Positive Multifunctionality ≠ All Functions Are Positive

Positive Multifunctionality ≠ All Functions Are Positive

I was dismayed this morning to read Bradford et al.’s recently accepted paper Discontinuity in the responses of ecosystem processes and multifunctionality to altered soil
community composition
in PNAS for several reasons.

The paper itself is really cool. They manipulated community complexity and nutrient conditions in the Ecotron, and then looked at five soil ecosystem functions. They then looked at whether complexity influenced multi functionality (as well as N), and found, indeed, it did! They went on and, as recommended in our paper on multifunctionality analyze single functions to understand what is driving that multifunctionality relationship, and then…

Then they fall off the boat completely.

Disappointment #1
They find that, while some functions were affected positively, some were not, and one more was affected negatively. They conclude, therefore, that multifunctionality metrics are not useful.

…multifunctionality indices may obscure insights into the mechanistic relationships required to understand and manage the influence of community change on ecosystem service provision.

The mismatch between our community and fertilization effects on multifunctionality and the individual processes, however, cautions against using the framework as a predictive tool for achieving desired levels of functioning for multiple, specified ecosystem services.

What is frustrating about this is that the authors completely miss what multifunctionality actually tells us.

I’m going to say this once very simply, and then in much more detail –

high multifunctionality ≠ every single function performing well

To quote from my own work, multifunctionality is “simultaneous performance of multiple functions.” No more, no less. A positive relationship between a driver and multifunctionality does not imply a positive relationship between that driver and every function being monitored. But rather that said driver will be able to increase the performance of more functions than are decreased.

Some More Detail
Indeed, in the example in Byrnes et al. 2014, we look at the data from the German BIODEPTH experiment. Some of the functions have a positive relationship with richness. Some do not. One has a trending negative relationship. But, put together, multifunctionality is a powerful concept that shows us that, if we are concerned with the simultaneous provision of multiple functions, then, yes, biodiversity enhances multifunctionality.

In our paper, we advise that researchers look at single functions – precisely because they are likely not all related to a driver in the same way. We state

The suite of metrics generated by the multiple threshold approach provide powerful information for analysing multifunctionality, especially when combined with analyses of the relationship between diversity and single functions.

We say this because, indeed, one has to ask – is the driver-MF relationship as strong as it could be? Why or why not? How can we pull the system apart into its component pieces to understand what is going on at the aggregate level?

The approaches are not in opposition, but rather utilizing both provides a much more rich picture of how a driver influences an ecosystem – both through an aggregate lens and a more fine-scale lens. The similarities and differences between them are informative, not discordant.

Disappointment #2
UPDATE: See comments from Mark and Steve below. This #2 would appear incorrect and a tale of crossed paths not scene. While I cannot find anything in my various inboxes regarding communication, it’s possible either a bad email address was used, or it went missing in my transition between nceas and umb. If this is the case, I’m in the wrong on this. An interesting quandry of how do we resolve these things outside of the literature, and worth pondering in this our modern age of email. I leave my comments below for the sake of completeness, and as there are still some ideas worth thinking about. But, wish that email hadn’t disappeared somewhere into the ether! Now the more my disappointment in technology!

Perhaps the bigger bummer is that, despite this being a big critique of the idea of multifunctionality that our group spent a *huge* amount of time trying to figure out how to quantify in a meaningful and useful way, as far as I know, none of us were contacted about this rebuttle. The experiment and analysis of the experiment is excellent, and it gets into some really cool stuff about soil biocomplexity and ecosystem multifunctionality. But the whole attacking multifunctionality as a useful concept thing?

That entire controversy could have been resolved with a brief email or two, tops. For this group to go so far off base is really kind of shocking, and dismaying.

Dismaying because the advice that would seem to stem from this paper is to go back to just looking at single functions individually and jettison the concept of multifunctionality (no other alternative is provided). That places us squarely back in 2003, with fragmented different types of analyses being used in an ad hoc manner without a unifying framework. Precisely what we were trying to avoid with our methods paper.

And all it would have taken to prevent is a little bit of communication.

References
Bradford, M. A., S. A. Wood, R. D. Bardgett, H. I. J. Black, M. Bonkowski, T. Eggers, S. J. Grayston, E. Kandeler, P. Manning, H. Setälä, and T. H. Jones. 2014. Discontinuity in the responses of ecosystem processes and multifunctionality to altered soil community composition. PNAS. link

Byrnes, J. E. K., L. Gamfeldt, F. Isbell, J. S. Lefcheck, J. N. Griffin, A. Hector, B. J. Cardinale, D. U. Hooper, L. E. Dee, and J. Emmett Duffy. 2014. Investigating the relationship between biodiversity and ecosystem multifunctionality: challenges and solutions. Methods in Ecology and Evolution. 5: 111-124. doi: 10.1111/2041-210X.12143

Getting it Right – after Publication: A Multifunctional Journey?

Sometimes, you have to publish a paper to have the hit-yourself-in-the-head revelation of the real right answer.

Earlier this year, along with a great cohort of colleagues, I birthed a really neat piece summarizing how you can look at simultaneous change in multiple ecosystem functions. We were interested in how changes in diversity can affect the simultaneous performance of multiple functions, but, really, anything can be put on the X axis – warming, fertilization, lemony-fresh-scentedness – it doesn’t matter.

This was a problem that had been vexing the field of biodiversity-ecosystem-function, or, really, anyone who wanted to look at multiple functions. Our group spent multiple sessions bashing our heads against a wall trying to derive a solid analytic strategy to look at changes in so-called multifunctionality and in the end came up with something that I’m pretty proud of. The basic idea of our approach was to look at the slope of the relationship between a predictor and the number of functions ≥ some threshold of their maximum – but then do it for lots of thresholds. Why is it important to looks at lots of thresholds? Well, if you look at the lines for each choice of threshold, you get something like this:

lineplot

Note, that’s from the multifunc R package vignette, and its the analysis from our paper.

Anyway, you can see how the slope and intercept change with different threshold choices. We eventually looked at how slope changes with threshold, and used that to divine a fingerprint of multifunctionality. But, a plot of threshold v. slope – it’s kind of abstract, and can be hard to parse. It was the best we could do, though, as we thought and thought about it.

While working on a recent analysis, I began to wonder – one of the key numbers we want is something like, how does multifunctionality change with the addition or removal of one species? We can look at how one function changes with diversity – but we still don’t have a good something with our predictor on the X-axis. And yet, the question I want to answer is key if we want to think about the consequences of, say, losing species for a multifunctional world.

Discussing this with Jon Lefcheck, I was suddenly struck by something he had done in a figure on a manuscript. He drew a plot like the one I showed above, only, he also put a line across the top at the maximum number of functions observed in an experiment. I noticed that as my eye moved across the plot – from low to high diversity – I could see the color change along the line, indicating that the maximum number of functions were able to hit progressively higher levels of function. At low levels, nothing was able to hit any threshold. At high levels, a few were. So…one could in theory plot diversity against the highest threshold that all of the observed functions could hit all at once.

Lines drawn on top of the same figure for 5 functions and for 2 functions. Let your eye wander along them and note the change in color. Also, note the weird optical illusion at F=2. Yes, that line is actually straight.

Lines drawn on top of the same figure for 5 functions and for 2 functions. Let your eye wander along them and note the change in color. Also, note the weird optical illusion at F=2. Yes, that line is actually straight.

Moreover, if I were to, say, draw a line at a lower number of functions, I would see the same pattern – but the relationship would change. Now, higher thresholds could be reached by a lower number of functions – but, eyeballing it, it looked like the linearity changed. In some ways, it made me think of, in the BEF literature, if we have only one function, we get a saturating curved relationship between diversity and function. But for multifunction, a few of us have long wondered if we might get a more linear relationship.

So, for the German example, I decided to whip up some simple code to explore the relationship between diversity, number of functions, and maximum threshold those functions can achieve. I used the fitted model, and then just calculated which thresholds could achieve some number of functions at a given level of diversity, and grabbed the maximum. The results are interesting.

newmf

You can see that fewer functions can simultaneously achieve a higher threshold – this is predictable. But there’s a suggestion that the curvilinearity of the relationship switches from linear to concave-up as more functions are considered. That’s it’s linear to begin with is notable, as with few functions I would expect more concave-down-ness. And you kinda get that if you run it down to one function, but, this site in general in earlier papers didn’t have an incredibly strong saturating relationship compared to some other classic examples.

Overall, while it takes a bit of a moment to realize what’s going on, I think this is a far more interpretable graph than what we presented in the paper. I haven’t subjected it to the same in-depth can-this-be-fooled simulations that we did for the MEE paper, but, I have to admit…I kind of like this, and think it might be the answer we tried to get at oh so long ago.