The U.S. Great Plains is arguably the planet's most important agricultural region, helping to feed the world. At the same time, this vast area is home to one of the planet's great animal migrations, the movement of tens-of-millions of water birds (ducks, geese, shorebirds, etc.) from wintering grounds in the south to breeding ponds in the Prairie Pothole Region (PPR) of the Northern Great Plains. Conservation of this North American Central Flyway while at the same time supporting global food security, represents a tremendous challenge for policymakers, farmers, and wildlife managers. The National Science Foundation's Macrosystems Biology program, part of NSF?s broader emphasis on "Emerging Frontiers", was conceived in part to develop the science needed to address such difficult questions in environmental sustainability. Such problems are so vexing because they: (1) Span very large land areas; (2) Include multiple interacting parts, in this case birds, farmers, wetlands, grasslands, climate change and national environmental policy; and (3) Involve interactions which take place across a range of spatial and time scales; all of which influence long-term sustainability. 

NSF?s Intellectual Merit Criterion addresses the potential of a project to advance knowledge and understanding within its own field, or across different fields. Working across fields, we adopted methods from the analysis of social networks to describe movements of waterbirds across a special type of spatial network called a "habitat network" (Fig. 1).  As an example, the best breeding habitat for ducks consists of groups of ponds in close proximity to one another. At this local scale, ponds are considered "connected" only if separated by 0.6 miles (1 km) or less (Fig. 1). At larger, regional scales we constructed habitat networks composed of hundreds-of-thousands of water bodies (Fig. 2). These networks represent potential migratory pathways for millions of birds as they navigate across millions of square miles. 

Each spring, as temporary ponds in the PPR appear and disappear in response to changes in precipitation patterns, migratory birds experience different habitat networks. We found that year-to-year changes in these networks explained where mallard duck populations congregated to breed each spring (Fig 2). Notably, we found that there is a critical wetland density, around 1 pond per .77 square-miles (1 pond per 2 square-kilometers), below which habitat networks begin to fall apart, or fragment. At local scales, fragmented wetland networks are less likely to maintain high rates of reproductive success. At larger regional scales, fragmented networks make it more difficult for migratory birds to find high-quality habitat. 

This critical wetland density may hold the key to long-term resilience and sustainability of the Central Flyway. In fact, it likely explains why the Prairie Pothole Region has maintained its status as North America's most productive waterfowl habitat, while also supporting intensive agricultural production. In general, it appears that regulatory protections and actions of individual farmers have maintained wetland densities above critical levels. Collaborators from other institutions have shown that such network resilience extends to the southern Great Plains, where playas (another type of temporary water body) form the early stages of the migratory network.

Next, we considered land use change. Grassland cover surrounding breeding ponds is an important component of waterbird breeding success, as it provides concealment from predators and maintains water quality. Across the Great Plains, we found high rates of grassland conversion to cropland; with cropland expansion concentrated around corn ethanol refineries in the Dakotas. These findings were featured in the U.S. Environmental Protection Agency's 2018 "Report to Congress on Biofuels and the Environment". Grassland losses raise the potential for a systematic reduction in wetland quality, which may imply a reduction in "effective" wetland density across the PPR.

Lastly, we considered climate-change resilience of the Central Flyway. Somewhat surprisingly, computer simulations of wetland hydrology showed that projected increases in air temperature were offset by increased spring precipitation, potentially leading to higher wetland densities under climate change. This led to projected increases in waterbird populations by 2100, contrary to predictions from other studies.

In sum, we found that Great Plains wetland landscapes, and the waterbirds that rely on those landscapes, may be surprisingly resilient. This resilience appears to be an emergent property of wetland habitat networks. Therefore the conservation challenge becomes maintenance of those networks. What appeared to be such a vexing challenge as described above, may be reduced to maintaining wetland densities above critical thresholds. Any number of factors influence wetland density on the landscape. But the flip-side is that any number of actions can be taken to maintain those densities. We cannot emphasize enough that such an insight would not have been possible without support from NSF's Emerging Frontiers Macrosystems Biology program.

Last Modified: 03/29/2020
Modified by: Christopher K Wright