More than 40 percent of the planet's land mass is covered in grasslands -- a semi-arid ecosystem dominated by grasses, which receives too little rainfall to enable tree growth. Known worldwide as prairies, savannas, steppes, veldts, rangelands and pampas, these grassland systems collectively represent Earth’s most altered and endangered ecosystem, due in large part to intensified agriculture and development. Grassland surface waters, most originating from rainfall or melting snow, vary in their hydrology and chemistry, but all provide essential wildlife habitat.
Globally, streams are a critical component of grassland ecosystems, providing an essential link to downstream habitat. Compared to other ecosystems, grasslands tend to have more intermittent or seasonal streams, flowing only part of the year. Highly variable flows resulting from periods of flooding and drying make prairie streams ideal models in disturbance ecology -- the study of how ecosystems respond to temporary, but intense, changes in environmental conditions. One of the planet’s most studied grassland streams is Kings Creek in Konza Prairie of northeastern Kansas. From nearly three decades of work at Kings Creek, scientists have learned that prairie streams have excellent water quality and low transport of contaminants, support unique and often endangered species, and control the movement and processing of materials, notably nitrogen, from land to downstream aquatic habitats.
Roughly 12,000 years ago, the last of the Pleistocene Epoch glaciers retreated from the northern Great Plains, leaving behind an area pockmarked by millions of depressions. These depressions, or potholes, cover a swath of land totaling more than 270,000 square miles, extending from northwest Iowa to central Alberta. In the spring, the potholes fill with rain and snow melt, often absorbing excess water that would otherwise contribute to flooding. More than half of North America's migratory waterfowl use these depressional wetlands to rest, nest or breed. Tewaukon National Wildlife Refuge, in the heart of North Dakota's prairie pothole region, supports upwards of 700,000 snow geese, 2,000 swans and 75,000 ducks during the fall migration.
Vernal pools are temporary wetlands that are formed in cooler months by precipitation, but dry during the summer months. They're distributed throughout the world in Mediterranean climates, but are particularly abundant in and around grasslands on the Pacific Coast. Vernal pools are particularly important to species that depend on habitat free of predators to raise their young, such as the rare freshwater fairy shrimp and many amphibian species. In southern Oregon and California, more than 20 threatened and endangered species co-occur in vernal pools, prompting the U.S. Fish & Wildlife Service to implement a broad strategy of protecting an entire ecosystem -- in this case all intact vernal pools -- as a way to conserve all of the threatened inhabitants.
Flat-bottomed basins known as "playas" can be found in the desert grasslands of Texas, New Mexico, Kansas, Oklahoma and Colorado. Found at the lowest point of a large drainage area, these seasonal, marsh-like ponds collect and store water from rain and runoff. Unlike vernal pools, playas generally fill when temperatures are unsuitable for plant growth, and often the water is salty and/or alkali. Colorado's eastern plains are home to more than 2,500 playas that range in size from one to 50 acres. More than 200 species of birds, in particular shorebirds and waterfowl, utilize these playa lakes.