What prevents predator overpopulation?

Question

I've often heard that a population, human or otherwise, will continue to grow as long as there is food available (assuming nothing else is killing them off). It makes sense: if you have food you can live, and if nothing is hunting you you'll survive to reproduce. I recently designed a piece of software to simulate an ecosystem, with groups of creatures of different species eating and hunting and reproducing alongside each other. It was very simplified (each animal had simple attack/defense/speed/stealth values, etc), but something became rapidly apparent: in every simulation the predators overwhelmed the prey, reproducing until their numbers could not be sustained by the herbivores, and leading to an inevitable die-off of both groups. I could delay the die-off by adjusting different values and initial population counts, but it would always happen eventually. The predators would eat and breed and eat and breed until the entire system collapsed. At first I thought it was just the product of my over-simplified system, but it got me thinking: **what prevents predators from overpopulating in real life?** It seems like the natural tendency would be for (for example) the sharks to continue breeding and eating until all the fish are gone, or the wolves to eat all the deer, etc. Obviously some predators have predators of their own, but that's just putting off the question: if the hyenas don't overpopulate because the lions eat them, then what's keeping the lions from overpopulating? I can't come up with anything that would prevent the apex predators from growing too numerous, then fighting each other over a dwindling prey population, then dying off entirely when there was no more food to find. Do predator populations self-regulate to prevent putting undo stress on their prey populations? Or is there some other mechanism to keep the predator hierarchy from becoming top-heavy?

Answer 1

Hello,

So there is a lot to this question, "What prevents predators from overpopulating in real life?"

Firstly, in your model you have oversimplified the relationships in the ecosystem. Like you pointed out, great for the software, but missing many interactions that you would normally have in an ecosystem. One predator does not just have one prey, and one prey does not have just one predator. In your software you are looking at one part of a food chain and missing the food web aspect of the ecosystem. Thus, a multitude of feeding relationships are normally occurring which limit population sizes. Furthermore, limiting factors (water, space, etc.) were not taken into account in your model. These also limit apex species (and all populations) as they compete with each other for more than food (mates, territory).

Secondly, when you start to think about apex species you also want to look at animal behavior. Think about a pride of lions and the hierarchy and social interactions between them that could limit population size. And the fact that they reproduce sexually adds genetic diversity into the mix, where some individuals of a population will be inherently stronger than others, and thus their ability to compete will be stronger. This too, limits population size. Also, more young will be born than survive, and I am not sure in your model if you are taking this into account. Also, have you added in a death rate into your simulation? Populations only grow when birth rate exceeds the death rate. Furthermore, looking at biomass, there is significant energy loss as you move up in trophic levels, this also limits population size in apex species.

Finally, I think it is important to also look at dynamic equilibrium. Populations will fluctuate around the carrying capacity of an ecosystem/environment. If food is abundant then the population can grow, but once there is cause for competition it will drop down again (due to limiting factors). I am not sure if in your model your populations are going extinct, or if they are just decreasing. If they are decreasing this is normal.

In any case, there are just a ton of variables that contribute to population size beyond food availability. I think you have a pretty cool tool with your software to model a predator/prey relationship, and I hope some of the topics I brought up help you think about population dynamics in a new light!