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How would an ecosystem comprising of grass land be effected if all carnivores were removed?
Question Date: 2013-12-13
Answer 1:

Good question - ecologists have been puzzling over this one for decades.

A common theory in ecology is that carnivores limit the population of herbivores, thereby allowing the plants to grow freely without being eaten. This works as long as there are carnivores in the ecosystem, and only one level of carnivores; if there are super-carnivores that eat other carnivores, then now the carnivores that eat herbivores are reduced, herbivores can eat freely, and the plants are in trouble. The same happens if your remove the carnivores altogether, or so goes the theory.

The problem with this theory is that it only works when you think about large mammals as the only animals in the ecosystem. There are many rodent and insect herbivores, too, and they live in a much more complicated system ecologically, and there are multiple levels of carnivores: for instance, aphids (herbivores) are preyed upon by ladybugs (carnivores), which are themselves eaten by spiders (super-carnivores), which are eaten by songbirds (super-super-carnivores), which are eaten by hawks (super-super-super-carnivores). Those same hawks also eat rodents (herbivores) and snakes (carnivores), of which the snakes eat the rodents as well.

Another theory that attempts to explain how there are still plant-dominated ecosystems is that plants are basically not good food because they contain so little nitrogen, since they're mostly cellulose which is poor in nitrogen (and unlike protein, which is what animal flesh is mostly composed of). The suggestion goes that plants just aren't nutritious enough to support a large enough population of herbivores that they could actually eat all of the plants, except in a few, exceptional, short-lived cases such as locust swarms.

The problem here is that even if cellulose doesn't have much nitrogen, it still contains carbon, and thus energy, which animals can still use. Moreover, what about all of the fungi that also eat plants? Not much eats fungi - they're made out of chitin, which is even worse nutrition-wise than cellulose. Why haven't molds and mushrooms rotted out every land ecosystem in the world?

There's a lot that we don't know yet, clearly.

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