Biome

A biome represents a major ecological unit characterised by a distinctive climate, predominant vegetation and associated animal life. Each biome embodies a broad biological community that has developed in response to regional climate patterns and environmental conditions. Across continents, similar environmental controls produce comparable vegetation structures and ecological functioning, allowing globally dispersed regions to be described under shared biome categories.

Concept and Development of the Biome Idea

The modern understanding of biomes emerged from early ecological work examining how climate and vegetation co-vary. During the 1930s, Arthur Tansley expanded the study of ecological communities by integrating soil and climatic factors, contributing to the ecosystem concept. Later, mid-twentieth-century international ecological projects helped popularise the biome as a functional worldwide classification, emphasising large-scale vegetation structures and climate influences.
The concept is not uniformly applied across all scientific traditions. In some German ecological literature, particularly that following Heinrich Walter’s framework, the term is closer to the notion of a biotope, referring to concrete geographical units. In these contexts, broader and more abstract international biome categories correspond to zonobiomes, orobiomes and pedobiomes, which reflect the role of climate zones, altitude and soil type respectively. In contrast, Brazilian ecological scholarship often uses the term biome to denote biogeographic or floristic provinces based primarily on species composition, or to refer to large morphoclimatic and phytogeographical domains characterised by shared landforms, climatic regimes and vegetation types. These regional interpretations frequently encompass multiple biomes under the international definition.

Difficulties in Global Ecological Zoning

Dividing the world’s ecological variation into clear, discrete units is inherently challenging. Ecosystems change gradually across space rather than at abrupt boundaries, and local variation in microclimate, soils and relief can create small-scale diversity within broad zones. Any global classification therefore relies on generalised climatic averages and dominant vegetation forms rather than sharply defined edges.
Despite these difficulties, attempts to classify the Earth’s ecological diversity have produced influential frameworks. A wide array of schemes highlights the complexity of determining universal biome boundaries and demonstrates that no single system perfectly captures global ecological variation. Nonetheless, these classifications are valuable for studying vegetation patterns, predicting ecological responses to climate, and comparing large-scale ecosystems across continents.

Climatic and Ecological Controls on Biomes

Biomes arise primarily from interactions between climatic factors—especially temperature and precipitation—and plant physiological responses. A classic example comes from studies on North American grasslands, which demonstrated that evapotranspiration is strongly correlated with aboveground primary productivity. Precipitation largely drives aboveground biomass, while solar radiation and temperature influence below-ground investment such as root growth. Climatic conditions also determine whether warm-season or cool-season plant species dominate. These principles underpin many biome classification frameworks, including those developed by Holdridge and Whittaker.

Holdridge’s Life-Zone System

In 1947, Leslie Holdridge proposed a life-zone scheme based on the biological consequences of temperature, precipitation and potential evapotranspiration. Holdridge’s model uses a triangular diagram with axes representing biotemperature, annual precipitation and potential evapotranspiration ratio. By combining these climatic variables, he identified thirty humidity provinces.
Although Holdridge’s system largely omits variables such as soil type, solar exposure and altitude, he acknowledged their ecological importance. His framework nevertheless provides a widely used approach for visualising relationships between climate and vegetation at global scales.

Mid-Twentieth-Century Biome Typologies

Several ecologists proposed alternative systems during the mid-1900s. Allee (1949) and Kendeigh (1961) outlined principal biometypes and biome groupings for both terrestrial and marine environments, reflecting growing interest in identifying global vegetation patterns. These systems tended to focus on vegetation structure but also accounted for animal communities and ecological functioning.

Whittaker’s Biome Classification

Robert Whittaker developed one of the most influential biome schemes during the 1960s and 1970s. His approach simplified previous models by using two dominant climatic variables: average annual temperature and annual precipitation. By plotting these as axes, Whittaker organised terrestrial biomes along gradients of moisture availability and thermal conditions.
Whittaker’s ecological framework drew on both theoretical principles and empirical data from worldwide vegetation studies. He also introduced distinctions between biomes, biometypes and formation types:

• Biome: a major vegetation community on a continent with characteristic structure, environment and associated fauna.
• Biometype: a grouping of similar biomes across continents, emphasising structural convergence.
• Formation type: a classification of plant communities only, focusing strictly on vegetation.

To understand global variation, Whittaker examined key ecoclines such as climatic moisture gradients, latitudinal temperature changes, altitudinal temperature gradients and intertidal wetness. These gradients produce systematic variations in plant physiognomy, structural complexity and biodiversity. Environments become less favourable for species and structural diversity as conditions move towards climatic extremes.
Whittaker’s scheme identifies major biometypes including tropical rainforests, temperate forests, taiga, grasslands, savannas, deserts, tundra and a broad category of wetlands and swamp forests.

Ecosystem-Based Classifications

The Ecosystems of the World series, edited by David Goodall, expanded this approach by providing detailed descriptions of major global ecosystem categories. This multivolume work synthesised ecological research to produce a comprehensive reference on biome types, drawing attention to global comparisons and regional variations.

Walter’s Zonobiome System

Heinrich Walter’s scheme emphasised seasonal climatic patterns. By analysing the interplay between temperature seasonality and precipitation regimes, Walter identified nine major zonobiomes. These reflect dominant climate-vegetation relationships and account for stress factors such as cold or water deficiency, which strongly influence plant form. Walter also recognised that extreme local conditions, such as persistent flooding, can create divergent ecological communities within a single biome.

Schultz’s Ecozone Framework

Schultz proposed another large-scale ecological classification distinguishing nine ecozones across the world. These include polar, boreal, humid mid-latitude, dry mid-latitude, subtropical winter-rain, subtropical year-round-rain, dry tropical, tropical summer-rain and tropical year-round-rain zones. Schultz’s ecozones align more consistently with biome definitions than with biogeographic realms, focusing primarily on climatic and ecological characteristics rather than species composition.

Bailey’s Ecoregion System

Robert Bailey developed a climatic classification for the United States, later expanded to global coverage. His system divides the Earth into four broad climate-based domains—polar, humid temperate, dry and humid tropical—and then subdivides these into regions based on additional climatic variables such as temperature ranges and continental or maritime influences. The Bailey framework has been widely used in environmental management and landscape-level planning due to its practical focus on climate controls and ecological responses.