Factors Influencing World Distribution of Plants and Animals

Species diversity among plants and animals — both terrestrial and aquatic — is found continuously from the Equator to the poles, and from foothills to mountain tops. Explaining this patterned diversity requires understanding two things: (1) the different distributional patterns that species can show (cosmopolitan, endemic, continuous, discontinuous), and (2) the full range of abiotic (non-living) and biotic (living) factors — increasingly supplemented by anthropogenic pressure — that determine those patterns.

Patterns of Distribution — Cosmopolitan, Endemic, Continuous, Discontinuous

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Cosmopolitan distribution — the same or similar species is found widely distributed all over the world.

Endemic distribution — the same or similar species occurs only in one specific geographical location, and nowhere else.

Definitions

Humans, animals, and plants are all globally distributed to varying degrees. Some species show a very narrow, continent-restricted range, while others are essentially planetary in their spread:

Discontinuous (Disjoint) Distribution

In some cases, the pattern of distribution of plants and animals is discontinuous or disjoint — meaning the species is found in two widely separated areas (say, Central America and Indonesia) and not in any of the territory lying in between. It is always the biogeographer’s task to explain how such distributions arose, given that the intervening distance often appears far too great for the species to have migrated directly from one area to the other.

Classic example — the Tapir: This animal shows a disjoint distribution, being found in South America and in Malaysia, with no intervening populations — a puzzle that has motivated several explanatory theories.

Questions about how such disjoint distributions came about have given rise to a number of theories:

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 components of the environment (rocks, soil, water, air, temperature, light, nutrients)
• Biotic factors — the living components of the environment (competition, predation, disease, and human activity)

Abiotic Factors Affecting Distribution of Plants and Animals

1. Rocks

The rocks of the lithosphere must first be weathered and converted into soil before a plant can grow. The immediate environmental factor affecting the plant is therefore the soil, and soil characteristics are largely dependent on the parent rock. Rocks favour certain plants over others — differences in rock type can adapt a locality to different species of lichens and mosses in particular, since these pioneer organisms are highly sensitive to substrate chemistry.

2. Food

All living species must have food to survive — without it, they die. This is one of the most basic foundations for the differences and limitations in plant and animal distribution across different locations of the world, since food availability directly sets the carrying capacity of a habitat.

3. Air

All living things need air to breathe and for survival, since air is required for respiration to take place in organisms. Living organisms thrive only where air is available in abundance; when air pressure is low, especially at higher altitudes, some organisms find it difficult to breathe because of the insufficient amount of oxygen present at such heights.

Oxygen and carbon dioxide are both critically important: oxygen is essential for respiration and is utilised during various growth and development processes, while carbon dioxide is needed for photosynthesis to take place.

4. Water

Water enters the ecosystem through precipitation — snow, drizzle, sleet, rain, and hail. Precipitation, along with mean temperature, is the principal determinant of the worldwide distribution of biomes.

Some animals are aquatic and must live in water; others — including desert rats — survive in arid areas where they are unlikely ever to drink water directly. Water is essential for vital functions, yet only animals capable of conserving it can survive in deserts:

• Desert mammals such as pocket mice and kangaroo rats (and their Old World counterparts, gerbils) obtain most of the moisture they need from the seeds and grains they eat.
• Reptiles have adaptations such as producing highly concentrated urine and nearly dry faeces, allowing elimination of body waste without losing precious moisture.

A similar case applies to desert plants:

• Xerophytes — such as acacia, the camelthorn tree, saguaro, prickly pear, and Joshua trees — have unique adaptations for storing and conserving water. They often have few or no leaves (reducing transpiration), and possess fleshy stems and swollen leaves that absorb large amounts of water during infrequent rain, swelling up before contracting again as moisture is slowly lost through transpiration.
• Phreatophytes — plants that grow extremely long roots, allowing them to acquire moisture at or near the water table.

5. Nutrients

Nitrogen is needed to make proteins, enzymes, nucleotides, and vitamins. Phosphorus is used in the formation of phospholipids and other cellular structures — both are essential limiting nutrients that shape which species can establish and thrive in a given habitat.

6. Soil

For plants, soil type is a major factor deciding the type and variety of species growing in a particular area, since minerals, water content, and microorganism presence all differ across soil types. Soil is a combination of various organic and inorganic matter, and with varying content, the water retention capacity, fertility, and mineral presence of the soil all change:

• Clay soil can retain more water but less air.
• Black soil is ideal for plant growth, offering a balance of air and water retention capacities.
• The soil’s pH helps determine the absorption of nutrients by plants — if soil is too acidic, desertification can take place and ruin the chances of plant habitat establishment.

7. Temperature

The ability to survive at temperature extremes varies widely among plants and animals. Animals respond to temperature variation both physiologically and behaviourally:

• Endotherms (hot-blooded animals — birds and mammals) maintain relatively high body temperatures using heat generated by their own metabolism.
• Ectotherms (cold-blooded animals — insects, reptiles, amphibians, fish) regulate body temperature using ambient/surrounding temperature, drawing on sources of heat such as direct and indirect solar radiation and conduction. Behavioural adjustments include changing the position of fur or feathers (e.g., in the Carolina Chickadee), sweating, shivering, panting, burrowing, hibernating, and seeking shade in trees or water. Some desert animals can even store water in their bodies as a buffer against heat stress.

Plants, unlike animals, cannot move away to escape high or low temperatures in their environment. Consequently:

• Photosynthesis slows down or stops when temperatures get too high or too low.
• Leaves lose heat through evapotranspiration (the loss of water through small holes/stomata in leaves).
• Some plants have hairy stems and leaves to help withstand low temperatures, and may carry more solutes in their cytoplasm to reduce the freezing point.
• Others exhibit short growth forms and grow very close together to collectively resist cold temperatures and wind — a strategy typical of alpine and tundra cushion plants.

8. Light

Light is an important climatic factor used for the production of chlorophyll and for photosynthesis; it exerts a strong influence on the daily and seasonal activity patterns of both plants and animals. Light is needed for photosynthesis to take place and is the main source of energy in almost all ecosystems — energy enters the ecosystem through this single ultimate source: the Sun.

Extended Abiotic Dimension — Relief, Tectonics and Historical Factors

• Relief and topography: Altitude, slope angle and slope aspect independently modify local temperature and moisture, producing altitudinal zonation — a compressed version of the equator-to-pole sequence within a single mountain slope (e.g., the Himalayan transition from tropical to nival belts).
• Rain-shadow effect: Windward and leeward slopes of mountain ranges show sharply contrasting vegetation due to differential orographic precipitation.
• Tectonic factors: Continental displacement and drift, plate movement, volcanicity and seismic events reorganise landmasses over geological time, directly producing the disjoint distributions (e.g., Nothofagus in South America, New Zealand and Australia — all once joined within Gondwanaland).
• Barriers to dispersal: Oceans, deserts, and mountain ranges act as partial or complete filters to species movement, explaining why biogeographic realms retain distinct faunal/floral identities despite occupying similar climatic belts.

Biotic Factors Affecting Distribution of Plants and Animals

1. Competition

Competitive interactions are one of the major factors that diminish populations of plants and animals in their main habitats. Plants and animals compete for space — needed for reproduction, exercise, and feeding — as well as for several resources such as food, water, and mates. All of this can affect how a species is distributed.

Example: Due to limited resources, populations may become evenly distributed to minimise competition — as seen in forest habitats, where competition for sunlight produces an even spatial distribution of trees.

2. Predation

Predation affects the global distribution and abundance of plant and animal species, the strength and direction of energy flow within a system, and the diversity and composition of communities. Predators also play an essential role in evolution, driving adaptive traits in prey populations through selective pressure.

3. Diseases

Plant diseases may be fungal, bacterial, viral, or of animal origin; they include insects/pests, plant diseases proper, and invasive weeds. These diseases affect food crops, causing significant losses to farmers and threatening food security.

Examples: Banana diseases, locusts, fruit flies, armyworm, cassava mosaic, and wheat rusts are highly destructive to plant life — their outbreaks and upsurges can cause huge losses to crops and pastures, threatening the livelihoods of vulnerable farmers and the food and nutrition security of millions at a time.

The plant population will obviously reduce in a disease-affected environment, and will thrive better in areas where such diseases are absent. Animals are equally affected by disease outbreaks, which are increasingly linked to global warming — severely affecting ecosystem balance and producing visible changes in the global distribution and behaviour of both plants and animals.

These effects cascade through the food web:

• If there are more plants than usual in an area, the populations of animals that eat that plant may increase.
• If one animal’s population increases, the population of animals that prey on it may also increase.
• Other changes in the community will cause a population to decrease.
• If a population becomes diseased, that population may decrease — and the population of animals that eat the diseased animals will also decrease correspondingly.

4. Humans

Humans influence animal and plant populations in various ways, causing them to migrate away from their natural habitat into a new environment. When humans develop land for houses and buildings, they cut down trees and change animal and plant habitats:

• Some animals, like the skunk and raccoon, can adapt to these altered conditions; other animals cannot adapt, and their populations are adversely affected.
• Pollution can also hurt animal and plant populations.
• Hunting can affect animal populations — for instance, whale populations have been lowered because of overhunting.

Man contributes to the global redistribution of plants and animals through urbanisation and agricultural activity; these developments have displaced both animals and plants from their natural habitats. Some plant and animal species are forced to move to new and unfamiliar environments since they cannot cope with the harsh new conditions, while others have gone into extinction altogether.

Indian Context

• Monsoon climate control: India’s own vegetation zonation — from the wet evergreen forests of the Western Ghats/North-East to the Thar Desert’s xerophytic scrub — is a direct demonstration of the water/precipitation factor discussed above.
• Phreatophyte example: Prosopis cineraria (Khejri), the state tree of Rajasthan, sends roots to great depths to reach groundwater in the Thar Desert — an Indian parallel to the Phreatophyte adaptation.
• Endemism: The Western Ghats and Eastern Himalayas — two of India’s four biodiversity hotspots — host high concentrations of endemic species (e.g., Nilgiri tahr, lion-tailed macaque), illustrating the “narrow endemic range” concept discussed for the California Redwood.
• Disease/pest impact: The 2019–2020 desert locust swarms across Rajasthan and Gujarat, and periodic wheat rust outbreaks in the Indo-Gangetic Plain, are contemporary Indian illustrations of the “diseases” biotic factor and its threat to food security.
• Human impact and conservation response: India’s Project Tiger (1973) and Wildlife Protection Act, 1972 represent direct policy responses to human-driven habitat displacement and hunting pressure.

International Framework and Comparative Notes