500 because of the 10% rule when moving up in food chains, for example; patch of grass-200J rabbit eats some of it but doesn’t eat all - 150J the rabbit runs around for a bit and loses energy, the fox doesn’t eat all of the rabbit - 50J ( just a rough example may not be completely right but the concept of the rule is there and the energy).
Very detailed and over documentation of what they were doing, their results, the outputs. And noting on everything that was going on and who was there as well.
A leech is an example of a parasite