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One of the recent attempts to address the Hubble and $S_8$ tensions is toconsider the Universe started out not as a de Sitter-like spacetime, but ratheranti-de Sitter-like. That is, the Universe underwent an "AdS-to-dS" transitionat some point. We study the possibility that there are two dark energy fluids,one of which gave rise to the anti-de Sitter-like early Universe. Theinteraction is modeled by the Lotka-Volterra equations, commonly used inpopulation biology. We consider "competition" models that are furtherclassified as "unfair competition" and "fair competition". The former involvesa quintessence in competition with a phantom, and the second involves twophantom fluids. Surprisingly, even in the latter scenario it is possible forthe overall dark energy to cross the phantom divide. The latter model alsoallows a constant $w$ "AdS-to-dS" transition, thus evading the theorem thatsuch a dark energy must possess a singular equation of state. We also considera "conversion" model in which a phantom fluid still manages to achieve"AdS-to-dS" transition even if it is being converted into a negative energydensity quintessence. In these models, the energy density of the late timeeffective dark energy is related to the coefficient of the quadraticself-interaction term of the fluids, which is analogous to the resourcecapacity in population biology.

An Effective Sign Switching Dark Energy: Lotka-Volterra Model of Two Interacting Fluids