Abiotic factors are the nonliving parts of an environment that influence which organisms can survive there: temperature, water availability, sunlight, soil chemistry, wind, salinity. Biotic factors are the living components and their interactions: predators, prey, competitors, parasites, decomposers, and the microbes in the soil. Together, these two categories account for essentially everything that shapes an ecosystem, but the boundary between them is less clean than it first appears, because living things constantly reshape the nonliving conditions around them.
The Abiotic Side
Temperature and precipitation are the two abiotic factors that matter most for determining where species live on land. They set the broad boundaries: tropical rainforests exist where it is warm and wet year-round, tundra exists where it is cold and dry, and grasslands fill the middle ground where rainfall is moderate but inconsistent enough to prevent forests from taking hold. Other abiotic factors refine those boundaries. Soil pH determines which nutrients are available to plant roots. Dissolved oxygen levels govern which fish and invertebrates can survive in a lake. Salinity dictates where freshwater species give way to marine ones. Altitude, wind exposure, and the angle at which sunlight strikes a slope all create patchwork differences even within a single mountainside.
In aquatic systems, the abiotic picture gets especially dynamic. A river flowing into an estuary passes through zones of changing salinity, turbidity, and current speed, and each change filters the community of organisms that can persist. Researchers studying a river-estuary system found that cyanobacteria blooms originating from upstream lakes collapsed in the zone where salinity exceeded roughly one part per thousand, but salinity stress alone did not explain the crash: changes in water movement and channel shape also contributed.1PubMed Central. Phytoplankton, dissolved oxygen and nutrient patterns along a eutrophic river-estuary continuum: Observation and modeling That layering of multiple abiotic stressors, rather than a single kill switch, is typical. Organisms rarely face one challenge at a time.
The Biotic Side
Biotic factors include every way that living things influence one another. Predation is the most obvious example: wolves limit elk populations, which in turn affects how heavily elk browse on willows, which affects streambank erosion. Competition matters just as much. Two plant species that need the same patch of sunlight or the same soil nutrients will eventually reach a point where one outcompetes the other, unless some disturbance resets the playing field. Parasitism, disease, mutualism (like pollination or the bacteria in your gut), and decomposition are all biotic factors too.
Even viruses count. In extreme environments like acid mine drainage sites, viral communities significantly shape which microbes thrive by infecting and killing certain hosts while leaving others alone. One study found that viral diversity was positively correlated with microbial diversity, and that viruses explained more of the variation in microbial community makeup than environmental factors did in that particular setting.2PubMed Central. Viruses Regulate Microbial Community Assembly Together with Environmental Factors in Acid Mine Drainage In other words, who eats whom (or who infects whom) can matter more than the chemistry of the water in determining which species dominate.
Why Every Organism Has a Comfort Zone
Each species can tolerate a certain range of any given abiotic factor and performs best somewhere in the middle of that range. Ecologists have long described this with a bell-shaped curve: performance rises as conditions approach the optimum, then declines on either side as conditions become too extreme. The concept, sometimes called Shelford’s law of tolerance, applies to virtually any measurable environmental variable, from temperature and pH to soil moisture and light intensity.3Journal of Forestry Research. Plant hormesis and Shelford’s tolerance law curve
In animals, the boundaries of this curve often come down to oxygen delivery. As temperatures climb toward the upper limit of an animal’s range, its ability to supply oxygen to tissues drops, leading to reduced aerobic capacity well before outright lethal temperatures are reached.4PubMed. Climate variations and the physiological basis of temperature dependent biogeography: systemic to molecular hierarchy of thermal tolerance in animals The animal does not suddenly keel over at a threshold; it gradually loses the capacity to feed, reproduce, and compete. That gradual decline is why species distributions in the real world do not have sharp edges on a map. They thin out and overlap with neighboring species in messy transition zones.
A complication worth knowing about is that no species experiences just one abiotic factor at a time. In the wild, the ideal conditions for a species are usually a combination of many factors, each of which may be slightly off its individual optimum. The best real-world habitat is not the one where every variable is perfect but the one where the overall combination is most favorable.3Journal of Forestry Research. Plant hormesis and Shelford’s tolerance law curve Climate variables like temperature and rainfall are routinely used to predict where a species will be found, but other regulating variables, like the density of competitors or the availability of a key nutrient, are often overlooked even though they can be just as important.5Theory-Based Ecology. Ecological tolerance and the distribution of species
Nutrients and the Question of What Limits Growth
One of the clearest places where abiotic factors constrain biotic ones is nutrient supply. Plant growth on land is often limited by whichever essential nutrient is scarcest relative to what the plant needs. For most terrestrial ecosystems, that bottleneck is either nitrogen or phosphorus. A classic way of thinking about this says that the single nutrient in shortest supply sets the ceiling for growth, period. But modeling work has shown that plants can adjust how they invest in acquiring different nutrients, creating a smoother transition between limiting elements rather than a hard switch from one to the other.6PubMed Central. Nutrient limitation on terrestrial plant growth–modeling the interaction between nitrogen and phosphorus If nitrogen is scarce, the plant puts more energy into root structures and microbial partnerships that help capture nitrogen. If phosphorus runs short, it shifts strategy accordingly. Growth is still limited, but the organism is not a passive victim of its abiotic environment; it responds and adjusts within its genetic toolkit.
This interplay matters because humans have massively altered nutrient availability worldwide. Since the mid-twentieth century, human activities have more than doubled the amount of biologically active nitrogen entering ecosystems, mainly through fertilizer use and fossil fuel combustion.7PubMed Central. Disruption of the global nitrogen cycle: A grand challenge for the twenty-first century That excess nitrogen flows into waterways and drives eutrophication, where algae grow explosively, die, and decompose in ways that strip oxygen from the water. The result is coastal dead zones, drinking water contamination, and biodiversity loss, all cases where changing one abiotic factor cascades through the biotic community.8PubMed Central. Consequences of human modification of the global nitrogen cycle: Impacts of nitrogen
When Living Things Reshape Their Physical Environment
The textbook distinction between abiotic and biotic factors implies two separate categories, but in practice, organisms constantly modify the nonliving conditions around them. Ecologists call species that do this in especially dramatic ways “ecosystem engineers.” The concept captures organisms that shape their environments by modulating the availability of resources for other species, directly through physical structures or indirectly through their biological activity.9Functional Ecology. Special feature on ecosystem engineers: Cross‐scale and cross‐system perspectives Beavers damming a stream turn flowing water into a pond, changing oxygen levels, sedimentation, water temperature, and which plants and fish can survive. Earthworms rework soil structure. Coral polyps build limestone reefs that create habitat for thousands of other species. In each case, a biotic factor is actively constructing the abiotic environment.
Forests provide one of the best-studied examples. A dense tree canopy does not just sit passively in a climate; it creates its own microclimate underneath. Across temperate forests globally, the canopy on average cools maximum air temperatures by about 2.7 °C and warms minimum temperatures by about 1.2 °C compared to open-air conditions outside the forest.10PubMed Central. Microclimate temperature effects propagate across scales in forest ecosystems That buffering effect intensifies under denser canopies and holds up even during extreme heat events, with the strongest cooling occurring right at the forest floor.11Environmental Research Communications. Forest canopy cover affects microclimate buffering during an extreme heat event The tree species involved matters too. A study comparing mature stands of sessile oak and Scots pine in central France found that species identity had a significant effect on buffering capacity even after accounting for how much light the canopy intercepted.12Agricultural and Forest Meteorology. Capacity of a forest to buffer temperature: Does canopy tree species matter?
For the organisms living beneath those canopies, the “abiotic” temperature they experience is not simply whatever the regional weather delivers. It is a product of the forest itself, a biotic structure acting as a thermal filter. This is why clearcuts and old-growth forests a few hundred meters apart can support strikingly different communities of insects, fungi, and ground-nesting birds. The regional climate is identical; the microclimate is not.
Top-Down Versus Bottom-Up Control
A classic debate in ecology asks whether ecosystems are shaped more by their abiotic base (nutrients and energy flowing up from the bottom of the food web) or by their biotic top (predators and grazers pushing down from above). The answer, unsurprisingly, is both, but the balance shifts depending on the ecosystem and the moment.
In food webs with many species, modeling work shows that as community complexity increases, a crossover emerges from top-down control, where predators keep prey populations in check, to bottom-up control, where the productivity of plants or algae at the base sets the limit for everything above.13PubMed Central. Emergent competition shapes top-down versus bottom-up control in multi-trophic ecosystems In marine planktonic systems, the balance between these two modes of control has been shown to depend on nutrient loading (an abiotic factor) and sea surface temperature (another abiotic factor), with the relative importance of each varying between bays and estuaries.14PubMed. Relative prevalence of top-down versus bottom-up control in planktonic ecosystem under eutrophication and climate change So even the question of whether abiotic or biotic forces dominate turns out to be mediated by abiotic conditions. The two categories are deeply entangled.
Invasive Species and Abiotic Regime Shifts
One of the most disruptive things a biotic factor can do is permanently alter the abiotic rules of an ecosystem. Invasive plants are sometimes capable of exactly this. When certain invasive grasses colonize a landscape that historically did not burn often, they produce fine, dry fuel that increases fire frequency. More fire kills native shrubs and trees, which opens more ground for the grass, which produces more fuel, and the cycle locks in. The ecosystem has shifted to a new state, and the abiotic fire regime has been fundamentally rewritten by a biological invader.
A review of invasive plant impacts found that feedback loops leading to these regime shifts were most commonly associated with changes to seed banks, fire cycles, and nutrient cycling.15Diversity and Distributions. Invasive plants as drivers of regime shifts: identifying high‐priority invaders that alter feedback relationships Nitrogen-fixing invasive plants, for instance, can enrich the soil in nitrogen, favoring other fast-growing species over natives that evolved in nutrient-poor conditions. Once the soil chemistry has shifted, removing the invader alone may not restore the original community because the abiotic baseline has changed. This is a case where a biotic factor has, in effect, become an abiotic one.
Which Matters More, Abiotic or Biotic?
People sometimes want a clean ranking, but the honest answer is that it depends on what you are measuring and at what scale. An experiment comparing the effects of soil biota (biotic) versus soil chemistry (abiotic) on plant growth and insect populations found a split result: soil organisms significantly affected root structure and plant biomass, but the insects feeding on those plants varied between regions due to differences in abiotic soil properties rather than the presence or absence of soil life.16PubMed Central. Relative importance of biotic and abiotic soil components to plant growth and insect herbivore population dynamics The plant responded to its biotic neighbors below ground; the herbivore responded to the chemistry of the dirt. Same system, different answer depending on which organism you ask.
At geographic scales, abiotic factors tend to dominate because climate sets the broadest limits on where life can exist.17Earth Surface Responses and Processes. Relationship between precipitation and species distribution – Section: Abstract A study of two island gecko lineages that evolved similar body shapes independently found that abiotic factors, particularly climate, drove the physical divergence between species more than competition between them did.18AEJ. Abiotic Factors and Competitive Exclusion Drive Assembly Patterns in Two Insular Gecko Adaptive Radiations Displaying Ecomorphological Convergence At local scales, though, biotic interactions can be decisive. Two species with nearly identical climate tolerances can end up occupying very different patches because one outcompetes the other wherever they overlap. And over deep evolutionary time, abiotic upheavals like tectonic shifts, sea-level changes, and mass extinctions have repeatedly redirected evolution in ways that biotic interactions alone cannot explain.19Palaeontology. A macroevolutionary expansion of the modern synthesis and the importance of extrinsic abiotic factors
Life at the Extremes
If you want to see how far organisms can stretch the concept of abiotic tolerance, look at extremophiles. These are microbes, and occasionally larger organisms, that thrive under conditions that would kill almost everything else: boiling hot springs, near-freezing Antarctic brine, battery-acid pH, crushing deep-sea pressure, or radiation-blasted rock surfaces. They possess specialized cellular machinery that lets them function where conventional biology breaks down.20PubMed Central. Extremophiles: the species that evolve and survive under hostile conditions
A key adaptation involves their cell membranes. Both bacteria and archaea (the two major groups of single-celled organisms without a nucleus) can rapidly adjust the chemical makeup of their membranes in response to shifting temperature, pH, or pressure. Despite fundamental chemical differences between the membrane lipids of bacteria and archaea, both groups manage to maintain functional membranes across a wide range of conditions. Researchers have found that certain molecular adjustments, like membrane-spanning lipids and specific chemical branches, show a striking relationship with the growth boundaries of both groups.21PubMed Central. Adaptations of archaeal and bacterial membranes to variations in temperature, pH and pressure Extremophiles are a reminder that the tolerance curve described earlier is not fixed. Evolution can push it into territory that seems impossible, given enough time and selection pressure.
Urban Heat Islands and Rapid Adaptation
You do not need to visit a volcanic vent to see abiotic factors reshaping biology in real time. Cities generate their own abiotic anomaly: the urban heat island, where pavement, buildings, and waste heat raise local temperatures several degrees above the surrounding countryside. For birds that live alongside humans, these heat islands create thermal stress that mirrors, on a small scale, the kind of warming many species will face in coming decades. Researchers have begun studying urban heat islands as natural laboratories for observing how quickly birds can adapt to new thermal conditions.22AEJ. Urban Heat Islands as Evolutionary Laboratories for Rapid Thermal Adaptation in Synanthropic Birds The city, in this framing, is an unplanned experiment in how a single abiotic shift can drive evolutionary change over just a few generations.
Satellite remote sensing has become an important tool for tracking these kinds of abiotic changes at large scales. It can deliver long-term data on ecosystem area, structure, and function that would be impossible to collect on the ground, making it increasingly central to assessing how ecosystems are responding to environmental shifts.23ScienceDirect. The role of satellite remote sensing in structured ecosystem risk assessments That capacity matters because the interaction between abiotic and biotic factors is not static. As human activity alters climate, nutrient cycles, and land cover, the abiotic stage on which ecological communities perform is shifting faster than many species can adapt through biotic interactions alone.