7.3 Ecology and Ecosystems
Key Takeaways
- Ecological organization runs organism, population, community, ecosystem, biome, biosphere; an ecosystem includes both living and nonliving factors
- Only about 10% of energy passes from one trophic level to the next, so 10,000 kJ at the producer level leaves just 100 kJ for secondary consumers
- Producers make food by photosynthesis, consumers eat other organisms, and decomposers like bacteria and fungi recycle nutrients from dead matter
- In the nitrogen cycle, nitrogen-fixing bacteria in legume root nodules convert atmospheric N2 into forms plants can absorb
- Mutualism benefits both species, commensalism benefits one without affecting the other, and parasitism benefits one while harming the other
Why Ecology Matters for the PUPCET
Ecology questions test whether you can place organisms in a food chain, compute energy transfer between trophic levels, identify symbiotic relationships, and connect environmental issues to Philippine settings — the Sierra Madre, Taal Lake, and the Coral Triangle. These items reward clean definitions and simple math.
Levels of Ecological Organization
From smallest to largest:
- Organism — one individual (a tarsier)
- Population — all members of one species in an area (all tarsiers in Bohol's forests)
- Community — all the different populations living together in that area
- Ecosystem — the community plus nonliving (abiotic) factors like water, soil, and sunlight
- Biome — a large region with a similar climate and life forms (tropical rainforest)
- Biosphere — the sum of all ecosystems on Earth
Remember: biotic means living; abiotic means nonliving.
Food Chains, Food Webs, and Trophic Levels
A food chain traces one path of energy: grass -> grasshopper -> frog -> snake -> Philippine eagle. A food web is the network of many interconnected chains — more realistic, and more stable when one species declines.
| Trophic Level | Role | Example |
|---|---|---|
| 1st | Producers (autotrophs) — make food by photosynthesis | Grass, rice, phytoplankton |
| 2nd | Primary consumers — herbivores that eat producers | Grasshopper, carabao |
| 3rd | Secondary consumers — carnivores or omnivores that eat herbivores | Frog |
| 4th | Tertiary consumers — top carnivores | Philippine eagle |
Decomposers — bacteria and fungi — break down dead matter from every level and return nutrients to the soil.
The 10% Energy Rule
Only about 10% of the energy at one trophic level passes to the next; the rest is lost as heat or used in life processes. Worked pyramid: if producers capture 10,000 kJ, herbivores receive about 1,000 kJ, secondary consumers about 100 kJ, and tertiary consumers only about 10 kJ. This is why food chains rarely exceed four or five levels, why top predators like the Philippine eagle are naturally rare, and why eating lower on the food chain (rice instead of beef) supports more people per hectare.
Producers, Consumers, and Decomposers Recap
Keep the three ecological roles straight: producers (green plants and algae) capture energy; consumers (herbivores, carnivores, omnivores) transfer it by eating; decomposers close the loop by releasing nutrients from dead organisms back into the environment. An exam item showing mushrooms on a fallen narra log is describing decomposers at work.
Biogeochemical Cycles
Matter cycles; energy flows one way. Three cycles are PUPCET favorites:
Water Cycle
Evaporation (driven by the sun) -> condensation (clouds form) -> precipitation (rain) -> collection and runoff back to seas, plus transpiration from plant leaves. This cycle powers the habagat rains and the typhoon season.
Carbon Cycle
Producers remove CO2 from the air through photosynthesis; all organisms return it through respiration; decomposition, burning of fossil fuels, and volcanic activity add more. Excess CO2 strengthens the greenhouse effect and drives climate change — the link exam items often test.
Nitrogen Cycle
Atmospheric nitrogen (N2) is useless to most organisms until nitrogen-fixing bacteria — often in the root nodules of legumes like munggo and peanuts — convert it into ammonia compounds. Nitrifying bacteria then turn these into nitrates that plants absorb, while denitrifying bacteria return N2 to the atmosphere. Synthetic fertilizers mimic this cycle, and their runoff can trigger algal blooms and fish kills, as seen in Laguna de Bay.
| Cycle | Key processes to recall |
|---|---|
| Water | Evaporation, condensation, precipitation, transpiration |
| Carbon | Photosynthesis removes CO2; respiration and combustion release it |
| Nitrogen | Fixation, nitrification, denitrification — all bacterial |
Producers in a rice-field ecosystem capture 10,000 units of energy from sunlight. Following the 10% energy-transfer rule, about how much energy reaches the secondary consumers (the third trophic level)?
Symbiosis: Living Together
| Relationship | Effect on Each Partner | Philippine-flavored Example |
|---|---|---|
| Mutualism | Both benefit (+/+) | Bees pollinating calamansi flowers; clownfish and sea anemone |
| Commensalism | One benefits, the other unaffected (+/0) | Egrets riding on a grazing carabao; barnacles on a whale |
| Parasitism | One benefits, the other is harmed (+/-) | Tapeworm in the intestine; dengue virus in humans |
Predation (a predator eats its prey) and competition (organisms vie for the same limited resource) also shape communities, though they are not symbiosis in the strict sense.
Philippine Ecosystems and Environmental Issues
The Philippines is one of the world's megadiverse countries and sits within the Coral Triangle, the global center of marine biodiversity. Frequently tested contexts:
- Tropical rainforests — the Sierra Madre shields Luzon from typhoons, but deforestation from logging and kaingin (slash-and-burn farming) causes soil erosion, flash floods, and habitat loss for species like the Philippine eagle and the tamaraw.
- Coral reefs — damaged by dynamite and cyanide fishing, sedimentation from eroded soil, and coral bleaching caused by warming seas. Healthy reefs shelter fish nurseries and buffer coastlines from waves.
- Mangroves — fish nurseries and natural storm-surge barriers, whose destruction worsened the devastation of Typhoon Yolanda; many stands have been cleared for fishponds.
- Climate change — stronger typhoons, sea-level rise threatening coastal barangays, and shifting rainy seasons that disrupt rice planting schedules.
Common Traps
- Population vs. community: a community includes all species in an area, not just one.
- The 10% rule applies level by level — after two transfers, 10,000 becomes 100, not 1,000.
- Commensalism is usually the answer when the question stresses that one organism is "neither helped nor harmed."
- Decomposers are not a rung of the food chain; they recycle nutrients from every trophic level.
Egrets perch on grazing carabaos and eat the insects stirred up from the grass, while the carabao is neither helped nor harmed. What kind of ecological relationship is this?