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.

Countdown to Publication: #5—Co-limitation in a Post-Liebigian world

With RSWP scheduled for publication in less than a month (at least that is what Amazon tells me), I thought it would be interesting to highlight some of what I think are the key advances of the book. A self-promoter might call this “Countdown to a New Paradigm”, I’ll just call it Countdown to Publication, I guess. [As an aside, if you ever want to promote your career, start an intellectual battle with someone on a different continent and agree not to attempt to resolve your differences. It works great every time.] I do think #5 represents a new paradigm. Not a paradigm I should be credited with, but a fundamental change in how we think of resource limitation.

The Law of the Minimum that describes the basic ecological concept of limitation was established at a time when little was known about nutrient cycling and was not applied initially to limitation by light. The three parts of Liebig’s Law of the Minimum are

1) Growth is limited by the resource that is supplied at the lowest rate relative to the demands of the plant.
2) Growth is proportional to the rate of supply of the most limiting resource.
3) Growth cannot be increased by increasing the supply of a non-limiting nutrient.

In the Liebigian world of limitation, generally only one nutrient could be limiting at a time. In a post-Liebigian world, it is likely that there is co-limitation to growth among different resources, not just serial limitation. Co-limitation is more likely to occur than would be predicted from Liebig’s Law of the Minimum because a) nutrients supplied in excess are more likely to be lost, b) species differ in the stoichiometry of demands for optimal growth, c) plants can store resources to balance temporally variable rates of supply, and d) plants can increase the availability of the most limiting resource. If there are tradeoffs in acquisition between different allocation strategies, whether for increasing supplies or acquiring a greater fraction of a given supply, co-limitation is likely to occur. Nutrients and light. Water and CO2. Nitrogen and phosphorus. I expand on this in the book, but these co-limitations fundamentally alter our approach to understand everything from global change to evolution.

The existence of co-limitation is not that novel, nor even the suggestion that it should be common. Yet, the consequences of this focus are reach far. Resource co-limitation in the post-Liebig world does not only manifest itself in responses in productivity to the addition of two resources with no increases if only one resource is added. Due to allocational tradeoffs, plants can be co-limited by multiple resources and respond to individual resources independently. Also, the only way to understand why one resource is limiting is by understanding which resource is co-limiting with it.

When resources are co-limiting, costs for different allocation strategies or the production of different structures should be evaluated with a dual-currency model. Evaluating costs with just one resource ignores the often great cost of the second resource. There is no money in ecology—it’s a bartering world with a few key staples.

Lastly, natural selection favors plants that adjust their allocation strategies such that multiple resources are co-limiting. As such, plants are also selected to balance their resource use efficiencies to match resource supply ratios. For example, plants that grow with high relative limitation of nitrogen should have higher ratios of nutrient to light use efficiencies than plants that grow in environments with greater relative limitation by light. Resource limitation provides a fundamental constraint on efficiency of resource use that makes the world predictable.

Colimitation indices

Resource limitation is a fundamental structuring force in ecosystems. I've written a lot about how we need better ways to analyze the simple factorial experiments that are used to determine limitation.  Boiling down patterns of limitation to a single number or two is going to be a big help understanding broader patterns of limitation. Below, I introduce a colimitation index, that separates out some key patterns. 

Let’s start with a basic limitation experiment. 4 treatments: Control (0), +N, +P, +NP.

B₀ is biomass of controls

B_N is biomass with +N

B_P is biomass with +P

B_NP is biomass with +N+P

To make matters simple, let’s assume that there is no effect of P added alone and that the biomass of plants with N and P added is greater than unfertilized biomass.

With this, we’re trying to separate three cases.

1) Classic co-limitation (co-limitation by supply) where there is no direct effect of N (or a relatively small one).

2) Primary limitation by N and secondary limitation by P

3) Single resource limitation where there is an N effect, but no effect of P on N-fertilized plants.

There are two ways to calculate a co-limitation index (CI) based on absolute or relative changes in biomass.

First we can compare the absolute changes in biomass relative to controls and compare the N effect to the NP effect. If we calculate the co-limitation index as

(BN-B0)-(BNP-B0)/2

then a plant that had a CI > 0 would be primarily limited by N and secondarily by P. A CI <>

The interpretation of the pattern would be that soils with low P availability are co-limited by N and P. As P availability increases plants are more likely to be primarily limited by N and secondarily limited by P.

That’s a pretty clean story, but the problem with this approach is that you cannot separate if a plant is secondarily limited by P or just limited by N.

To get around this we can calculate a co-limitation index as:

(BN-B0)/(BNP-B0)

With this index, 0≤CI<0.5><1> 

Here are the patterns for the 100 soils data:

Pretty much the same story. Plants start out co-limited by N and P at low P availability. Then as P availability increases P limitation becomes more secondary until ~30 ppm available P at which we’re into strict single limitation by N. The problem with this approach is that the relative index is sensitive to BNP. For the graph at left I had to exclude two points that had |CI|>10.  

With these approaches statistical significance relative to threshold values, e.g. CI = 0 for the first index, are possible. I’m not sure how to extract them from JMP yet.

Note with a factorial resource addition experiment there are something like 9 different basic responses when you include inhibition and responses by individual resources. There will be no way to boil, but we might be able to get the most important patterns down to 2.

 

Arbuscular mycorrhizal fungi and nitrogen acquisition

How plants acquire nitrogen is one of the fundamental questions in understanding plant adaptations to low nutrient availability. For many years, plants were understood to associate with mycorrhizal fungi in order to acquire P. Later it was understood that some types of mycorrhizal fungi (ectomycorrhizal and ericoid) acquired organic N and transferred that to the host plant. Yet, for many years it was unknown whether arbuscular mycorrhizal fungi were important in the N nutrition of plants.

Leigh, Hodge, and Fitter (2009) take a big step in showing that arbuscular mycorrhizal fungi acquire N from the soil and transfer the N to plants. Building on earlier work by Hodge, the authors grew Plantago lanceolata in microcosms with two compartments. In some, plant roots had access to the second compartment which contained 13C and 15N enriched shoots. In others, only arbuscular mycorrhizal fungi could access the compartment. The authors show that the arbuscular mycorrhizal access inorganic N derived from the shoot material and transfer a large fraction of it to the plants. In one case, ~20% of the plant’s N came from the AM fungi.

In showing the ability of AM to acquire N and transfer large amounts of it to the plant, the study is an important one. The degree to which this potential is ecologically important remains to be seen. In this study, the AM fungi had access to a large, N-rich patch that the roots did not. In nature, it doesn't seem too often that an N-starved plant find's itself on the other side of a goretex barrier from a rotting carcass. Yet, if there ever is a race for a patch, can AM fungi find it before roots? Do they proliferate and therefore compete better than roots? Can they access small, rich patches that roots cannot? The experiment by Leigh, Hodge, and Fitter is an important step in our understanding. The next experiment might be the really important step.

Leigh, Hodge, and Fitter. 2009. “Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material” New Phytologist, 181: 199-207.