nitrogen transfer efficiency, used as a proxy for overall energy transfer, by up to 56%. Both phytoplankton and zooplankton biomass dropped in the heated ponds, meaning less energy was getting captured by photosynthesis and less of what was captured made it up the food chain.11Nature. Warming impairs trophic transfer efficiency in a long-term field experiment If that pattern scales to real-world warming, it means more of the carbon fixed by plants and algae will be lost as heat to the atmosphere rather than fueling animals higher up the food web.
Nutrient Cycles That Bind It All Together
Energy flows through an ecosystem in one direction, from the sun (or chemical sources) outward as heat. Nutrients, by contrast, cycle. Carbon, nitrogen, phosphorus, sulfur, and water move between the living and non-living pools in loops that can take anywhere from hours to millennia to complete. These biogeochemical cycles are what make ecosystems self-sustaining over time, as opposed to a one-way machine that would stall the moment its fuel ran out.
The nitrogen cycle illustrates how tightly bound the living and non-living parts of an ecosystem really are. Nitrogen gas is abundant in the atmosphere but useless to most organisms until bacteria convert it into forms plants can absorb. This process of nitrogen fixation, along with nitrification, denitrification, and ammonification, is carried out almost entirely by soil microbes.12Nitrogen. Ecosystem and Climate Change Impacts on the Nitrogen Cycle and Biodiversity When those microbial communities are altered by climate change or pollution, the whole cycle shifts. Interestingly, not every ecosystem runs all parts of the nitrogen cycle at the same intensity. In some prairie soils, researchers found that genes for nitrogen fixation were present but barely active, while nitrification and denitrification genes were running normally, creating a gap in the local nitrogen budget.13Nitrogen. A Gap in the Prairie Nitrogen Cycle: Nitrogen Fixation Is Low, Despite Presence of Diverse Nitrogen Fixing Bacteria
When Living Things Reshape Their Non-Living Surroundings
The abiotic-to-biotic direction of influence gets the most attention: temperature limits where species can live, soil pH dictates which microbes dominate, water availability controls productivity. But the reverse direction is just as real. Organisms routinely alter the non-living environment around them, sometimes profoundly.
Ecosystem engineers are species that physically modify habitats. A Mediterranean harvester ant studied in dry grasslands is a striking example. By building nests and clearing vegetation, the ant changes soil chemistry, alters the plant community on the surface, and reshapes the invertebrate and microbial communities below ground. The engineering activity creates habitat heterogeneity at a local scale, which increases the number of ecological niches available and can boost overall biodiversity.14PubMed Central. Above- and below-ground effects of an ecosystem engineer ant in Mediterranean dry grasslands When multiple engineers coexist in the same system, their combined effects can compound. Research on grasslands with cattle, vegetation, and macrodetritivores (large soil invertebrates that shred dead plant material) found that all three and their interactions modified the physical environment of soil fauna, particularly through changes in soil porosity and litter availability, shifting the composition of tiny soil-dwelling arthropods called springtails.15PubMed Central. Impact of three co-occurring physical ecosystem engineers on soil Collembola communities
These feedback loops mean that ecosystems are not just collections of organisms sitting on an abiotic stage. The organisms are constantly remodeling the stage itself, which then favors different organisms, which remodel it further. This is one reason ecosystems can be surprisingly resistant to disturbance: the living components have reshaped the non-living environment in ways that reinforce the system’s current state.
Where One Ecosystem Ends and Another Begins
Ecosystems rarely have sharp borders. The transition zone between a forest and a grassland, or between a coral reef and the open ocean, is itself a distinct environment. Ecologists call these zones ecotones, and they play a disproportionate role in controlling the flow of organisms, materials, and energy across landscapes.16Ecosystems. Toward Conceptual Cohesiveness: a Historical Analysis of the Theory and Utility of Ecological Boundaries and Transition Zones
A common assumption is that transition zones are hotspots of biodiversity because they blend species from both sides. The reality is more complicated. Field studies of the boundary between cereal fields and grassland found no increase in species richness in the transition strip; instead, only a few species could tolerate the distinct environmental conditions there, making the zone poorer in species than either adjacent area.17Acta Oecologica. The status of transitions between cultivated fields and their boundaries: ecotones, ecoclines or edge effects? In coastal tropical seascapes, the boundaries between mangroves, seagrass beds, and open water are fuzzy, and carbon dynamics across those transitions remain poorly understood, making it hard to account for them in climate models.18Limnology and Oceanography. Edge effects impact blue carbon dynamics across coastal ecotones in a tropical seascape
Scale also matters in surprising ways. You might expect that bigger habitat patches automatically harbor more species, and for free-living fungi in bromeliad “islands” that pattern holds weakly, with area explaining only about 10-12% of the variation in fungal richness. But host-associated bacteria and fungi showed no relationship with island size at all, suggesting that for microbes living inside another organism, the habitat patch that matters is the host, not the landscape.19The ISME Journal. Microbiome spatial scaling varies among members, hosts, and environments across model island ecosystems Meanwhile, arthropod diversity in alpine tundra is best explained not by broad landscape features but by fine-scale differences in soil moisture and soil temperature at specific locations.20PubMed Central. Arthropod diversity in the alpine tundra using metabarcoding: Spatial and temporal differences in alpha- and beta-diversity The upshot is that “an ecosystem” can be meaningfully defined at almost any spatial scale, from a rotting log to the entire Amazon basin, depending on the question you are asking.
How Human Activity Rewires Ecosystems
When people alter the non-living conditions of an ecosystem, the living components respond, and the effects propagate through the web of interactions. In European terrestrial vertebrate food webs spanning over 800 species, direct exploitation (hunting, fishing, logging) and agricultural intensification were the two most damaging threats, affecting about 34% and 31% of species respectively. Because those species interact with others through predation and competition, the threatened species connected to roughly 85% and 69% of all trophic interactions across Europe.21PubMed. Vulnerability of terrestrial vertebrate food webs to anthropogenic threats in Europe Losing even a fraction of species can unravel a disproportionate share of the food web’s connections.
Climate change adds another layer. In marine mesocosm experiments, warming combined with ocean acidification reduced energy flow from the base of the food web to secondary consumers, while acidification alone actually increased energy flow at the lowest levels.22PLOS Biology. Climate change could drive marine food web collapse through altered trophic flows and cyanobacterial proliferation In lake ecosystems, warming generally shifts the balance between top-down control (predators controlling prey) and bottom-up control (resource availability controlling consumers), with resources increasingly controlling consumers under warmer conditions, especially when phosphorus levels are high.23Nature Climate Change. Disruption of ecological networks in lakes by climate change and nutrient fluctuations
Ecosystems do have some capacity to absorb shocks. In China’s drylands, ecosystem resilience to drought increased from arid to sub-humid regions, and grasslands showed higher resilience but lower resistance than forests, meaning grasslands were more likely to lose vegetation during a drought but bounced back faster afterward. Systems that experienced frequent droughts actually developed a kind of “drought memory” that boosted their resilience over time.24Agricultural and Forest Meteorology. Evaluation of ecosystem resilience to drought based on drought intensity and recovery time Plant traits also matter: species that can resprout from roots after a fire are more resilient to shortened fire intervals than species that depend entirely on seeds, though even resprouters need successful seedling recruitment to sustain populations over the long run.25Journal of Ecology. Resistance and resilience to changing climate and fire regime depend on plant functional traits
Ecosystems Without Sunlight
Nearly all the ecosystems you encounter day to day run on solar energy, but some run entirely on chemistry. Deep-sea hydrothermal vents, first discovered in 1977, host thriving communities powered by chemosynthetic bacteria and archaea that exploit the chemical contrast between hot, mineral-rich vent fluid and cold, oxygen-rich seawater. These microbes fix inorganic carbon into biomass the way plants do with photosynthesis, except their energy source is chemical reactions involving hydrogen sulfide, methane, or hydrogen rather than light.26Nature Reviews Microbiology. The microbiomes of deep-sea hydrothermal vents: distributed globally, shaped locally The abiotic components here look nothing like a forest or a lake: extreme pressure, temperatures ranging from near-freezing to over 400°C just centimeters apart, acidic fluids loaded with heavy metals. Yet the basic ecosystem logic remains the same. Energy enters the system, producers convert it into organic matter, consumers eat the producers, and decomposers recycle the remains.
Putting a Dollar Value on Ecosystem Parts
One practical reason to understand what makes up an ecosystem is that each component provides services people depend on, whether or not we notice. A global synthesis drawing on over 1,300 studies and more than 9,400 value estimates organized economic values for 23 ecosystem services across 15 biomes, standardized into comparable monetary units.27Ecosystem Services. Economic values for ecosystem services: A global synthesis and way forward Pollination, water purification, climate regulation, flood buffering, and soil formation all have price tags when you calculate what it would cost to replace them with human-built infrastructure.
Even species that seem unremarkable can carry surprising economic weight. Dung beetles in the United Kingdom, by burying cattle dung, reduce pest flies, lower gastrointestinal parasite loads in livestock, prevent pasture fouling, and increase soil nutrients. One preliminary estimate suggested these beetles save the U.K. cattle industry roughly £367 million per year.28Ecological Entomology. The application of an ecosystem services framework to estimate the economic value of dung beetles to the U.K. cattle industry That is a single group of insects performing services that span the biotic and abiotic divide: living organisms altering soil chemistry, suppressing other living organisms, and recycling nutrients back into the non-living substrate.
Building Ecosystems from Scratch
If you really want to test whether you understand what makes up an ecosystem, try building one. The European Space Agency’s MELiSSA project is a bioregenerative life-support system designed to recycle gas, liquid, and solid wastes during long-distance space missions. It uses microbial cultures in bioreactors, a plant compartment, and a human crew, essentially recreating the producer-consumer-decomposer loop of a natural ecosystem inside sealed hardware.29PubMed. Microbial ecology of the closed artificial ecosystem MELiSSA A Chinese research team took a similar approach with a closed system called the Bioregenerative Life Support System (BLSS), designed for potential lunar habitation. Their tests showed the system achieved a material closure degree of 98.2%, meaning only about 2% of materials needed to be resupplied from outside. Oxygen, carbon dioxide, and trace gases stayed within ranges safe for both the human crew and the plants growing inside.30bioRxiv. Establishment of a closed artificial ecosystem to ensure human long-term survival on the moon
These projects confirm a point that runs through every section of this topic: you cannot maintain the living parts without carefully controlling the non-living conditions, and the living parts immediately begin altering those conditions through their own metabolism. Even inside a sealed metal chamber, the same abiotic-biotic feedback loops that operate in a forest or a lake emerge within days. The engineers designing these systems are, whether they frame it this way or not, doing applied ecology, balancing pH, managing gas concentrations, tuning nutrient flows, and hoping the microbial communities cooperate.