The following diagram shows an oceanic food web. At what trophic level do the killer whales primarily feed?

The killer whales primarily feed on the secondary consumers in the oceanic food web.

Explanation:

Killer whales are the tertiary consumers and occupy the highest position in the ocean food web. They are predating carnivores and feed on secondary consumers like other fishes, marine animals like leopard seals, sea turtles, dolphins, sea birds, etc. These secondary consumers in turn feed on primary consumers like zooplanktons, small fishes etc which feed on the primary producers like phytoplanktons. The energy flow is highest at the primary producer level and decreases as it goes up to the consumer levels.

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

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