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Ecoregion

An ecoregion is defined as a large unit of land or water containing a geographically distinct assemblage of , communities, and ecological processes that share a large majority of their species composition, dynamics, and environmental conditions, and that interact in ways essential for their sustained viability. This emphasizes differentiation from adjacent areas due to barriers to species dispersal or unique abiotic factors such as , , and soils. The concept of ecoregions gained prominence through the work of the , which published a comprehensive global classification of terrestrial ecoregions in 2001, delineating 867 such units nested within 14 biomes and 8 biogeographic realms. This system builds on earlier ecological regionalizations but provides a standardized tool for assessing patterns at a scale larger than individual habitats yet smaller than continents. A parallel marine classification, the Marine Ecoregions of the World (MEOW), was developed in 2007, identifying 232 ecoregions across 12 realms and 62 provinces to address coastal and shelf . A corresponding freshwater classification, the Freshwater Ecoregions of the World (FEOW), was developed in 2008 by and , delineating 426 ecoregions. Ecoregions serve as foundational units for planning, enabling prioritization of areas like the WWF's ecoregions, which highlight exceptional and for targeted protection efforts. They integrate biotic and abiotic elements to map ecosystems holistically, supporting strategies that maintain ecological integrity amid threats like and . In practice, organizations such as the U.S. Environmental Protection Agency apply ecoregion-based approaches to evaluate environmental resources and restoration across North America's 15 level I ecoregions.

Definition and Classification

Definition

An ecoregion is a relatively large unit of land or water containing a distinct assemblage of natural communities and , with boundaries that approximate the aggregated extent of those communities sharing a large of their , dynamics, and environmental conditions. These units are shaped by recurring patterns of ecosystems influenced by major factors such as , , , , and evolutionary history, which together determine the characteristic and ecological processes within the region. Key attributes of ecoregions include distinct assemblages of and species, ecological communities, and interacting environmental factors, often facing similar disturbance regimes like fire or flooding that maintain their structure and function. Boundaries between ecoregions are typically non-abrupt and fuzzy, reflecting gradual ecological transitions rather than sharp demarcations, as species distributions and environmental gradients blend across adjacent areas. Ecoregions differ from related concepts in scale and focus: s represent broader, climate-dominated zones that encompass multiple ecoregions, such as the temperate broadleaf biome containing various ecoregions adapted to local variations, while ecosystems denote smaller-scale interactions of and their immediate within an ecoregion. Typically spanning hundreds of thousands to millions of square kilometers, ecoregions provide a mesoscale framework for understanding global biodiversity patterns, for example, the ecoregion covering about 1.8 million square kilometers.

Classification Systems

Classification systems for ecoregions employ a variety of approaches to delineate areas of relative ecological homogeneity, ranging from hierarchical frameworks based on environmental drivers to comprehensive global schemes informed by patterns. These systems typically integrate abiotic factors such as , , and with biotic elements like and species distributions to create nested hierarchies that facilitate ecological analysis and management. One foundational algorithmic approach is Robert G. Bailey's hierarchical system for North American ecoregions, first mapped in 1976 and expanded in subsequent editions. This framework classifies ecoregions into domains, divisions, provinces, sections, and districts based primarily on macroclimatic zones (using the Köppen-Trewartha system), physiographic features (such as landforms from mountains to plains), and potential natural vegetation (e.g., , forests, or steppes). Bailey's method emphasizes the interaction of these factors to define complexes, with mountains often treated separately due to , enabling a scalable structure for resource assessment across the continent. In contrast, expert-driven holistic methods, such as James M. Omernik's ecoregion framework developed for the U.S. Environmental Protection Agency, rely on integrative analysis of multiple environmental variables to delineate nested levels (I through IV). Omernik's system identifies broad Level I ecoregions based on similarities in , physiography, , , soils, potential natural , and , then refines them into finer sublevels through expert judgment and spatial data overlays. This approach, first outlined in and refined over decades, prioritizes ecological similarity across resource mosaics rather than rigid boundaries, supporting applications in and . Major global frameworks have emerged to standardize ecoregion delineation worldwide, notably those from the . The Terrestrial Ecoregions of the World (TEOW), published in 2001, divides the planet's land surface into 867 ecoregions nested within 14 biomes and 8 realms, drawing on expert consultations, existing regional classifications, and GIS overlays of , landforms, and historical to capture distinct assemblages of and habitats. Complementing this, the Marine Ecoregions of the World (MEOW, 2007) establishes 232 coastal and shelf ecoregions across 12 realms and 62 provinces, using biogeographic data on distributions, , and larval dispersal to define units of relative homogeneity. Similarly, the Freshwater Ecoregions of the World (FEOW, 2008) identifies 426 ecoregions aligned with watersheds, primarily based on compositions, patterns, and hydrological barriers, incorporating evolutionary history and regional expert input for conservation prioritization. Contemporary ecoregion mapping increasingly integrates advanced data sources, including Geographic Information Systems (GIS) for , for vegetation and monitoring, and biodiversity databases for species occurrence data. For instance, GIS platforms enable overlaying layers of climatic models, satellite-derived imagery (e.g., Landsat or MODIS), and databases like the (GBIF) to refine boundaries and assess ecological transitions, as seen in the development of TEOW and updates to national systems. This enhances accuracy and , allowing for dynamic updates in response to environmental changes.

History

Early Developments

The foundational concepts underlying ecoregions trace back to the early , when explored the relationships between vegetation distribution and environmental gradients such as altitude, temperature, and humidity during his expeditions in the . Humboldt's observations, detailed in works like Essai sur la géographie des plantes (1807), established the idea of biogeographic zones where plant communities form distinct belts influenced by climatic and physiographic factors, laying the groundwork for later ecological classifications. In the mid-20th century, Russian ecologist Viktor Sukachev advanced these ideas through his development of biogeocenology, formalizing the concept of biogeocenosis—a spatially discrete unit integrating living organisms, their , and interactions—in the 1940s and 1950s. This approach emphasized holistic units beyond isolated , influencing global ecological thought by highlighting biotic-abiotic interdependencies. The term "ecoregion" itself was coined in 1962 by Canadian forest ecologist Orie L. Loucks in his classification of forest in the Maritime Provinces, where he defined ecoregions as areas with relatively homogeneous , physiography, and suitable for management purposes. Key milestones in the United States included A. W. Kuchler's 1964 map of potential natural across the conterminous U.S., which delineated approximately 116 vegetation types based primarily on climax communities and their environmental controls, providing a vegetation-centric for planning. Building on this, Robert G. Bailey introduced a more comprehensive ecoregions in 1976 for the U.S. Forest Service, dividing the country into 53 provinces (the finest-level ecoregions) within a of 9 domains and 31 divisions that integrated , physiography, , soils, and potential to support ecological land and management. Internationally, the Man and the (MAB) Programme, launched in 1971, spurred global mapping efforts, with its 1973 publication on the International Classification and Mapping of offering a standardized physiognomic system for delineating major units at scales useful for reserves and planning. In the , the Fund () began emphasizing -level classifications in its strategies, such as campaigns targeting tropical forests and other major , which paved the way for finer-scale ecoregion delineations by integrating patterns with habitat types. Over this period, ecoregion concepts shifted from predominantly vegetation-based mappings, like Kuchler's, to integrated biotic-abiotic models exemplified by Bailey's , which incorporated climatic regimes and geomorphic processes to better capture dynamics and . This reflected growing recognition that ecoregions must account for multifaceted environmental drivers to inform sustainable .

Modern and Recent Advances

In the early , the World Wildlife Fund (WWF) advanced ecoregion classification through its Terrestrial Ecoregions of the World (TEOW) , initially published in 2001, which delineated 867 terrestrial ecoregions based on biogeographic analyses of species distributions and environmental factors. This system was refined in the 2017 RESOLVE Ecoregions dataset, updating the count to 846 ecoregions by incorporating higher-resolution data on , , and to better capture ecological boundaries and reduce overlaps from the original mapping. The revised dataset has facilitated integration with the International Union for Conservation of Nature (IUCN) threat assessments, enabling ecoregion-scale evaluations of risks such as habitat loss and , as demonstrated in WWF's Living Planet Reports that overlay data onto ecoregion maps for global conservation prioritization. In the United States, the Environmental Protection Agency (EPA) refreshed its Level III and IV ecoregion delineations in early 2025, incorporating updated satellite-derived data to refine boundaries for 105 Level III and over 1,000 Level IV units, enhancing applications in monitoring and restoration. Concurrently, the Federal Geographic Data Committee (FGDC) released Ecosystem Vegetation Classification (EcoVeg) Version 3.0 in October 2025, aligning with the U.S. National Vegetation Classification (USNVC) standard to introduce a restructured eight-level from biomes to local associations, peer-reviewed for consistency and integrated with global typologies like the IUCN Global Ecosystem Typology. This update improves accessibility through interactive maps and portals, supporting finer-scale ecoregion mapping for inventories and . Globally, the One Earth initiative introduced its Bioregions 2020 framework in the early 2020s, defining 185 bioregions as intermediate units between biogeographic realms and ecoregions to emphasize ecological connectivity and human impacts, building on the 846 ecoregions for a hierarchical view of planetary life-support systems. Complementing this, a 2025 global landscape pattern assessment analyzed land use/land cover (LULC) changes from 1992 to 2020 using the HILDA+ dataset, revealing trends such as the expansion and fragmentation of unmanaged grass/shrublands alongside declines in forest and pasture/rangeland areas, informing ecoregion vulnerability to human-induced alterations. Technological integrations have further propelled ecoregion advancements, with AI-driven mapping leveraging for automated boundary delineation and change detection; for instance, models applied to multispectral Landsat data since the enable real-time monitoring of vegetation shifts within ecoregions, improving accuracy over traditional methods. Open-access datasets, such as the Ecoregions 2017 hosted at ecoregions.appspot.com, provide downloadable shapefiles under a CC-BY 4.0 license, fostering collaborative research and integration with platforms like Engine for scalable analyses.

Importance

Ecological Role

Ecoregions serve as fundamental units of evolutionary distinctiveness, often encompassing hotspots that host unique assemblages of with high levels of , where a significant proportion of and fauna are found nowhere else on Earth. For instance, the WWF's ecoregions were selected based on criteria including exceptional and rates. Within ecoregion-specific biomes, ecosystem dynamics are regulated by localized processes such as nutrient cycling, where decomposers and plants recycle essential elements like and , maintaining and primary . is similarly influenced, with ecoregions shaping water flow patterns through cover and , as seen in forested ecoregions that moderate runoff and . occurs prominently in these units, with biomes like tropical rainforests and temperate forests storing vast amounts of carbon in and soils; for example, the ecoregion in the sequesters approximately 3.3 million metric tons of carbon per year through changes. Ecoregions enhance by facilitating across landscapes, supporting metapopulations where subpopulations exchange individuals to buffer against local extinctions. This promotes , preserving ; in large-scale ecoregions like the boreal forest, migratory pathways enable seasonal movements that sustain population viability and adaptive potential. Ecoregions provide critical ecosystem services, including by native insects and birds that sustain and , through accumulation and microbial activity, and provisioning for diverse taxa. These functions are often quantified using indices, such as the Shannon diversity index, indicating robust ecological support for these services in high-biodiversity ecoregions like the .

Conservation Applications

Ecoregions serve as fundamental units for designing and implementing strategies, enabling the prioritization of areas that capture the full spectrum of while addressing threats at ecologically meaningful scales. By delineating regions with relatively homogeneous assemblages and ecological processes, ecoregions facilitate targeted interventions that enhance representation in networks and support policy frameworks aimed at halting . The launched the initiative in to identify priority ecoregions for , selecting 238 terrestrial, freshwater, and ecoregions that collectively represent the planet's major types and harbor exceptional . This framework prioritizes these areas for action to ensure the preservation of evolutionary processes and , guiding 's ecoregion-based programs worldwide. Although the core list remains foundational, has integrated updates through ongoing assessments, such as incorporating new threat data and aligning with global targets like the UN . Ecoregions play a key role in aligning protected areas with international biodiversity targets, including the Convention on Biological Diversity's (CBD) 30x30 goal under the Kunming-Montreal Global Biodiversity Framework, which aims to conserve at least 30% of terrestrial and marine areas by 2030 through effective and equitable systems. As of 2024, global protected areas cover approximately 17% of land and 8% of oceans, highlighting gaps addressed through ecoregion-based planning. Gap analyses conducted by the International Union for Conservation of Nature (IUCN) and WWF utilize ecoregion boundaries to evaluate representation gaps, identifying underprotected areas where species and habitats are inadequately covered by existing reserves. For instance, these analyses reveal that only a fraction of ecoregions achieve 30% protection, informing expansions of networks like the World Database on Protected Areas to better safeguard biodiversity hotspots. In regional contexts, ecoregion-based planning enhances localized conservation efforts. In the United States, NatureServe and partners like conduct ecoregional assessments to map priorities, assess threats, and recommend expansions across western North American ecoregions, such as deserts and grasslands, to maintain ecological . Similarly, in , the (EEA) employs biogeographical regions—analogous to ecoregions—as a framework under the EU to evaluate and guide the network, ensuring habitat types are represented proportionally across regions like the Atlantic and Boreal zones. Monitoring ecoregion health relies on indices that track trends and integrity. WWF adapts tools like the Index, which measures vertebrate population changes, to ecoregion scales in regional reports to detect declines and evaluate conservation outcomes, such as in freshwater ecoregions where pressures from are acute. These metrics support , allowing conservationists to refine strategies based on verifiable indicators of vitality.

Ecoregion Types

Terrestrial Ecoregions

Terrestrial ecoregions encompass the diverse land-based ecosystems of , classified hierarchically by the into eight biogeographic realms, such as the Nearctic and Afrotropical, which reflect broad evolutionary histories and continental-scale patterns. These realms are further subdivided into 14 biomes, including tropical and subtropical moist broadleaf forests, temperate grasslands and savannas, and , capturing major vegetation and climate zones. In total, this system delineates 867 terrestrial ecoregions, each defined by relatively homogeneous environmental conditions, species assemblages, and ecological processes that distinguish them from neighboring areas. These ecoregions span approximately 149 million square kilometers of Earth's land surface, representing the full extent of habitable terrestrial habitats excluding polar ice caps and barren rock. Biodiversity within terrestrial ecoregions peaks in the , where factors like stable climates and high support exceptional ; for instance, the Neotropical realm's ecoregion hosts over 40,000 plant species and 3,000 vertebrate species, far exceeding temperate zones. In contrast, the Palearctic realm's Siberian , the world's largest contiguous forest, covers about 12 million square kilometers but features lower diversity with dominant coniferous trees and adapted mammals like and wolves. Key characteristics of terrestrial ecoregions include dominant structures that shape complexity, with forests comprising six of biomes and covering roughly 31 percent of land area through dense canopies of broadleaf and needleleaf trees. Soil types vary distinctly by , such as the nutrient-poor, acidic podzols in forests or the fertile, clay-rich alfisols in temperate woodlands, influencing and plant adaptations. Faunal assemblages are -specific, featuring high in tropical realms—e.g., and felids in Afrotropical savannas—while arid ecoregions support specialized reptiles and ungulates resilient to .

Marine Ecoregions

Marine ecoregions are delineated through the Marine Ecoregions of the World () , a hierarchical biogeographic classification system developed to capture patterns of marine in coastal and shelf areas. This system organizes the world's marine environments into 12 realms, 62 provinces, and 232 distinct ecoregions, focusing primarily on waters shallower than 200 meters where most coastal and shelf ecosystems occur. The framework emphasizes both coastal zones, including intertidal and benthic habitats, and the overlying pelagic zones within shelf areas, providing a nested structure for analyzing ecological similarity and distinctiveness. Key characteristics of ecoregions are shaped by oceanographic processes such as currents, gradients, and , which drive nutrient distribution, temperature variations, and species assemblages. Ocean currents transport larvae and influence connectivity between regions, while variations—often resulting from freshwater inflows or —create unique hydrographic conditions that support specialized communities. brings nutrient-rich deep waters to the surface, fostering high productivity in certain ecoregions and enabling diverse food webs. These factors distinguish ecoregions from terrestrial ones by prioritizing dynamic water mass movements over static land-based features like or . Representative examples illustrate these influences. The Coral Triangle, located within the Central realm, exemplifies a high-diversity hotspot driven by the Indonesian Throughflow—a major current system that mixes Pacific and waters, promoting exceptional coral and fish diversity through enhanced nutrient and larval dispersal. In contrast, the California Current ecoregion in the Cold Temperate Northeast Pacific province (part of the Temperate Northern Pacific realm) features seasonal along temperate shelves, supporting productive fisheries and forests adapted to cooler, nutrient-laden waters. Similarly, the Agulhas Bank ecoregion in the Agulhas province (Temperate Southern Africa realm) is influenced by the swift , which generates intermittent and eddies, sustaining subtropical to temperate biodiversity in a transition zone between tropical and polar influences. These ecoregions collectively span the global , which covers approximately 70% of Earth's surface, though specifically targets the biologically rich coastal and areas representing about 7.7% of total ocean extent. This distribution ensures comprehensive coverage of nearshore habitats critical for global productivity, from polar ice-influenced realms to equatorial tropical systems. priorities for ecoregions highlight their role in protecting hotspots, with many exhibiting high levels of due to isolation by currents or bathymetric barriers. The framework facilitates integration with marine protected areas (MPAs) by enabling systematic gap analyses and representativeness assessments, allowing planners to prioritize underrepresented ecoregions for protection and enhance global MPA networks. For instance, it has been used to evaluate how MPAs align with ecoregional boundaries, promoting more effective ecosystem-based management.

Freshwater Ecoregions

Freshwater ecoregions represent distinct biogeographic units within inland aquatic systems, delineated to capture patterns of freshwater and support efforts. The primary framework for their classification is the Freshwater Ecoregions of the World (FEOW), developed by the World Wildlife Fund and collaborators, which identifies 426 ecoregions spanning nearly all non-marine areas of the globe. These ecoregions are organized into 12 major habitat types, including large lakes, large river deltas, montane freshwaters, xeric freshwaters and endorheic basins, temperate coastal rivers, temperate upland rivers, temperate rivers and complexes, tropical and subtropical coastal rivers, tropical and subtropical upland rivers, tropical and subtropical rivers and complexes, polar freshwaters, and oceanic islands. This classification emphasizes ecological and evolutionary processes unique to freshwater systems, distributed across major biogeographic realms such as the Nearctic, Afrotropical, and Indomalaya. These ecoregions are primarily defined by variations in —such as flow regimes, seasonal flooding, and water residence times—along with water chemistry factors like , , , and nutrient levels, which together shape assemblages of organisms. Endemic often serves as a key indicator of ecoregional boundaries, reflecting historical isolation and adaptation. For instance, the Laurentian ecoregion (ID 116) in the encompasses the watersheds of Lakes Superior, , , Erie, and , holding over 20% of the world's surface freshwater across 245,000 km² of lake area. Its oligotrophic waters, influenced by glacial origins and underlying acidic bedrock, support a diverse cold-water community, including endemic cisco species flocks such as the bloater (Coregonus hoyi) and the extinct blue pike (Sander vitreus ). Globally, freshwater ecoregions are concentrated within major river basins, where they form complex networks of interconnected s that facilitate species dispersal and ecological processes. In the Indomalaya realm, the ecoregion (ID 729) exemplifies a large , encompassing the lower River and tributaries like the Tonle Sap, with extensive peat swamp forests and seasonal flooding that sustains high . This area features lowland tropical with bimodal flood pulses, supporting endemic fishes such as Akysis similis and Hemibagrus spilopterus amid diverse complexes. Similarly, in the , the Upper Congo ecoregion (ID 540) within the vast highlights tropical upland river characteristics, with the Lualaba River's slow-flowing reaches and rapids transitioning from to equatorial forest, fostering endemics like Barbus papilio and Chiloglanis marlieri in its riverine and swamp s. A critical aspect of freshwater ecoregion dynamics is , which enables and among aquatic but is increasingly threatened by barriers. Dams fragment habitats by blocking longitudinal river , altering flow regimes and essential for downstream ecosystems. Global analyses indicate that only 37% of rivers longer than 1,000 km remain free-flowing over their entire length, with being the primary cause of this loss, underscoring the vulnerability of many ecoregions to such alterations.

Challenges

Climate Change Effects

Climate change is driving significant shifts in the distribution and composition of ecoregions worldwide, primarily through biome migrations as and ecosystems respond to warming temperatures and altered patterns. A seminal study identified 89 globally exceptional ecoregions, noting that even under moderate warming scenarios, their entire ranges would experience extreme monthly temperatures exceeding historical norms with less than 2°C of local warming by 2070. Recent projections using Phase 6 (CMIP6) models indicate more pronounced biome shifts than earlier estimates in , with widespread novel climate conditions emerging across terrestrial s by mid-century under high-emission scenarios. These migrations are expected to accelerate in mountainous and polar regions, where of — the rate at which must move to track suitable conditions—could exceed natural dispersal capabilities. Vulnerability hotspots are particularly evident in Arctic and tropical ecoregions, where warming facilitates biological invasions that disrupt native assemblages. A 2023 analysis found that alters invasion vulnerability across global ecoregions, with tundra facing heightened risks from non-native establishment due to expanded thermal tolerances and reduced winter barriers. In , projections highlight potential refugia—areas of relative climatic stability—such as certain high-elevation and coastal areas, which may persist as analogs to current conditions amid broader ecoregion displacement. These refugia could serve as critical havens for , though their efficacy depends on and protection from secondary stressors. Ecoregion functionalities are also compromised, with notable impacts on ecosystem services like and water regulation. In forest ecoregions, climate-induced stressors such as droughts and fires have led to reduced carbon storage, with western U.S. forests showing declines in live carbon across multiple ecoregions since the early 2000s. Globally, up to 32.6% of ecoregions are projected to be highly vulnerable under high-emission pathways, exacerbating net carbon losses. In North American freshwater ecoregions, altered , including shifts in timing and magnitude, disrupts cycling and availability in riverine systems. Recent assessments underscore the cascading effects on . A 2021 study revealed that climate-driven ecoregion displacements could undermine targets, with 4.6% of global protected lands experiencing novel climates by the mid- to late under 2°C warming, reducing their representativeness of original ecoregion types. In the Landscape of the Hindu Kush Himalaya (HKH) region, UNFCCC reports from the early 2020s document rapid ecoregion shifts, including upward migration of montane forests and degradation of alpine meadows. These changes highlight the need for dynamic strategies that account for projected displacements, particularly in regions like the HKH where they threaten for over 240 million downstream.

Human Impacts and Management

Human activities have profoundly altered ecoregions through habitat loss, primarily driven by , , and . In tropical moist forests, an estimated 17% of the original area has been lost since 1990, with over 420 million hectares of global forest cover converted worldwide by 2020, more than 90% of which occurred in tropical regions. This loss disrupts structure and reduces , often converting biodiverse habitats into plantations or urban developments. Habitat fragmentation exacerbates these effects by creating barriers to and in terrestrial ecoregions. Approximately 70% of remaining global is located within 1 km of an , exposing it to like increased human access and altered microclimates that hinder wildlife movement. Fragmented landscapes also facilitate the spread of , as disturbed patches provide entry points and reduced native competition allow invasives to establish and proliferate, further degrading native . Pollution from industrial emissions, agricultural runoff, and urban waste introduces contaminants that acidify soils and water bodies, impairing functions across ecoregions. Atmospheric deposition of and , for instance, leads to and toxicity in forests and wetlands, reducing plant growth and altering composition. , including in marine ecoregions and excessive or in terrestrial ones, depletes key at rates exceeding natural replenishment. Examples include the collapse of in coastal ecoregions due to industrial trawling and the decline of large mammals in ecoregions from , threatening trophic balances. Management strategies emphasize ecoregion-scale to mitigate these impacts and promote . The EcoVeg approach, a vegetation-based classification system, supports by mapping types for targeted efforts, such as replanting in fragmented U.S. landscapes to reconnect habitats. International agreements like the provide frameworks for wetland ecoregion protection, designating sites for conservation and wise use to maintain hydrological and functions through regulated land practices. Ongoing monitoring relies on global land use and land cover (LULC) datasets to track human-induced changes at fine resolutions. Updated 2025 datasets, such as the 10 m global land cover reference aligned with imagery, enable detection of habitat alterations and evaluation of management effectiveness across ecoregions. These tools support data-driven interventions, like prioritizing anti-deforestation measures in high-risk tropical areas.