The Ecology & Ecosystems Dictionary.

An ecosystem can persist in two or more structurally distinct configurations under the same external conditions.

Alternative stable states (ASS) theory (Holling 1973; May 1977) holds that ecosystems can have multiple basins of attraction: under the same environmental conditions, different starting points or perturbation histories lead to different stable community configurations. A critical threshold (regime shift) separates the domains. Recovery from one state to another often requires crossing the threshold by much more than the original perturbation that caused the shift (hysteresis), making restoration costly. Examples include clear vs. turbid lake water, coral reef vs. algal mat, savanna vs. closed woodland, and kelp forest vs. urchin barren. CORRECTION OF MYTH: this explicitly refutes the 'balance of nature' concept of a single natural equilibrium — ecosystems can and do have multiple valid stable states, and 'natural' is not uniquely defined.

The study of ecological processes and patterns in an epoch fundamentally shaped by human activity at the planetary scale.

Anthropocene ecology is the emerging field concerned with understanding ecological dynamics in a world where human activities are the dominant driver of environmental change at the planetary scale — altering climate, biogeochemical cycles, species distributions, and evolutionary trajectories. The Anthropocene (proposed as an epoch beginning ~1950 with the Great Acceleration of industrial production) is characterized by: unprecedented rates of species loss and invasion, transformation of >50% of Earth's land surface, alteration of the nitrogen and phosphorus cycles beyond natural variation, ocean acidification, and climate warming exceeding the Pleistocene–Holocene transition rate. Anthropocene ecology must integrate human agency explicitly into ecological models rather than treating human effects as external perturbations.

The marine process by which photosynthesis in surface waters transfers carbon to the deep ocean as sinking particles.

The biological pump is the suite of biological and physical processes that fix carbon dioxide in sunlit surface waters via phytoplankton photosynthesis and transfer a fraction of that carbon to the deep ocean as sinking particles (dead cells, fecal pellets, marine snow) or via active transport by migrating organisms. Only ~10–25% of surface NPP sinks below 100 m; a fraction of that reaches the seafloor. The pump sequesters carbon on timescales of decades to millennia (deep water circulation timescales), making it critical to the global carbon cycle and to climate regulation. Iron fertilization experiments have tested the idea of enhancing the pump to sequester atmospheric CO₂ — results show real but limited and variable carbon export.

Global biome types are determined primarily by temperature and precipitation, with local soil, disturbance, and history modifying the pattern.

Biomes are large-scale ecosystem types characterized by dominant vegetation form and adapted animal communities, distributed globally in patterns controlled mainly by mean annual temperature and annual precipitation (the Whittaker biome diagram). Temperature determines latitudinal and altitudinal belts; precipitation determines whether tropical temperatures produce rainforest, savanna, or desert. However, biome distribution is not purely deterministic from climate: disturbance regime (especially fire), soil type, and historical contingency (glacial refugia, dispersal barriers) generate significant departures from climate-predicted patterns. Many biome boundaries are not sharp — they are broad ecotones where one biome grades into another, sensitive to climate variability.

The process by which human activities cause communities worldwide to become more similar in species composition.

Biotic homogenization is the increasing similarity in species composition among ecological communities over time, driven by the global spread of generalist, invasive, and human-adapted species alongside the local decline of endemic and specialist species. It operates at multiple scales: globally (cosmopolitan species replace regional endemics), regionally (urban and agricultural species replace interior specialists), and locally (disturbance-tolerant functional types replace sensitive ones). The result is a more uniform biosphere with reduced beta-diversity (between-community diversity) even as local alpha-diversity may remain stable or increase. Homogenization reduces ecosystem resilience by lowering functional diversity and genetic variation between populations.

Boreal forests (taiga) store disproportionate amounts of global terrestrial carbon in peat and slow-decomposing soils.

The boreal forest (taiga) is the largest terrestrial biome by area (~12 million km²), spanning northern North America, Russia, and Scandinavia. It stores an estimated 30–40% of global terrestrial carbon, mostly in peat and mineral soils rather than in biomass — because low temperatures slow decomposition and allow organic matter to accumulate over millennia. Permafrost underlies much of the boreal zone, physically locking away ancient carbon. Climate warming is increasing decomposition rates, permafrost thaw, and fire frequency in boreal systems, potentially converting parts of the boreal from a net carbon sink to a net carbon source — a major positive climate feedback. The slow-cycling nature of boreal carbon makes its loss functionally irreversible on human timescales.

The fraction of carbon taken up by an organism or community that is allocated to growth rather than respired.

Carbon use efficiency (CUE) is the ratio of net carbon gain (growth or production) to gross carbon uptake (GPP for plants; uptake for microbes). For ecosystems, plant CUE (also called growth respiration ratio) is ~0.5 on average, meaning plants respire ~50% of fixed carbon and allocate ~50% to biomass. Microbial CUE in soil is central to soil carbon storage: high microbial CUE means more carbon enters stable organic matter; low CUE means more is respired as CO₂. Temperature shifts CUE downward (warming raises respiration relatively more than uptake), which has implications for soil carbon feedbacks to climate change.

A framework partitioning coexistence into equalizing mechanisms (fitness similarity) and stabilizing mechanisms (niche differences).

Modern coexistence theory (Chesson 2000) asks why many species coexist when competitive exclusion predicts one winner per niche. Two classes of mechanism are distinguished: stabilizing mechanisms (niche differences that make intraspecific competition more intense than interspecific, e.g. storage effect, frequency-dependent advantage) and equalizing mechanisms (fitness similarities that reduce competitive asymmetry and slow exclusion). Long-term coexistence requires stabilizing differences; equalizing ones merely slow loss. The framework unifies classical concepts (niches, trade-offs, environmental variation) under a single mathematical structure.

The processes that determine which species from a regional pool establish and persist in a local community.

Community assembly theory asks why a given local community contains the species it does, from the larger regional species pool. Filters operate at multiple scales: dispersal limitation (which species can reach the site), abiotic filtering (which arrivals can tolerate local conditions), and biotic filtering (which can establish against competitors and natural enemies). Contemporary community ecology integrates deterministic filtering with stochastic processes (neutral drift, priority effects) — the relative importance of each is an active research front. Assembly history (order of arrival) can cause the same regional pool to produce different final communities (priority effects).

Two species competing for identical resources cannot coexist indefinitely; one excludes the other.

Competitive exclusion (Gause's principle) holds that if two species compete for exactly the same limiting resource with no other niche difference, the superior competitor will eventually drive the inferior to local extinction. In practice, complete niche identity is rare; real communities exhibit niche partitioning, coexistence through trade-offs, or oscillating dynamics. The principle functions as a null hypothesis: observed coexistence implies niche differentiation or non-equilibrium dynamics.

Breakdown of dead organic matter by microbes and detritivores, returning nutrients to inorganic form for plant uptake.

Decomposition is the physical fragmentation and chemical transformation of dead organic matter (detritus) into inorganic nutrients by a community of bacteria, fungi, and detritivores (e.g. earthworms, isopods, millipedes). It is the dominant pathway of nutrient return in most ecosystems — often exceeding the path through herbivory. Decomposition rate depends on temperature, moisture, oxygen availability, and litter quality (C:N ratio, lignin content). High-N, low-lignin litter (tropical leaves, fresh plant material) decomposes fast; low-N, high-lignin material (conifer needles, wood) decomposes slowly. Decomposition couples the carbon cycle to nitrogen, phosphorus, and other nutrient cycles, making it central to global biogeochemistry.

The characteristic frequency, intensity, extent, and seasonality of disturbances that shape an ecosystem.

A disturbance regime is the statistical summary of disturbance events in an ecosystem — their type (fire, flood, wind, herbivory), frequency (return interval), intensity (energy released or biomass removed), spatial extent (patch size), and seasonality. Many ecosystems are adapted to a particular historical disturbance regime; significant departures from that regime (anthropogenic fire suppression, altered flood timing, invasive herbivores) alter community composition and function. The regime concept emphasizes that disturbance is not an anomaly but a recurring process that is part of ecosystem dynamics — some communities are maintained by, not merely disrupted by, disturbance.

Whether species diversity promotes ecosystem stability is nuanced, context-dependent, and contested — not a simple positive relationship.

The diversity–stability relationship asks whether more diverse communities are more stable. Classical theory (MacArthur 1955) suggested more diverse food webs were more stable; May's (1972) mathematical work showed the opposite (random large communities are inherently less stable). Modern empirical and theoretical work finds nuance: functional diversity (not species richness per se) promotes resistance and resilience; redundancy among species with similar functions provides insurance against species loss; diversity stabilizes ecosystem-level productivity even as it may destabilize individual populations; temporal stability is more consistently related to diversity than resistance or resilience. The relationship is not universal, and context (type of stability, type of ecosystem, type of diversity measured) determines outcome. MYTH CORRECTED: 'more diversity always means more stability' is false as a universal claim.

Drylands (arid and semi-arid biomes) cover ~41% of Earth's land area and support 2 billion people, with water as the master limiting factor.

Drylands are areas where potential evapotranspiration exceeds precipitation on average, encompassing hot deserts, cold deserts, semi-arid grasslands, and shrublands. They cover ~41% of the terrestrial surface and range from hyperarid (~no rainfall) to dry-subhumid (~rainfall that slightly exceeds evapotranspiration in some years). Ecological processes in drylands are dominated by resource pulses: brief periods of rainfall trigger rapid plant growth and microbial activity, followed by long dry periods. Communities are adapted to water limitation through drought tolerance, deep or lateral roots, CAM and C4 photosynthesis, and dormancy. Soil biotic crusts (cyanobacteria, mosses, lichens) are key ecosystem engineers in many drylands, fixing nitrogen and stabilizing soil against erosion.

A measure of the biologically productive land and sea area required to sustain a population's resource use and absorb its waste.

The ecological footprint (Rees & Wackernagel, 1990s) expresses human resource demand in terms of the biologically productive area needed to supply those resources and absorb wastes — measured in global hectares (gha). It aggregates demands for cropland, grazing land, fishing grounds, forest (for timber and carbon sequestration), and built land. The global average footprint is ~2.8 gha per person; global biocapacity is ~1.6 gha per person (2023 estimates), implying Earth Overshoot — humanity is consuming 1.7 Earths' worth of biocapacity per year. Footprint analysis is useful for communicating aggregate demand but has methodological limitations: it is not a precise scientific measurement, and its carbon-sequestration component is contested. It should be treated as an indicator, not a precise accounting tool.

Multiple species performing the same functional role, providing an insurance buffer against species loss.

Ecological redundancy refers to the presence of multiple species that perform similar ecological functions (e.g. similar pollination, seed dispersal, or nitrogen fixation roles). Redundant species are often thought to provide an 'insurance effect': when one species declines, others performing the same function maintain ecosystem processes. Redundancy is greater in species-rich systems. However, apparent redundancy may conceal functional uniqueness that only becomes apparent under novel conditions; and species considered redundant for one function may be non-redundant for another. The concept informs the debate over minimum viable species numbers and ecosystem function in degraded systems.

Directional, sequential change in community composition following disturbance or on new substrate.

Ecological succession is the process of community change over time following a disturbance (secondary succession) or colonization of bare substrate (primary succession), driven by species modifying their environment in ways that alter competitive outcomes for later colonizers. Classical Clementsian theory described succession as deterministic progression toward a single regional climax community. Modern ecology rejects the single-climax view: multiple stable states are possible, stochastic events (priority effects, disturbance timing) create path dependence, and 'climax' communities are themselves dynamically maintained by recurring disturbance. The intermediate disturbance hypothesis proposes maximum diversity at moderate disturbance frequency, though empirical support is mixed and context-dependent.

The benefits that functioning ecosystems provide to humanity, from food and water to climate regulation and cultural values.

Ecosystem services are the direct and indirect contributions of ecosystems to human wellbeing (Millennium Ecosystem Assessment 2005). They are classified as: (1) provisioning services (food, freshwater, fiber, fuel); (2) regulating services (climate regulation, flood control, air quality, disease regulation, pollination); (3) cultural services (recreation, aesthetic value, spiritual significance); and (4) supporting services (nutrient cycling, primary production, soil formation — the processes enabling the others). The framework made explicit that ecosystem degradation imposes real economic costs that markets do not capture. CAUTION: the ecosystem services framing is useful but contested — some ecologists argue it risks reducing nature to its instrumental value to humans, potentially devaluing intrinsic worth and non-anthropocentric conservation goals.

Altered biotic and abiotic conditions at the boundary between two habitat types, with consequences for species composition.

Edge effects are changes in species composition, microclimate, and ecological process at the boundary (ecotone) between two habitat types. Physical edge effects include increased light penetration, wind, temperature variation, and desiccation along forest edges. Biotic edge effects include increased nest predation (as generalist predators concentrate at edges), brood parasitism (cowbirds follow edges), invasion of edge-adapted and generalist species, and reduced abundance of interior specialists. The depth of edge penetration (edge depth) varies from a few meters (light) to hundreds of meters (predation effects on birds). As fragmentation increases edge length relative to interior area, edge effects can permeate entire small patches.

Nutrient enrichment of a water body that causes excessive algal growth, oxygen depletion, and loss of biodiversity.

Eutrophication is the process by which excessive nutrient input (primarily nitrogen and phosphorus from agricultural runoff, sewage, and atmospheric deposition) stimulates algal and cyanobacterial growth in aquatic ecosystems, leading to algal blooms, shading of submerged vegetation, oxygen depletion (hypoxia) as blooms die and decompose, release of toxins, and loss of fish and invertebrate diversity. Eutrophication is one of the leading causes of freshwater and coastal marine ecosystem degradation globally. Hypoxic 'dead zones' now exceed 400 globally, including the large dead zone in the Gulf of Mexico fed by Mississippi River drainage. Alternative stable state dynamics (turbid vs. clear) make eutrophied systems difficult to restore.

The time-lagged extinction of species predicted to be lost following habitat reduction, but not yet observed.

Extinction debt is the difference between the current number of species in a fragmented landscape and the equilibrium number predicted by species–area relationships or metapopulation theory, given the current (reduced) habitat area. Species that will inevitably go extinct given current conditions are 'committed to extinction' but have not yet disappeared — often because long-lived individuals persist as the population fails to recruit. Extinction debt means that current species counts underestimate the ecological damage already done by past habitat loss; conservation that maintains current richness is not maintaining current viability. Debt is paid over decades to centuries, depending on species' longevity and minimum viable population size.

The study of fire as an ecological process that shapes communities, nutrient cycles, and evolutionary adaptations.

Fire ecology examines fire as an ecosystem process rather than purely a disturbance event to be prevented. Fire transfers energy, mineralizes nutrients (releasing them from organic matter), resets succession, and maintains fire-adapted community types (savannas, chaparral, boreal forest). Fire-adapted species have evolved serotinous cones that open with heat, thick bark that insulates cambium, resprouting root crowns that survive crown removal, and seeds that require smoke or heat for germination. The 20th-century U.S. policy of total fire suppression increased fuel loads in fire-adapted systems, producing larger and more severe fires when fire eventually occurs — the 'fire debt' concept.

The range and variety of functional traits present in a community, more directly linked to ecosystem function than species richness alone.

Functional diversity measures the variety of functional traits (morphological, physiological, behavioral) present in a community — rather than simply counting species. Traits relevant to ecosystem processes include leaf economics (specific leaf area, leaf nitrogen), body size, feeding mode, root depth, and phenology. Functional diversity predicts ecosystem functioning (productivity, decomposition, resource use) better than species richness in many contexts because it captures what organisms are doing, not just how many are present. Functional richness, evenness, and divergence are distinct components. Loss of functional diversity (through biotic homogenization or filtering toward a trait optimum) can reduce ecosystem resilience even when species richness is maintained.

Division of continuous habitat into isolated patches, reducing patch size, increasing edge, and interrupting connectivity.

Habitat fragmentation is the process by which continuous habitat is divided into smaller, more isolated patches through land conversion or human infrastructure. It has three separate but interacting effects: (1) habitat area loss (less total habitat); (2) edge effects (increasing ratio of edge to interior); (3) isolation (reducing dispersal and gene flow between patches). Fragmentation effects are strongest for interior specialists (species sensitive to edge conditions), large-area requirements (large home ranges), and poor dispersers. Area and isolation together determine local extinction probability (via island biogeography and metapopulation dynamics). Connectivity corridors can partially compensate for isolation.

Systems with hysteresis return to a prior state via a different path — and sometimes require much larger reversal of the driver than caused the shift.

Ecological resilience (Holling 1973) is the capacity of a system to absorb disturbance and reorganize while undergoing change so as to retain essentially the same function, structure, identity, and feedbacks. Hysteresis is the phenomenon in which the path of recovery differs from the path of degradation: when a driver is increased, the system shifts at one threshold; decreasing the driver to its original value is insufficient to shift the system back, because internal feedbacks stabilize the degraded state. The dual concepts together explain why ecosystems that appear robust can collapse suddenly and recover slowly. Engineering resilience (return time after a small perturbation near a single equilibrium) is distinct from ecological (or 'latitude') resilience.

Non-native species whose establishment and spread cause measurable ecological harm — with important caveats about framing.

An invasive species is a non-native (introduced) species that spreads beyond initial introduction sites and causes documented ecological or socioeconomic harm. CAUTION: 'invasive' describes ecological impact, not origin alone — many introduced species are non-invasive and some become locally significant without causing net harm. The claim that all non-native species are harmful is empirically false (most introductions fail or remain benign); the claim that no non-natives cause harm is also false (a minority cause severe impacts). Ecological effects depend on recipient community composition, evolutionary history with the invader, and landscape context. Novel ecosystems may include non-native species performing important ecological functions.

Species richness on islands is determined by a dynamic equilibrium between immigration from a source pool and local extinction.

The theory of island biogeography (MacArthur & Wilson 1967) proposes that species richness on an island (or habitat island) reaches a dynamic equilibrium where immigration rate from the mainland source pool equals local extinction rate. Richness increases with island size (larger islands support larger populations, reducing extinction) and decreases with distance from the source (reducing immigration). The theory predicts species turnover: the identity of species changes even as total richness remains approximately constant. Extended to terrestrial 'habitat islands' (forest fragments, reserves, mountain tops), island biogeography informs reserve design (larger and better-connected reserves retain more species). NOTE: the theory predicts equilibrium richness, not a stable fixed community — species composition turns over continuously.

The degree to which the landscape facilitates or impedes movement of organisms between habitat patches.

Landscape connectivity is a landscape-scale property measuring how readily organisms can move through the landscape matrix between patches of suitable habitat. Structural connectivity describes the physical arrangement of habitat (patch proximity, corridor presence); functional connectivity describes actual movement rates, which depend on species-specific perceptual abilities, dispersal distances, matrix resistance, and behavioral responses to matrix types. High connectivity allows dispersal rescue of declining local populations, genetic exchange, and range shifts in response to climate change. Connectivity is not binary: the same matrix may be traversable by some species and impermeable to others.

A population of semi-isolated sub-populations linked by dispersal.

A metapopulation is a set of spatially distinct local populations of the same species that interact through occasional migration (dispersal) between patches of suitable habitat. Local populations may go extinct and be re-founded via immigration — the classic 'blink-out and recolonize' dynamic described by Levins (1969). Regional persistence depends on the balance between local extinction rates and colonization rates, not on any single population's size. The concept is central to conservation planning for fragmented landscapes.

The rate at which plants fix carbon via photosynthesis minus their own respiration costs.

Net primary productivity (NPP) is gross primary productivity (GPP) minus autotrophic respiration: the carbon fixed by plants that is available to heterotrophs. It is typically expressed in grams of carbon per square meter per year (g C m⁻² yr⁻¹). NPP differs from standing biomass: a tropical rainforest and a productive estuarine marsh may have similar NPP but vastly different standing biomass because turnover rates differ. Globally, terrestrial NPP is estimated at ~56 Pg C yr⁻¹ (petagrams of carbon per year); marine NPP at ~50 Pg C yr⁻¹. Climate change is already altering NPP patterns — some high-latitude systems show increased NPP, many tropical systems show declining or shifted seasonality.

Species coexist by partitioning resources, time, or space to reduce direct competition.

Niche differentiation is the process (or evolved outcome) by which competing species use different subsets of available resources, activity times, microhabitats, or prey size classes sufficiently to allow stable coexistence. Modern coexistence theory (Chesson 2000) formalizes this as 'stabilizing mechanisms' that make intraspecific competition stronger than interspecific competition. Differentiation may be constitutive (fixed morphology) or plastic (behavioral partitioning when competitors are present).

Human-modified ecosystems with new species combinations and no historical analogue, which may be self-sustaining without active management.

Novel ecosystems (Hobbs et al. 2006) are systems altered by human action to have species compositions and abiotic conditions with no close historical analogue — often arising from combinations of habitat disturbance, introduced species, and altered nutrient regimes. Unlike hybrid or historic ecosystems, novel ecosystems may be self-sustaining and may actively resist restoration to a prior reference state. The concept challenges classical conservation's focus on restoring pre-disturbance states: in many places, the reference state is no longer achievable or even desirable given changed climate and species pools. Novel ecosystems may provide ecosystem services and biodiversity conservation value, but their long-term trajectory and stability are uncertain.

In streams and rivers, nutrients cycle through biotic and abiotic pools while being transported downstream.

Nutrient spiraling describes how a nutrient atom (e.g. nitrogen or phosphorus) cycles between dissolved inorganic form, uptake by algae or biofilm, transit through invertebrates, and return to dissolved form via decomposition — all while being transported downstream by current. The 'spiral length' (distance traveled per full cycle) integrates uptake efficiency and current velocity: short spirals indicate tight biological retention; long spirals indicate leaky, inefficient systems. Nutrient spiraling is a system-scale integration of decomposition, uptake, and hydrology specific to lotic (flowing water) ecosystems. The concept was developed by Elwood and Webster in the early 1980s.

Ecosystems as collections of patches at different successional or resource states, with dynamics driven by disturbance and colonization.

Patch dynamics treats ecosystems as heterogeneous mosaics in which local patches are disturbed, recover, and are re-disturbed on different schedules, creating a shifting landscape of resource states. The framework is relevant across scales: tree-fall gaps in forests, beaver ponds in riparian systems, mole mounds in grasslands, and oyster beds in estuaries all generate fine-grained disturbance patches. Regional diversity is maintained by patch heterogeneity (the intermediate disturbance hypothesis applies at patch scale). The temporal dynamics of individual patches (establishment, growth, senescence, disturbance) determine the landscape-scale distribution of habitat states at any time.

Early successional species improve conditions (soil, shade, moisture) that enable later species to establish.

Pioneer facilitation is a mechanism of succession in which early colonizing species modify the abiotic environment in ways that promote the establishment of later-stage species — at the cost of eventually being replaced by those species. Classic facilitators include nitrogen-fixing plants (alder, legumes) that enrich nutrient-poor substrates; nurse plants that shade the soil surface and reduce temperature extremes; and physical engineers that stabilize substrate against erosion. Facilitation-dominated succession is most common in harsh or nutrient-poor environments. In more productive conditions, competitive inhibition (where early arrivals suppress later species) and tolerance (where later species simply outlast earlier ones) are more common mechanisms.

Nine Earth-system processes with estimated safe operating limits; transgressing them risks pushing Earth out of the stable Holocene state.

The planetary boundaries framework (Rockstrom et al. 2009; updated Steffen et al. 2015; Richardson et al. 2023) identifies nine Earth-system processes critical to maintaining conditions resembling the stable Holocene epoch in which human civilizations developed: climate change, biosphere integrity (biodiversity), land-system change, freshwater use, biogeochemical flows (N and P), ocean acidification, stratospheric ozone depletion, aerosol loading, and novel entities (chemicals, plastics). Quantitative limits are proposed for each; transgressing multiple boundaries simultaneously may trigger non-linear Earth-system shifts. As of 2023, at least 6 of 9 boundaries are assessed as transgressed. The framework is scientifically influential but contested on quantification, spatial heterogeneity, and the assumption that global limits translate to local harm.

Community development on substrate that has never supported life or has been stripped to bare rock or sediment.

Primary succession begins where no biological legacy exists: newly formed volcanic islands, retreating glaciers exposing bare rock, freshly deposited sand dunes, or landslide scars. The process is slow (decades to centuries) because pioneers must build soil organic matter from scratch. Nitrogen fixers (lichens, cyanobacteria, alder) are often critical early colonists because bare substrates are nitrogen-poor. Facilitation is more important in primary than secondary succession: early colonists modify the substrate in ways that enable later species establishment. The trajectory is strongly controlled by parent material, climate, and the regional species pool — identical 'bare' substrates in different biomes produce entirely different climax communities.

The order in which species colonize a site affects which community ultimately assembles, independent of final fitness.

Priority effects occur when early-arriving species alter conditions (resources, microhabitats, soil microbiome) in ways that favor or inhibit later colonists. In positive priority effects (facilitation), early colonizers make the site more suitable for successors. In negative priority effects (founder control), early arrivals preempt resources and resist replacement. Priority effects contribute to contingency in community assembly: identical regional pools can produce different communities depending on the order and timing of colonization, complicating prediction and restoration.

A sudden, persistent shift in ecosystem state when a threshold is crossed, often with hysteresis making reversal difficult.

A regime shift is a rapid, large, and persistent reorganization of ecosystem structure and function following a relatively small change in a driver, once a threshold (tipping point) is crossed. The transition is often nonlinear: the system resists change until a threshold is passed, then switches abruptly. Regime shifts are predicted to become more common as human pressures on ecosystems increase. Early warning signals have been proposed (slowing return rate, increasing variance, autocorrelation) but their reliability in practice is contested. Key examples: lake eutrophication, coral bleaching-to-algal mat, savanna-to-shrubland, Atlantic cod collapse, and some proposed changes in the Atlantic Ocean Meridional Overturning Circulation.

Large-scale restoration strategy focused on restoring self-regulating ecosystems through the return of apex consumers and ecological processes.

Rewilding is an approach to ecological restoration that emphasizes restoring self-sustaining ecosystems by reintroducing keystone species (especially large predators and herbivores), removing artificial management interventions, and reconnecting landscape fragments to allow natural ecological processes. Distinct from traditional conservation (which often manages for particular species or habitat states), rewilding aims to restore process: predation, herbivory, disturbance, nutrient dispersal by animals. Major rewilding projects range from European wolf and lynx reintroductions to Pleistocene rewilding proposals (reintroducing functional analogues of extinct megafauna). Benefits claimed include trophic cascades, increased biodiversity, carbon sequestration, and ecosystem resilience — though empirical evidence for each is context-dependent and some claims are contested.

Savannas are maintained by the interplay of rainfall, fire, herbivory, and tree–grass competition, not by climate alone.

Savannas are biomes characterized by a continuous grass layer with scattered trees or shrubs, covering ~20% of the Earth's land surface and supporting large proportions of global terrestrial biodiversity. Their structure is maintained by a tree–grass balance determined by rainfall seasonality, fire return interval, herbivory intensity, and soil properties — not by any single factor alone. The Jeltsch model and related frameworks show that fire and herbivory together prevent savannas from converting to closed-canopy woodland even at rainfall levels that climatically 'should' support forest. The dynamism of savannas means that the tree–grass balance can shift markedly with changes in fire frequency or herbivore community.

Community recovery following disturbance when soil and seed banks remain, proceeding faster than primary succession.

Secondary succession occurs when disturbance (fire, agriculture, logging, windstorm) removes much of the plant community but leaves soil organic matter, seed banks, root systems, and microbial communities largely intact. Recovery is faster than primary succession because the biological legacy accelerates recolonization. The trajectory depends heavily on disturbance severity (a low-intensity fire may leave most below-ground plant parts intact; cultivation destroys seed banks); on the composition of the surviving seed bank; and on the proximity of propagule sources. Unlike classical succession theory, modern models recognize that secondary succession can converge on multiple endpoint communities, not a single deterministic climax.

A landscape in which patches cycle through successional stages independently, maintaining overall heterogeneity at the landscape scale.

The shifting mosaic model (Bormann & Likens 1979) describes landscapes where recurring disturbances create patches in different successional stages, each cycling through initiation, maturation, and transition. At any moment, the landscape is a mosaic of ages and structure; overall compositional 'steady state' exists only at the landscape scale, not within any individual patch. The model is an explicit rejection of the older idea of a stable regional climax — the landscape is in perpetual dynamic flux, not a fixed end-state. This has important implications for biodiversity: landscape-level heterogeneity supports more species than a uniform climax would, because different successional stages provide different habitats.

Productive source habitats subsidize declining sink populations via dispersal.

In a landscape of heterogeneous habitat quality, source populations in high-quality patches have birth rates exceeding death rates (positive net growth) and export individuals into adjacent sink populations, where mortality exceeds reproduction and the population would decline to extinction without the immigration subsidy. The distinction matters for management: apparent population stability in a sink can mask dependence on a distant source, so protecting the sink alone is insufficient.

A power-law relationship: larger areas contain more species, with species number scaling as A raised to a constant exponent (z ≈ 0.2–0.35).

The species–area relationship (SAR) describes the empirical pattern that larger areas contain more species, following a power law: S = cAᶻ, where S is species richness, A is area, c is a constant (intercept on log-log plot), and z is the slope (typically 0.20–0.35 for oceanic islands and ~0.12–0.17 for samples within continuous habitat). The SAR is one of ecology's most general patterns and underlies predictions about habitat loss and species extinction: a 90% reduction in habitat area predicts approximately 50% species loss (at z = 0.30). However, the SAR is not a precise extinction calculator — extinction debt, refugia, and edge effects complicate prediction; and z varies with taxon, region, and sampling method.

Species that overlap in resources can coexist if each performs best in a different environmental condition and can buffer bad years via a 'storage' life stage.

The storage effect is a stabilizing coexistence mechanism arising from three ingredients: species-specific responses to environmental variation, buffered population growth (via long-lived adults, seed banks, or dormancy that store past good conditions), and covariance between competition and environment. When species A thrives in wet years and species B in dry years, each escapes competition when the other is most vigorous, and buffering prevents elimination in bad years. The storage effect can maintain diversity even when species share limiting resources.

Predators or herbivores regulate community structure from higher to lower trophic levels.

Top-down control describes food-web regulation driven by consumer pressure rather than resource availability. When predators limit herbivore populations, plant biomass increases; when herbivores limit producers, carnivore removal leads to vegetation loss. The concept is contrasted with bottom-up control, where nutrient or primary production limits shape the whole system. Most real food webs exhibit both, in proportions that vary by ecosystem type: strongly top-down in simple aquatic food chains, more bottom-up in complex terrestrial systems with many herbivore species.

Top predators indirectly control lower trophic levels, with effects rippling down the food web.

A trophic cascade occurs when a top predator suppresses its prey population sufficiently to release the prey's own food source from herbivory or predation pressure. The effect propagates down trophic levels: predator removal → prey increase → vegetation/plankton decrease (or vice versa). Trophic cascades demonstrate that community structure is not solely bottom-up (resource-driven) but also top-down (predator-driven). Strength of cascades varies with food-web complexity: simple food chains show strong cascades; complex webs tend to dampen them. NOTE: Trophic cascades represent system-level regulation, not a 'balance of nature' in the sense of a fixed equilibrium — the system is dynamic, and predator removal does not simply reverse to a pristine prior state.

Global loss of apex consumers that destabilizes ecosystem structure and function.

Trophic downgrading (Estes et al. 2011) refers to the widespread removal of top predators and large apex consumers worldwide — through hunting, habitat loss, and persecution — and the cascade of consequences that follows: increases in mesopredators, collapse of trophic cascades, altered nutrient cycling, increased disease, and homogenization of ecosystems. Unlike individual trophic cascades, trophic downgrading is a planetary-scale reorganization of ecosystem control. Recovery requires restoring apex consumers, not just stopping their removal.

The fraction of energy transferred from one trophic level to the next, approximately 10% on average.

Trophic efficiency (also Lindeman efficiency) is the fraction of production at one trophic level that becomes production at the next higher level. The commonly cited value is ~10%, meaning herbivores typically incorporate ~10% of plant NPP into their own biomass, carnivores ~10% of herbivore biomass, and so on. The ~10% figure (Lindeman 1942) is a rough average with wide natural variation (range roughly 5–20%). Energy losses occur through respiration, excretion, unassimilated food (feces), and material not consumed. The low efficiency explains why apex predators are rare relative to their prey and why carnivorous diets require more land/ocean area than plant-based diets. NOTE: trophic efficiency is not identical to assimilation efficiency or production efficiency; these are distinct ratios measured at different points in the energy budget.

Tropical forests are high-diversity, high-NPP systems driven by gap dynamics, nutrient cycling on poor soils, and strong biotic interactions.

Tropical rainforests occur where mean annual precipitation exceeds ~1600–2000 mm and is distributed throughout the year (or with a short dry season). They contain more than 50% of all described species despite covering ~10% of land area. Many tropical forest soils are old, deeply weathered, and nutrient-poor (especially oxisols and ultisols of the Amazon and Congo) — the high biomass and diversity is supported by efficient internal nutrient cycling rather than soil fertility. NPP is high (~1000 g C m⁻² yr⁻¹), decomposition is rapid, and little organic matter accumulates in soil. Diversity is maintained by a complex of mechanisms: gap dynamics, Janzen–Connell effects (host-specific pests limiting seedling survival near parents), niche differentiation, and historical refugia from Pleistocene climate fluctuations.