Ecology, populations, homeostasis

What this reading is about

This block covers three big areas that MCAS tests together:

1. Ecosystems — food webs, energy flow, and species interactions.
2. Populations — how populations grow, shrink, and level off.
3. homeostasisThe body's ability to keep internal conditions stable. — how organisms keep their bodies stable.

These topics connect: an ecosystemAll the living and non-living things in one area, working together. is the big picture (all organisms + their environment), populationAll the organisms of one species living in one area at one time.s are groups within that ecosystem, and homeostasis is how individual organisms stay alive inside it all.

Ecosystems: who eats whom?

An ecosystemAll the living and non-living things in one area, working together. includes all the bioticA living factor in an ecosystem (plants, animals, fungi, bacteria). (living) and abioticA non-living factor in an ecosystem (water, sunlight, temperature, soil). (non-living) factors in an area. Energy flows through the ecosystem in one direction: from the sun → producerAn organism that makes its own food from sunlight (like a plant).s → consumerAn organism that gets energy by eating other organisms.s → decomposerAn organism that breaks down dead things and recycles nutrients.s.

Food web rules:

• Arrows point from the organism being eaten TO the eater (the direction energy flows).
• Energy decreases at each level — about 90% is lost as heat. Only ~10% passes up.
• Matter (nutrients) is recycled by decomposers. Energy is NOT recycled.

The correct food chain order

Producer → primary consumer → secondary consumer → tertiary consumer.

Example: grass → rabbit → hawk → eagle.

Every food chain starts with a producerAn organism that makes its own food from sunlight (like a plant).. decomposerAn organism that breaks down dead things and recycles nutrients.s connect to every level because they break down dead organisms from all trophic levels and return nutrients to the soil.

Species interactions: who helps whom?

Species in an ecosystem interact in different ways. MCAS tests three types:

Interaction	Species A	Species B	Example
mutualismA relationship where both species benefit (+/+).	Helped (+)	Helped (+)	Bee pollinates flower; bee gets nectar
commensalismA relationship where one species benefits and the other is unaffected (+/0).	Helped (+)	Unaffected (0)	Bird nests in tree; tree is fine
parasitismA relationship where one species benefits and the other is harmed (+/−).	Helped (+)	Harmed (−)	Tick feeds on dog; dog loses blood

Biotic vs abiotic — the MCAS trap

bioticA living factor in an ecosystem (plants, animals, fungi, bacteria). = living. abioticA non-living factor in an ecosystem (water, sunlight, temperature, soil). = non-living. Most of the time this is obvious. But watch for: fungi are biotic (living). They don't move, they don't look like animals, but they are alive. If a question asks "which is NOT abiotic?" and fungi is a choice, pick fungi.

Populations: growth and limits

A populationAll the organisms of one species living in one area at one time. is all the members of one species in one area.

The population change formula:

Change = (births + immigrants) − (deaths + emigrants)

• If births + immigrants > deaths + emigrants → population grows.
• If deaths + emigrants > births + immigrants → population shrinks.
• If they're equal → population stays the same.

Carrying capacity and the S-curve

carrying-capacityThe maximum population size an environment can support long-term. is the maximum population an environment can support. On a graph, you see it as the flat top of an S-curve (logistic growth).

At carrying capacity, birth rate = death rate. The population stops growing — not because organisms stop reproducing, but because deaths balance births.

Limiting factors that determine carrying capacity: food, water, space, predators, disease. If resources decrease, carrying capacity drops and the population may decline.

Homeostasis: keeping the body stable

homeostasisThe body's ability to keep internal conditions stable. is the body's ability to maintain stable internal conditions. Your body has setpoints — target values it tries to maintain. When something pushes a value away from the setpoint, a feedback-loopA system that detects change and responds to correct or amplify it. brings it back.

Feedback loops: negative vs positive

Negative feedback = the most common type. It brings the system back to the setpoint. This is how homeostasisThe body's ability to keep internal conditions stable. works. Think of it as a balancing loop.

Positive feedback = rare. It pushes the system further from the starting point. Example: blood clotting — one clot factor activates more clot factors until the wound is sealed.

Two systems that deliver oxygen

MCAS often asks: "Which TWO systems deliver oxygen to cells?" The answer is always respiratory + circulatory. The respiratory system (lungs) brings oxygen into the body. The circulatory system (heart + blood) delivers it to every cell.

The carbon cycle

Carbon cycles through ecosystems:

1. producerAn organism that makes its own food from sunlight (like a plant).s absorb CO₂ from the air during photosynthesis.
2. consumerAn organism that gets energy by eating other organisms.s eat producers and use the carbon.
3. decomposerAn organism that breaks down dead things and recycles nutrients.s break down dead organisms and release carbon back to the soil and air.
4. Cellular respiration by all organisms releases CO₂ back to the atmosphere.

Key point: energy flows one way (sun → producers → consumers → lost as heat). Matter cycles (carbon, nitrogen, water move in loops).

Why this matters for MCAS

Ecology and population questions make up about 30% of MCAS Biology (Reporting Categories 3 and 4 combined). The most common question types:

1. "Food web arrows point from ___?" → from eaten to eater.
2. "Loss of producers → what happens?" → all consumers decrease.
3. "Which is NOT abiotic?" → fungi (living!).
4. "Population rising — why?" → births + immigrants > deaths + emigrants.
5. "What maintains stability?" → negative feedback / homeostasis.
6. "Two systems deliver oxygen?" → respiratory + circulatory.
7. "Two species, one helped, one unaffected?" → commensalism.