One of the reasons for any organism to be a simultaneous hermaphrodite is that they may live in habitats/areas where it's difficult to find a mate/the gender ratio is skewed toward one gender/the organisms are sessile and cannot move to find a mate of the opposite gender. The ability of a species to develop hermaphroditism allows any two individuals that come into contact to mate.
For sequential hermaphroditism, there's usually a benefit to being one gender or the other (larger size for protecting the smaller members of the opposite gender, able to produce more eggs).
Unfortunately, I don't think there's any one condition/niche that works for all hermaphroditic species.
Plants have tissues to transport water, nutrients and minerals. Xylem transports water and mineral salts from the roots up to other parts of the plant, while phloem transports sucrose and amino acids between the leaves and other parts of the plant.
If there wasn't a lot of organisms and energy in the productive part of the triangle, then there wouldn't be a lot of third level organisms. The energy piramid would be a lot smaller then what it is here, and maybe if the energy source was low then there might not be such a vast varity of animals in the ecosystem.
