GS Notes

5. New Environment Notes 05

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Elaborate Notes

Rainforest Biome

Rainforests are characterized by high rainfall and are among the most biodiverse biomes on Earth. They are broadly classified into temperate and tropical rainforests based on their latitudinal location and climatic characteristics.

Temperate Rainforest

  • Climatic and Geographical Context: Temperate rainforests are found in mid-latitudes, typically along the western margins of continents where they are influenced by oceanic climates. These regions are subject to the year-round influence of on-shore Westerlies, which bring abundant moisture from the oceans. This results in high precipitation, often exceeding 140 cm annually, distributed throughout the year. The climate is mild, with cool summers and winters that are not excessively cold. This climatic type is often categorized under the British Type or Cool Temperate Western Margin Climate (Cfb in the Köppen classification).
  • Distribution: Prominent examples of temperate rainforests are found in the Pacific Northwest of North America (from Southern Alaska to Northern California in the USA and Western Canada), parts of Western Europe (e.g., Norway, Ireland, Scotland), Southern Chile, Southeastern Australia (Tasmania and Victoria), and New Zealand.
  • Vegetation Characteristics: The vegetation is luxuriant, dense, and marked by tall coniferous trees such as Douglas Fir, Western Red Cedar, and Sitka Spruce in North America, and various species of Eucalyptus in Australia. Deciduous trees can also be present. The understory is rich with mosses, lichens, and ferns, which thrive in the damp, shaded conditions. The constant moisture and mild temperatures mean the growing season is long.
  • Biodiversity: While rich in life, the biodiversity of temperate rainforests is considerably lower than that of their tropical counterparts. The cooler temperatures limit the diversity of reptiles and amphibians. The faunal assemblage typically includes large mammals like bears and deer, as well as a variety of birds and smaller mammals. Ecologist Alwyn Gentry’s work (1988) highlighted the latitudinal diversity gradient, explaining why species richness generally decreases as one moves away from the equator.

Tropical Rainforest

  • Climatic and Geographical Context: Located in equatorial regions, typically between 10°N and 10°S latitude, tropical rainforests experience an equable climate characterized by high temperatures (averaging 25-27°C) and high humidity throughout the year. There is minimal seasonal variation in temperature. Annual precipitation is exceptionally high, typically exceeding 200 cm, and is primarily of the convectional type. This daily cycle of heating, evaporation, and afternoon thunderstorms is a hallmark of the equatorial climate (Af in the Köppen classification).

  • Distribution: The three major blocks of tropical rainforest are the Amazon Basin in South America, the Congo Basin in Central Africa, and the Indo-Malayan region covering parts of Indonesia, Malaysia, and Papua New Guinea. Smaller pockets exist in regions like Venezuela and Central America.

  • Soil Characteristics:

    • Leaching and Laterization: The incessant heavy rainfall leads to intense weathering and leaching, a process where essential mineral nutrients (like calcium, magnesium, and potassium) are washed down deep into the soil profile, beyond the reach of plant roots. This process results in the formation of laterite soil.
    • Nutrient Profile: The soil is inherently infertile and acidic due to the removal of alkaline salts. The topsoil is rich in insoluble iron and aluminum oxides, which give it a reddish colour and can form a hard crust (lateritic duricrust) when exposed to the sun.
    • Humus Content: Despite the massive amount of biomass, the humus layer is thin. The high temperature and humidity accelerate the decomposition of organic matter by bacteria and fungi. Nutrients released are rapidly absorbed by the dense vegetation, creating a highly efficient but fragile nutrient cycle concentrated in the biomass rather than the soil. This was extensively studied by scholars like Carl F. Jordan in “Nutrient Cycling in Tropical Forest Ecosystems” (1985).
    • Soil Erosion: Once the protective forest cover is removed, the soil is highly susceptible to severe erosion by heavy rainfall.
    • Note on Indonesian Soil: The soil in parts of Indonesia, particularly on islands like Java and Sumatra, is comparatively more fertile. This is due to volcanic activity, which regularly deposits nutrient-rich ash, replenishing the soil and counteracting the effects of leaching.

  • Flora (Vegetation):

    • Structure: Tropical rainforest vegetation is evergreen, with trees shedding leaves gradually throughout the year rather than seasonally. The forest is characterized by a distinct vertical stratification or layering, an evolutionary adaptation to compete for sunlight. Typically, five layers can be identified: emergent layer (tallest trees), canopy, understory, shrub layer, and forest floor.
    • Sunlight as a Limiting Factor: The dense canopy blocks most of the sunlight; only about 2% reaches the forest floor. This limits the growth of undergrowth, making the forest floor relatively clear in mature rainforests.
    • Adaptations: Plants have developed specific adaptations. ‘Drip tips’ on leaves facilitate the shedding of excess water, preventing the growth of fungi. Lianas (woody creepers) and epiphytes (plants like orchids and ferns that grow on other trees) are common, adapting to reach sunlight without needing tall trunks.
    • Tree Characteristics: The trees are predominantly hardwood species like Mahogany, Ebony, Rosewood, and Rubber. This density makes them commercially valuable but also difficult to log and transport.
    • Biodiversity: Tropical rainforests are the planet’s most biodiverse terrestrial ecosystems. While covering only about 6% of the Earth’s land surface, they are estimated to contain over 50% of all terrestrial plant and animal species. Carnivorous plants like the Venus flytrap and pitcher plants have adapted to nutrient-poor soils by trapping insects.

  • Fauna (Animals):

    • Diversity: The ecosystem supports an immense diversity of fauna, including insects, reptiles, amphibians, birds, and mammals.
    • Apes: All great apes are native to rainforests: Gorillas and Chimpanzees in Africa, and Orangutans in Southeast Asia (Borneo and Sumatra). Gibbons, the lesser apes, are also found here, including the Hoolock gibbon, which is the only ape species found in India (Northeastern states).
    • Adaptations: Animals exhibit remarkable adaptations for arboreal (tree-dwelling) life and intense competition. Monkeys have prehensile tails and strong grips. Gliders like flying squirrels and flying lizards have evolved membranes to move between trees. Camouflage is highly developed, as seen in insects like leaf mimics and predators like the green mamba. Birds like toucans and hornbills have large, strong beaks to crack nuts and fruits. Many species are brightly coloured, which can be for mating displays (e.g., birds of paradise) or to warn predators of toxicity (e.g., poison dart frogs).

  • Threats: The primary contemporary threat to tropical rainforests is deforestation, driven largely by commercial agriculture, particularly the expansion of palm oil plantations in Southeast Asia and cattle ranching and soybean cultivation in the Amazon.

Aquatic Ecosystems

An aquatic ecosystem includes any water-based environment, from oceans to ponds. They are classified based on several criteria, including salinity, water flow, and light penetration.

  • Classification by Salinity: Salinity is measured in parts per thousand (ppt).

    • Saline Ecosystems: Salinity is greater than 35 ppt. This includes oceans and seas.
    • Brackish Ecosystems: Salinity is intermediate, between 5 and 35 ppt. These are found where freshwater mixes with saltwater, such as in estuaries, salt marshes, and mangrove swamps.
    • Freshwater Ecosystems: Salinity is less than 5 ppt. This includes rivers, lakes, ponds, and wetlands.

  • Classification of Freshwater Ecosystems:

    • Lotic Ecosystems: Characterized by flowing water, such as in rivers and streams. Organisms here are adapted to withstand currents.
    • Lentic Ecosystems: Characterized by still or standing water, such as in lakes, ponds, and swamps. Lentic systems, due to their contained nature, can experience more drastic fluctuations in temperature and oxygen levels but can often buffer environmental changes better than a constantly flowing system.

  • Classification by Light Penetration (Vertical Zonation):

    • Photic (or Euphotic) Zone: The upper layer of a water body where sufficient sunlight penetrates to allow for photosynthesis. This zone is the primary site of food production.
    • Aphotic Zone: The lower layer where light penetration is too low for photosynthesis. Life in this zone is dependent on organic matter sinking from the photic zone or chemosynthesis. Oxygen levels are lower.
    • Thermal Stratification: In deeper lentic bodies, thermal layers can form:
      • Thermocline: A transitional layer where temperature changes rapidly with depth. Organisms here may be almost transparent to evade predation in the clear water.
      • Hypolimnion: The deep, cold, and relatively undisturbed layer below the thermocline. In the aphotic part of this zone, some predators have evolved bioluminescence (producing their own light) to attract prey.

  • Horizontal and Depth-based Zonation:

    • Littoral Zone: The shallow, near-shore area where light reaches the bottom. It is rich in sunlight, oxygen, and nutrients from land runoff, making it the most productive and life-rich zone in many lakes and ponds.
    • Pelagic Zone: The open water area away from the shore. It is further divided vertically into photic and aphotic zones.
    • Benthic Zone: The bottom substrate of the aquatic ecosystem, regardless of depth. Organisms here (benthos) live in or on the sediment. Sunlight and oxygen are often limited, especially in deeper parts.

  • Organisms in Aquatic Ecosystems:

    • Plankton: Microscopic organisms that float or drift in the water column.
      • Phytoplankton: Autotrophic, plant-like plankton (e.g., diatoms, cyanobacteria) that form the base of most aquatic food webs through photosynthesis.
      • Zooplankton: Heterotrophic, animal-like plankton that feed on phytoplankton or other zooplankton.
    • Nektons: Actively swimming organisms that can move against water currents, such as fish, turtles, and whales.
    • Benthos: Organisms living at the bottom (benthic zone), such as crabs, worms, and clams.
    • Neuston: Organisms that live on the water’s surface (the air-water interface), such as water striders and some beetles.
    • Periphytons: Organisms (like algae and bacteria) that cling to submerged surfaces like rocks, stems, and leaves, typically in the photic zone.
    • Hydrophytes: Aquatic plants adapted to live in water or water-logged soil. Examples include rooted plants like lotus and floating plants like water hyacinth. Mangroves are specialized hydrophytes adapted to saline, brackish conditions.

Ecotones

  • Definition and Distinction: In ecology, an ecotone is a transition area between two adjacent but different ecological communities (ecosystems), such as the zone between a forest and a grassland. It is distinct from an ecocline, which is a gradual, continuous change in environmental conditions and species composition across a landscape without a sharp boundary. An ecotone is a physical place, while an ecocline is the gradient of change.
  • Examples: Wetlands (transition between terrestrial and aquatic ecosystems) and grasslands (often a transition between forests and deserts) are classic examples of large-scale ecotones. Estuaries, where rivers meet the sea, are another prime example.
  • Edge Effect: This is a key characteristic of ecotones. The term, popularized by ecologist Eugene P. Odum in his foundational text “Fundamentals of Ecology” (1953), refers to the tendency for an increased variety and density of species in the ecotonal zone compared to the adjoining ecosystems. This happens because the ecotone contains resources and niches from both adjacent communities, and also provides unique conditions of its own.
  • Edge Species: Some species are specifically adapted to and may be restricted to the conditions of the ecotone. These are known as edge species. For instance, certain water birds and amphibians are specifically adapted to the shallow water and vegetation structure found in wetlands, thriving in a zone that is neither fully terrestrial nor fully aquatic.

Wetlands

  • Definition: A wetland is a distinct ecosystem that is inundated or saturated by surface or groundwater at a frequency and duration sufficient to support vegetation typically adapted for life in saturated soil conditions. As per the Ramsar Convention on Wetlands (1971), wetlands are defined as “areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres.” They represent a classic example of an ecotone.
  • Characteristics of Wetlands:
    • (a) High Primary Productivity: Wetlands are among the most productive ecosystems in the world. The high productivity is due to the abundance of key resources: water, sunlight (in shallow areas), and a high concentration of nutrients that accumulate from surrounding land through runoff.
    • (b) Dominant Vegetation (Hydrophytes): The plant life in wetlands consists of hydrophytes, which are specifically adapted to survive in water-logged, anaerobic (low oxygen) soil conditions. Examples include mangroves, cattails, and water lilies.
    • (c) Hydric Soil: Wetland soils are classified as hydric soils. These soils are saturated with water for long enough periods during the growing season to develop anaerobic conditions in the upper part, which influences the types of plants that can grow and the chemical processes within the soil.

Prelims Pointers

  • Temperate Rainforests: Found in cool temperate western margins (Western Europe, Western Canada). Receive rainfall from on-shore westerlies.
  • Tropical Rainforests: Located in equatorial regions. Climate is hot and humid with high convectional rainfall (>200 cm).
  • Soil Type: The typical soil in tropical rainforests is Laterite, which is acidic, poor in nutrients, and rich in iron and aluminum oxides.
  • Nutrient Cycling: In tropical rainforests, most nutrients are stored in the biomass, not the soil.
  • Volcanic Soil: The soil in parts of Indonesia (e.g., Java) is fertile due to enrichment from volcanic ash.
  • Sunlight Limitation: Only about 2% of sunlight reaches the floor of a tropical rainforest.
  • Vegetation Layers: Tropical rainforests have a distinct vertical stratification (emergent, canopy, understory, etc.).
  • Plant Adaptations: Examples include ‘drip tips’ on leaves, lianas (creepers), and epiphytes.
  • Hardwood Trees: Tropical rainforest trees like Rosewood, Ebony, and Mahogany are hardwoods.
  • Ape Distribution: Gorillas/Chimpanzees (Africa), Orangutans (South-East Asia).
  • Hoolock Gibbon: The only ape species found in India (Northeast India).
  • Threat to Rainforests: Major threat is deforestation for palm oil cultivation and commercial plantations.
  • Aquatic Ecosystem Salinity:
    1. Freshwater: < 5 ppt
    2. Brackish: 5 - 35 ppt
    3. Saline: > 35 ppt
  • Lotic vs. Lentic: Lotic refers to flowing water (rivers); Lentic refers to still water (lakes).
  • Aquatic Zones:
    • Photic Zone: Upper layer with sunlight for photosynthesis.
    • Aphotic Zone: Lower layer with little or no sunlight.
    • Littoral Zone: Shallow, near-shore area.
    • Pelagic Zone: Open water area.
    • Benthic Zone: The bottom substrate.
  • Aquatic Organisms:
    • Plankton: Free-floating (Phytoplankton - producers; Zooplankton - consumers).
    • Nekton: Active swimmers (e.g., fish).
    • Benthos: Bottom-dwellers (e.g., crabs).
    • Neuston: Surface-dwellers.
    • Periphyton: Organisms attached to submerged surfaces.
    • Hydrophytes: Aquatic plants (e.g., Lotus, Mangroves).
  • Ecocline: A gradual change between two ecosystems.
  • Ecotone: A transition zone between two ecosystems (e.g., wetlands, grasslands).
  • Edge Effect: Increased species diversity and density in an ecotone.
  • Wetland Definition: Transitional land between terrestrial and aquatic systems where water depth generally does not exceed 6 meters.
  • Wetland Soil: Characterized by hydric soil, which is saturated with water and low in oxygen.

Mains Insights

GS Paper I (Geography) & GS Paper III (Environment)

  1. Human-Environment Interaction in Rainforests:

    • Cause-Effect: The demand for commodities like palm oil, soy, and beef in developed and emerging economies directly drives deforestation in tropical rainforest regions. This highlights a cause-effect relationship between global consumption patterns and localized environmental degradation. For instance, the expansion of palm oil plantations in Indonesia and Malaysia has led to massive habitat loss for species like the orangutan.
    • Debate: A significant debate exists between conservation and development. Developing nations with rainforests often argue for their sovereign right to utilize their natural resources for economic growth, while developed nations and environmental groups advocate for conservation. This raises questions of environmental justice and the historical responsibility of industrialized nations for climate change.
    • Geopolitical Angle: The control and management of resources in the Amazon and Congo basins have geopolitical implications, involving indigenous rights, national sovereignty, and international climate agreements like the Paris Accord.

  2. The Fragility of Tropical Ecosystems:

    • Analytical Point: The apparent lushness of tropical rainforests masks an underlying fragility. The ecosystem’s health is dependent on a rapid and efficient nutrient cycle within the biomass. Once the forest is cleared, this cycle is broken. The nutrient-poor laterite soil cannot support sustained agriculture without massive artificial inputs, and is highly prone to erosion, leading to land degradation.
    • Historiographical View: Early colonial perspectives often viewed tropical lands as endlessly fertile and ripe for exploitation. Modern ecological science, pioneered by figures like Eugene Odum, has completely overturned this view, emphasizing the interconnectedness and fragility of these ecosystems.

  3. Ecotones and Wetlands as Critical Ecological Hubs:

    • Ecological Significance: Ecotones, particularly wetlands, are not merely transitional zones but are critical ecosystems in their own right. The “edge effect” makes them hotspots of biodiversity. Wetlands provide a disproportionately high number of ecosystem services.
    • Ecosystem Services of Wetlands:
      • Provisioning: Provide food (fish, rice), freshwater, and raw materials.
      • Regulating: Act as “nature’s kidneys” by filtering pollutants, and as natural sponges by absorbing floodwaters and recharging groundwater. They are also significant carbon sinks.
      • Cultural: Hold aesthetic, recreational, and spiritual value for many communities.
    • Policy Implications: The recognition of these services is central to conservation efforts like the Ramsar Convention. In India, the Wetlands (Conservation and Management) Rules, 2017, aim to conserve and manage wetlands, but their implementation faces challenges related to urban encroachment, pollution, and jurisdictional conflicts between different government bodies.

  4. Biodiversity and Climate Regulation:

    • Interlinkage: The high biodiversity of rainforests is directly linked to their role in climate regulation. The vast number of trees contributes to regional and global climate patterns through transpiration (influencing rainfall) and by acting as a massive carbon sink. Deforestation not only reduces biodiversity but also releases vast amounts of stored carbon, accelerating climate change.
    • Ethical Dimension (GS Paper IV): The irreversible loss of species due to habitat destruction raises profound ethical questions about humanity’s stewardship of the planet. It represents an intergenerational equity issue, as future generations will be deprived of the biological and potential scientific resources (e.g., new medicines) that these ecosystems hold.

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