Neotropical realm
The Neotropical realm is one of Earth's eight biogeographic realms, defined by distinctive patterns in the distribution of terrestrial organisms and encompassing tropical ecoregions from central Mexico southward through Central and South America, the Caribbean islands, and portions of southern North America including Florida.[1][2][3] This vast region, originally delineated as a zoogeographical division by Alfred Russel Wallace in the late 19th century, spans approximately 19 million square kilometers and features diverse biomes such as tropical rainforests, savannas, montane forests, and deserts.[4][3] It stands out for its unparalleled biodiversity, with the Amazon basin serving as the primary source of species richness and high endemism across taxa, exemplified by 96% of native amphibian species being unique to the realm.[5][6] The realm's evolutionary history, shaped by geological events like Andean uplift and climatic fluctuations, has fostered exceptional diversification in groups such as primates, birds, and insects, though ongoing habitat loss poses significant threats to this ecological treasure trove.[5][7]
Definition and Boundaries
Core Definition
The Neotropical realm constitutes one of Earth's eight biogeographic realms, a classification system delineating major regions of distinct evolutionary biota shaped by historical barriers to dispersal and vicariance events.[8] This realm encompasses the tropical and subtropical terrestrial ecoregions of the Americas, extending from southern Mexico through Central America, the entirety of South America including its temperate zones, and the Caribbean islands, with some inclusions in southern Florida due to shared tropical affinities./The_Physical_Environment_(Ritter)/13:_Earth_Biomes/13.01:_Patterns_of_the_Biosphere) Its boundaries are primarily defined by faunal and floral discontinuities, such as the absence of Nearctic elements north of the Isthmus of Tehuantepec and the prevalence of endemic taxa like marsupials, edentates, and platyrrhine primates, reflecting isolation since the Mesozoic era.[2] Characterized by exceptional species richness, the Neotropical realm hosts over 3,000 species of freshwater fish across 32 families, surpassing other realms in diversity for this group, alongside dominant vegetation of tropical rainforests, savannas, and montane forests.[2] The Amazon Basin alone accounts for more than half of the realm's forested area, representing the world's largest contiguous tropical rainforest and a key driver of global biodiversity, with endemism rates exceeding 50% in many taxa.[9] These features underscore the realm's role as a primary center of speciation, influenced by diverse climates ranging from equatorial wet to semi-arid, and topographic variation from Andean peaks over 6,000 meters to lowland plains./The_Physical_Environment_(Ritter)/13:_Earth_Biomes/13.01:_Patterns_of_the_Biosphere)Geographic Boundaries
The Neotropical realm encompasses the tropical and subtropical regions of the Americas, extending from central and southern Mexico southward through Central America, the Caribbean islands, and the entire South American continent to Tierra del Fuego. This area covers approximately 19 million square kilometers, bounded laterally by the Pacific Ocean to the west and the Atlantic Ocean to the east.[10][11] The northern boundary with the Nearctic realm is transitional, primarily located in central Mexico around the Isthmus of Tehuantepec and extending into southern Florida for certain taxa, reflecting historical faunal and floral discontinuities rather than a strict latitudinal line near 23° N. This transition zone features overlapping distributions of Nearctic and Neotropical species, with the exact limit varying by organism group; for instance, mammalian analyses place it variably in northern to central Mexico based on distributional patterns.[12][13] To the south, the realm reaches the subantarctic zones of Patagonia, approximately 55° S, including the Andean cordillera and Patagonian steppes, where biotic assemblages maintain Neotropical affinities despite cooler climates. The Caribbean subregion integrates volcanic and limestone islands from the Greater and Lesser Antilles, fully within the realm due to shared evolutionary histories with continental faunas. Oceanic islands like the Galápagos are sometimes affiliated but primarily exhibit unique insular endemism; however, continental shelf extensions confirm the core maritime boundaries.[14][7]Subregional Divisions
The Neotropical realm's subregional divisions are delineated based on cladistic biogeographic analyses of terrestrial organism distributions, emphasizing historical vicariance and dispersal barriers such as the Andes uplift and climatic oscillations. Contemporary classifications, such as those proposed by Morrone, recognize three primary subregions—Antillean, Brazilian, and Chacoan—supplemented by two transition zones where Neotropical and Nearctic faunas intermix: the Mexican Transition Zone and the South American Transition Zone. These divisions encompass 53 biogeographic provinces, reflecting distinct evolutionary histories and endemic taxa concentrations.[15][16] The Antillean Subregion includes the Caribbean islands from the Bahamas to Trinidad and Tobago, isolated by marine barriers that fostered high endemism, particularly among reptiles, birds, and amphibians; for instance, Cuba hosts over 50 endemic bird species representing about 20% of its avifauna. This subregion's biota derives largely from South American colonization via overwater dispersal, with subsequent adaptive radiations.[17] The Brazilian Subregion dominates the tropical lowlands of South America, encompassing the Amazon Basin, Guianas, eastern Brazil, and parts of the Orinoco, characterized by vast rainforests supporting unparalleled diversity, such as the 10,000+ tree species in Amazonia alone. It features provinces like Amazonian and Atlantic Forest, where stable tropical climates preserved ancient lineages, evidenced by fossil pollen records dating to the Eocene indicating continuous humid conditions.[18] The Chacoan Subregion covers semi-arid to subtropical areas of southern South America, including the Gran Chaco, Pampas, and parts of Uruguay and Paraguay, with biota adapted to seasonal droughts and fires; notable endemics include the Chacoan peccary, discovered in 1975 and restricted to this area. This subregion exhibits influences from both tropical and temperate zones, with floristic elements showing Miocene origins.[7] Transition zones facilitate biotic exchange: the Mexican Transition Zone spans from central Mexico southward to Nicaragua, blending Nearctic temperate species like pines with Neotropical elements, as seen in the Trans-Mexican Volcanic Belt's mixed conifer-oak forests. The South American Transition Zone, extending from southern Brazil to Patagonia, integrates Andean and austral elements, with provinces like the Espinal hosting hybrid assemblages responsive to Pleistocene glaciations. These zones, comprising about 10-15% of the realm's area, underscore dynamic faunal gradients rather than sharp boundaries.[19][11]Historical Classification
Alfred Russel Wallace's Framework
Alfred Russel Wallace established the Neotropical realm as one of six primary zoogeographical regions in his seminal 1876 work, The Geographical Distribution of Animals, based on empirical analysis of terrestrial vertebrate distributions, emphasizing discontinuities in mammalian and avian faunas attributable to historical barriers and isolation.[20] This framework prioritized faunal assemblages over mere continental geography, identifying the Neotropical as a distinct unit due to its archaic and endemic elements, such as the persistence of pre-placental mammals and the absence of many Old World placental orders.[21] Wallace's delineation reflected causal factors like the region's Gondwanan origins and subsequent tectonic separation, which fostered unique evolutionary trajectories unsupported by recent faunal interchange with northern realms.[22] The realm's boundaries, as defined by Wallace, extended across South America from Tierra del Fuego northward through Central America and the Isthmus of Panama, incorporating the West Indies and southern Mexico up to transitional zones near the Tropic of Cancer, where Neotropical forms like armadillos and opossums intermingle with Nearctic species.[21] Unlike rigid latitudinal lines, these limits were drawn from distributional data, noting gradual faunal blending in Mexico but sharp distinctions southward, with oceanic barriers isolating Caribbean islands.[23] Wallace excluded temperate southern South America only insofar as subantarctic elements overlapped minimally, affirming the region's tropical core as the dominant biogeographic signal.[22] Within the Neotropical, Wallace subdivided into four subregions to capture intra-regional variation: the Brazilian (encompassing Amazonian lowlands and adjacent plateaus, richest in endemic primates and insects); the Peruvian (high Andean cordilleras, with altitudinal zonation yielding specialized highland forms); the Mexican (Central America and southern Mexico, blending tropical humidity with drier elements); and the Antillean (West Indian archipelago, marked by insular endemism and fewer mammals but diverse birds and reptiles).[21] [23] These divisions highlighted gradients in humidity, elevation, and isolation, with the Brazilian subregion exemplifying maximal diversity—over 100 endemic mammalian genera, including all Platyrrhine monkeys and Edentata—while the Antillean showed adaptive radiations constrained by island size.[22] Wallace underscored the realm's mammalian poverty in advanced orders (e.g., scant ungulates, no elephants or true ruminants) contrasted by abundance in lower forms, attributing this to ancient barriers preventing placental colonization post-Cretaceous, evidenced by fossil records of early marsupials and xenarthrans.[23] Avian highlights included 30+ endemic families, such as trogons and toucans, with distributional patterns reinforcing subregional integrity; for instance, hummingbird genera clustered distinctly across divides.[21] Reptilian and amphibian faunas further validated the framework, showing low overlap with Nearctic taxa south of Panama, underscoring Wallace's emphasis on verifiable zoographic breaks over speculative diffusion.[22] This structure has endured as a benchmark, its empirical grounding in specimen-based catalogs enabling falsifiability against later phylogenetic data.[20]20th-Century Refinements
In the early 20th century, Ángel L. Cabrera and Arturo R. Yepes refined Wallace's framework by analyzing mammalian distributions, proposing subregions such as the Guianan-Brazilian and Patagonian, along with 11 districts defined by physiographic features and species assemblages.[24] These divisions highlighted faunal discontinuities driven by geographic barriers, building on vertebrate data to delineate areas of high endemism in eastern South America and southern extremes.[24] Philip J. Darlington's 1957 monograph Zoogeography: The Geographical Distribution of Animals integrated extensive taxonomic data across vertebrate and invertebrate groups, confirming the Neotropical realm's integrity while emphasizing evolutionary stasis in tropical lineages and the isolating effects of the Andean uplift and Amazonian drainage on internal diversification.[25] Darlington argued that the realm's ancient Gondwanan affinities persisted due to limited inter-realm exchanges, refining boundaries northward to exclude predominantly Nearctic faunas beyond central Mexico.[26] Gonzalo Halffter's 1962 conceptualization of the Mexican Transition Zone marked a significant advancement, identifying this band from southwestern United States to central Mexico as a dynamic overlap area where Neotropical biotas progressively dominate southward through successive colonization waves, termed cenocrons, primarily evidenced in scarab beetle distributions.[27] This zone, spanning approximately 1,000–1,500 km latitudinally, incorporated biotic mixing without sharp demarcation, challenging Wallace's simpler provincial lines and attributing patterns to historical climate shifts and topographic barriers like the Sierra Madre.[27] Philip Hershkovitz's analyses in 1969 and 1972 of Neotropical mammals delineated three core subregions—Brazilian (encompassing Amazonia and eastern lowlands), Patagonian (southern grasslands and steppes), and West Indian (Caribbean islands)—while classifying Mesoamerica as an extension of the Brazilian subregion based on shared archaic taxa like platyrrhine primates and xenarthrans.[24] These refinements quantified endemism rates, with the Brazilian subregion hosting over 60% of Neotropical mammal genera, underscoring vicariance from Andean orogeny.[24] Ángel L. Cabrera and Abraham Willink's 1973 Biogeografía de América Latina provided a hierarchical system for the region, recognizing five dominions—Caribbean, Amazonian, Guianan, Chacoan, and Andean-Patagonian—subdivided into 25 provinces using integrated floral, faunal, and vegetational data.[7] This classification excluded the southernmost Patagonian tip due to transitional affinities with subantarctic elements, emphasizing climatic gradients and riverine barriers in provincial boundaries, such as the Paraná River separating Chacoan from Amazonian dominions.[28] Their work facilitated quantitative assessments, revealing the Amazonian dominion's dominance with over 40% of provincial endemics.[18]Contemporary Adjustments
In the early 21st century, biogeographic classifications have increasingly incorporated quantitative analyses of species distributions alongside phylogenetic relationships to reassess Wallace's realms, emphasizing evolutionary turnover rather than solely distributional overlaps. Holt et al. (2013) applied a clustering approach to data from 21,037 species of tetrapods (mammals, birds, amphibians, and reptiles), identifying 20 distinct zoogeographic regions globally by quantifying both species composition and phylogenetic divergence across grid cells. This method revealed boundaries defined by abrupt shifts in evolutionary history, updating Wallace's framework with finer resolution informed by molecular phylogenies and georeferenced occurrence records. Applied to the New World, the analysis upheld the core Neotropical realm—encompassing most of South America, Central America, and parts of southern North America—but delineated a distinct Mesoamerican region spanning southern Mexico, Central America, and northern South America as a transitional zone of elevated phylogenetic turnover. This adjustment reflects the region's role as a historical dispersal corridor during the Great American Biotic Interchange around 3 million years ago, where northern Holarctic lineages mingled with southern Gondwanan ones, fostering unique assemblages not fully aligned with either the Nearctic or traditional Neotropical faunas. The northern Neotropical boundary was refined southward from Wallace's original placement, excluding arid northern Mexican deserts more akin to Nearctic patterns, based on empirical turnover metrics exceeding global medians in Mesoamerica. These proposals sparked debate over methodological validity. Kreft and Jetz (2013) critiqued the turnover-based clustering for generating scale-dependent boundaries that fragment established realms, arguing it prioritizes local endemism over broader historical signals and lacks validation against alternative data like plants or insects.[29] In response, Holt et al. (2013) defended the approach through extensive sensitivity tests across taxa, spatial resolutions, and turnover definitions, demonstrating consistency with independent phylogenetic studies and superior capture of deep-time divergence compared to species-only metrics. Subsequent works have built on this phylogenetic emphasis, though the Neotropical realm's core extent remains stable in most classifications, with Mesoamerica often treated as a subregion rather than fully independent.[30]Physical Geography and Climate
Topography and Landforms
The Neotropical realm exhibits profound topographic variation, influenced primarily by Andean orogeny, ancient cratonic stability, and fluvial sedimentation. Western South America is defined by the Andes, the longest exposed mountain chain above sea level, extending roughly 7,240 kilometers from Venezuela to Tierra del Fuego with widths of 200 to 700 kilometers and an average elevation near 4,000 meters.[31][32] This range, formed by ongoing subduction of the Nazca Plate beneath the South American Plate, features parallel cordilleras, high plateaus like the Altiplano (elevations 3,500–4,500 meters), and active volcanism, culminating in peaks such as Aconcagua at 6,961 meters.[33] Contrasting the Andean uplift, the eastern and central portions comprise low-relief basins and shields. The Amazon Basin, the largest riverine lowland globally, covers approximately 6 million square kilometers of nearly flat terrain (slopes under 0.1%), dominated by alluvial plains, meandering rivers, and seasonal flood zones that expand up to 150,000 square kilometers annually.[34][35] Flanking this are the Precambrian Guiana and Brazilian shields, eroded into rugged highlands and tablelands; the Brazilian Plateau, spanning over 3 million square kilometers in eastern and southern Brazil, rises to averages of 750–1,000 meters with isolated peaks exceeding 2,800 meters, characterized by escarpments like the Great Escarpment and inselbergs.[36][37] Central America and the Caribbean add volcanic and tectonic complexity. Central America's cordilleras, part of a volcanic arc from subduction along the Middle America Trench, include over 100 volcanoes (e.g., in Guatemala and Costa Rica) reaching 4,000 meters, interspersed with grabens, karst plateaus, and coastal lowlands.[38] Caribbean landforms blend subduction-driven volcanoes (e.g., in the Lesser Antilles), folded ranges from tectonic collisions, and limestone platforms from coral reef buildup, with elevations from sea-level cays to peaks like Pico Duarte at 3,098 meters. These features collectively create steep elevational gradients, driving habitat diversity across the realm.[39]Climatic Patterns
The Neotropical realm is dominated by tropical climates, where mean monthly temperatures in lowland areas consistently exceed 18 °C, supporting minimal seasonal thermal variation near the equator. Annual temperatures typically range from 24–28 °C in Amazonian lowlands, with diurnal fluctuations often greater than annual ones due to the region's proximity to the equator and stable solar insolation. These patterns reflect the influence of Hadley cell circulation and the persistence of warm air masses, though altitudinal effects in the Andes impose adiabatic cooling at rates of approximately 6.5 °C per 1,000 m elevation gain, creating cooler paramo and puna zones above 3,000 m.[2][40] Precipitation exhibits high spatial and temporal variability, primarily modulated by the seasonal migration of the Intertropical Convergence Zone (ITCZ), which shifts northward in boreal summer (May–October) to deliver heavy rains to Central America and northern South America, and southward in austral summer (November–March) for southern regions like the Gran Chaco. This results in bimodal or unimodal wet-dry cycles across savannas and deciduous forests, with annual totals ranging from 250–2,000 mm in seasonally dry areas to over 2,000 mm in equatorial rainforests, where convection and orographic uplift sustain persistent cloud cover. Topographic features, such as the Andes, generate pronounced rain shadows, fostering arid conditions on leeward slopes, exemplified by the Atacama Desert's annual precipitation below 10 mm in coastal valleys.[41][42][40] Subtropical zones in southeastern South America, including parts of Argentina and Uruguay, transition to more temperate patterns with cooler winters (averaging 10–15 °C) and occasional frosts, influenced by polar air incursions from the south. Interannual variability is amplified by phenomena like El Niño-Southern Oscillation (ENSO), which can induce droughts reducing precipitation by 20–50% in the Amazon during warm phases or excessive rains during cool phases, underscoring the realm's sensitivity to ocean-atmosphere teleconnections. These climatic gradients underpin the region's biome diversity, from hyperhumid lowlands to xeric highlands.[43][44]Hydrological Features
The Neotropical realm's hydrology is dominated by expansive tropical river basins, seasonal flooding regimes, and Andean-influenced highland water bodies, shaped by high annual precipitation exceeding 2,000 mm in many areas and orographic effects from the Andes. These systems collectively drain over two-thirds of the continent's land area through four primary basins: the Amazon, Orinoco, La Plata (including Paraná and Paraguay rivers), and São Francisco. River discharges exhibit strong seasonality, with peak flows during the wet season (November to April) driven by convective rainfall and snowmelt from Andean headwaters, contributing to nutrient transport and sediment loads that sustain downstream wetlands and deltas.[45] The Amazon River basin, encompassing roughly 7 million km² across nine countries, represents the world's largest drainage system by volume, with an average discharge of 209,000 m³/s—accounting for about one-fifth of global riverine freshwater input to oceans. Its hydrology features low-gradient floodplains (várzeas) that inundate up to 150,000 km² annually, facilitating sediment deposition and blackwater rivers like the Rio Negro, which contrast with whitewater tributaries such as the Solimões due to differing soil leaching and organic content. The Orinoco basin, spanning approximately 880,000 km² primarily in Venezuela and Colombia, delivers a mean discharge of 36,000 m³/s, third globally, with bimodal flooding from both equatorial rains and Guiana Shield runoff; unique features include the Casiquiare canal, a natural waterway linking it to the Amazon via bifurcation.[46][47][48] The La Plata basin, the second-largest at 2.8 million km², integrates the Paraná River (4,880 km long) and Paraguay River, with discharges averaging 18,000 m³/s at the confluence; its hydrology supports the Pantanal, the globe's largest contiguous tropical wetland (140,000 km²), where pulse flooding from the Upper Paraguay River—peaking in March–July—creates a retention time of 6–9 months, modulating flood peaks and recharging aquifers. Highland lakes like Titicaca (3,812 m elevation, 8,300 km² surface area) maintain balance through inflows from 25 rivers and direct precipitation (1,200 mm/year), offset by evaporation (1,500 mm/year equivalent) and outflow via the Desaguadero River, resulting in slight endoreic tendencies amplified by recent droughts. Central American contributions, such as the Usumacinta (480 km, draining Mayan lowlands) and Lempa (422 km), feature steeper gradients and volcanic influences but smaller basins under 100,000 km² each, feeding Gulf of Mexico and Pacific outflows.[49][50][51][52]| Major River/Basin | Approximate Basin Area (km²) | Average Discharge (m³/s) | Key Hydrological Note |
|---|---|---|---|
| Amazon | 7,000,000 | 209,000 | Largest global volume; extensive floodplains[46][47] |
| Orinoco | 880,000 | 36,000 | Bimodal flooding; inter-basin linkage[48] |
| Paraná (La Plata) | 2,800,000 | 18,000 (at confluence) | Supports Pantanal pulsing[49][50] |