If having large numbers of offspring results in high fitness, why don't all females have very large numbers of offspring?

"They are limited by the amount of energy they can devote to reproduction in any one season." is the best answer. 

Having children comes with a cost, lowering the female's survival skills due to time and energy spent in raising offspring. A trade-off between reproduction, longevity and parental investment exists.

Depending on the ecosystem, various things could happen. For example, if you introduced it to a forest/mountain area (Northern USA) a large cycle would change. Within 15 years, the mountain lions would prey on deer, mice and rabbits. The decrease in those populations would lead to an increase in plant wildlife. The variety in new plants could bring new animals like wild bison or even beavers.

Hope this helped.