While looking into the organization of ecosystems and how evolution shapes it, I've been reading "Ecology" (1975) by Eugene Odum. In chapter 6, p. 167, Odum speaks about the mutualistic relationship between bullhorn acacia and acacia ant, and writes:

As one species evolves to take a selective advantage of the association, the other species comes under selective pressure to strengthen the interdependence. We can see how a genetic feedback process of this sort could shape the evolution of a whole ecosystem.

I don't understand the first sentence. So, the acacia increases its fitness through the association with the ant, and the ant would come "under selective pressure to strengthen the interdependence" with the acacia? What is Odum trying to say here?

  • 2
    $\begingroup$ Welcome to Biology.SE. Please take the tour and then go through the help center pages starting with How to Ask questions effectively on this site and edit your question accordingly. In particular, your title question is much too broad and doesn't reflect the body. It would also strengthen your question if you referenced more current material (or at least sources available online) that you had used to try to work this out yourself. $\endgroup$
    – tyersome
    Mar 11, 2022 at 22:58
  • $\begingroup$ Thank you for your welcome. I read what you recommended and I edited my question, making the title and the question more specific, with the detailed question about Odum's own answer being better integrated with the question. I also explained why I read this old source. $\endgroup$
    – Martin P.
    Mar 12, 2022 at 9:49
  • $\begingroup$ Can you add the page number for your quote of Odum? $\endgroup$
    – arara
    Mar 16, 2022 at 16:08
  • $\begingroup$ Sure, it's page 167. I see you also suggested an edit of my question. Thanks, however the phrasing is too specific and doesn't match my real interest. That being said I would welcome someone simply explaining the Odum quote. Odum does reply to the question I'm asking, so explaining his reply is a way of replying to my question. (Reposted for more concision and clarity.) $\endgroup$
    – Martin P.
    Mar 17, 2022 at 23:51
  • $\begingroup$ I have approved the edit (with minor adjustments). I think that the edit at the least clarifies an answerable question. It is not obvious what else beyond this was of interest (except perhaps that you are looking for more general principles of the phenomenon). I encourage you to a) further edit or b) ask a new question building on the specifics in this one if you are not satisfied with the edit. $\endgroup$ Mar 19, 2022 at 20:21

2 Answers 2


I don't claim that this is a fully fleshed out answer, but it's too long for a comment.

I suggest taking a look at some of the classic models for the evolution of cooperation. One example would be Axelrod and Hamilton (1981) in Science, who formulate a series of theoretical conditions under which cooperation 1) arises, 2) is a robust strategy, and 3) stabilizes. The paper was then treated at book length by Axelrod.

Briefly, they argue that cooperative strategies are more successful when the frequency of cooperating partners is higher (and obviously, when the benefit of cooperation is higher).

In a very simple example, cooperation in the Prisoner's Dilemma becomes a better strategy as the game is iterated, and you learn whether your partner is trustworthy or not.

Extended to the evolution of associations like ant-acacia, if one species (say acacia) "takes selective advantage of the association", then necessarily the frequency of acacia individuals willing and able to cooperate will increase. This could be either because there are more acacias or because more of a stable population of acacias will cooperate with ants.

Therefore, the benefits of cooperation will increase for ants, because the frequency of cooperating acacias is higher. This would then be the "selective pressure" for cooperation that Odum writes about.

Increased fitness of cooperators relative to non-cooperators is "selective pressure" to cooperate, meaning more cooperation in the long term under assumptions of a stable environment.

The paper itself is light on equations, using instead mostly simulations. Others have gone so far as to build more modern simulation packages implementing a variety of strategies, in case that is helpful.

  • $\begingroup$ I read the article you suggested (Axelrod 1981). They do show how, within their game-theoretic model, constant defection is evolutionarily stable relative to any kind of cooperation, and therefore cooperation can only prosper in an environment with some cooperators -- which I guess is what you are alluding to. Still I'm not convinced that you are explaining the quote satisfactorily. I think Odum is making (in 1975) a general statement about how mutualistic relationships arise from coevolution and end up shaping whole ecosystems. But I don't understand his sentence. It may be a phrasing issue. $\endgroup$
    – Martin P.
    Mar 29, 2022 at 9:42
  • $\begingroup$ @MartinP. I guess I have a very different reading of that paper. I see the paper as making a series of arguments about why cooperation evolves even in the presence of incentives for defection (title: "The Evolution of Cooperation"). To me, all that Odum seems to be saying is that cooperation is a comparatively better bet when there are lots of cooperators, relative to when there are few cooperators. In other words, it only makes sense to have a credit card when the stores you shop at mostly accept credit cards. $\endgroup$ Mar 29, 2022 at 17:52
  • $\begingroup$ I was not making a summary of the paper, but of the part that seemed to justify your reference to it in your answer. My impression is that the paper's model has very specific assumptions and is not much help for ants and acacias. One such assumption is that all individuals can cooperate with each other. Acacias can't cooperate with each other. $\endgroup$
    – Martin P.
    Mar 29, 2022 at 22:42
  • $\begingroup$ In your bank and credit card example, how are the credit cards holders pressured "to strengthen the interdependence", as Odum writes? I'm thinking more of an employee-company analogy. The employee is happy with their job, so they go the extra mile to make sure the company is happy with them and keeps them. That is "strengthening the interdependence". See my own answer. $\endgroup$
    – Martin P.
    Mar 30, 2022 at 7:06
  • $\begingroup$ @MartinP. Imagine that I sell tacos out of a truck in 1970. 0% of my customers want to use a credit card to buy tacos, I have no reason to accept cards. However in 2022, probably 80-90% of my customers will want to use a card. Likewise, over those years those customers went through a change where more and more businesses accepted cards (and now some don't accept cash). Both agents adapted to each others' behavior to strengthen the interdependent behavior (where behavior == "credit card transactions"), driven by increasing frequency of cooperating behavior. $\endgroup$ Mar 30, 2022 at 20:49

My guess is that Odum is trying, and possibly failing, to provide a general reasoning about how stable mutualistic relationships might naturally evolve and shape ecosystems. Consider ants and acacias.

At first, a particular mutually beneficial exchange happens by happy coincidence between some ants and some acacias: the ants find shelter in the acacia's thorns, and their natural reaction to intruders serves as a defence for the acacia. The acacias figure out through evolution that if they can thus count on the ants then they can stop the costly production of the bitter alkaloids that acacias usually have for their defense, which gives them an edge relative to other acacias that bear that cost. To make sure the ants do keep coming and the happy coincidence becomes a way of life, they also evolve to produce (less costly) Beltian bodies on which ants can feed.

The ants are glad to have have found a provider of free food and shelter (free, because it was a happy coincidence at first and by hypothesis the ant hasn't yet evolved to specially adapt to it). However, now that they have found such a worthwhile partner, they don't want to lose it and therefore evolve to "strengthen the interdependence" (as Odum writes). Perhaps they become even better defenders of the acacia by making their pheromones even more repellent to the acacias' ennemies. And that is how what was a happy coincidence becomes a stable mutualistic relationship.

I'm not sure this second part makes sense as it is (we'd probably need a more precise explanation of why the ant needs to do anything more), but it may be the logic Odum was suggesting in 1975.

It may be that, contrary to what Odum suggests (and to what I assumed as well), there is in fact no generally accepted evolutionary mechanism that explains mutualism, only particular stories for particular cases. Someone in fact writes, in an article that I should probably read, "Simple, general principles governing the evolution of mutualism have proved elusive" (The evolution of mutualism, 2012).


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