Question

Within any ecosystem, there is a fixed amount of energy available for organisms to use. Since organisms usually produce more offspring than a given ecosystem can support, this causes _______

Answer 1

Competition

For organisms are going to fight for the limited food or resource

Answer 2

I'm going to say carrying capacity since carrying capacity is the maximum amount of organisms that can live in an ecosystem. So the reason for there being a maximum of organisms is because, like the question says, organisms usually produce more offspring than the ecosystem can support.