This article argues that organizational ecology would benefit from comparative studies since the trajectories of organizational populations depend on the resource conditions under which these dynamics unfold. By comparing different settings, the boundary conditions of theories are determined and explanations sharpened. The article reports the results of a comparative study that starts from resource-partitioning processes that explain the counter-intuitive association between market concentration and the rise of specialist organizations. The authors set up an empirical study in the belgian audit industry, comparing the findings with those of a study of the dutch audit industry. Contrary to the dutch setting, this study finds that the failure rate of small (large) organizations increases (decreases) with market concentration in the belgian setting. The findings suggest that the shape of the resource space and the strength of exploitation economies mould market structure dynamics in predictable ways, and clarify the conditions necessary to trigger resource-partitioning.