Figure 55.11 Competitive Exclusion or Coexistence? G. F. Gause grew three species of Paramecium in tubes filled with medium containing bacteria and yeast. (A–C) When grown alone, each species reached a stable carrying capacity. (D, E) When grown in pairs, P. caudatum faced two possible outcomes—extinction or coexistence—depending on whether P. aurelia or P. bursaria was present. P. caudatum and P. aurelia both fed mainly on floating bacteria, while P. bursaria fed mainly on yeast cells.