Factors Influencing World Distribution of Plants and Animals

• Plants and animals, both terrestrial and aquatic, occur continuously from the Equator to the poles and from foothills to mountain tops, yet no species lives everywhere.
• Explaining this patterned diversity needs two things:
  • the patterns of distribution a species can show: cosmopolitan, endemic, continuous or discontinuous;
  • the factors that produce them: abiotic (non-living) and biotic (living) controls, historical controls acting over geological time, and rising human (anthropogenic) pressure.
• A range reflects the species’ tolerance of its environment, its ability to reach a place, and its interactions with other organisms there.

Patterns of Distribution

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Cosmopolitan and endemic distribution

• Cosmopolitan distribution: the same or similar species is found widely all over the world, as are the house sparrow, the brown rat and bracken fern, mostly spread by people or by very efficient dispersal.
• Endemic distribution: the same or similar species occurs only in one specific geographical area and nowhere else, whether a continent or a single hill or island; endemism peaks on old, isolated islands and mountains.
• Between the two lie wide but bounded ranges: pantropical (throughout the tropics) and circumboreal (round the northern coniferous belt).

Continuous and discontinuous (disjunct) distribution

• A continuous distribution occupies one unbroken range, as the moose does across the northern coniferous forest.
• A discontinuous (disjunct) distribution is found in two or more widely separated areas and not in the territory between, often across a gap that looks far too wide to cross. Explaining such ranges is a central task of biogeography.
  • Classic case, the tapir: three species live in Central and South America and the Malayan tapir in South-East Asia.
    • Tapirs arose in the Northern Hemisphere about 50 million years ago and spread across Eurasia and North America. They entered South America only about 3 million years ago, over the new Isthmus of Panama.
    • They then died out in between. The survivors are the ends of a once-continuous range, a relict distribution, not a Gondwana split.
• Other disjunctions:
  • Lungfishes (South America, Africa, Australia) and the southern beech Nothofagus (southern South America, Australia, New Zealand, New Guinea, New Caledonia) trace the break-up of Gondwana, though molecular dating suggests some Nothofagus lineages crossed the sea later.
  • The tulip tree (Liriodendron) has one species in eastern North America and one in southern China; like many genera split the same way, it is a relict of a warm forest that once ringed the Northern Hemisphere.
  • Camels (Asia, Africa) and llamas (Andes) descend from North American ancestors that died out at home.

• India’s best-known disjunction is the Malayan element shared by the Western Ghats and the north-east: the great hornbill, torrent fishes and many wet-forest plants occur in both but not in the dry Deccan between.
  • Sunder Lal Hora (1949) proposed the Satpura hypothesis: the Satpura–Vindhya hills once carried a wet-forest bridge for Malayan fauna.
  • Many of his torrent fishes proved to be unrelated forms with convergent adaptations; a wetter past climate that let rainforest cross central India is now favoured.

Factors Controlling Distribution

The two broad categories of factors

• The global distribution of plants and animals depends on two broad categories of controlling factors:
  • Abiotic factors: the non-living environment (rocks, soil, water, air, temperature, light, nutrients, relief).
  • Biotic factors: the living environment (food, competition, predation, disease, mutualism and human activity).
• In finer detail, plant distribution has eight controls: climatic, edaphic (soil), biotic, physiographic (relief, slope, aspect), tectonic (drift, volcanicity, earthquakes), fire, dispersal and human interference.
• Animal distribution is more complex because animals are mobile, depend on plants for food and can escape bad seasons; no animal species is universally distributed.
• Factors act at different scales: climate sets the broad belts, relief and soil the local mosaic, history the identity of a region’s biota, and biotic interactions the exact edges.

Limiting factors and tolerance

Liebig’s law of the minimum

• Justus von Liebig (1840), a German agricultural chemist, showed that growth is set not by the total of resources but by the one in shortest supply relative to need. Carl Sprengel had said the same in 1828, so it is also the Sprengel–Liebig law.
  • Barrel analogy: a barrel of unequal staves holds water only up to its shortest stave; adding a plentiful nutrient does nothing until the scarce one is supplied.
• Frederick Frost Blackman (1905) applied it to rates: photosynthesis runs only as fast as its slowest factor (light, carbon dioxide or temperature) allows.
• Limits: the law treats factors singly and only their minimum. Factors interact, and excess also limits: waterlogging, salinity, heat.

Shelford’s law of tolerance

• Victor Ernest Shelford (1911), an American ecologist, added the maximum: each species can live and reproduce only between a minimum and a maximum of every factor, with an optimum between.
  • Performance forms a bell-shaped tolerance curve: the optimum range at the centre, zones of physiological stress on either side, and zones of intolerance beyond the limits of tolerance.
  • A species’ range maps its tolerance onto geography: it lives where every factor stays within its limits.
• Breadth of tolerance is named with the prefixes eury- (wide) and steno- (narrow):

• Corollaries (Eugene Odum): species tolerant of many factors are the most widespread; the narrowest tolerance sets the range; and the reproductive stage (seed, egg, larva, seedling) has the narrowest limits.
• Tolerance matters more than seed output or dispersal power: many seeds reach new ground, but only those whose tolerance fits the new habitat establish and spread.

Abiotic factors affecting distribution of plants and animals

1. Rocks

• Rocks must be weathered into soil before plants can grow, so soil is the immediate factor; but soil characteristics depend largely on the parent rock.
• Rock type favours certain plants, especially pioneer lichens and mosses, which are highly sensitive to substrate chemistry.
  • Limestone carries lime-loving (calcicole) plants; acid granite carries lime-avoiding (calcifuge) ones such as rhododendrons.
  • Serpentine rocks, rich in magnesium and nickel, carry sparse, highly endemic floras.

2. Food

• All living species need food, so food availability is one of the most basic causes of differences in distribution: it sets the carrying capacity of a habitat.
• Stenophagous animals are restricted most: the giant panda to bamboo forests, the koala to eucalyptus woodland.

3. Air

• All organisms need air for respiration. At high altitude low air pressure leaves too little oxygen: at about 5,500 m it is roughly half that at sea level.
• Oxygen is used in respiration and growth; carbon dioxide in photosynthesis.
• Wind dries and chills, leaving stunted, flag-shaped trees near the treeline.

4. Water

• Water enters ecosystems through precipitation: rain, snow, drizzle, sleet and hail. Precipitation, with mean temperature, is the principal determinant of the world distribution of biomes.
• Some animals are aquatic; others, such as desert rats, survive without ever drinking. In deserts only water-conserving animals survive:
  • Desert rodents (pocket mice, kangaroo rats and their Old World counterparts, gerbils) live on metabolic water released when dry seeds are digested.
  • Reptiles produce highly concentrated urine and nearly dry faeces, removing waste without losing water.
• Desert plants are similar:
  • Xerophytes (acacia, camelthorn, saguaro, prickly pear, Joshua tree) have few or no leaves to cut transpiration, and fleshy stems or swollen leaves that soak up water after rare rain and shrink as it is slowly lost.
  • Phreatophytes grow very long roots that reach moisture at or near the water table.
• Xerophytes form the dry end of the classification of plants by water need, which runs through mesophytes to hydrophytes, with halophytes on saline ground.

5. Nutrients

• Nitrogen is needed for proteins, enzymes, nucleotides and vitamins; phosphorus for phospholipids, DNA and energy transfer. Both are common limiting nutrients that decide which species can establish.
• Calcium limits shelled snails, which are scarce on acid soils.

6. Soil

• Soil type is a major factor deciding which plants grow in an area, because minerals, water content, air and micro-organisms differ between soils; these soil properties vary with the mix of organic and inorganic matter.
  • Clay soil retains more water but less air; sandy soil drains fast.
  • Loam gives the best balance of air and water. India’s clay-rich black soil (regur) holds moisture through the dry season, which suits cotton.
• Soil pH controls nutrient uptake; most plants grow best at about pH 6–7.5.
  • In strongly acid soils aluminium becomes toxic and phosphorus is locked up; in alkaline soils iron and zinc become unavailable.
• Special soils carry special floras: halophytes (mangroves, Salicornia) on saline soils, heath on podzols.

7. Temperature

• Survival at temperature extremes varies widely. Temperature acts through thresholds:
  • the poleward and upper treeline lies near the line where the warmest month averages about 10 °C;
  • frost bars many tropical plants, and mangroves stop roughly where winter sea temperature falls below about 20 °C.
• Ectotherms (cold-blooded: insects, reptiles, amphibians, fish) take body heat from their surroundings, by solar radiation and conduction, so reptiles and amphibians thin out sharply poleward.
• Endotherms (warm-blooded: birds and mammals) keep a high body temperature from their own metabolism, which lets them live in polar and alpine zones.

How temperature and precipitation shape plant growth and distribution

• Plants cannot move away from heat or cold:
  • Photosynthesis slows or stops when temperature is too high or too low; each species has cardinal temperatures (minimum, optimum, maximum).
  • Leaves lose heat by transpiration, the loss of water vapour through the stomata.
  • Some plants have hairy stems and leaves against cold, and more solutes in their cells to lower the freezing point.
  • Others grow as short cushions packed close together against cold and wind, typical of alpine and tundra plants.
• Seasonal temperature sets phenology, the timing of leafing, flowering and fruiting.
  • Vernalisation: winter wheat flowers only after a cold spell.
  • Chilling requirement: in Himachal Pradesh, apple growing has shifted upslope towards Kinnaur and Lahaul-Spiti as winters warm.
• Precipitation acts through its total, season and reliability:
  • total and effectiveness (rain minus evaporation) decide forest, grassland or desert;
  • a long dry season forces leaf fall and gives deciduous forest;
  • unreliable rain favours drought-escaping annuals and deep-rooted shrubs.
• India’s monsoon gradient: evergreen forest above about 200 cm of rain, moist deciduous at 100–200 cm, dry deciduous and thorn forest at 50–100 cm, desert scrub below about 50 cm.

How animals cope with variations in climate

• Behaviour: basking and shade-seeking (lizards), burrowing and nocturnal activity in deserts, and fluffing fur or feathers to trap warm air, as the Carolina chickadee does.
• Physiology: endotherms sweat, pant or shiver; some desert frogs store water in their bladders.
• Escape in time: hibernation in winter (marmots, bears) and aestivation in heat or drought (lungfishes, desert snails).
• Escape in space: migration; Amur falcons roost in Nagaland each autumn between East Asia and southern Africa.
• Body form: Bergmann’s rule (larger bodies in the cold) and Allen’s rule (shorter limbs and ears in the cold).

8. Light

• Light is needed to make chlorophyll and for photosynthesis; the Sun is the main energy source of almost all ecosystems.
• It controls daily and seasonal rhythms: photoperiod (day length) triggers flowering in short-day plants (many rice varieties) and long-day plants (wheat), and the breeding and migration of birds.
• In the oceans photosynthesis is confined to the photic zone, the upper 200 m or less.

Physiographic, historical and fire factors

Relief and topography

• Altitude, slope angle and aspect change local temperature and moisture.
  • Temperature falls about 6.5 °C per 1,000 m, so a mountain repeats in a few kilometres of height the belts that span thousands of kilometres of latitude: altitudinal zonation.
  • Himalaya: sal forest in the Shiwaliks; chir pine at about 1,000–2,000 m; oak, rhododendron and conifers to about 3,300 m; birch and juniper near the treeline; alpine meadows (bugyals) to about 4,500–5,000 m; then the nival zone.
• Aspect: in the western Himalaya the shaded, moister north-facing slopes carry dense forest and the sunny south-facing slopes chir pine and grass.
• Rain-shadow effect: the Western Ghats carry evergreen forest on the windward side and dry deciduous to thorn forest on the leeward Deccan.

Barriers and vagility

• Barriers block or filter movement: seas for land animals, land for aquatic ones, salinity for freshwater fish, mountains such as the Himalaya, Rockies and Andes, deserts and ice sheets.
  • Barriers are not permanent: mountains wear down and oceans open and close, so ranges change over geological time.
  • They explain why biogeographic realms keep distinct faunas and floras despite occupying similar climatic belts.
• Vagility, a species’ power of movement, decides how fast it disperses, actively or passively (by wind, currents, animals or people).
• Oceanic islands never joined to a continent have unbalanced biotas: Hawaii has no native amphibians, land reptiles or land mammals except one bat.

Tectonic and historical factors

• Continental drift, plate movement, volcanicity and earthquakes reorganise landmasses over geological time and directly produce disjunct distributions.
  • The fossil fern Glossopteris occurs in South America, Africa, India, Antarctica and Australia: evidence of continental drift.
  • Land bridges let biotas mix: Bering joined Asia and North America; Panama (about 3 million years ago) set off the Great American Biotic Interchange.
• Pleistocene glaciations pushed species towards the Equator and downslope; they survived in refugia and recolonised after the ice, leaving many relict ranges.

Fire

• Lightning and human-set fires decide the balance between trees and grass. Fire-adapted (pyrophytic) plants have thick bark or underground buds: savanna grasses, eucalypts and the Himalayan chir pine. Regular fire keeps savannas open.

Biotic factors affecting distribution of plants and animals

• A long-standing idea, traced to Charles Darwin and restated by Robert MacArthur (1972): climate fixes the harsh (cold or dry) edge of a range and biotic interactions fix the mild edge.

1. Competition

• Competition is a major factor that reduces populations in their habitats. Plants and animals compete for space (for breeding, movement and feeding) and for food, water, light and mates.
• Limited resources can make populations evenly spaced, as with forest trees competing for sunlight.
• Competitive exclusion principle (Georgy Gause, 1934): two species with identical needs cannot coexist for long.
  • Joseph Connell (1961) showed that one barnacle could live lower on the shore but was crowded out there by another; its lower limit was set by competition, its upper limit by drying.

2. Predation

• Predation affects the distribution and abundance of species, the flow of energy, and the diversity and composition of communities. Predators also drive evolution in prey.
• Biological control uses it: the moth Cactoblastis cactorum, released in Australia in 1926, destroyed most of the prickly pear that had overrun millions of hectares.

3. Diseases and parasites

• Plant diseases may be fungal, bacterial, viral or of animal origin; with pests and invasive weeds they destroy crops and threaten food security.
  • Banana diseases, locusts, fruit flies, armyworm, cassava mosaic and wheat rusts cause huge losses to crops and pastures, threatening millions of vulnerable farmers.
• Plant populations shrink where a disease is present and thrive where it is absent. Animals suffer outbreaks too, increasingly linked to global warming; the chytrid fungus has driven declines in over 500 amphibian species.
• Effects cascade through the food web: more plants can raise the numbers of animals that eat them, and in turn of their predators; when a population is diseased and declines, so do the animals that eat it.

4. Amensalism

• Amensalism is an interaction in which one species is harmed and the other is unaffected (0/−). It fixes range edges because the suppressed species cannot establish where the other dominates.
• Allelopathy, the release of chemicals that inhibit other plants, is its commonest form:
  • black walnut releases juglone, which kills many plants beneath it;
  • aromatic Salvia shrubs in Californian chaparral leave a bare belt 1–2 m wide in the surrounding grassland;
  • Lantana camara and Parthenium suppress native ground plants in India.
• Antibiosis (the mould Penicillium killing bacteria) and trampling by large grazers are other forms.

5. Mutualism and commensalism

• Mutualism (+/+) ties a species to the range of its partner: figs need their own fig wasps; pines planted in the tropics failed until their mycorrhizal fungi were introduced; legumes rely on nitrogen-fixing bacteria.
• Commensalism (+/0): epiphytic orchids and ferns live on forest trees.

Human factor

• Humans are now the most powerful modifier of distribution, deliberately or by accident, an age of change that Vladimir Vernadsky called the noosphere.
• Habitat change: developing land for houses, farms and roads cuts trees and alters habitats. Urbanisation and agriculture displace plants and animals; some move to unfamiliar places and others become extinct.
  • Some animals adapt, such as the skunk and raccoon in North America and the rhesus macaque in India; others cannot, and decline.
  • Pollution harms plant and animal populations, and hunting cut whale populations until the 1986 moratorium on commercial whaling.
• Disturbed habitats (fields, roadsides, railway lines) favour camp-follower weeds such as dandelion and shepherd’s purse; cultigens (domesticated crops) depend wholly on people to spread.
  • Maize, potato and tobacco went from the Americas to Europe; rubber and cinchona from South America to Asia.
• Introductions, intentional (European rabbits to Australia, 1859; rabbits and red deer to New Zealand, whose grazing caused soil and gully erosion) or accidental (brown rats on ships), can turn invasive: prickly pear in Australia, kudzu in the USA, and Lantana*, Prosopis juliflora, Parthenium and *Ipomoea carnea in India.
  • The IPBES assessment (2023) counts over 37,000 alien species moved by people. Invasives were a factor in about 60% of recorded extinctions and cost at least US$423 billion a year (2019).
• Climate change is now shifting ranges poleward by a median of about 17 km and upslope by about 11 m per decade, while habitat fragmentation cuts many ranges into isolated pieces.

Indian Context

Factors at work in India

• Monsoon climate control: India’s vegetation zones, from the wet evergreen forests of the Western Ghats and the north-east to the xerophytic scrub of the Thar, demonstrate the water factor directly.
• Phreatophyte example: Prosopis cineraria (khejri), the state tree of Rajasthan, sends roots to great depths to reach groundwater in the Thar.
• Endemism: the Western Ghats–Sri Lanka and Himalaya biodiversity hotspots, two of the four in India, hold endemics such as the Nilgiri tahr and lion-tailed macaque, narrow ranges like that of the coast redwood.
• Disease and pests: the 2019–20 desert locust upsurge across Rajasthan and Gujarat and recurrent wheat rust in the north-west show the pest factor and its threat to food security.
• Invasive species: Lantana camara has invaded about 154,000 km², over 40% of the forests in India’s tiger range (2020 study); Prosopis juliflora has overrun the Banni grasslands of Kachchh.
• Human impact and conservation response: Project Tiger (1973) and the Wildlife (Protection) Act, 1972 answer human-driven habitat loss and hunting. The Asiatic lion’s shrinkage to Gir shows how far human pressure can contract a range.

Wild plants of India: controls and economic significance

• India holds about 7–8% of the world’s recorded species on 2.4% of its land. Its flora mixes Indo-Malayan, Himalayan (Palaearctic) and African elements, because three realms meet here.
• Controls: rainfall amount and season (over 1,000 cm at Mawsynram, under 15 cm in western Rajasthan); temperature and altitude; soils (laterite, black soil, alluvium, saline coastal mud); history (Gondwanan and Malayan elements); and grazing, shifting cultivation, fire and invasive weeds.
• Economic significance:
  • Timber: teak, sal, deodar, rosewood.
  • Non-timber forest produce: tendu leaves for bidis, mahua, sal seeds, bamboo, lac and gum karaya, which support millions of forest dwellers.
  • Medicinal plants: Rauvolfia serpentina, Himalayan yew (Taxus, source of taxol), Picrorhiza.
  • Wild relatives of crops: wild rice and wild citrus, protected in the Citrus Gene Sanctuary of the Garo Hills, a store of genes for breeding at India’s gene pool centres.
  • Fodder, fuel and ecosystem services such as soil and water conservation.

International Framework and Comparative Notes