Unit 25: Ecology — Long Questions

A biogeochemical cycle is the cyclic movement of chemical elements such as carbon, nitrogen, oxygen, and phosphorus between the living (biotic) and non-living (abiotic) components of an ecosystem. Based on their primary reservoir, these cycles are classified as gaseous cycles (reservoir is air or oceans, e.g., water, nitrogen, oxygen and carbon cycles) or sedimentary cycles (reservoir is Earth's crust, e.g., phosphorus, sulfur, calcium cycles).

The water (hydrological) cycle is the continuous movement of water between abiotic components (air, soil, sea) and biotic components, and has two loops:

Abiotic Loop 1. Evaporation: Solar radiation heats surface water of oceans and lakes, turning it into vapor; about 86% of global evaporation occurs from the oceans (in Pakistan, chiefly the Arabian Sea).
2. Condensation: As vapor rises, it cools and condenses into liquid droplets or ice crystals that cling to dust particles, forming clouds.
3. Precipitation: When droplets become too heavy, they fall as rain, snow, or sleet.
4. Infiltration and Runoff: Water either infiltrates (percolates) into the soil to recharge groundwater, or flows as surface runoff into streams, rivers (e.g., the Indus), and eventually the ocean.

Biotic Loop 1. Water Entry: Plants absorb water through root hairs for photosynthesis and turgor pressure; animals obtain water by drinking and through food.
2. Water Exit: Water returns to the environment through transpiration (via stomata in plants), respiration (metabolic water vapor), excretion and egestion (urine, feces), perspiration (sweat), and decomposition of dead organisms.

The nitrogen cycle circulates nitrogen between the biotic and abiotic components of the ecosystem, converting atmospheric N₂ (78% of the atmosphere) into usable compounds and back again, through five steps:

1. Nitrogen Fixation: Converts N₂ into ammonia (NH₃). It occurs in three ways — Biological fixation (over 90% of all fixation) by free-living bacteria (cyanobacteria, Azotobacter, Clostridium) and symbiotic bacteria (Rhizobium, in legume root nodules); Atmospheric fixation (5-10%), where lightning combines N₂ with oxygen to form nitrogen oxides and then nitrates; and Industrial fixation, where N₂ and hydrogen are combined synthetically to manufacture fertilizers.

2. Nitrification: Soil bacteria Nitrosomonas convert ammonia to nitrite (NO₂⁻), and Nitrobacter convert nitrite to nitrate (NO₃⁻), a form easily absorbed by plants.

3. Assimilation: Plants and microbes absorb nitrates from soil to synthesize proteins, DNA, and chlorophyll; animals assimilate nitrogen by consuming plants and other animals.

4. Ammonification: When organisms die or release nitrogenous waste, decomposers (fungi and bacteria) break down the complex proteins and convert the organic nitrogen back into ammonia.

5. Denitrification: In waterlogged soils, denitrifying bacteria (Pseudomonas, Thiobacillus) convert nitrates back into nitrogen gas (N₂), which is released into the atmosphere, completing the cycle.

In an ecosystem, organisms are arranged into feeding groups called trophic levels. Trophic level 1 consists of producers (green plants, algae, photosynthetic bacteria). Trophic level 2 consists of primary consumers/herbivores (e.g., insects, rabbits) that feed on producers. Trophic level 3 consists of secondary consumers/carnivores and parasites (e.g., frog, snake) that feed on herbivores. Trophic level 4 consists of tertiary consumers (e.g., wolf, hawk) that eat carnivores. Decomposers and detritivores (e.g., crabs, vultures, jackals) form a separate level, feeding on detritus and dead matter from all levels.

Energy flows unidirectionally through these levels and is progressively lost. Only about 1-2% of solar energy falling on producers is converted into the bond-energy of organic matter, and at each subsequent transfer only about 10% of the energy is passed on to the next trophic level, the remainder being lost mainly as heat during metabolism. For example, if producers fix 10,000 kcal, primary consumers incorporate about 1,000 kcal, secondary consumers about 100 kcal, and tertiary consumers only about 10 kcal of the original energy. Because of this progressive loss, short food chains (fewer trophic levels) are more energy-efficient than long food chains.

Since the early 1700s, advances in technology have given humans greater control over food supply, better weapons against predators, and remedies against diseases, reducing vulnerability to climatic uncertainties and expanding the carrying capacity of human habitats. This has driven rapid population growth: the global population reached 8 billion in 2022 (after 12 years to grow from 7 to 8 billion) and is projected to reach 9 billion by 2037 and nearly 10 billion by 2050. Pakistan's population was recorded at 241.49 million in the 2023 census, growing at an official rate of 2.55% annually, making it the world's 5th most populous country.

This rapid growth is perhaps the greatest challenge to the future of the biosphere, placing severe strain on the environment through: habitat destruction and changes in land use; depletion of natural resources such as freshwater and fossil fuels; poverty and inflation; increased waste generation and elevated pollution; climate change; deforestation and loss of biodiversity; and increased chances of new epidemics and pandemics, starvation and malnutrition. Notably, this burden is unevenly distributed — the wealthiest 20% of the world's population consumes 86% of global resources and produces 53% of global CO₂ emissions, while the poorest 20% consumes only 1.3% of resources and emits just 3% of CO₂.

Causes The main cause of rising greenhouse gas (GHG) concentrations is human activity — burning fossil fuels (coal, oil, gas) for machinery, vehicles, heating and electricity generation, along with deforestation, releases large amounts of Carbon Dioxide (CO₂), the most significant GHG, contributing about 80% of global warming (CO₂ has risen from about 280 ppm before industrialization to 428 ppm in 2026). Methane (CH₄) is produced by coal/gas production, livestock (enteric fermentation), and agriculture (about 11% of global warming). Nitrous Oxide (N₂O) comes from agricultural, industrial activities and fossil fuel combustion (about 6%). Fluorinated gases (CFCs) are released by industrial processes like old refrigerators and air conditioners (about 3%).

Impacts The enhanced greenhouse effect causes Global Warming — global surface temperature has risen by about 1.1°C since the pre-industrial era (IPCC, 2023). This drives intense Heatwaves (e.g., Jacobabad and Sibi crossing 50°C, causing heatstroke and livestock heat stress); more erratic and intense Floods from turbo-charged monsoons (e.g., the June 2025 floods in Pakistan, which killed over 1,000 people and displaced over 3 million); Glacial Melting leading to Glacial Lake Outburst Floods (GLOFs) in Gilgit-Baltistan and Chitral; Sea Level Rise and Coastal Erosion, threatening areas like Thatta and Badin with seawater intrusion; Food Insecurity through reduced crop yields and desertification; and Ecosystem Disruption, forcing species to migrate and increasing extinction risk.

The Tropical Thorn Forest is the most widespread terrestrial ecosystem of the plains in Pakistan, characterized by xerophytic (drought-resistant) plants bearing thorns that deter herbivores and reduce water loss.

Location The Indus Plains, covering much of Punjab (Bahawalpur, Khanewal, Mianwali, Dera Ghazi Khan), Sindh (southern regions bordering the Indus River), and Balochistan (western regions).

Abiotic Profile The climate is arid to semi-arid, with summer temperatures reaching up to 50°C, and scanty annual rainfall of 200 mm to 750 mm.

Key Flora Prosopis cineraria (Jhand/Kandi), Salvadora oleoides (Peelu/Wan), Capparis aphylla (Karir), and Vachellia nilotica (Kikar).

Key Fauna Camels, houbara bustard, chinkara, golden jackal, Balochistan gerbil, porcupines, and rattlesnakes.

Current Status Heavily degraded — most of these natural jungles have been cleared for human settlements and the massive canal-irrigation agriculture of Punjab.

Temperate Deciduous Forests are found in moist mountain regions with distinct seasons, including a cold winter that causes trees to shed their leaves (deciduous).

Location Northern areas including Shogran, Neelam Valley, Murree, and parts of Kaghan.

Abiotic Profile The soil is rich, grayish-brown with high organic matter (humus); the climate has moderate temperatures (4°C to 30°C) and annual rainfall between 750 mm and 1500 mm.

Key Flora Taxus baccata (Yew), Pinus wallichiana (Blue Pine), Cedrus deodara (Himalayan Cedar), and shrubs like Berberis lyceum (Sumbal).

Key Fauna Rhesus monkey (Macaca mulatta), Black bear, Leopard cat, wolves, and various bird species.

Current Status Degraded — many large predators have been wiped out, and the forest is under continued pressure from the lumber industry and agricultural expansion.

Human activities are the dominant driver of recent climate change. Burning fossil fuels (coal, oil, gas) for vehicles, machinery, heating and electricity generation releases large quantities of CO₂, which has risen from about 280 ppm before industrialization to 428 ppm in 2026 — a change entirely tied to the industrial era, not natural cycles. Deforestation removes vegetation that would otherwise absorb CO₂. Livestock farming and agriculture release methane and nitrous oxide, while industrial processes (old refrigerators, air conditioners) release fluorinated gases. Together these human-driven emissions have intensified the natural greenhouse effect, raising global surface temperature by about 1.1°C since the pre-industrial era (IPCC, 2023) — a trend that coincides directly with industrialization, population growth, and rising energy consumption. Furthermore, the unequal distribution of emissions (the wealthiest 20% of the world's population produces 53% of global CO₂ while the poorest 20% produces only 3%) shows that this warming stems from specific patterns of human consumption and production rather than from natural variability.