Monday, April 27, 2009

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.

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