What you'll learn
This topic examines the relationships between living organisms and their physical environment, focusing on how energy flows through ecosystems and how matter cycles through biological and non-biological components. You will study food chains, nutrient cycles, population dynamics, and human impact on Caribbean and global ecosystems. This section typically accounts for 10-15% of Paper 02 questions.
Key terms and definitions
Ecosystem — a community of living organisms interacting with each other and their physical environment (soil, water, air, light) in a defined area.
Habitat — the specific place where an organism lives within an ecosystem, characterized by particular physical conditions.
Population — all the organisms of one species living in the same area at the same time.
Community — all the populations of different species living and interacting in the same area.
Food chain — a linear sequence showing the transfer of energy from one organism to another through feeding relationships.
Food web — interconnected food chains showing multiple feeding relationships in an ecosystem.
Trophic level — the position an organism occupies in a food chain (producer, primary consumer, secondary consumer, tertiary consumer).
Biodiversity — the variety of different species of organisms living in an ecosystem or on Earth.
Core concepts
Energy flow in ecosystems
Energy enters ecosystems through photosynthesis when producers (green plants and algae) convert light energy into chemical energy stored in glucose. This energy flows through the ecosystem but cannot be recycled—it is eventually lost as heat.
Producers (autotrophs):
- Manufacture their own food using light energy
- Form the first trophic level
- Caribbean examples: mangrove trees, seagrass, sugarcane, banana plants, phytoplankton
Consumers (heterotrophs):
- Primary consumers: herbivores that feed on producers (iguanas, parrotfish, cattle, mongoose)
- Secondary consumers: carnivores that feed on herbivores (boa constrictors, barracuda, hawks)
- Tertiary consumers: carnivores that feed on other carnivores (sharks, eagles)
- Omnivores: organisms that feed on both plants and animals (humans, pigs, land crabs)
Decomposers:
- Bacteria and fungi that break down dead organic matter
- Release nutrients back into the soil
- Essential for nutrient recycling
- Operate at all trophic levels
Energy transfer efficiency:
- Only 10% of energy passes from one trophic level to the next
- 90% is lost through respiration (heat), movement, excretion, and undigested material
- This explains why food chains rarely exceed 4-5 trophic levels
- Pyramids of number, biomass, and energy illustrate these relationships
Caribbean food chain example: Seagrass → Parrotfish → Barracuda → Shark
Nutrient cycles
Unlike energy, matter is recycled continuously through ecosystems. Two critical cycles for CSEC examination are the carbon cycle and nitrogen cycle.
The Carbon Cycle:
Carbon moves between the atmosphere, living organisms, and Earth's crust:
- Photosynthesis removes CO₂ from the atmosphere; plants convert it to glucose
- Respiration by all living organisms returns CO₂ to the atmosphere
- Combustion of fossil fuels and wood releases CO₂
- Decomposition of dead organisms releases CO₂ through bacterial and fungal respiration
- Fossilization stores carbon in coal, oil, and natural gas over millions of years
- Ocean absorption and release regulates atmospheric CO₂
Caribbean context: Deforestation for agriculture (banana plantations, sugarcane farming) reduces carbon storage. Coastal mangroves are particularly important carbon sinks in the region.
The Nitrogen Cycle:
Nitrogen is essential for protein synthesis but atmospheric nitrogen (N₂) cannot be used directly by most organisms:
- Nitrogen fixation: Lightning and nitrogen-fixing bacteria (Rhizobium in legume root nodules) convert N₂ to nitrates
- Absorption: Plants absorb nitrates through roots and build proteins
- Feeding: Animals obtain nitrogen by consuming plants or other animals
- Decomposition: Decomposers break down proteins in dead organisms into ammonia
- Nitrification: Nitrifying bacteria convert ammonia → nitrites → nitrates
- Denitrification: Denitrifying bacteria convert nitrates back to N₂, returning it to the atmosphere
Agricultural importance: Caribbean farmers practice crop rotation with legumes (pigeon peas, beans) to restore soil nitrogen naturally, reducing fertilizer costs.
Population dynamics
Populations change in size based on four factors:
Factors increasing population:
- Births (natality)
- Immigration
Factors decreasing population:
- Deaths (mortality)
- Emigration
Population growth patterns:
- Exponential growth occurs when resources are unlimited (J-shaped curve)
- Logistic growth occurs when populations reach carrying capacity (S-shaped curve)
- Carrying capacity: the maximum population size an environment can sustainably support
Limiting factors:
Factors that restrict population growth:
Density-independent factors:
- Temperature extremes
- Hurricanes (highly relevant to Caribbean ecosystems)
- Drought
- Floods
Density-dependent factors:
- Food availability
- Disease transmission (increases with population density)
- Competition for resources
- Predation
- Accumulation of waste products
Caribbean example: Lionfish populations in Caribbean waters have grown exponentially since introduction because they face few natural predators and outcompete native species for food resources.
Ecological succession
Succession describes the gradual change in species composition in an ecosystem over time.
Primary succession:
- Occurs on bare rock or volcanic lava (relevant to Soufrière Hills volcano, Montserrat)
- Pioneer species (lichens, mosses) colonize first
- Soil gradually forms from weathered rock and dead organisms
- Grasses and small plants establish
- Shrubs and small trees follow
- Climax community (stable ecosystem) eventually develops
Secondary succession:
- Occurs after disturbance (fire, hurricane, farming abandonment)
- Soil already present
- Faster than primary succession
- Caribbean example: Abandoned sugarcane fields recolonize with grasses, then shrubs, eventually secondary forest
Human impact on the environment
Deforestation:
Causes in the Caribbean:
- Agricultural expansion (banana, cocoa, coffee plantations)
- Urbanization and tourism development
- Timber harvesting
- Charcoal production
Consequences:
- Soil erosion and landslides (especially on mountainous islands)
- Loss of biodiversity
- Disruption of water cycles
- Increased atmospheric CO₂
- Loss of watershed protection
Pollution:
Water pollution:
- Agricultural runoff (pesticides, fertilizers) causes eutrophication
- Sewage discharge increases bacterial contamination
- Oil spills damage coral reefs and mangroves
- Industrial waste (bauxite processing in Jamaica)
Air pollution:
- Vehicle emissions in urban areas (Kingston, Port of Spain, Bridgetown)
- Industrial emissions
- Burning of agricultural waste
Land pollution:
- Improper waste disposal
- Plastic accumulation in coastal areas
- Mining activities (bauxite, limestone quarrying)
Eutrophication process:
- Excess nitrates and phosphates enter water bodies from fertilizer runoff
- Algae population explodes (algal bloom)
- Algae block sunlight, preventing photosynthesis by underwater plants
- Plants die; decomposers increase
- Decomposers consume oxygen through respiration
- Oxygen depletion kills fish and other aquatic organisms
Climate change impacts:
Caribbean-specific effects:
- Rising sea levels threaten low-lying coastal areas and islands
- Coral bleaching from increased ocean temperatures
- More intense hurricanes
- Changes in rainfall patterns affecting agriculture
- Saltwater intrusion into freshwater aquifers
Conservation strategies
Sustainable practices:
- Reforestation programmes
- Marine protected areas (Belize Barrier Reef Reserve System)
- Fishing quotas and seasonal restrictions
- Renewable energy development (geothermal in St. Lucia, wind and solar)
- Waste management and recycling programmes
- Organic farming and integrated pest management
- Ecotourism rather than mass tourism
Legislation:
- Environmental protection acts
- Wildlife protection laws (sea turtle conservation)
- Marine park regulations
- Building codes for coastal development
Community involvement:
- Beach clean-up initiatives
- Mangrove restoration projects
- Education programmes in schools
- Local monitoring of environmental conditions
Worked examples
Example 1: A farmer notices his crop yields declining each year on the same plot of land.
(a) Suggest TWO reasons why crop yields are declining. (2 marks)
Mark scheme answer:
- Nutrients in the soil have been depleted by continuous farming (1 mark)
- Soil erosion has removed topsoil / soil structure has deteriorated (1 mark)
(b) Describe ONE sustainable farming practice the farmer could use to improve soil quality. (3 marks)
Mark scheme answer: Practice: Crop rotation with legumes (1 mark) Explanation: Legumes contain nitrogen-fixing bacteria in root nodules (1 mark) which convert atmospheric nitrogen into nitrates that enrich the soil (1 mark)
Example 2: The diagram shows a food web in a Caribbean coastal ecosystem:
Seagrass → Parrotfish → Barracuda Seagrass → Sea urchin → Lobster Phytoplankton → Zooplankton → Small fish → Barracuda
(a) Identify the producer(s) in this food web. (1 mark)
Mark scheme answer: Seagrass and phytoplankton (both needed for 1 mark)
(b) Explain what would happen to the sea urchin population if parrotfish were overfished. (2 marks)
Mark scheme answer: Sea urchin population would increase (1 mark) because there would be more seagrass available / less competition for food (1 mark)
Example 3: Explain how deforestation contributes to increased atmospheric carbon dioxide levels. (4 marks)
Mark scheme answer:
- Fewer trees means less photosynthesis occurring (1 mark)
- Therefore less CO₂ is removed from the atmosphere (1 mark)
- Burning of cleared trees releases CO₂ through combustion (1 mark)
- Decomposition of wood waste releases CO₂ through bacterial respiration (1 mark)
Common mistakes and how to avoid them
Confusing food chains with food webs: A food chain is linear (A→B→C); a food web shows multiple interconnected chains. Always identify specific chains within a web when asked.
Reversing arrow direction: Arrows in food chains show energy flow from food source to consumer, NOT the direction of eating. Write: Grass → Cow → Human (not Cow → Grass).
Stating energy is recycled: Energy flows through ecosystems and is lost as heat. Only nutrients (matter) are recycled through decomposition and nutrient cycles.
Ignoring the 10% energy rule: When calculating energy transfer, remember only approximately 10% passes to the next trophic level. If producers have 10,000 kJ, primary consumers receive only 1,000 kJ.
Incomplete explanations of pollution effects: Don't just state "pollution is bad." Explain the mechanism: fertilizer runoff → eutrophication → algal bloom → oxygen depletion → fish death.
Confusing nitrogen fixation with nitrification: Nitrogen fixation converts N₂ gas to nitrates. Nitrification converts ammonia to nitrates. These are different processes performed by different bacteria.
Exam technique for "Living Organisms and the Environment"
"State" vs "Explain": "State" requires a simple fact (1 mark each). "Explain" requires a reason or mechanism (usually 2-3 marks) with linking words like "because," "therefore," "this causes."
Diagram interpretation: Food webs and nutrient cycles appear frequently. Practice tracing pathways and identifying what happens if one component is removed. Always use organism names from the diagram.
Use Caribbean examples: When questions ask for examples of conservation, pollution, or species, use relevant Caribbean context (mangroves, coral reefs, hurricanes, local agriculture) to demonstrate applied knowledge.
Link structure to mark allocation: A 4-mark question needs four distinct points. If you only write two sentences, you cannot score full marks. Plan your answer length based on available marks.
Quick revision summary
Energy flows unidirectionally through ecosystems from producers through consumers (only 10% transfers between trophic levels), while nutrients cycle continuously through carbon and nitrogen cycles involving respiration, photosynthesis, and bacterial action. Populations grow until limited by density-dependent and density-independent factors, with Caribbean populations particularly affected by hurricanes and resource competition. Human activities including deforestation, pollution, and unsustainable farming reduce biodiversity and disrupt nutrient cycles, but conservation strategies and sustainable practices can protect Caribbean ecosystems for future generations.