Ecosystems & Ecosystems & Nutrient CyclesNutrient Cycles

AP Biology

Trophic Relationships

A Review of Trophic Levels• Primary Producers = Autotrophs create

their own energy (through photosynthesis or chemical methods)

• Primary Consumers = Herbivores, Heterotrophs (can’t make own energy)

• Secondary & Tertiary Consumers = Heterotrophs, Carnivores

• Detritivores/Decomposers = get nutrients from nonliving organic material (detritus)

Ecosystem Energetics

• Gross Primary Productivity = Total amount of light energy converted to chemical energy in a fixed time (by photosynthesis)

• Net Primary Productivity = Amount of chemical energy actually available to pass on to consumers (accounts for energy lost through cellular respiration)

• Net Primary Productivity varies between ecosystems• Net Primary Productivity depends on nutrients available

Secondary Productivity

• the rate at which consumers convert the chemical energy of the food they eat into their own biomass - Biomass = mass of living tissue (dry weight)

- Limited by Net Primary Production

Transfer of energy between trophic levels

• A considerable amount of energy is lost between trophic levels– Energy used up in

respiration– Energy released in

feces

• Usually, about 10% is transferred between levels

Biomass pyramids

• Most pyramids narrow rapidly because energy transfer across levels is so poor– Cannot support large

number of organisms at higher levels

• Why would one pyramid be upside down?

• Because producers are consumed too quickly to ever develop a large population size

Green World Hypothesis• Why don’t all the herbivores eat all the

green plants?

• Herbivore populations are kept in check by– Plant defenses against herbivory– Limited nutrients supplied by plants– Unfavorable environmental conditions/changes– Competition within the species – Other interactions like predation, parasites,

disease.

Nitrogen Cycle

• Nitrogen cycle converts N2 gas into forms that plants can use

• Through a series of conversions by specialized bacteria, N2 is converted into nitrates

• Plants take in nitrates through assimilation