Primary consumers in a food chain are ________.

Energy enters ecosystems through photosynthesis (producers fix ~1-5% of solar energy). Flows through trophic levels via feeding. Lindeman's Law (10% rule): ~10% transferred per trophic level. Ecological pyramids show: pyramid of numbers (can be inverted), pyramid of biomass (usually upright), pyramid of energy (always upright — never inverted).

Grazing food chain: starts with living green plants. Detritus food chain: starts with dead organic matter. Most energy in terrestrial ecosystems flows through the detritus chain. Aquatic: more energy through grazing chain. Both chains are interconnected in a food web.

Grassland food web: Grass → Grasshopper → Frog → Snake → Eagle. Trophic levels: T1=Grass, T2=Grasshopper, T3=Frog, T4=Snake, T5=Eagle. Pond: Phytoplankton → Zooplankton → Small fish → Large fish → Osprey. Sea: Seaweed → Sea urchin → Starfish → Sea otter.

Persistent chemicals (DDT, PCBs, mercury) accumulate in organisms' fat tissue (bioaccumulation). Concentration INCREASES at higher trophic levels (biomagnification). Classic: DDT → thinning of bird eggshells (especially eagles, pelicans, ospreys). Fish → human mercury toxicity from industrial pollution. Top predators most affected.

Energy enters an ecosystem as sunlight, is fixed by producers, and passes up the trophic levels — but it never returns. At each transfer most of it is lost as heat during respiration, and heat cannot be re-used to do biological work. This is a direct consequence of the second law of thermodynamics, and it is why an ecosystem needs a continuous external energy input while nutrients can simply cycle round.

The grazing food chain starts with living producers and runs through herbivores to carnivores. The detritus food chain starts with dead organic matter and runs through decomposers and detritivores. In a terrestrial ecosystem the detritus chain actually carries far more of the total energy flow than the grazing chain, which surprises students who assume grazing dominates. The two are linked, since detritivores can themselves be eaten.

Standing crop is the biomass present at a given moment, while productivity is the rate at which it is produced. Gross primary productivity is the total energy fixed by photosynthesis; net primary productivity is what remains after the producers' own respiration, and only NPP is available to the next trophic level. Secondary productivity is the rate at which consumers build new organic matter. A question asking what is "available to herbivores" is asking for NPP, never GPP.