Indigenous communities rely extensively on plants for food, shelter, and medicine. It is still unknown, however, to what degree their survival is jeopardized by the loss of either plant species or knowledge about their services. To fill this gap, here we introduce indigenous knowledge networks describing the wisdom of indigenous people on plant species and the services they provide. Our results across 57 Neotropical communities show that cultural heritage is as important as plants for preserving indigenous knowledge both locally and regionally. Indeed, knowledge networks collapse as fast when plant species are driven extinct as when cultural diffusion, either within or among communities, is lost. But it is the joint loss of plant species and knowledge that erodes these networks at a much higher rate. Our findings pave the road toward integrative policies that recognize more explicitly the inseparable links between cultural and biological heritage.