Reducing the quality of the niche available for colonisation by lowering O₂ flux reduces Wrinkly Spreader fitness. Oil overlays of 10, 20, and 40 mm were used to reduce O₂ flux or supply to the A-L or oil- interface in static microcosms. Microcosms were incubated for 3 days before assay and the relative competitive fitness of the Wrinkly Spreader was determined compared to the non-biofilm-forming P. fluorescens SBW25. Here, the relative fitness () is provided where the reference microcosms with no oil have relative fitness of one (marked by the horizontal dashed line); the curve indicated here is illustrative only. Means ± SE are shown (), and means not linked by the same letter are significantly different (TK-HSD, ). The standard microcosm (no oil) data are also shown in Figure 3.