It’s the population, stupid

Life is too short to get it right. We’re fallible. It’s often an accident if you do. And it’s relative. What’s “right” when, where, how, and why?

Sometimes we Dawdlers say things in these episodes and I cringe because I am pretty sure there’s a decent answer to a question, but I have not really bothered to look it up ahead of time. Yet the piece of information becomes relevant in conversation.

The last time this happened was a couple times in episode 11: “I just wanna get off the bus”. A particular point was brought up early concerning the way I talked about carrying capacity. You can listen here.

I said, “The population’s carrying capacity” to which Harland corrected as “The environment’s carrying capacity.” I never outwardly disagreed but I think it highlights the difficulties in a two hour podcast episode when even here there’s not enough time or perhaps interest to explicate these matters. So I thought I would share, for better or worse, what I think of carrying capacity and why I said “The population’s carrying capacity” and not the “environment’s”. I aim to rectifah.

And for the record, Harland’s not wrong, he’s just an asshole. See the difference?

We can conceptualize carrying capacity like a bucket. Fill up the bucket to the brim and the volume of water is at capacity. The bucket’s carrying capacity is that volume. Any more floweth over. And it’s not the water’s carrying capacity. All good in this version.

But, there is something inherent in water that isn’t in the bucket’s vacuity. Water has a density. So does air and dirt. The volume may be the same between these different substances while other measures are quite different; other measures like mass and density. So, as far as volumetric thinking goes, the bucket/environment is the sole holder of the capacity. But we don’t always think of this volumetric aspect when considering organisms. And moreover, what limits a population’s size is not just a boundary imposed from outside.

If carrying capacity is only a function of the environment than carrying capacity would be just a thing like albedo. But it isn’t. “Environment,” which I’ll guess means something akin to the outside world, is just part of it.

But “environment” is a confusing word in the present context. Where is our focus? It’s the population, stupid. No offense if anybody is upset at being called stupid. It’s a modification of a throwback phrase to Bill Clinton’s first presidential campaign. Back then it was, “It’s the economy, stupid.”  My play on it is not out of disdain for the reader. I mean, I have a disdain for you. It’s just not in that particular passage. Ok, it is.

Moving on.

I think of carrying capacity in a mathematical way because I learned that for me I can think more clearly with math since it can…sometimes compartmentalize the world more simply and concisely than words, sentences, and paragraphs.

Classically, carrying capacity is denoted as K. I’ll just jump the gun and show you a simple formulation derived in a 1971 paper by ecologist Michael L Rosenzweig.

Here, R indicates resources in, let’s say, kilojoules per day (kj/d) and F represents the field metabolic rate (i.e., how much energy [kj/d] an organism uses in its daily activities and let’s also say for simplicity how much energy would likely be best for it to get back). α is the positive, unitless measure representing the influence intraspecifics (sometimes aka, “conspecifics”—members of the same population) have on the organism’s ability to consume. But I don’t see why these density-dependent effects couldn’t be the influence of all other regulating factors like, for instance, interspecific factors such as competition or predation, and so on.

Now, R and F being constant, a relatively large α will contribute to a smaller carrying capacity and a relatively small α to a larger capacity. Consider a founding population on an unpopulated island versus some stray organisms landing on one that’s teeming.

So for organisms, resources allow them to survive by replenishing their energy supply and by providing enough energy for them to produce another generation. However, if one considers where these facets of the environment intersect, it becomes difficult to find the bucket’s vacuity exactly. How fast is an important resource (R) replenished independent of a consuming organism (F)? How much space does the organism need to get that requisite amount (α)? Can it vary? Is there a threshold? This suggests a certain density of intraspecifics and even interspecifics is important.

Once a certain number of organisms is reached per unit area, there may be a greater daily struggle to survive or density-dependent dispersal could be a solution. There may be a number of solutions to this problem of density at capacity. This is inherent to an organism’s evolutionary history and it’s not imposed from the outside. It comes from “within” because it was inherited regardless of what the past environments were.

Within what? The population, stupid. (How many “stupids” does a blogger get per post?)

So here I think the population has some ownership of the carrying capacity. There’s a feedback between organism and environment that isn’t easily taken apart. Consider the oxygenic product of photosynthesis and the carbonic product of animal respiration. Sure, there’s a point when it mattered who threw the first evolutionary punch, but it doesn’t matter now. I just gotta breathe, man!

This is why I say the “population’s carrying capacity”. It is a value not solely assigned to the environment, but also to a given population based upon the characteristics inherent to the organism and the members of its relevant spatiotemporally delineated reproductive community. Not only that, but we might also talk about an ecological community’s capacity, a continent’s, and so on (see del Monte-Luna et al., 2004). It acknowledges that whenever we pull focus on a unit of “biological system”, we can’t ignore what the system brings to the table itself.