Ecology

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Ecology — Quick Facts

Key Definitions:

• Ecology: The scientific study of interactions among organisms and their environment
• Biosphere: The global sum of all ecosystems — the zone of life on Earth
• Biome: A major regional group of distinctive plant and animal communities
• Ecosystem: Community of living organisms interacting with their physical environment
• Habitat: The natural environment of an organism
• Niche: The role and position a species has in its environment

Energy Flow:

• Sun → Producers (plants) → Primary Consumers (herbivores) → Secondary Consumers (carnivores) → Decomposers
• Energy decreases by ~10% at each trophic level (10% law of Raymond Lindemann)
• Food chains rarely exceed 4-5 trophic levels

⚡ Exam Tips for MDCAT:

• The Pyramid of Energy is always upright — energy cannot be created or destroyed
• Pyramid of Numbers can be inverted (e.g., one tree → many insects)
• Biogeochemical cycles (Carbon, Nitrogen, Water) are frequently tested
• Pakistan’s major biomes: Tropical rainforest (Thar desert), Temperate (northern mountains), Arid (Balochistan)

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Ecology — Study Guide

Levels of Ecological Organisation:

1. Species: Group of similar organisms that can interbreed
2. Population: Organisms of same species in a given area
3. Community: Multiple populations interacting
4. Ecosystem: Living + non-living components
5. Biome: Regional climate-specific community
6. Biosphere: Global living system

Biogeochemical Cycles:

Carbon Cycle:

• CO$_2$ absorbed by photosynthesis → returns via respiration, combustion, decomposition
• Key reservoirs: Atmosphere, oceans, fossil fuels
• Human impact: Burning fossil fuels → increased CO$_2$ → global warming
• Carbon is stored in limestone (CaCO$_3$), coal, oil, methane hydrates

Nitrogen Cycle:

• N$_2$ (78% of atmosphere) → Ammonia (NH$_3$) → Nitrification → Nitrate (NO$_3^-$) → Denitrification → N$_2$
• Nitrogen fixation: Rhizobium (in legume root nodules), Azotobacter, cyanobacteria
• Lightning also fixes atmospheric nitrogen
• Nitrification: NH$_3$ → NO$_2^-$ → NO$_3^-$ (by Nitrosomonas and Nitrobacter)

Water (Hydrological) Cycle:

• Evaporation → Condensation → Precipitation → Runoff/Infiltration → Transpiration
• Transpiration: Loss of water vapour through stomata (~90% of water absorbed by roots)

Population Interactions:

Interaction	Species 1	Species 2	Example
Predation	+	−	Lion and zebra
Competition	−	−	Two species competing for same food
Parasitism	+	−	Tapeworm and human
Mutualism	+	+	Bee and flower
Commensalism	+	0	Barnacle on whale
Amensalism	0	−	Penicillin killing bacteria

Ecological Succession:

• Primary Succession: Starts on bare rock (no soil) — lichens → mosses → grasses → shrubs → climax community
• Secondary Succession: Starts on disturbed area with soil — faster, e.g., after forest fire
• Climax Community: Stable, self-perpetuating end stage

Common Student Mistakes:

• Confusing habitat (where an organism lives) with niche (what it does)
• Thinking energy is recycled like nutrients — energy flows one way and is lost as heat
• Mixing up decomposers and detritivores

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Ecology — Comprehensive Notes

Population Ecology:

Population Growth Models:

1. Exponential Growth: $dN/dt = rN$ (unlimited resources)

   • J-shaped curve
   • $r$ = intrinsic rate of increase

2. Logistic Growth: $dN/dt = rN(K-N)/K$

   • S-shaped curve
   • $K$ = carrying capacity
   • Growth slows as $N$ approaches $K$

Population Dynamics:

• Carrying Capacity (K): Maximum population size the environment can sustain
• Population Density: Number of individuals per unit area
• Density-dependent factors: Predation, competition, disease (effect increases with density)
• Density-independent factors: Natural disasters, temperature extremes, floods (effect independent of density)

Species Interactions in Detail:

Predation:

• Controls prey population below carrying capacity
• Can cause “predator-mediated coexistence”
• Character displacement in prey species
• Arms race (predator-prey coevolution)

Competition — Competitive Exclusion Principle (Gause, 1934):

• Two species competing for the same limited resource cannot coexist indefinitely
• One will always outcompete the other
• Resource partitioning: Species divide resources to reduce competition
• Example: Gause’s experiments with Paramecium species

Mutualism:

• Mycorrhizae: Fungi + plant roots (phosphorus exchange)
• Lichen: Fungus + algae/cyanobacteria
• Pollination: Insect + flowering plant
• Nitrogen fixation: Rhizobium + legume

Ecological Pyramids:

Pyramid Type	Always Upright?	What It Shows
Numbers	No	Number of organisms at each level
Biomass	Usually	Total dry mass at each level
Energy	Always	Energy content at each level

Energy pyramids are always upright because:

• Energy is lost as heat at each level (~90%)
• 10% rule: Only ~10% of energy transfers to next level

Biodiversity and Conservation:

Threats to Biodiversity:

1. Habitat destruction (biggest threat — ~85% of species at risk)
2. Overexploitation
3. Invasive species
4. Pollution
5. Climate change

Conservation Strategies:

• In-situ conservation: Protected areas, national parks, wildlife sanctuaries
• Ex-situ conservation: Zoos, botanical gardens, seed banks, cryopreservation
• Keystone Species: Disproportionate effect on ecosystem (e.g., sea otters controlling sea urchin populations)
• Indicator Species: Reflect environmental health (e.g., amphibians)

Pakistan’s Ecological Highlights:

• Thar Desert: Rhesus monkey, desert fox, spiny-tailed lizard
• Himalayan region: Snow leopard, markhor, western tragopan
• Coastal mangroves: Indus Delta — vulnerable to sea level rise
• Khewra and Salt Range: Unique biodiversity

Ecosystem Services:

• Provisioning: Food, water, timber, medicines
• Regulating: Climate control, flood control, disease control
• Cultural: Recreation, tourism, spiritual values
• Supporting: Nutrient cycling, soil formation, photosynthesis

⚡ MDCAT High-Yield Patterns:

• The carbon and nitrogen cycles are most frequently tested
• Always remember the 10% energy transfer rule
• Ecological relationships table is a common question format
• Know the difference between primary and secondary succession

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