Looking at one issue of overpopulation can illustrate this, the issue of human sewage. (This would also apply to the overpopulation of any species.)
The planet evolved into systems that recycle the waste from each/all species; recycling it and using it to regenerate the soil, providing a continuation of food sources.
Human waste is not the ideal fertilizer, lacking in the amounts of necessary nutrients, bc of our heavily laden meat diets.
This is one reason meat isn't added to home compost systems.
For every living thing that excretes waste, there are natural systems that evolved to recycle waste back into the environment in a time consuming manner.
Humans invented methods of recycling our waste, which mostly worked for a time.
But now, with 8 billion ppl pooping and peeing huge quantities every minute of every day, even our invented systems are overwhelmed, and raw sewage is regularly being released into the environment by cities and towns across the globe.
This is unsanitary, carrying it's own health threats, as well as leading to an unhealthy environment all around.
These threats would be far lesser, and more easily managed with fewer humans on the planet.