Have you ever wondered why prairies thrive without any human intervention? No pesticides, no fertilizer, no irrigation, and yet they come back year after year, come drought or flood. The soil remains healthy, full of nutrients and swarming with organisms.
Simply put, it’s the complexity. Living things survive by taking advantage of resources as soon as they become available. A bared patch of soil doesn’t stay that way for long. (Reserving it for just one plant species – say, corn – takes a lot of chemicals and tillage.) In time, plants and organisms colonize the area, essentially divvying up available resources.
The majority of the plants will be perennials; they have more extensive root systems than annuals, and so can reach deeper for water and nutrients. They also tend to out-compete annuals for sunlight because they are active for more days every year.
They form a complex ecosystem that is radically different from a field planted with an annual crop. Consider rainfall. Raindrops that strike the bare earth of single-crop fields break up the soil, encouraging erosion and crusting. If the field is not entirely flat, lack of vegetation allows surface water to flow more freely, eroding topsoil. Shallow root systems fail to bind saturated soil.
By contrast, a pasture or prairie is covered by perennial plants that absorb the impact of raindrops. They form a physical barrier that slows the flow of surface water, giving it more time to be absorbed by the soil. And the extensive root systems of perennials not only hold the soil better, they also keep it looser, allowing greater infiltration of water.
A less obvious, more complicated characteristic of polycultures is the resilience and fertility that emerge with the complexity. This web of checks and balances, predator and prey that make up complex ecosystems make it difficult for any single species to dominate. Instead, a self-regulating equilibrium sets in.
It is this quality that gives prairies and rain forests their resilience. When this complexity is destroyed (think, plowed) other measures – chemicals, energy – must be used.