If there is one central element to RSWP, it is not about how to quantify importance, the nature of resource limitation, or the mechanisms of competition. These fundamental questions all serve a higher purpose in the book: to understand the resource strategies of wild plants.
The broadest theories of plant strategies have differed on whether there was a common general strategy to succeeding when resources were low, or whether there were fundamentally different strategies associated with success when different resources are low. As I describe in RSWP, the most parsimonious conclusion is that there are four major strategies for growth when limiting resources are supplied uniformly over time. There is less support for the theory of a general “low-resource” strategy with variations associated with limitation by different resources than separate strategies for succeeding when water, nutrients, light, or CO2 are strongly limiting. The availabilities of resources are independent enough from one another and there are physiological and evolutionary tradeoffs in producing traits for success for each resource availability. Consequently, there is no one general strategy that covers low availability of all resources. Being built to perform well under low light precludes being competitive for nutrients which precludes acquiring water when soil water potential is low. All of these strategies might share a low maximal relative growth rate, but this appears to be a consequence of convergence. A fifth strategy is associated with success when the availabilities of all resources are high.
The five strategies I outline in RSWP are the most fundamental and widespread with regards to resources, yet it is important to recognize that no one set of traits works best across all environments that share having low availability of a given resource. For example, although both limited by nutrients, phosphorus limitation in the fynbos of South Africa has selected for plants that fundamentally different from those that dominate nitrogen-limited grasslands in Minnesota. How the multitude of environmental stresses and disturbances have shaped the world's flora are some of the most subtle questions about the forces that make our complex world beautiful. Yet, the skill of the ecologist is to appreciate the complex while seeking the simple. When we collapse the diversity of the world into its most fundamental units, we are left with the five resource strategies of wild plants.