Some nutrients are lost by erosion from overland flow, leaching from percolating soil water, or harvesting crops. Since energy conversions are never entirely efficient, heat is generated at all steps along the way. In an ecosystem, metabolic energy is dissipated as heat energy. The inefficiency of energy conversions throughout the food web produces a biomass pyramid . 1000 lbs of green plants can support about 100 lbs of herbivores, which can support 10 lbs of carnivores, etc.
Top carnivores are rare animals in any ecosystem.
They need to forage over large areas to harvest enough food to survive, so numbers and density remain low.