Ecosystems & Nutrient 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 N 2 gas into forms that plants can use Through a series of conversions by specialized bacteria, N 2 is converted into nitrates Plants take in nitrates through assimilation