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 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
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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