Why do some transnational advocacy groups adopt radical, confrontational tactics whereas others focus on ‘inside’ lobbying and information provision? Why do some advocacy groups seek to appeal to large global audiences while others approach decision-makers behind closed doors? Bringing together interest group studies and population ecology theory, this article examines how population ecological dynamics affect strategic specialization among transnational advocate groups. I argue that growing resource competition resulting from ‘organizational crowding’ along with the introduction of new legal and technological tools has led to growing strategic differentiation among transnational advocates, and has prompted a strategic division-of-labor whereby some groups (mainly larger, well-established and resource-rich groups) specialize in gaining political access and media attention, while others (mainly smaller, less established groups) develop ‘niche’ agendas and strategies including, inter alia, radical protest, monitoring and enforcement, and litigation. I illustrate my argument with quantitative data and comparative cases from the realm of transnational environmental conservation advocacy.