Notes
Responses to the environment
Organisms respond to changes with behavior and physiology: taxis and kinesis, migration, hibernation, and communication by signals (visual, audible, chemical like pheromones, tactile). Cooperative behaviors such as altruism can increase the fitness of relatives.
Energy flow
- Endotherms use metabolic heat to keep body temperature steady; ectotherms rely on the environment. Smaller endotherms have higher metabolic rates per unit mass.
- Producers capture energy; consumers eat them. Only about 10% of energy passes from one trophic level to the next; most is lost as heat from respiration.
- Gross primary productivity is total photosynthesis; net primary productivity = GPP − energy used for respiration.
Biogeochemical cycles
- Carbon: photosynthesis removes $\mathrm{CO_2}$; respiration, decomposition and burning fossil fuels return it.
- Nitrogen: nitrogen-fixing bacteria convert $\mathrm{N_2}$ into ammonia that plants can use; nitrification and denitrification move it further.
- Phosphorus: no atmospheric form; it comes from weathering rock and is often a limiting nutrient.
- Water: evaporation, transpiration, condensation and precipitation.
Populations
- Exponential growth: $\displaystyle \frac{dN}{dt} = r_{max}N$, a J-shaped curve when resources are unlimited.
- Logistic growth: $\displaystyle \frac{dN}{dt} = r_{max}N\left(\frac{K - N}{K}\right)$, an S-shaped curve that levels off at carrying capacity $K$.
- Density-dependent factors (disease, competition) hit harder as density rises; density-independent factors (natural disasters, weather) don't depend on density.
Communities and ecosystems
- Interactions: competition (−/−), predation and herbivory (+/−), mutualism (+/+), commensalism (+/0), parasitism (+/−).
- Simpson's diversity index $\displaystyle D = 1 - \sum\left(\frac{n}{N}\right)^2$: closer to 1 means more diverse.
- Keystone species have a big effect relative to their numbers. Ecosystems with more diversity are usually more resilient to disturbances.
- Human impacts: invasive species, habitat loss, climate change, and eutrophication from fertilizer runoff.
Topics in this unit
Each topic has its own short notes and practice.
Practice
10% to primary consumers (2,000), then 10% of that to secondary (200).
Nitrogen fixation turns $\mathrm{N_2}$ into ammonia.
Logistic growth levels off at $K$.
Disease spreads faster in dense populations.
Both species benefit.
Their removal has a huge effect on the whole community.
This is eutrophication: decomposition lowers oxygen.
Diverse ecosystems have other species to fill the role.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A population of deer on an island starts at 50, and the island's carrying capacity is 500. The maximum per capita growth rate is $r_{max} = 0.4$ per year.
(a) Calculate the population's growth rate $\frac{dN}{dt}$ when $N = 250$.
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$\displaystyle \frac{dN}{dt} = 0.4(250)\left(\frac{500 - 250}{500}\right) = 100(0.5) = 50$ deer per year.
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(b) Describe what happens to the growth rate as $N$ approaches 500. Explain.
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It approaches 0. Resources like food and space become limited, so births and deaths balance near carrying capacity.
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(c) Wolves are introduced to the island. Predict the effect on the deer population and on the plants the deer eat.
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The deer population would decrease due to predation, so pressure on the plants would decrease and plant populations would likely increase.
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