Among-individual variation in resource use is pervasive and may have ecosystem-wide effects. This variation between individuals can affect population niche breadth. My study determined if niche breadth was best explained by dispersal of individuals from locations with different prey resources driven by ecosystem level disturbance regimes or competition among individuals inhabiting a site, resulting in dietary partitioning. I used structural equation modeling to examine the direct and indirect effects of several environmental variables spanning gradients of disturbance, competition strength, and food availability on niche breadth of the Eastern Mosquitofish (Gambusia holbrooki). I evaluated two complementary models 1) allowing for only direct effects of disturbance on niche breadth and 2) limiting effects of disturbance on niche breadth to indirect effects via food availability and competitor densities. The partitioning hypothesis excluding direct effects of disturbance on niche breadth was best supported by my data.