UPCAT General Science (Extended) — Ecology, Biogeochemical Cycles & Species RelationshipsRevision Notes
Condensed revision notes for Ecology, Biogeochemical Cycles & Species Relationships, built for the final weeks before the UPCAT 2026. These are the distilled key points you need when there is no time left for full study notes — just the concepts, formulas, and traps University of the Philippines tests.
Exam context
The University of the Philippines College Admission Test is conducted by University of the Philippines and is scheduled for Mid-2026 (announced by UP Admissions). The General Science (Extended) subtest is marked as "Extended coverage for UP Science programs" in the official pattern, and Ecology, Biogeochemical Cycles & Species Relationships appears in position 6th of 6 in the UPCAT General Science (Extended) review rotation. Passing mark: UPG ≤ 2.2 typical. Recent UPCAT 2026 papers have drawn roughly 20 questions from this subject.
Ecology, Biogeochemical Cycles & Species Relationships - Revision notes
Ecology is the scientific study of how organisms interact with each other and their environment. This chapter covers the fundamental principles of ecological organization, biogeochemical cycles that sustain life on Earth, and the various relationships between species. Understanding these concepts is crucial for comprehending how ecosystems function and maintain balance in nature.
Sections
Exam Tips
- Remember the hierarchy: Organism → Population → Community → Ecosystem → Biosphere
- Practice identifying biotic vs abiotic factors in given scenarios
- Know specific examples for each organizational level
Key Points
- Biosphere: The broadest level encompassing all life on Earth and its atmosphere
- Ecosystem: Units containing all organisms and non-living environment in a particular area
- Community: All interacting organisms living in a particular area (thousands of species)
- Population: Group of individuals of the same species in a specific area with breeding capability
- Organism: Individual level focusing on adaptations to overcome environmental challenges
Definitions
Term
Ecology
Definition
The scientific study of how organisms interact with each other and their environment, including both biotic and abiotic factors
Importance
Foundation for understanding ecosystem function and environmental conservation
Term
Biotic factors
Definition
All living factors in the environment that affect organisms
Importance
Essential for understanding species interactions and community dynamics
Term
Abiotic factors
Definition
Non-living factors essential to living organisms (water, temperature, soil, sunlight)
Importance
Determine what organisms can survive in specific environments
Section Title
Levels of Ecological Organization
Common Mistakes
- Confusing ecosystem with community - ecosystem includes non-living components
- Thinking population refers to all organisms in an area - it's only one species
- Mixing up biotic and abiotic factors in examples
Exam Tips
- Know the energy transfer rule: only 10% passes to next trophic level
- Practice drawing food chains and webs from given organisms
- Remember: Producers → Primary → Secondary → Tertiary consumers
Key Points
- Producers (Autotrophs): Organisms that make their own food using energy (plants, photosynthetic organisms)
- Consumers (Heterotrophs): Organisms that obtain energy by consuming other organisms
- Herbivores: Primary consumers that eat plants only
- Carnivores: Secondary/tertiary consumers that eat other consumers (meat eaters)
- Omnivores: Organisms that eat both producers and consumers
- Decomposers (Saprovores): Break down dead organic material, recycling nutrients
Definitions
Term
Food Chain
Definition
The feeding relationship between different organisms showing energy flow from producers to consumers
Importance
Shows how energy moves through ecosystems in a linear path
Term
Food Web
Definition
A network of interconnected food chains showing complex feeding relationships
Importance
More realistic representation of ecosystem feeding relationships
Term
Trophic Level
Definition
Each step in a food chain representing the organism's position in energy transfer
Importance
Helps understand energy flow and ecosystem structure
Section Title
Feeding Relationships and Energy Flow
Common Mistakes
- Confusing autotrophs with heterotrophs - remember 'auto' means self-making
- Thinking all carnivores are at the same trophic level
- Forgetting that decomposers are essential for nutrient cycling
Formulas
Example
Plants use CO₂ from atmosphere to make glucose, releasing oxygen
Formula
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
Variables
CO₂ = carbon dioxide, H₂O = water, C₆H₁₂O₆ = glucose, O₂ = oxygen
Application
Photosynthesis in carbon cycle
Exam Tips
- Draw and label each cycle showing the main reservoirs and processes
- Remember which cycles have atmospheric phases (C, N, S) vs which don't (P)
- Know the role of decomposers in returning nutrients to soil
Key Points
- Elements are continuously recycled through biogeochemical cycles involving biology, geology, and chemistry
- Water Cycle: Evaporation → Condensation → Precipitation → Collection
- Carbon Cycle: Photosynthesis removes CO₂, respiration and combustion release CO₂
- Nitrogen Cycle: Nitrogen fixation → Nitrification → Assimilation → Denitrification
- Phosphorus Cycle: No atmospheric phase, cycles through rocks, soil, and living organisms
- Sulfur Cycle: Involves atmospheric sulfur compounds and biological processes
Definitions
Term
Biogeochemical Cycle
Definition
The movement and recycling of chemical elements through living organisms and the physical environment
Importance
Essential for maintaining life on Earth by recycling nutrients
Term
Nitrogen Fixation
Definition
Process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) that plants can use
Importance
Makes nitrogen available to living organisms for protein synthesis
Section Title
Biogeochemical Cycles
Common Mistakes
- Thinking energy is recycled like matter - energy flows one way, matter cycles
- Confusing the direction of processes in carbon cycle
- Forgetting the role of bacteria in nitrogen cycle
Exam Tips
- Use the +/0/- system to quickly identify relationship types
- Know specific examples for each type of relationship
- Remember: Intraspecific = same species, Interspecific = different species
Key Points
- Mutualism (+/+): Both species benefit (bee and flower)
- Commensalism (+/0): One benefits, other unaffected (orchid on tree)
- Parasitism (+/-): One benefits, other harmed (lice on human)
- Predation (+/-): Predator kills and eats prey (cat and mouse)
- Competition (-/-): Both species harmed by limited resources (lion and cheetah)
- Amensalism (-/0): One harmed, other unaffected (algal bloom affecting fish)
- Neutralism (0/0): Neither species affected
- Protocooperation (+/+): Both benefit but relationship not obligatory
Definitions
Term
Symbiosis
Definition
Close relationship between organisms of different species where at least one benefits
Importance
Fundamental to understanding species interactions and ecosystem stability
Term
Ecological Niche
Definition
The ecological role of an organism, including conditions, resources, and interactions needed for survival
Importance
Determines how species coexist and compete in ecosystems
Term
Competitive Exclusion Principle
Definition
Two species with identical niches cannot coexist indefinitely with limited resources
Importance
Explains species distribution and the importance of niche differentiation
Section Title
Species Relationships and Interactions
Common Mistakes
- Confusing mutualism with protocooperation - mutualism is obligatory
- Mixing up parasitism and predation - parasites usually don't kill host immediately
- Thinking competition only occurs between different species
Exam Tips
- Remember: Primary = pristine/new, Secondary = second chance after disturbance
- Know examples of what causes each type of succession
- Understand the role of pioneer species in preparing the environment
Key Points
- Ecological succession: Gradual changes in ecosystem structure and composition over time
- Primary succession: Development in previously uncolonized areas (volcanic islands)
- Secondary succession: Development in previously colonized but disrupted areas (after forest fire)
- Pioneer species: First organisms to colonize new or disturbed areas
- Climax community: Final stable community in succession sequence
Definitions
Term
Primary Succession
Definition
Development of a new community in an area never colonized before, starting from bare rock or soil
Importance
Shows how life establishes in completely new environments
Term
Secondary Succession
Definition
Series of changes in a previously colonized community that has been damaged or disrupted
Importance
Shows how ecosystems recover from disturbances
Term
Pioneer Species
Definition
First species to colonize new or disturbed areas, usually hardy and fast-reproducing
Importance
Start the succession process and prepare environment for other species
Section Title
Ecological Succession
Common Mistakes
- Confusing primary and secondary succession - primary starts from scratch
- Thinking succession always leads to forests - depends on climate
- Assuming succession happens quickly - it takes decades to centuries
Connections
- Connects to Chemistry: Understanding chemical cycles and molecular processes in ecosystems
- Connects to Physics: Energy flow principles and thermodynamics in biological systems
- Links to Environmental Science: Human impact on ecological systems and conservation
- Relates to Agriculture: Nitrogen cycle understanding for fertilizer use and crop rotation
- Connects to Medicine: Understanding parasitic relationships and disease ecology
- Links to Climate Science: Carbon cycle's role in global warming and climate change
Exam Strategy
Focus on memorizing the specific examples for each concept (bee-flower for mutualism, orchid-tree for commensalism). Practice drawing and labeling biogeochemical cycles from memory. Use the +/0/- system to quickly identify species relationships. Create concept maps linking ecological organization levels. Review the differences between primary and secondary succession with specific examples. Understand that energy flows one-way while matter cycles through ecosystems.
Quick Review Questions
What is the correct order of ecological organization from smallest to largest?
Each level becomes more inclusive, with organisms being individuals and biosphere encompassing all life on Earth.
In a mutualistic relationship, how do both species benefit?
Example: Bees get nectar from flowers while flowers get pollination services - both depend on this relationship.
What is the main difference between primary and secondary succession?
Primary starts from bare rock (like volcanic islands), secondary starts from existing soil (like after forest fires).
Why is the nitrogen cycle important for living organisms?
Most organisms cannot use atmospheric nitrogen directly, so bacteria fix it into ammonia and nitrates.
What happens to energy as it moves through trophic levels?
This is why food chains are usually limited to 4-5 trophic levels and why there are fewer top predators.
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