Are some general ecological concepts TOO general?

Many ecologists, including me, want to discover generalities. We want to see the forest for the trees. That often means abstracting away from certain details so as to focus on features shared by all cases of interest.

But is there such a thing as too much generality, or the wrong kind of generality? It’s a good thing to step back and see the forest for the trees, but what if you step back too far (into deep space, say)? Don’t you lose sight of the forest, or end up mistaking the forest for something else?

I think so. In seminars, I sometimes use the hypothetical example of a ‘general theory of growth’. Try to imagine a general theory of growth that would apply to everything that can be said to ‘grow’. Not just individual organisms, but populations, economies, egos, the entire universe…The point is that just because two things can both be said to ‘grow’ doesn’t mean they’re comparable in any interesting or useful sense. Note that one could statistically compare the growth rates of, say, organisms and egos. But that doesn’t make the comparison scientifically meaningful. I use this example because it’s obviously silly–at least, I thought it was obviously silly. Then I attended a philosophy seminar at which the speaker argued, in all seriousness, that major disciplinary boundaries (e.g., between astronomy and biology) inhibit scientific progress because they inhibit the development of general theories–such as a general theory of growth that might apply to everything from growing organisms to the expanding universe. So at least to some, there’s no such thing as too much generality!

Moving away from hypothetical examples and the sort of philosophers who focus exclusively on them, there are real-world examples of overly-general theories or concepts in ecology and evolution. The obvious examples are theories which were empirically false due to their over-generality. An example is the pre-Darwinian idea that development of individual organisms and evolution of species are closely analogous. Darwin showed that that’s just false–development is directed towards a pre-determined goal, evolution isn’t. So the notion of ‘development’ doesn’t apply nearly as generally as some pre-Darwinians thought.

But more interesting cases are when theories or concepts are over-general without necessarily being empirically false. Over-generality in this sense is trickier to pin down, but can involve such features as:

  • vaguely-defined terms
  • loose analogies
  • lack of a general mathematical version of the theory or concept. Thereby forcing reliance either on purely verbal models, or else on less-general mathematical models of specific cases, from which over-general conclusions are drawn
  • reliance on purely statistical tools to compare and contrast different case studies in a phenomenological way (as in statistical comparison of the growth rates of organisms and egos)

All these features have the effect of facilitating comparisons of ‘apples to oranges’–comparisons that highlight comparatively superficial commonalities among different cases, while obscuring deeper distinctions that need to be drawn in order for explanatory progress to be made.

I’ve argued that the idea of ‘biodiversity affecting ecosystem function’ (BDEF) is over-general in this way (Fox 2006, Fox and Harpole 2008). The terms ‘biodiversity’ and ‘ecosystem function’ are like the term ‘growth’–they embrace many phenomena which really need to be kept separate in order for explanatory progress to be made. I have used the Price Equation to suggest what distinctions ought to be drawn between different classes of BDEF problems (a ‘divide and conquer’ approach, if you like). This formal mathematical approach forces precise definition of terms, and highlights not only important distinctions but also important but unrecognized commonalities. For instance, ‘community variability’ is not generally regarded as an ‘ecosystem function’, but effects of biodiversity on community variability can be analyzed within the Price Equation framework (Fox 2010).

One very prominent ecological idea which has been criticized as over-general is a citation classic: Jones et al. (1994), ‘Organisms as ecosystem engineers’ (>2100 WoS citations). Jones et al. define ecosystem engineers as “Organisms that directly or indirectly modulate the availability of resources [other than themselves] to other species, by causing physical state changes in biotic or abiotic materials. In so doing, they modify, maintain and create habitats.” The concept has subsequently been broadened to include essentially any effect of organisms on their physical environment (e.g., Harmon et al. 2009). Ecologists now have a huge range of case studies of ecosystem engineering (reviewed in Wright and Jones 2006), mathematical models of a few specific examples (e.g., Wright et al. 2004), and an admirable attempt to unify these disparate examples within a general (but unfortunately, not fully specified) mathematical framework (Jones et al. 2010). Despite all this, even ecosystem engineering’s staunchest advocates recognize that they have yet to present a fully-convincing argument that they’re not comparing apples to oranges (Jones et al. 2010). That some specific models of ecosystem engineering (e.g., Wright et al. 2004), and some prominent case studies (e.g., Harmon et al. 2009) appear not to fit within the general mathematical framework of Jones et al. 2010 is a little worrisome. I honestly admire the imagination and ambition of the idea of ecosystem engineering–big ideas like these can drive real progress in science. But I remain to be convinced that there is a general, non-trivial theory that covers everything from the creation of aquatic habitat by beavers (Wright et al. 2004) to the effects of stickleback fish on the chemical composition of DOC (Harmon et al. 2009).*

None of which is a criticism of anyone whose worked on BDEF, ecosystem engineering, or other ideas that may be overbroad. Probably, if we’re not overgeneralizing sometimes, we’re not trying hard enough to generalize in the first place.

*And based on a quick glance at citation data, I wonder if the field as a whole is starting to agree with me. Jones et al. 1994 has only been cited 165 times this nearly-completed year, way down from 211 last year. And before this year, citations of Jones et al. 2002 had been essentially flat since 2012 (203-215 citations annually). Which means that citations of Jones et al. 2002 have effectively been declining since 2012, given the rapid exponential growth of total citations. Of course, there are lots of reasons why interest in ecosystem engineering might be starting to decline. So I’m being a facetious in suggesting the whole field agrees with me that ecosystem engineering is an overbroad concept. 🙂


Note: this is a lightly-edited rerun of a post that first ran on the Oikos blog back on May 11, 2011.

15 thoughts on “Are some general ecological concepts TOO general?

  1. I think the term competition is overly general in a detrimental way. All that binds it together is a mutual negative effect on the population growth rate of the other species. Which means all its really useful for is in lotka volterra competition equations. Anything that wants to get more mechanistic is all of a sudden looking at consumption of depletable resources (food), consumption of fixed resources (space, nesting cavities), and multiple forms of interference competition and probably other forms. There is little in common in mechanisms, outcomes, or dynamics of these different forms.

    • Hmm…presumably you’d say the same thing about enemy-victim interactions (+-) and mutualism (++)?

      “There is little in common in mechanisms, outcomes, or dynamics of these different forms.”

      Hmm…what about “they all lead to competitive exclusion, in the absence of sufficiently-strong stabilizing mechanisms”?

      Also, nitpicky aside: not sure what you mean by contrasting “fixed” resources with “depletable” ones. Space and nesting cavities are depletable. If a bird occupies a nesting cavity, there’s now one fewer nesting cavity available for other birds. Just as, if a bird eats a worm, there’s now one fewer worm available for other birds.

      • I’d definitely say that about mutualisms and enemy-victim.

        Perhaps a better wording for types of resources would be renewable vs non-renewable. Bottom line though is competition for food is different than competition for living space.

        Re: ““they all lead to competitive exclusion, in the absence of sufficiently-strong stabilizing mechanisms” – you’ve just moved the ball for me – “stabilizing mechanisms” is such a broadly sweeping word that means entirely different things in the 3 different contexts I mentioned.

        Also, are there good models showing that interference competition alone leads to competitive exclusion?

      • “Bottom line though is competition for food is different than competition for living space.”

        Is it? I’m not so sure about that. Tilman has a whole chapter in Resource Competition and Community Structure on “Space as a resource”.

        “Also, are there good models showing that interference competition alone leads to competitive exclusion?”

        Hmm…not quite sure what you mean. If you wanted to, you could interpret the Lotka-Volterra model as a model of interference. Or are you looking for a more mechanistic model, in which the only intra- and interspecific interactions are fighting or whatever?

        “‘stabilizing mechanisms’ is such a broadly sweeping word that means entirely different things in the 3 different contexts I mentioned.”

        It’s looking like pretty much every general concept I like is on your list of overly-general concepts! Next you’ll tell me that “selection” is an overly-general concept.

        In the interests of moving the conversation forward, I’d emphasize that general concepts like “stabilizing mechanisms” and “competition” are complements to narrower concepts, not substitutes. You need both an overarching general framework, and detailed models of specific cases or subsets of cases within that overarching framework. Do you agree with that general point, and only disagree with me about which cases to which it applies?

      • To support Brian’s dichotomy, one big difference between food and space/sunlight/whatever is that food adapts. For quickly-reproducing groups with large effective population sizes (e.g., arthropods, many parasites), this difference could play out fast enough to be relevant for ecological timescales.

        In less-quickly evolving situations, prey polymorphism could also change the game. If you have a resource population that has, say, two behavior variants. One behavior makes the resource population more vulnerable to predator A, and the other behavior makes the resource population more vulnerable to predator B, and those behaviors are genetically controlled, then the relative abundance of each variant could fluctuate from year-to-year as a direct result of predator efficiency.

        Examining “competition” between predator A and B in a situation where their behavior has an impact on resource abundance year-to-year (polymorphic or adapting prey) strikes me as distinct from examining competition between those same two predators over a resource that varies year-to-year in a manner unrelated to the predators (e.g., # of nest cavities).

  2. Ludwig von Bertalanffy wrote a book called General System Theory. He claimed it applied to a wide variety of biological and social systems. I believe that he also claimed that the Bertalanffy equations were a general model of growth.

    • Interesting, I didn’t know that.

      Lotka’s Elements of Physical Biology argues for very abstract general principles that purportedly apply across biology, chemistry, and physics. And Herbert Spencer had a grand unified theory of the evolution of pretty much everything–species, human societies, the entire universe.

      I’d be curious to read more history of late 19th and early 20th century science to see if there was something about the general tenor of thought at the time that led so many people to propose “theories of everything”.

    • YES, It goes by the name ‘system theory’,’ general systems theory’, and many others. It bloomed post WW2, and has a vast literature, its own Society [ over 50 yrs old]. It is live and well, and lives at places like the SANTA FE INSTITUTE (FOR THE STUDY OF COMPLEXITY). A useful write-up is in Wikepedia [].
      The metabolic theory of ecology came out of that world [Geoff West was eventually president of SFI ] in talks between ecologists and physicists. Centers for the study of complexity now dot many NA universities. Much more here; just search the internet with the many key words in the Wikpedia article.

  3. As you note, it seems to be generality without careful formalisation/mathematics that causes many of these problems. Group theory is very general, but very useful. Differential equations are a bit less general but extremely useful for many people. And group theory helps you understand differential equations.

    The problem seems to be trying to be too general and too specific at the same time. Which is something that many, many mathematical models suffer from. Personally I think it pays to think of using a formal model and providing an interpretation of a formal model as explicitly separate ‘modes’ of analysis, while also trying to take both seriously.

    It can be hard to be a both decent mathematician and a decent scientist as required for proper generalisation (and specialisation), however.

  4. I’m late to the party, but I’m trying to teach “disturbance” in pop bio today (the Forestry program that requires my course seems to think “disturbance” is important). And I don’t even know what the word means – everything is a disturbance, or maybe nothing is. And I’ve been teaching it for years 😦

  5. Pingback: How far can the logic of shrinkage estimators be pushed? (Or, when should you compare apples and oranges?) | Dynamic Ecology

Leave a Comment

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.