Ecologists often want to study the relative importance or strength of different variables or factors. Which is stronger: top down or bottom up effects? Where does community X fall on a continuum from “drift dominated” to “niche dominated”? What’s the relative importance of density-dependent vs. density-independent factors in explaining temporal variance in species’ abundances? What’s the relative importance of ecological vs. evolutionary determinants of species’ range limits? Etc.
Questions about the relative importance of different variables or factors often are very sensible questions to ask in a multicausal world (see also). But if you’re not careful, it’s very easy to ask what seems like a sensible question, but actually makes no sense at all. In general, just because dependent variable Y is affected by more than one thing does not mean that it makes sense to ask about the relative strength or importance of those things!
Here are some common pitfalls in studies of the relative importance or strength of different variables in ecology:
- Arbitrary or contestable definition of “importance”, when the answer is sensitive to one’s choice of definition. See Rees 2013 Eco Letts for a great critique of one instance of this problem. More broadly, see this old post and the excellent comment thread for discussion of different measures or indices of the “same” thing (here, the “strength” or “importance” of some variable).
- Asking where natural systems fall along some purported continuum that is not actually a continuum. See: all attempts to treat “niches” and “drift” as the two ends of a continuum. That’s like trying to treat loud sounds and high pitched sounds as the two ends of a continuum. There’s a continuum from neutrality to selection, and from drift to, um, lack of drift. But there’s no continuum from drift to selection.
- Asking about the relative importance of things that do not combine additively. Rather, they combine in some non-additive way, so there’s an interaction term. In which case the effect of any given variable (either in an absolute sense, or relative to the effects of the others) depends on the values of the other variables. For instance, the strength of top down effects depends on the strength of bottom up effects: the change plant biomass following top predator removal depends on resource enrichment. (Aside: in general, asking whether variables A and B have additive effects is a bad scientific question if you define “additive” statistically. Whether or not there’s a statistical interaction term between two variables is very sensitive to your data transformation and other factors having nothing to do with how the world actually is. Statistics is a tool to help you answer your scientific questions, it shouldn’t define your scientific questions. See #6 in this post.)
- Asking about the relative importance of different things that, when combined, yield “emergent” effects. Example: in population dynamics, the combination of deterministic and stochastic forces often yields surprising emergent outcomes. Think of quasi-cycles: regular or nearly regular population cycles that emerge when a population that would exhibit damped oscillations to a stable equilibrium in the absence of stochasticity is subjected to ongoing, non-cyclic stochasticity. Asking about the relative importance of determinism vs. stochasticity in generating quasi-cycles is like asking about the relative importance of sugar, flour, mixing time, and oven temperature to the baking of a cake. It doesn’t make sense. And any attempt to try and force it to make sense is going to be highly artificial and unhelpful. Much the same could be said about attempts to partition the relative strength or importance of determinism and stochasticity in many other contexts in ecology, such as certain approaches to metacommunity dynamics.