Notes
Levels and factors
Organism → population → community → ecosystem → biome → biosphere. Ecosystems include biotic (living) and abiotic (nonliving) factors. A niche is an organism's role.
Energy flow
- Sunlight → producers → primary consumers → secondary consumers → tertiary consumers. Decomposers recycle nutrients.
- Only about 10% of energy passes to the next level; the rest is lost mostly as heat.
- Toxins biomagnify up the food chain.
Cycles of matter
Water, carbon, nitrogen and phosphorus are recycled. Photosynthesis removes $\text{CO}_2$; respiration, decomposition and burning fossil fuels add it. Nitrogen-fixing bacteria make nitrogen usable.
Populations
- Exponential (J-curve) vs logistic (S-curve) growth. Logistic growth levels off at carrying capacity.
- Density-dependent limits: food, disease, competition, predation. Density-independent: fires, floods, weather.
Interactions
| Relationship | Species 1 | Species 2 |
|---|---|---|
| Mutualism | + | + |
| Commensalism | + | 0 |
| Parasitism | + | − |
| Predation | + | − |
Succession: primary (bare rock, starts with lichens) and secondary (soil remains).
Human impact
Habitat loss, pollution, invasive species, overharvesting and climate change reduce biodiversity. Conservation helps protect ecosystems.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Removing a predator lets its prey population grow.
Each level supports less biomass because most energy is lost as heat.
Toxins become more concentrated at each higher trophic level.
At carrying capacity the population levels off.
One species benefits and the other is unaffected.
Lichens are pioneer species in primary succession; they begin forming soil.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Scientists counted a population of moose on an island over 40 years. The population grew from 20 to 1,000 in 15 years, then crashed to 400 during a hard winter with deep snow. Wolves arrived later, and the moose population then went up and down in cycles with the wolf population.
(a) Describe the type of growth the moose showed in the first 15 years and explain why it happened.
Show the answer and scoring guide
The moose showed rapid, exponential growth. With plenty of food and no predators, births greatly outnumbered deaths.
Check off the points you earned:
(b) Classify the hard winter and the wolves as density-dependent or density-independent limiting factors. Explain one.
Show the answer and scoring guide
The hard winter is density-independent, because deep snow affects moose no matter how many there are. The wolves are density-dependent, because predation has a bigger effect as the moose population becomes larger and easier to find.
Check off the points you earned:
(c) Explain why the wolf population rises and falls a little after the moose population.
Show the answer and scoring guide
When moose are plentiful, wolves have more food and their numbers increase. More wolves then eat more moose, so moose decline. With fewer moose, wolves starve or have fewer pups, and their numbers fall, which lets moose recover.
Check off the points you earned: