Brain dump: Finding the “successful” plant

Alright, my first blog post in a long time! Now that I have had a full year of Professoring under my belt, I am finally starting to feel like I’m settling into the role and have some spare time and energy for pursuits like blogging.

As my research lab is getting off the ground, I am coming to realize that there is a lot about plants that I “know” at more of an intuitive level than a practical one. How should I share what I know about plants with my students in an efficient and effective way? For now, my plan is to do these little “brain dumps” as an exercise in trying to organize and order my thoughts on various subjects–by seeing all the pieces lying somewhere, maybe I can start to see how I can best share them with others. Hopefully, this will be a benefit for you as well!

Today’s subject: “What does it mean to be a “successful” plant?”

Ok–this question is precisely answerable in evolutionary terms: A successful plant is one that is fit. A successful plant species is one that has many fit individuals. But measuring fitness is a challenge in practice. Plus, this is a rather unsatisfying answer in the sense that it essentially just says a successful plant is one that survives and reproduces well. While useful, it begs the question: “Well, yes, but how do successful plants do that??”

On this front, there are some long-standing models that might be useful to think about. While these models feel distinct on the surface, they overlap a surprising amount with one another, such that I would consider them to all be more or less valid ways of looking at essentially the same thing. The lesson they all share is this: “There is no such thing as a perfect plant, but there are such things as perfect plants.” In other words, there are multiple, equally valid ways to be successful as a plant (all else being equal), and they are divergent such that no one plant could successfully nail them all.

To start, let’s review some life history theory (my favorite!) Life history theory suggests that basically every organism on Earth has to do a mix of 4 things to be successful:

  1. Grow (so one can acquire resources)
  2. Defend (so one can keep the resources acquired)
  3. Reproduce (so one can persist long-term as a species)
  4. Store (so one can endure hard times)

Of these, #s 1 and 3 are generally considered the least “optional.” If you acquire no resources or if you do not reproduce, you will have no fitness and you will not be successful on evolutionary terms. #4 is generally considered the most “optional.” If conditions are basically ideal or if your life is likely to be very short,  there may be no need for storage. However, only a small subset of organisms fall into either or both of these camps, so it is generally rare for an organism to be able to ignore the need for storage entirely. Theory goes on to state, more generally, that while all of these functions are necessary at some level, they are not all equally necessary and some can and should be prioritized over others, depending on the circumstances. More on that in a sec!

It’s important to note that these functions have some clear relationships with one another and even some dependencies on one another. For example, one cannot do anything else if one has not grown and thereby acquired resources. Also, storing resources would be pointless without the ability to defend those stored resources at least some amount. It’s also important to note that these functions are largely (though not strictly entirely) in tension, meaning that resources spent towards one function often cannot then be spent on another. If I store a sugar molecule, that sugar molecule is not going into a leaf for use as energy. More generally, if I spend a lot of resources growing taller, I probably have less left over to then immediately spend on reproduction.

There is a notable caveat here: This tension exists in the short term, but longer-term, it may not. If I store resources now, I cannot reproduce with them now, but I may be able to reproduce even better in the future than I otherwise would have been able to because I stored resources now. The same logic works for resource acquisition and even defense: If I spent some resources now to defend myself effectively, I might not be dead in the future! That obviously increases my reproductive potential in the future over the alternative! The only one of the four functions that doesn’t facilitate the others over the long-term is reproduction, which is noteworthy. In this way, the one thing plants most need to do also places the most limits on how they can live!

Anyhow, so the argument goes, there are likely optimal ways to allocate one’s resources between these 4 (or maybe 3) functions to achieve maximal success, and which way is optimal will depend on the environment in which a plant lives (as well as how the plant has evolved, but we’ll ignore that for now).

One useful model for thinking about this: I forget what it’s called or who pioneered it (embarrassing!), but I’ll call it the “disturbances vs. resources” model. The model essentially argues that there are four classes of environment on Earth:

  1. Those where disturbance is frequent and resources are scarce
  2. Those where disturbance is frequent and resources are abundant
  3. Those where disturbance is rare and resources are scarce
  4. Those where disturbance is rare and resources are abundant.

AKA a 2 by 2 set of possibilities: High/low disturbance x high/low resources. The argument then goes that each of these four environment types rewards a very different set of allocations between our four life history functions.

First, a highly disrupted, unstable environment where resources are scarce (#1) would be inhospitable to basically all lifeforms, so this one of the four would not “reward” any strategy–these are our barren landscapes.

By contrast, the very stable, high-resource environments (#4) would be practically ideal for most organisms. In these environments, the most likely thing to kill you (which would end your success!) is not resource scarcity or disturbance but other organisms muscling you out. As such, the optimal strategy here becomes focusing on growth so that you can maximize your resource acquisition relative to rivals. A little storage is also often valuable because you can hoard resources above what you may immediately need and thus prevent your rivals from getting enough, giving you an edge. Reproduction, it is argued, can be delayed in these environments until one’s place in the environment is secure. Defense is also less important because damage from disturbances should be rare anyhow and because resources are abundant, so it is more strategic to just regrow if needed than to defend against an attack or disruption that may never come.

In environments where disturbance is rare but resources are scarce (#3), conditions are far from ideal, but they are at least predictable. Here, a very different strategy prevails. The most likely thing to kill you is not other organisms (the conditions are not ideal, so there should be fewer rivals here), nor is it disturbances. Instead, it is poorly spending what little you have or losing it to someone else. As such, storage and defense are strongly favored in these environments; the best approach, then, is to be slow, steady, and cautious. Growth is only invested in to the minimum extent needed–any excess growth just means extra maintenance that might be prohibitive. Reproduction is delayed here as well until either conditions become suddenly better or until the plant feels they simply no longer have anything to lose (“going out with a bang, pun absolutely intended!”)

Lastly, we have environments with a lot of resources but frequent disturbance (#2). Here, plants can assume they will encounter abrupt change often, but resources will be available. The most likely thing to kill you here is the next big change! Growing a lot will just make you that much more susceptible to change (a hurricane easily blows over trees). Similarly, storage and defense may be wasted effort if the environment changes so abruptly that you can’t live there any longer afterward anyhow. As such, these environments are all about reproduction–make as many of you as you can to replace yourself when the inevitable change comes. Perhaps the seeds of the plant can survive the disturbances, or perhaps they can spread far enough away from the parent plant that they land in an environment in which the next disturbance won’t happen for a bit. (Sidenote: I feel like this is the most confusing one to explain–am I making my point here?)

At this point, it pays to think: Do we know any environments that sound like these three model ones described above? And can we think of any stereotypical plants that seem to match the needed priorities for each environment? (I can! But I won’t pollute your mind with my thoughts here.)

The above model essentially leads to the conclusion that there are three successful ways to be a plantWhich one makes the most sense just depends on the environment. Not coincidentally, maybe the most popular model of “successful ways to be a plant” is Grime’s Triangle, which argues the same thing!  Grimes argued that plants can invest in traits that make them competitive for resources (a large root system, e.g.), good at evading disturbances (the ability to form a large seed bank to regrow after a fire, e.g.), or good at tolerating stress (having a large taproot for storage, e.g.), but it would be basically impossible to be really good at all three at once. Why? Because you can’t have both a big taproot and a giant root system, for example–those two things would preclude each other. These three plant “types” often get called competitors, ruderals (weeds), and tolerators, respectively.

…The savvy mind might see parallels here between Grime’s three plant types and the three optimal strategies for environments I described earlier!

  • Competitors are our low disturbance, high resources species–ones that need to fend off others to be successful.
  • Tolerators are our low disturbance, low resources species–one that need to not overspend their means and that need to often survive hard times.
  • Ruderals are our high disturbance, high resources species–ones that need to complete their business quickly before the next round of chaos comes.

I personally find it comforting that these two seemingly distinct models reach similar conclusions about what makes for “ideal” plants! However, one thing that both models acknowledge but that Grime’s Triangle makes easier to understand (I think) is that life isn’t quite as black-and-white as either model makes it seem at first. In other words, there are “medium” environments, right?? Grimes and those who developed his model further recognized this by noting there are “hybrid” strategies that exist in between the extremes listed above. For example, it might be beneficial in a high-resource, medium-disturbance environment to be a “competitive ruderal, by storing a little and delaying reproduction a little to get an edge on similarly minded species before the need to reproduce arises.

Thus, we might speak of “generalist” strategies and “specialist” strategies. A generalist species can maybe hang ok in more extreme environments but can only excel in more moderate environments, whereas only a specialist strategy works in more extreme environments. So, if one is wondering what strategy is “best” for a plant, one must also consider the availability of different environments in the landscape--if most environments are “moderate,” generalist species  that do a mix of all four things but none of them as well as a specialist would are favored. This is relevant because, it can be argued, we are “specializing” the world right nowwe are increasing disturbance and increasing resources, perhaps rewarding the ruderal strategy over all others! I dunno about you, but this makes me sad–I don’t want to live in a world where weeds are the clear winners!

Other ecological models that you may have been taught also line up with these two models remarkably well. For example, ruderals would be true r-selected species–they “live fast and die young.” Competitors, meanwhile, would be true K-selected species–they live to play a long game and be a highly successful adult. Tolerators are generally considered K-selected species too, since they are very cautious. In between, we have our generalists, who live or die by their ability to be flexible and to survive a mixture of unpredictable challenges.

Even the plant growth forms that you may be familiar with (annual, biennial, perennial, etc.) actually line up pretty well with Grime’s Triangle. Trees invest heavily into growth and defense, making them a competitor-tolerator hybrid. Mid-successional plants like shrubs and large perennial herbs tend to be the truest competitors. Lichens and cacti are hardcore tolerators. Rubus species, like those I study, are often considered ruderals, but I would argue they are closer to generalists–they need to be able to tolerate shade and outcompete other mid-sized plants, which they do by forming clones. They also defend themselves with sharp prickles. So, I would call them “mildly tolerant competitive ruderals.” How’s that for a mouthful??

Did you find this an interesting way of thinking about this? What was confusing? Please let me know!