Consider a closed ecosystem which contains only mosquitoes and frogs. the mosquitoes feed on the blood of the frogs, and the frogs eat the mosquitoes. this ecosystem is impossible because:

It is impossible as there is a loss of energy between the transfer of energy between flogs and mosquitoes. This ecosystem would never be sustainable or in equilibrium. There is also no primary producer within this ecosystem. No ecosystem can exist without some initial source of energy that would be obtained using solar or chemical energy. For terrestrial ecosystems, plant primary producers would normally provide the initial energy into the ecosystem.