PageRank Algorithm Applied To the Food Web

An anonymous reader brings word of a new application for PageRank, Google's link analysis algorithm: monitoring the food web in an ecosystem. A team of researchers found that a modified version of PageRank can predict with great accuracy which species are vital to the existence of others. Quoting: "Every species is embedded in a complex network of relationships with others. A single extinction can cascade into the loss of seemingly unrelated species. Investigating when this might happen using more conventional methods is complicated, as even in simple ecosystems, the number of combinations exceeds the number of atoms in the universe. So, it would be impossible to try them all. Co-author Dr. Stefano Allesina realized he could apply PageRank to the problem when he stumbled across an article in a journal of applied mathematics describing the Google algorithm. 'First of all, we had to reverse the definition of the algorithm. In PageRank, a web page is important if important pages point to it. In our approach, a species is important if it points to important species.'"

Comments

Why is this surprising?

[Anonymous Coward]

Pagerank is just a repeated application of a transformation matrix. It has the effect of running a Markov model (a way to model discrete states) until there is convergence. He just used a Markov model the way that it is supposed to be used...

I dont get it... what's notable here?

Re:Why is this surprising?

[xenocide2]

It's remarkable because a biologist discovered math and possibly statistics.

Re:Why is this surprising?

[DeadDecoy]

Because it's a novel use of an existing method? It was published in PLoS and not some mathematics journal. So, while the algorithms are not new, they may be new to the intended audience. The actual claim of the article is that it can offer a predictive analysis of extinction rates of a species and validated them on some in-silico experiments. This could be useful for bench-scientists, as they could figure out what might happen in an experiment before running expensive tests. This might be useful for conservations trying to make sure whole ecosystems don't die out due to the removal of a species; the 'might' is significant as real-world ecosystems are generally more complex. But anyways, it's interesting because the models have practical application outside of theory to help us understand the world.

Re:Why is this surprising?

[jpmorgan]

Yes, it is notable that this wasn't published in 'some mathematics journal'. Pagerank is computing the limiting distribution of a discrete-time markov chain applied to webpages using a certain statistical model of hyperlinks. It makes no sense to talk of applying 'pagerank' to things other than webpages, because that's what makes pagerank special! As soon as you take pagerank out of the web-context, it's just a steady state analysis of a markov chain, which is a standard statistical technique covered in undergraduate statistics courses. It's like saying applying bayesian inference to a problem in ecology is using a 'spam filter.'

For me, this tells me that perhaps these researchers should wander over to their local mathematics department more often. They might find all sorts of goodies that mathematicians have developed in the past few centuries. Dr. Allesina might have discovered that there was no need to reverse engineer the algorithm, since the underlying mathematical principles have been well understood for over a hundred years. We might have a better understanding of the world if most sciences took mathematical models as seriously as physicists do.

Re:Importance

[JorDan Clock]

The model helps determine if a species is important. That's the whole point. Previously, we didn't have an easy way to determine a particular species impact on an ecosystem until it was almost extinct or already gone. Now by using "PageRank" to determine their importance, we can model what will happen if specific species are no longer part of the food web.

Re:More than atoms in the universe?

[MyLongNickName]

What does your tweezers and removing atoms have to do with combinations? It is trivial to come up with a situation where there are more possible combinations that atoms of the universe. The number of possible chess games starts to get close (magnitude of 50 versus 80 for the atoms in the universe. Slightly more complex scenarios would easily go past 10^80. The trick is to find a way to model the complexity with a much simpler algorithm.

Re:Importance

[Hadlock]

NO! Page Rank is not named after webPage. It's named after Larry Page who created it. Arrrgh.
 
http://en.wikipedia.org/wiki/PageRank
 
http://en.wikipedia.org/wiki/Larry_Page