The formal access of transnational actors (TNA) to international organizations (IO) has increased steadily over the past five decades, and a growing body of literature is at the moment concerned with the theoretical and normative implications of these developments. However, very little is known as of yet about who the TNAs in global governance are, where they come from, which issue areas they focus on, and when and where they choose to participate. Using analytical tools from interest group theory, in particular a subfield called population ecology, this study describes and explains the chronological development of two populations of TNAs in global governance, namely the observer communities of the United Nations Framework Convention on Climate Change and the United Nations Convention on Biological Diversity. TNAs’ financial resources and their geographical proximity to global governance venues emerge as important factors influencing their capacity to participate, causing these TNA populations to be stratified and volatile.