Overpopulation of these primary consumers would cause a shortage of their food source, or primary producers. This means there could be endangerment of the food source, plants, and increased competition among the consumers. This would impact humans in that there could be less plant food sources, and yet a surplus of deer and other rodents (which holds some positives and negative results). This is why it is important to have consistent balance in an ecosystem.
I haven’t had biology in about two years but I’m confident in the answer and hope it’s enough.