In many political systems, the set of interest groups with a regular lobby presence is only a fraction of all mobilized interest groups. However, we know relatively little about how particular levels of organizational mobilization translate into actual lobby presence. First, major theories of interest representation propose competing expectations. They either posit a positive relationship between (biased) mobilization and (similarly biased) lobby presence, or expect a negative relationship between mobilization and lobbying, due to free-riding. In contrast, recent population-ecology approaches suggest a density dependent curvilinear relationship. Second, empirical research has been inconclusive because research either examines aggregate interest group populations or lobby presence in concrete lobby campaigns associated with specific institutions. In this paper we examine how the densities of interest communities within the EU predict actual lobby presence. The paper relies on a dataset linking lobby presence across 125 different legislative proposals with population data of EU organized interest.
