15. which organism in this food web has the greatest influence on the ecosystem?

Secondary consumers

Explanation:

With out secondary consumers there would be nothing for primary consumers to eat which will lead to the whole ecosystem dying out.

plants

Explanation:

This is a really tough one to answer. In reality, they are all equally important, and time should be spent going over all possibilities.  Most students will guess plants since all life depends on the energy produced by plants.  The teacher should also point out that, if a top-level consumer is killed off, the organisms it eats will increase in population, which might mean that more grass will be eaten, etc.  All attempts should be made to show the interdependence of organisms in an ecosystem and to show that they are, therefore, all important.

could you go a bit more in depth with you question? it's quite hard to understand. as soon as you do that i will be glad to : )