Ecology & Environment

1. The Organisation of Life

Ecology is the scientific study of the interactions between living organisms and their environment. To understand these complex relationships, we organise the living world into several levels.

• Habitat: The specific place where an organism lives, providing it with food, shelter, and conditions for survival. For example, the mangrove forests of the Indus Delta are a habitat for the mudskipper fish.
• Population: A group of organisms of the same **species** living in the same habitat at the same time. For example, all the snow leopards in the Karakoram National Park constitute a population.
• Community: All the different populations of different species living and interacting in the same habitat. The community of a pond in Punjab would include populations of fish, frogs, water lilies, and dragonflies.
• Ecosystem: A community of living organisms (**biotic factors**) interacting with their non-living physical environment (**abiotic factors** like sunlight, water, soil pH, and temperature). An ecosystem is a dynamic, self-sustaining unit.
• Biodiversity: The variety of different species of organisms within an ecosystem. High biodiversity is a sign of a healthy and stable ecosystem, such as the diverse juniper forests of Ziarat.

2. Energy Flow in Ecosystems

Energy flows in one direction through an ecosystem, starting from the sun.

Food Chains & Food Webs:

A food chain illustrates the linear flow of energy. Each stage is a trophic level.

* Producers: Organisms, typically plants and algae, that produce their own food using light energy through photosynthesis. They form the first trophic level.

* Primary Consumers: Herbivores that feed on producers (e.g., a goat eating grass).

* Secondary Consumers: Carnivores or omnivores that feed on primary consumers (e.g., a fox eating a rabbit).

* Tertiary Consumers: Carnivores that feed on secondary consumers (e.g., a golden eagle eating a fox).

* Decomposers: Organisms like bacteria and fungi that break down dead organic matter (dead plants, animals, faeces). They are crucial for recycling nutrients but are not typically shown in a food chain.

A food web is a more realistic model, showing interconnected food chains within an ecosystem.

Energy Transfer & Pyramids:

Energy transfer between trophic levels is inefficient. Only about 10% of the energy from one level is incorporated into the next. The remaining 90% is lost primarily as:

1. Metabolic Heat: Lost during respiration to maintain body temperature.
2. Movement & Life Processes: Energy used for hunting, growth, and reproduction.
3. Indigestible Waste: Excreted as faeces or urine.
4. Incomplete Consumption: Not all parts of an organism are eaten.

This energy loss can be represented by ecological pyramids:

• Pyramid of Numbers: Shows the number of individual organisms at each trophic level. *Exam Trap:* This pyramid can be inverted. For example, a single large oak tree (producer) can support thousands of caterpillars (primary consumers).
• Pyramid of Biomass: Represents the total **biomass** (dry mass of organisms) at each level. It is measured in units like **g/m²**. This pyramid is almost always upright, as the total mass of producers must be greater than the consumers they support.
• Pyramid of Energy: Shows the total energy transferred through each trophic level over a period. Measured in **kJ/m²/year**. This pyramid is *always* upright, as energy is always lost at each successive level.

3. Nutrient Cycles

Unlike the one-way flow of energy, nutrients like carbon and nitrogen are recycled within an ecosystem. Decomposers are vital for this process.

The Carbon Cycle:

1. Photosynthesis: Plants absorb carbon dioxide (CO₂) from the atmosphere to make glucose.
2. Respiration: All living organisms respire, releasing CO₂ back into the atmosphere.
3. Consumption: Carbon is transferred along the food chain when animals eat plants or other animals.
4. Decomposition: Decomposers respire as they break down dead organisms, releasing CO₂.
5. Combustion: The burning of fossil fuels (coal, oil, gas) and wood releases vast amounts of stored carbon as CO₂, disrupting the natural balance.

The Nitrogen Cycle:

Nitrogen is essential for making proteins. 78% of the atmosphere is nitrogen gas (N₂), but it is unreactive and cannot be used directly by plants.

1. Nitrogen Fixation: The conversion of N₂ gas into nitrogen compounds. This is done by:

• Nitrogen-fixing bacteria in the soil and in the root nodules of leguminous plants (like peas and lentils, widely grown in Pakistan).
• Lightning strikes.

1. Nitrification: Nitrifying bacteria in the soil convert ammonia (from decomposition) into nitrites (NO₂⁻) and then into nitrates (NO₃⁻).
2. Assimilation: Plants absorb nitrates from the soil through their roots to synthesise proteins.
3. Decomposition: Decomposers break down dead organisms and waste products (like urea), returning nitrogen to the soil as ammonia.
4. Denitrification: Denitrifying bacteria (in anaerobic conditions, like waterlogged soil) convert nitrates back into N₂ gas, which returns to the atmosphere.

4. Human Impact on the Environment

Human activities often have severe negative consequences for ecosystems.

• Deforestation: The large-scale removal of forests for timber, agriculture, or urbanisation. In northern Pakistan, this leads to **soil erosion**, landslides, **loss of biodiversity**, and disruption of the water and carbon cycles (fewer trees to absorb CO₂).
• Water Pollution (Eutrophication): The contamination of water bodies, often by sewage and agricultural fertilisers. This leads to **eutrophication** in places like the Ravi River:

1. Fertilisers (containing nitrates and phosphates) wash into the river.
2. This causes a rapid growth of algae on the surface (algal bloom).
3. The bloom blocks sunlight, killing plants below.
4. Bacteria decompose the dead algae and plants, using up large amounts of dissolved oxygen in the process.
5. The severe depletion of oxygen kills fish and other aquatic animals.

• Air Pollution: The release of harmful substances into the atmosphere.
• Acid Rain: Sulfur dioxide (from burning coal) and nitrogen oxides (from car exhausts) dissolve in rainwater to form sulfuric and nitric acid. This damages buildings, forests, and lakes.
• Enhanced Greenhouse Effect: The burning of fossil fuels increases the concentration of greenhouse gases like CO₂. This traps more heat in the atmosphere, leading to **global warming**, climate change, and the melting of glaciers in the Himalayas and Karakorams.