Dynamic populations, dynamic landscapes: conservation science case studies of colonial waterbirds in the North American Great Lakes

Abstract

Colonial animals concentrate locally, presenting different conservation challenges and opportunities than more broadly-distributed species. The Double-crested Cormorant (Phalacrocorax auritus) and the Black Tern (Chlidonias niger) in the North American Great Lakes provide two contrasting case studies of the challenges and opportunities for conservation and management of colonial waterbirds. I used a combination of historical datasets, new field studies, and statistical modeling to investigate issues surrounding conservation and management of these two birds in Great Lakes. The Double-crested Cormorant, focus of the first case study, has experienced extreme population growth and is now subject to population management at a majority of U.S. Great Lakes colony sites; effects of cormorants and their management on co-nesting waterbird species have remained largely unknown. I observed that nesting among Double-crested Cormorants increased frequency of agonistic interactions for Great Blue Herons (Ardea herodias), particularly when nesting on the ground. My research also showed that Black-crowned Night-Heron (Nycticorax nycticorax) colony growth was negatively associated with Double-crested Cormorant abundance and implementation of management, while Herring Gull (Larus argentatus) and Ring-billed Gull (Larus delawarensis) colony growth was positively associated. The Black Tern, subject of the second case study, has been declining for over half a century due to unknown causes, although wetland loss and degradation have likely contributed. I found a positive relationship between increasingly clumped vegetation in Great Lakes coastal wetlands and historical Black Tern colony abandonment. Under current conditions in the region, wetland type and area were critical parameters in delineating wetlands that were unsuitable for Black Terns from potentially suitable ones. In the ever-changing landscape of the North American Great Lakes, my investigations provide important results to inform future conservation and management actions for these two very different species.