This has to do with the fact that at every step in the food chain only 10 percent of the available energy gets passed on (starting with the energy of the sun incident on the plants).
(only 10 percent of the available energy; gets; passed on (starting with the energy of the sun incident on the plants) (only 10 percent of the available energy; gets passed on; )
only 10 percent of the available energy only 10 percent of the available energy
gets gets passed on
passed on (starting with the energy of the sun incident on the plants