IBS 2017: Weak case for experimental macroecology, dynamic macroecology on the rise, and the problem of process vs mechanism

By | January 17, 2017

I’ve returned from IBS meeting in Tucson. Here are my thoughts on experimental and dynamic macroecology, the two big issues discussed this year:

Experimental macroecology needs better justification

One entire morning was dedicated to experimental macroecology. Presented were results from small-grain manipulative experiments, sometimes replicated over large extents, sometimes not. However, it all felt like a session at a regular ecological (or botanical) meeting: I missed connections to large-scale biogeography and to the currently unresolved issues in macroecology.

One of the justifications for experimental macroecology was that it directly tests mechanisms, which can improve macroecological predictions. However, I haven’t seen a plausible demonstration that this really works, and I remain skeptical – I still believe that macroecological patterns emerge as a result of a whole array of idiosyncrasies, and I find it more useful to study large-scale (and especially large-grain) behavior of ecological systems using state variables and statistical models, rather than using reductionism and experiments.

I am not saying that experimental macroecology is bullshit. I just think that it needs to more directly demonstrate (theoretically and empirically) that it can actually deliver.

Dynamic macroecology takes off, but it needs better terminology

Another major block of talks involved dynamic macroecology. John Harte announced a new dynamic version of maximum entropy theory of ecology, there were talks on both present and deep time extinctions, on invasions, and on local-scale community dynamics. There was a lot of emphasis on processes and mechanisms.

In his earlier talk Brian McGill tried to clarify the role of grain and extent in experimental macroecology. This distinction is absolutely crucial for the progress of the field, and we need to get this right in order to understand each other. I suggest that similar service should be done to the terms process and mechanism, which are key for dynamic macroecology. We should learn the exact difference between processes and mechanisms; plus, when thinking along the temporal dimension, we should learn how processes and mechanisms link to concepts such as pattern, event, causality, correlation, and drivers.

There is always excitement when temporal macroecological data are presented – such data are rare and there is the promise that they will get us close to processes and mechanisms, which ecologists consider more important than patterns or correlations. But are they really?

To me, a process is, quite simply, a temporal change of any kind. A process can be deterministic or stochastic, it can involve causal chains but it does not have to. Moreover, having a process captured at a single location does not necessarily enable to map it in space (similarly, a spatial pattern may indicate little about the temporal process behind). Hence, I don’t see processes as more important or fundamental than, for example, static spatial gradients. They seem kind of similarly important. I also don’t see much of an added value in process-based models over statistical models.

This contrasts with mechanisms (“machines”), which somehow invoke the notion of causality – in a machine you push a piston, it turns the crankshaft, which turns the wheel, action, reaction. In ecology, a population hits carrying capacity, this reduces individual fecundity and increases mortality, and population growth slows down. Hence, mechanisms seem to have an added value over processes: mechanisms always involve causes and forces.

Yet even mechanisms are potentially treacherous: one person’s mechanism is another person’s correlation, or pattern. For example, macroecologists may perceive a tendency of a species to maintain viable populations only in a given temperature as a mechanism driving species' distribution, but for physiologists it’s a correlation (between temperature and population viability).

All in all, it’s a bit of a mess, and a clarification is due.

 

Acknowledgements: Some of the ideas presented here are based on discussions with David Currie, and with members of Center for Theoretical Study in Prague.

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