Why aren’t there rabbits everywhere?

Question

I live in a small city in the US, where it’s grass everywhere. There’s lots of rabbits, but why aren’t there more? They eat grass, and there’s clearly more grass than they can eat at their current population size. There’s no significant predators to speak of, I don’t think. They breed legendarily quickly, there’s even an expression about it. So if food isn’t a constraint, predators aren’t a constraint, what is the constraint? I would think they should just increase population until we don’t have to cut our grass anymore.

In: Biology

Answer 1

This actually reflects a really deep and interesting ecological question: to sum up, why is the world green?

Here’s a review paper on the topic which may not help much because i’m on ELI5 not askscience

[Full article: Why is the world green? The interactions of top–down and bottom–up processes in terrestrial vegetation ecology (tandfonline.com)](https://www.tandfonline.com/doi/full/10.1080/17550874.2016.1178353)

Anyway, to *heavily* sum up, it’s not immediately obvious why herbivores (all herbivores, not just rabbits) don’t increase until most green vegetation gets eaten not long after it grows. This is kind of how things work in the ocean, after all. Algae normally gets eaten as fast as it grows.

But there are several possible reasons, all of which probably play a role. First off, and perhaps most importantly, plants are not undefended. Plants contain all sorts of toxins and poisons and spines and indigestible matter than makes eating them time consuming and costly for herbivores. A field of grass may look all the same to you, but a rabbit is going to be focusing on certain plants and certain parts of leaves to maximize nutrition vs digestion effort. If there are too many rabbits, the food available won’t be as high quality. Second, predators and disease play a role. The more dense rabbits get, the more their predators and disease populations grow, which in turn limits the number of rabbits. Finally, herbivores can be limited by other environmental factors. So for example, rabbits want to dig burrows, but not all ground is equally suited for that.