Unit 25: Ecology — Short Questions

Biogeochemical cycles are classified into two types based on their primary reservoir (the long-term storage pool): (1) Gaseous cycles, where the reservoir is the air or oceans (e.g., nitrogen cycle, oxygen cycle, carbon cycle, water cycle), and (2) Sedimentary cycles, where the reservoir is the Earth's crust (e.g., phosphorus cycle, sulfur cycle, calcium cycle).

Denitrification To keep the nitrogen cycle balanced, nitrogen must return to the atmosphere. In waterlogged soils, denitrifying bacteria such as Pseudomonas and Thiobacillus convert nitrates (NO₃⁻) back into nitrogen gas (N₂), which escapes into the atmosphere. Ammonification: When plants and animals die, or when animals excrete nitrogenous waste, decomposers (fungi and bacteria) break down the complex proteins present in them and convert this organic nitrogen back into ammonia (NH₃), recycling it into the soil.

Energy flows unidirectionally from the sun through the trophic levels of an ecosystem — producers, primary consumers, secondary consumers and tertiary consumers — and is never recycled. 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 passes to the next trophic level, the rest being lost mainly as heat. For example, if producers fix 10,000 kcal, primary consumers get about 1,000 kcal, secondary consumers about 100 kcal, and tertiary consumers only about 10 kcal. Because of this loss, short food chains are more energy-efficient than long ones.

Population density is the number of individuals of a population per unit area or volume, e.g., the number of mulberry trees per square kilometer or E. coli per milliliter of culture. As density approaches carrying capacity, competition for resources intensifies, leading to lower birth rates, higher mortality, and increased pollution. Population growth is the increase in the number of individuals in a population over a specific time period. It follows two models: the exponential model, in which the population grows without limit at its maximal rate, increasing by the same ratio per unit time; and the logistic model, in which growth slows as resources (space, light, water, nutrients) become limiting, and the population stabilizes at its carrying capacity, producing a sigmoid (S-shaped) growth curve.

Rapid human population growth is perhaps the greatest challenge to 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.

Greenhouse gases (CO₂, CH₄, N₂O, fluorinated gases) allow sunlight to pass through to Earth's surface but absorb and re-emit the heat radiated back from it, preventing it from escaping into space. Human activities such as burning fossil fuels, deforestation, livestock farming and industrial processes have sharply increased the concentration of these gases (CO₂ rose from about 280 ppm before industrialization to 428 ppm in 2026). This enhanced, unnatural greenhouse effect traps more heat than the natural level, raising the Earth's average surface temperature — global warming (about 1.1°C increase since the pre-industrial era, per IPCC 2023).

Long-term effects of global warming include: intensified heatwaves (e.g., Jacobabad, Sibi crossing 50°C, causing heatstroke and heat stress in livestock); more erratic and intense floods from turbo-charged monsoons; accelerated glacial melting leading to Glacial Lake Outburst Floods (GLOFs); sea level rise and coastal erosion (e.g., seawater intrusion in Thatta and Badin); food insecurity due to reduced crop yields and desertification; and ecosystem disruption, forcing species to migrate and increasing the risk of extinction.

The Temperate Deciduous Forest has rich, grayish-brown soil with high organic matter (humus), moderate temperatures (4°C to 30°C), and relatively high annual rainfall (750 mm to 1500 mm). In contrast, the Tropical Thorn Forest has an arid to semi-arid climate with summer temperatures reaching up to 50°C, and much scantier rainfall (200 mm to 750 mm annually), favoring drought-resistant (xerophytic) vegetation rather than humus-rich soils.

A biome is a large, distinct geographical region characterized by a specific climate, soil, altitude, and the types of plants and animals adapted to it (e.g., the Coniferous Forest Biome of Murree and Swat). The biosphere is the highest, most inclusive level of ecological organization — the global sum of all biomes, encompassing every part of the Earth where life exists.

A biogeochemical cycle is the cyclic movement of chemical elements (such as carbon, nitrogen, oxygen, phosphorus) between the biotic and abiotic components of an ecosystem — matter is continuously recycled and reused. Energy flow, in contrast, is the unidirectional passage of energy from the sun through producers and successive trophic levels, being progressively lost as heat and never recycled back into the system.

Population growth is the increase in the number of individuals within a population over a specific time period, following exponential or logistic models. Population density is the number of individuals of a population present per unit area or volume at a given time, e.g., bacteria per milliliter.

A convex (Type I) population survivorship curve shows that most individuals survive to an old age and then die rapidly, with environmental factors having little impact on mortality (e.g., humans and mammals). A concave (Type III) population survivorship curve shows very high juvenile mortality, but individuals that reach adulthood have a much lower mortality rate (e.g., fishes and many invertebrates).

Nitrogen fixation is the conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃), carried out biologically by bacteria such as Azotobacter, Clostridium and Rhizobium, as well as by lightning and industrial processes. Nitrification is a later step in which soil bacteria Nitrosomonas and Nitrobacter convert this ammonia first to nitrite (NO₂⁻) and then to nitrate (NO₃⁻), a form readily absorbed by plants.

Nitrification is the conversion of ammonia into nitrite and then nitrate by bacteria such as Nitrosomonas and Nitrobacter, making nitrogen available for plant uptake. Denitrification is the reverse process, in which denitrifying bacteria such as Pseudomonas and Thiobacillus convert nitrates back into nitrogen gas (N₂) in waterlogged soils, releasing it into the atmosphere and completing the cycle.