The influence of stochastic fluctuations on population dynamics: An in-silico approach

Current non-deterministic models predict that stochastic fluctuations have a greater impact on the dynamics of small populations, and therefore are more prone to extinction than large populations. However, here we have developed a computer model that demonstrates that the effects of these fluctuations, combined with the effects of other mechanisms inherent to natural selection, have a significant impact on the selection, evolution, and population dynamics of digital species, regardless of population size. We also studied the necessary conditions to apply the findings to natural systems. Especially in fragmented systems, we observe that intraspecific interactions are more frequent than interspecific interactions because stochastic fluctuations limit the maximum number of interspecific interactions to the population frequency of the smallest interacting population. If these interactions are adaptive, they result in greater average fitness for presocial and eusocial species that do not have this limitation. In addition to inclusive fitness, we have also observed that the direct/indirect fitness ratio is essential for measuring species competitiveness. Stochastic fluctuations equally affect the density variability of all populations, regardless of their fitness. Organisms that cannot interact due to these fluctuations can only have their direct fitness. In species where direct fitness is higher, these “ singles ” increase the average fitness of the species. Stochastic fluctuations affect the evolution of neutral and complex traits. Depending on the population distribution, we observe that the successive effects of these fluctuations, the Allee effect, and competitive exclusion can lead to fixation of a single trait. This same combination of effects creates a mechanism that has not been considered until now in interspecific interactions, whereby, under equal conditions and fitness, a single species selected at random prevails and excludes the rest of the competing species. When reaching maximum carrying capacity, competing species reach a population level close to their own unstable Allee range, with densities fluctuating due to demographic stochasticity. The sum of all these frequencies equals the carrying capacity. Therefore, if by chance one species increases, it comes at the expense of the others, inevitably reaching a point where the Allee inflection point is surpassed, either below or above, resulting in exclusion or prevalence over its competitors.