Countdown #4—competition and supply preemption

Dave Tilman’s great advance for understanding competition over 30 years ago was to introduce the idea that competitive outcomes were determined by resource reduction. Phytoplankton that could lower the concentration of a limiting nutrient to the lowest level would become competitively superior. When applied to plants in soil, the concentration reduction hypothesis assumed that soil solutions were well-mixed and it was the average soil solution concentration that determined the rate at which a plant grew. As such, lowering the average soil solution concentration of a limiting nutrient was the key to displacing other species.

Leaving aside the assumptions of the nature of the nutrient supply, for plants in soil, resource reduction is still the mechanism for competitive displacement. Yet, it’s not concentration reduction, but supply preemption that determines competitive superiority. When nutrients are limiting, a plant competes for a limiting nutrient supply by attempting to preempt the nutrient supply from other plants. Due to the relatively slow diffusion of nutrients in soil, as roots acquire nutrients from the soil solution, nutrient supplies are partitioned among plants based on the relative amount of root length they hold in a particular soil volume. The key to acquiring the majority of a given nutrient supply is root length dominance, which reduces the availability of nutrients to others.

Although the magnitude of this conceptual shift is open, supply preemption is the proper application of resource reduction to plants growing in soil. What about other resources? As I discuss in RSWP, supply preemption is the best concept for understanding light competition, too. Water is notoriously pulsed in availability, but supply preemption rules here, too. Just a bit different than nutrients that are supplied evenly over time.

Competition research can be summarized as “who wins and why?” The secret to competing for resources is to get them before your neighbors do. Took about 30 years to nail down how that works on land, and it'll probably take another few decades likely to nail down the details.