A couple of weeks ago, I posted a simulation tool for thinking about environmental change and population competition and replacement, along with some results that built on recent work on Neanderthals by Timmerman and a few others. The basic conclusion was that a population is more likely to replace a competing population that has a lower dispersal rate when the environment changes infrequently but abruptly, and less likely when the environment changes frequently but subtly.

Beyond the influence of the environmental change regime on replacement, I looked at network radius and found, not surprisingly, that favourable replacement conditions are improved further when the replacing population can maintain more spatially extensive social networks.

In a follow up set of experiments (for details of the model and the methods, see previous post here), I now consider what happens when the replacing population has an advantage in mobilizing carrying capacity, either through technology or some other means. Very briefly, the simulation tracks two populations in a variable environment, each with its own growth rate, diffusion rate, social network extent, and efficiency at mobilizing carrying capacity.

Here I focus on what happens, for example, when population A can sustain only half the size of population B on the same resource base? In terms of the simulation that would mean population A (APop) has a carrying capacity multiplier of 2, and population B (BPop) has a multiplier of 1. Not surprisingly, it turns out that an advantage in mobilizing carrying capacity helps one population replace another, but it isn’t as simple as saying that more carrying capacity mobilization is always better.

I used favourable replacement conditions as determined in the first set of experiments. Both populations have the same growth rate (1.02). BPop has a diffusion rate of 0.01, but APop does not diffuse beyond overflow due to population pressure. BPop can maintain a network radius of 4 patches, but APop has a network radius of 1.

Figure 1: Average BPop proportion at the end of 5 runs for each combination of environmental change frequency and amplitude, when BPop has no carrying capacity mobilization advantage (APop mult =1), and when it has a double or four-fold advantage (mult = 2 and mult = 4).

In the first set of experiments, I found that these conditions tend to lead to replacement mostly when there is infrequent but abrupt environmental change. In this new set of experiments, I find that giving BPop an advantage in mobilizing carrying capacity allows replacement even more easily when environmental change is infrequent and abrupt, but also makes it easier when environmental change is frequent and subtle.

For example, BPop proportion goes from 0.09 to 0.81 when environmental change frequency is 90% and amplitude is 10% (frequent small changes) and BPop is four times as efficient as APop at mobilizing carrying capacity in the environment. In other words, greater efficiency at mobilizing carrying capacity opens up new environmental change regimes to possible replacement scenarios, and increases the dominance of BPop when the combination is either frequent and small change or infrequent abrupt change.

On the other hand, and perhaps more interestingly, it closes other environmental change regimes that can generate replacement when BPop does not have an advantage in mobilizing carrying capacity. At intermediate values of change frequency and amplitude, efficiency in mobilizing carrying capacity doesn’t seem to help at all. At 50 frequency and 50 amplitude, BPop proportion at the end of a run goes from 0.94 when there is no advantage, to 0.11 when BPop has a four-fold advantage. Even at when frequency is 30 and amplitude is 50, BPop proportion goes from 0.92 when there is no advantage, to 0.04 when there is a four-fold advantage.

I think the mechanism at work here is that a locally expanding APop rapidly uses up all the carrying capacity in a patch with its small population, but the environment is too unpredictable and violent for the arriving BPop to establish itself and fully express its advantage at mobilizing that carrying capacity. That advantage expresses itself much more easily in more congenial environmental change regimes, which are either step-like with relatively long periods of stability, or very gradually changing.

This is further suggested by the fact that the high frequency, high amplitude quadrant is dominated by BPop proportions near 0.50, regardless of whether BPop has an advantage in mobilizing carrying capacity, suggesting that outcomes are due to chance. With change frequency and amplitude both at 90, BPop proportion at the end of the run varies between 0.48 and 0.52, regardless of the advantage.

Being better at mobilizing available carrying capacity, and so sustaining a larger population on a given resource base is helpful under several environmental change regimes, but doesn’t necessarily help a small arriving population when environmental conditions are very challenging.

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