The US Long Term Ecological Research (LTER) program began in 1980 to address large scale ecological phenomena through research at individual sites and through comparative and synthetic activities among sites. We applied network science measures to assess how the LTER program has performed as a coherent system using joint publications among sites as the measure of collaborations across sites. We identified three periods of increasing collaboration. As the LTER program has grown from 6 to 26 study sites over the past quarter century, it has evolved from a collection of individually operating sites of multiple ephemerally connected groupings to a collaborative, densely connected network. While numerous factors have fostered collaboration, site similarity and history, actions by the Network office and the National Science Foundation correspond to the coalescing of the network.