What determines the carrying capacity of an ecosystem is simply a number out of the population in the ecosystem; whether is be a quarter of it, half of it, all of it, it's a population that can be supported by the the needs of the organisms living in the ecosystem. Say "food", say there is over 300,000 fish in a swamp with 300 gators. Each gator eats 2000 fish a year. There is not enough fish to feed all 300 gators. So let's say the carrying capacity of this ecosystem is 200.