Wednesday, December 4, 2013

How many species for a model species set?

Angela Moles and I were discussing how many species would be necessary for a model species set.

Think of the model species set as the Arabidopsis of plant functional traits. Just like with a model organism, if we restrict the species that we work with, we can build synergy faster for different types of measurements. If I measure drought tolerance on my species and you measure tannin concentrations on your species, we don't learn much. But if we measure the same species, we learn more than we did before.

So, if we select just 100 species to represent the global diversity of grasses, herbaceous eudicots, or trees, would that be enough species to capture the global diversity of the functional group?

There are 100,000 tree species in the world. Can we possibly capture enough of the global diversity with just 100 randomly-selected species.

There is no way to really test the idea, but we talked about whether 100 species would have been enough to delineate the leaf economic spectrum? That paper had 2548 species. If we had just measured leaf traits like leaf longevity and N concentrations on 100, would we have discovered the leaf economic spectrum.

To explore this, I took the data from the paper and downsampled down to 100 species for some of the major relationships.

The LES looked like this with all the species:

What if the Amass-Nmass relationship just had 100 species? If I randomly select just 100 species from that pool...pattern is just as strong.

What about one of the weaker relationships, like LMA vs. Leaf Longevity?

Still there.

Some caveats here...the LES is not built on a random subset of the world's flora. I'm subsampling a stratified sampling of the global flora. Also, I'm working across a number of functional groups. Relationships might be weaker if we sampled just 100 species of a functional group. 

There are philosophical points to work through about the nature of plant strategies, between vs. within functional or phylogenetic groups for example. 

But, the key is that growing and measuring 100 species is not too hard. If we randomly select them so that there is a broad diversity of functional traits, we should be able to represent the global patterns of functional trait relationships.

The leaf economic spectrum has already been described (though there is still more to learn), but the other spectra have not. 

It might just take growing the right 100 species to quantify the root economic spectrum, the wood economic spectrum, or some other strategy we just aren't even aware of. 

Should we worry about the randomly selecting 100 species that end up in the same genus or all just come from Madagascar? We can test for representativity of the randomly selected species set to make sure that chance didn't screw things up too much. Odds are against it, but it is possible. No one would quibble too much if we had to randomly select a second time (20 times, maybe).

In all, it's encouraging.

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