Countdown #3: Competition selects for the sub-optimal

A typical eucalyptus: lots of wood, not a lot of leaves. Why aren't more plants this efficient? 

In economic and ecological theory, selection for efficiency among competing firms or organisms is a ruling paradigm. Yet, competition, among firms or plants, does not always select for the most efficient. All one needs to see this in nature is to take a shovel to their front lawn or take a walk in a forest.

When nutrients are limiting, a plant competes for a limiting nutrient supply by attempting to preempt the nutrient supply from other plants. As such, the key to acquiring the majority of a given nutrient supply is root length dominance. A consequence of supply preemption being the mechanism of competition for nutrients and root length dominance the key to preempting nutrient supplies is that plants face an evolutionary tragedy of the commons in their allocation patterns to roots. When the nutrient supply is limiting and a given plant is grown in isolation, a relatively low root length density optimizes growth. Yet, in the presence of competitors, plants that can maintain higher root length densities than are optimal in the absence of competition are able to acquire a larger fraction of the total nutrient supply and therefore would have been favored by natural selection. As such, high root length densities are more evolutionarily stable than the lower root length densities that would optimize growth in the absence of competition. This easy to see by taking a shovel to most temperate lawns. Most temperate grasses appear to have an order of magnitude more roots than is optimal for growth in the absence of competition.

The supply of light differs fundamentally from nutrient supplies in that it is largely supplied directionally. Yet, competition for light can select for plants that have canopies that are suboptimal in the absence of competition. The trunks of trees do not help a forest acquire more light—they only help an individual acquire more light than a competitor. As I discuss in RSWP, research has also shown that many plants also hold more leaves than is optimal and the leaves are held too flat. Plants that are optimizing canopy photosynthesis would hold their leaves at a higher angle in order to allow more light to penetrate deeper into the canopy where it can be used more efficiently. What would a tree with an efficient canopy look like? Probably something like a eucalyptus with its pendulous leaves and sparse canopies. It probably is no coincidence that eucalyptus are known by foresters to produce wood at some of the highest rates.

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.