Over at The Monkey Cage, Andrew Gelman asks what are the (relatively) settled matters in social science? What do we really know, no doubt about it? He notes that it’s difficult to come up with any, since matters that some people think are settled often aren’t considered settled by others.
He also notes that just because something is settled doesn’t necessarily mean we should stop studying it. For instance, once everyone is satisfied that phenomenon X happens, that sets the stage for follow-up studies examining why phenomenon X happens, how to manipulate phenomenon X, etc.
There are lots of important bits of ecology that are settled, I think (and I suspect the same is actually true in the social sciences, if you restrict attention to sufficiently-basic claims). Just listing bits of ecology that are settled would result in a pretty long and boring list. So how about we try to list bits of ecology that were once controversial but are now settled?
In some ways, this gets back to my old post asking readers to name examples of productive and unproductive debates in ecology. But it’s not exactly the same, I don’t think, since a debate can be productive because it clarifies issues, reveals unspoken assumptions, etc., even if it doesn’t lead to a settled resolution.
I suspect one challenge in coming up with this list will be in deciding the level of precision at which any given claim can be considered “settled”. For instance, when we do predator removal experiments to look for trophic cascades, we mostly find trophic cascades (Shurin et al. 2002). So broad qualitative questions like “Do trophic cascades occur?” have settled answers (“Yes, at least everywhere we’ve looked so far, and we’ve looked in lots of different places”). But more quantitative or refined versions of the same question may not have settled answers. For instance, “Are trophic cascades typically stronger in aquatic than terrestrial systems?” might not be considered settled.
Of course, as many have joked, the settled answer to controversial questions in ecology often turns out to be “it depends.” But I’m hopeful that we can come with some examples where the settled answer is not “it depends”, or at least is a more elaborate version of “it depends” (“it depends on factors X, Y, and Z, in the following ways…”)
So, what once-controversial matters in ecology are now settled?
p.s. Even things that seem settled can occasionally turn out to be wrong. Think of classical Newtonian physics being overturned by Einsteinian relativity.
Dynamic Ecology 
Maybe Neutral vs. Niche dynamics? Sometimes goes under the radar, but it seems to me Adler, HilleRisLambers, and Levine 2007 (“A niche for neutrality”, http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2006.00996.x/abstract) pretty much solved that problem (at least theoretically/conceptually) by considering neutrality a special case where there are no stabilizing differences (niche differences) and species have near-equivalent fitness.
Isn’t the solution to every either/or dichotomy in ecology that it’s sometimes one, sometimes the other, but usually both? Niche vs. neutral, top-down vs. bottom-up, biotic vs. abiotic, chance vs. necessity…
In general I agree with you. But here it is not necessarily a “both” conclusion, in terms of both operating simultaneously (like the top-up, bottom-down example). Rather, neutrality is a special (limiting) case of a coexistence theory that explicitly recognizes the importance of niche differences. This is, of course, dependent on how one defines “neutrality”.
The necessity of density-dependence (over some range of densities, at some time) to regulate populations (prevent them from blowing up to infinity)? At least I haven’t seen this controversy bubble up in the last decade, so can we declare it settled?
The “settled matters” that communities are not strictly neutral (all individuals of all species are not equal) and that populations cannot grow infinitely, these are trivial and were always non-controversial. Hubbell never said that trees in forests have equivalent niches – he just pointed out to the interesting fact that coexistence, variation in abundances (and species-area relationships and realistic phylogenies) can be predicted WITHOUT the need of including niche into the game.
Similarly, the fact that we have models of unbounded population dynamics does not mean that anybody actually seriously thought that they really exist. Yet they are useful approximations in some situations.
If we settle on the matter the trivial fact that simplistic models are (by definition) unrealistic, we realize that the big questions of the specific fields are still opened. There still is a little consensus on what drives patterns of diversity, what is the most important mechanism promoting coexistence, or what regulates populations. And I am left unable to bring a single example of a big matter in ecology that have been settled… bloody hell (and sorry).
While no one believes in infinite populations, the role of density dependence in preventing them has been wildly controversial. Check out:
Murray, B. G. J. (1999). Can the population regulation controversy be buried and forgotten ? Oikos, 84(1), 148–152.
White, T. C. R. (2001). Opposing paradigms : regulation or limitation of populations? Oikos, 93, 148–152.
Berryman, A. A. (2002). Population regulation, emergent properties, and a requiem for density dependence. Oikos, 99, 600–606.
White, T. C. R. (2007). Resolving the limitation – regulation debate. Ecological Research, 354–357.
I’d agree with lowendtheory on density dependence. Whether it even occurs (at least often enough to be worth worrying about) really was once very controversial. Nowadays everyone agrees that it’s common or even ubiquitous, though perhaps often weak enough that (given the limits of the available data) our estimates of its strength aren’t significantly different from zero.
Similarly, I’d list interspecific competition as another once-controversial matter that’s now clearly settled. When you do removal experiments to look for interspecific competition, you mostly find it (and if you don’t find it, you mostly find facilitation instead).
As I think I commented on an old post, I think ecologists are actually pretty good at settling controversies that can be settled by the collection of data, as long as the required data is fairly straightforward to collect, analyze, and interpret. I think that’s what the cases of density dependence, interspecific competition, and trophic cascades all have in common.
Re: Andrew’s comments on neutrality (what it is, what we learned from the controversies over it), Brian and I have various old posts and comment threads on this:
That last post also is related to Petr’s comments about how the world isn’t literally neutral, but various aspects of it (like the shape of the species-abundance distribution) are more or less as they would be if the world were literally neutral. In that old post I talk about the circumstances in which it is, or isn’t, helpful to have a simple “null” or “neutral” model that reproduces certain aspects of the world without being literally true.
Hi Jeremy, was there ever really a controversy about whether interspecific competition existed or whether the presence of one species could affect the abundance or growth of another? I wouldn’t have thought so. On the other hand, I think the question of whether interspecific competition plays an important role in the population dynamics of species in natural communities isn’t even close to decided. Best, Jeff H
Here are a few:
1. Life-history characteristics associated with different modes of reproduction (semelparous and iteroparous) seem to be fairly consistent and settled now for some time.
2. That the characteristic growth pattern exhibited by most animals is caused by maximum energy assimilation or transport rates(Mass^ 2/3-3/4ths) scaling with body size to a power less than maintenance rates (M ^ 1).
3. Consumer-resource or predator-prey systems can cycle even in the absence of external forcing due to non linearities in the functional response
4. The sampling effect in community ecology is also a pretty settled matter.
5. Or also in community ecology the fact that diversity does not by default beget stability seems settled
How about “Time-scales should not be separated between ecological and evolutionary dynamics.”
I’m a little confused by Benjamin’s point 3: non-linear functional responses (at least of types II and III) are generally stabilising. I.e., they’re likely to dampen the instabilities/cycles that might arise in an ‘equivalent’ linear (Lotka-Volterra) system. What am I missing? Are you saying that this has been settled on the side of non-linearities being stabilising, or have I gone wrong somewhere?
You lost me a bit Mike: type II functional responses are destabilizing compared to linear ones. For instance, a predator with a linear functional response, constant conversion efficiency, and density-independent mortality consuming a logistically growing prey gives you an equilibrium that’s globally asymptotically stable if feasible. Change the functional response to type II and you’ve got the Rosenzweig-MacArthur model, which of course can exhibit stable limit cycles. I’m guessing I’m misunderstanding you?
Re: separation of time scales between ecology and evolution, I’d probably agree with the caveat that we’re talking about microevolution. Speciation is mostly a slow process, as far as I know–too slow to balance even slow rates of exclusion due to neutral drift.
Yeah – you’re right about type II in that case. Not sure what I was thinking. Perhaps it was type III vs II that was stabilising (prey refuge at low densities)*. That could be wrong too though. Maybe I was thinking of a different sort of stability*.
Not sure about your caveat on eco vs evo dynamics though. Some might say that there’s no meaningful difference between the processes that drive micro- and macro- evolution (whether that equals speciation is another thing). But that’s more of a “controversy” driven by religious ideology than science.
* It’s late here, though, and my brain is kid-addled. The days when I used to think “don’t drink and blog” are long gone. Now I should remember “don’t post on blogs when you should be recovering from the parenting”.
Just to be clear, I didn’t mean to imply that I think macroevolution is driven by different processes than microevolution. There can be a separation of timescales between speciation and ecology/microevolution even if speciation is itself one outcome of exclusively-microevolutionary processes.
True, forgot for a second the LV was neutrally stable (the cycles completely depend on initial conditions). Type II is generally de-stabilizing, unless the K of prey is small.
I would like to rephrase number 3: Consumer-resource or predator-prey systems can cycle even in the absence of external forcing
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