Table of Contents
Overview: How the Biosphere Is Zoned
The biosphere is not uniform. Because temperature, moisture, light, and other conditions change over the surface of the Earth and with altitude and depth, life is organized into large-scale “zones.” These zones can be described in several ways:
- By climate and vegetation (e.g., tropical rainforest, desert)
- By altitude (mountain belts)
- By latitude (from equator to poles)
- By water depth and distance from shore (in oceans and lakes)
In this chapter, the focus is on recognizing and understanding these large spatial patterns, not on the detailed functioning of each individual ecosystem (covered elsewhere).
Key ideas:
- Environmental factors change in a gradual (continuous) way.
- Organisms often respond in a zonal (banded) way: certain communities recur in similar conditions.
- Similar zones may occur in distant regions if the environmental conditions are comparable (e.g., deserts on different continents).
Latitudinal Zonation: From Equator to Poles
Because the Earth is spherical and tilted, solar radiation and climate change with latitude. This leads to broad belts of climate and vegetation, often called climatic zones or biomes.
Main Climatic and Vegetation Zones
The names and boundaries may vary slightly between classification systems, but typical belts from equator to poles are:
- Tropical Zone (Equatorial and Subtropical Regions)
- Climate: Warm year-round, small annual temperature variation.
- Moisture variation:
- Very wet areas → tropical rainforests (evergreen, multilayered canopies, extremely high biodiversity).
- Seasonal rainfall → savannas (grassy landscapes with scattered trees, pronounced dry season).
- Very low rainfall → hot deserts (sparse vegetation, drought-adapted plants and animals).
- Biological features: High productivity where water is sufficient, many specialized species.
- Subtropical/Transition Zones
- Mediterranean-type climates (mild, wet winters; hot, dry summers) with sclerophyllous (hard-leaved) shrubs.
- Often strong human use and land transformation.
- Temperate Zone
- Climate: Moderate temperatures, pronounced seasons.
- Vegetation types:
- Temperate deciduous forests: Trees lose leaves in winter.
- Temperate coniferous forests: Conifers dominate in cooler or nutrient-poor areas.
- Grasslands/steppes/prairies: In continental interiors or rain shadows where precipitation is too low for closed forest but higher than in deserts.
- Biological features: Clear seasonal rhythm in growth, reproduction, and animal behavior.
- Boreal / Subpolar Zone (Taiga)
- Climate: Long, cold winters; short, cool summers; moderate to low precipitation.
- Vegetation: Vast coniferous forests (spruce, fir, pine), with mosses and lichens.
- Biological features: Short growing season, many migratory animals, freeze-tolerance strategies.
- Polar Zone (Tundra and Polar Deserts)
- Climate: Very cold, long winters, short cool summers; low precipitation; permafrost.
- Vegetation: Tundra (low shrubs, mosses, lichens; no trees), and almost vegetation-free polar deserts on ice.
- Biological features: Extreme adaptations to cold and short growing seasons; low primary productivity.
These latitudinal zones are sometimes grouped into biomes, each characterized by:
- A typical large-scale climate pattern
- A corresponding dominant vegetation structure
- Characteristic animal communities
Altitudinal Zonation: From Lowlands to Mountain Tops
Mountains show changes in environmental conditions similar to those from equator to pole, but over short horizontal distances and increasing altitude.
Why Conditions Change with Altitude
With increasing altitude:
- Air temperature decreases (on average about 0.5–1 °C per 100 m, depending on conditions).
- Air becomes thinner and drier, and wind exposure often increases.
- Soils tend to become shallower and rockier.
- UV radiation increases.
Thus, as you go up a mountain, you often move through altitudinal vegetation belts that roughly correspond to the latitudinal zones, but compressed vertically.
Typical Altitudinal Belts (Conceptual Scheme)
Names differ between regions and mountain ranges, but a typical sequence (from bottom to top) might be:
- Colline Zone (Lowland/Foot Zone)
- Climate similar to surrounding lowlands.
- Land often heavily used by humans (settlements, agriculture, mixed forests).
- Montane Zone
- Cooler, more precipitation than lowlands.
- Extensive montane forests (deciduous, mixed, or coniferous, depending on latitude and region).
- Subalpine Zone
- Near the upper limit of closed forest.
- Open coniferous forests, forest line, and krummholz (stunted, deformed trees due to harsh conditions).
- Alpine Zone
- Above the closed forest line.
- Open, low-growing vegetation: alpine meadows, dwarf shrubs, cushion plants.
- Very short growing season, strong winds, high UV, often snow cover much of the year.
- Nival Zone
- Highest elevations.
- Almost no permanent vegetation.
- Glaciers, perennial snowfields, bare rock.
Not every mountain range contains all belts; the sequence and elevation of belts depend on latitude (mountains in the tropics have different communities at a given altitude than mountains in the temperate zone) and regional climate.
Similarities Between Latitudinal and Altitudinal Zonation
Altitudinal zonation is often described as a “compressed pole journey”:
- Climbing from foothills to summit in the tropics can resemble traveling from a tropical lowland up to “temperate” and even “polar-like” conditions.
- Many ecological patterns repeat: reduced temperature, shorter growing season, and increasing stress with increasing altitude or latitude.
However, the species composition is not literally the same: a “high mountain tundra” in the tropics has different species than an Arctic tundra, even if their environmental challenges are similar.
Zonation in Aquatic Systems
Aquatic environments also exhibit zonation, both horizontally (e.g., from shore to open water) and vertically (e.g., from surface to depth). Only the basic patterns are outlined here; the detailed functioning of lake and marine ecosystems is dealt with in their respective chapters.
Vertical Zonation by Light: Photic vs. Aphotic
In both marine and freshwater systems, light availability declines with depth, influencing where photosynthesis is possible.
- Photic (euphotic) zone: Upper water layer with enough light for photosynthesis.
- Dominated by primary producers (phytoplankton, algae, aquatic plants).
- High importance for oxygen production and food webs.
- Aphotic zone: Deeper water where light is insufficient for photosynthesis.
- No net primary production from photosynthesis.
- Life depends on organic material coming from above or on special chemolithoautotrophic processes in particular environments.
The exact depths of these zones vary with water clarity, season, and suspended particles.
Horizontal Zonation in Lakes
Lakes show spatial differentiation from shore to open water, as well as from surface to bottom.
Common horizontal zones (in a conceptual sense):
- Littoral zone: Near-shore, shallow area where light reaches the bottom.
- Rooted aquatic plants can grow.
- High structural complexity; habitat for many invertebrates, fish, and amphibians.
- Limnetic (pelagic) zone: Open water area away from the shore, above the deeper bottom.
- Dominated by plankton and free-swimming (nektonic) organisms.
Vertical zones related to the bottom:
- Profundal zone: Deep water region beyond the reach of effective light penetration; often low oxygen, especially in summer stratification.
- Benthic zone: The bottom itself (sediment surface and upper layers), home to benthic organisms (worms, insect larvae, mollusks, bacteria).
The arrangement and relative size of these zones depend on lake depth, shape, and water clarity.
Zonation in Marine Environments
The ocean is broadly divided:
By Distance from Shore
- Littoral (intertidal) zone: Area between high and low tide marks.
- Strongly changing conditions (submersion and exposure, salinity, temperature).
- Organisms are highly adapted to fluctuations.
- Neritic zone: Shallow sea above the continental shelf.
- Usually within the photic zone; often highly productive due to nutrient inputs from land and upwelling.
- Oceanic zone: Open ocean beyond the continental shelf.
- Vast areas, often nutrient-limited in surface waters.
By Depth and Light (Simplified)
- Epipelagic: Surface to ~200 m; photic zone in the ocean; main site of marine photosynthesis.
- Deeper pelagic zones (meso-, bathy-, abyssopelagic): Increasing depth, decreasing light, pressure increases. Adapted communities with various strategies for dealing with darkness and scarcity of food.
- Benthic zones (continental shelf, slope, deep-sea floor): Life on or in marine sediments, with zonation according to depth, substrate, and nutrient supply.
Ecotones and Transition Zones
Between clearly identifiable zones (latitudinal, altitudinal, aquatic), there are typically transition areas rather than sharp borders.
- These transitions are called ecotones.
- Characteristics:
- Mix of species from neighboring zones.
- Sometimes higher species diversity than in the adjacent zones.
- Particularly sensitive to environmental changes and human impacts.
Examples:
- Gradual transition from forest to grassland in semi-arid regions.
- The treeline zone between closed forest and alpine meadows.
- Brackish water zones at river mouths (estuaries) between freshwater and marine systems.
Ecotones illustrate that zonation is often a continuum, not a set of rigid, sharply separated belts.
Human Influence on Zonation
Human activities have strongly altered natural zonation patterns:
- Land use change (agriculture, urbanization, deforestation) can fragment or erase natural belts, especially in desirable lowland and montane zones.
- Climate change shifts climatic zones:
- Many vegetation belts and associated species are moving poleward and upward in altitude.
- Alpine and polar zones have limited “space” to move to, making some communities particularly vulnerable.
- Pollution and eutrophication can modify aquatic zonation (e.g., shrinking of oxygenated zones in lakes and coastal seas).
Understanding natural zonation patterns is essential for:
- Predicting the responses of ecosystems to climate and land-use change.
- Planning protected areas that adequately cover different ecological zones.
- Interpreting large-scale biodiversity patterns on Earth.