Two Invasive Wetland Plant Species and the Relative Availability of Sandhill Crane Roosting Habitat on the Central Platte River
Resources provided by the Platte River and its riparian habitats are the key to a wide range of species’ survival. The Platte River has been altered to meet human needs, such as urban development, irrigation, and the introduction of non-native vegetation. In 1866, the Platte River channel in Nebraska, between Kearney and North Platte, was 1,200 to 2,000 meters wide, but has decreased dramatically since. Coincidently many species that rely on the river’s natural processes often run into challenges.
Many organizations are known for their collaborative conservation efforts along the Platte River to ensure suitable habitats are available and to maintain the natural system as much as possible. Locations such as Audubon Society’s Rowe Sanctuary protect the river and surrounding habitats to aid species survival. Rowe Sanctuary is owned and managed by the National Audubon Society and is located in the Platte River Valley near Gibbon, Neb. It is a great place for people from all over the world to learn about the river and wildlife, including the special experience observing the sandhill crane spring migration from blinds along the riverbank.
Sandhill Crane Migration
Between late February and mid-April, sandhill cranes migrate north to breeding grounds in the Canadian Arctic, western Alaska, and northeastern Siberia. The migration path used to fulfill this long journey is known as the Central Flyway, in which the pinch in its hourglass shape consists of a small section of the central Platte River. About 80 percent of the world’s population of sandhill cranes stage along this 80-mile stretch, between North Platte and Grand Island, to rest and refuel for up to a month.
Each night, from dusk to dawn, sandhill cranes gather on the river to communally rest, also referred to as a roost. A suitable and preferred roosting habitat for sandhill cranes is a maximum of 50 meters from riverbanks. A former study observed more sandhill cranes roosting on wide river channels compared to narrow river channels, so maintaining the river’s width increases roosting site availability. It is important sandhill cranes rest throughout the night, so areas surrounded by water are chosen to increase predatory detection. Sandhill cranes prefer short vegetation along the riverbank in order to limit predatory risk; therefore tall vegetation along riverbanks can cause unwanted obstruction to available roosting sites. When severe weather occurs or when the river channel is unsuitable for roosting, the cranes use wetlands as secondary roosting sites to limit disturbance. If they roost on the river during these times dense roosting occurs.
Invasive wetland plant species, such as common reed (Phragmites australis) and purple loosestrife (Lythrum salicaria) have negative impacts on roosting availability on the central Platte River. In recent years they have been encroaching on riverbanks and anchoring sandbars on the central Platte River, causing drastic changes in the river’s hydrology and surrounding habitats. When common reed and purple loosestrife start to invade, plant diversity is reduced until an eventual monoculture exists. The number of bird species found in invaded riparian areas and wetlands decrease due to the limited plant diversity and alteration of the river’s natural processes.
Managing and preserving the Platte River in south-central Nebraska is crucial for the survival of millions of migratory bird species, including the sandhill crane. The sandhill crane is not currently listed as an endangered or threatened species, although important staging habitat used during spring migration continue to dwindle. Land management and control treatments need to be applied annually to decrease the abundance of common reed and purple loosestrife. Methods conducted in former studies, such as applying herbicide or a combination of herbicide and burning have been found to decrease the species’ prevalence, but do not eliminate them.
Aerial infrared photography, remote sensing, and GIS mapping have been used in the past to document control methods and to study sandhill crane roosting habitat. For this study, time-lapse imagery was used to observe the relationship between common reed and purple loosestrife presence and the availability of roosting habitat at Audubon’s Rowe Sanctuary.
In 2011, water levels were high due to the flood year, but shallow water and bare sandbars were present during spring migration. The first sandhill cranes to arrive roosted primarily along the south bank on sandbars with limited vegetation in close proximity to dead common reed. The closest distance observed was approximately 10 feet away from a small stand of dead common reed. As sandhill crane abundance increased in 2011, roosting also occurred in the middle of the river channel and in close proximity to the north bank upstream from the Rowe Tower camera location.
Water levels were high with few visible sandbars during the 2012 spring migration. The 2011 flood dispersed purple loosestrife and common reed seeds downstream, so sandhill cranes were rarely seen roosting next to purple loosestrife or common reed. When roosting occurred along the south bank, cranes were at least 30 feet away with standing water between the roost site and riverbank for predatory detection. The first sandhill cranes to arrive to the river began their roost on the northern side of the river channel, but as more arrived they began occupying available sites throughout the wide channel. Roosting occurred primarily in the middle of the river channel in shallow water or bare sandbars surrounded by water. They were also observed roosting near the north bank upstream from the Rowe Tower camera location.
In 2013, water depth fluctuations occurred during spring migration. Shallow water and visible sandbars were present throughout the channel, but were occasionally limited. Sandhill cranes roosted primarily in the middle of the river channel like the previous years. When water was high and sandbars were no longer visible, roosting habitat availability decreased, dense roosting increased, and use of vegetated sandbars on the south bank occurred. The water depth fluctuation impacted roosting habitat availability significantly. When water was high, dense roosting occurred at available roosting sites and when water was low, roosting was dispersed throughout the river channel. Dense roosting is not desirable because it can lead to risk of disease and decrease affective rest for sandhill cranes. Occasional roosting within 30 feet of purple loosestrife and common reed was observed. Sandhill cranes began to roost later in the evening and stayed longer in the morning when water was high and during severe weather events.
Both invasive species pushed out native wetland plant species from the observed location and created thick monocultures along the south riverbank at Rowe Sanctuary. Purple loosestrife and common reed take up a large amount of water. During the 2012 drought, purple loosestrife was prevalent and fully bloomed, while native plant species died from lack of water. Locations on the river in which sandhill cranes were observed roosting varied due to water depth, tall and thick monocultures of invasive plant species, and weather events. The time-lapse imagery supports the findings of previously conducted research related to common reed, purple loosestrife, and sandhill crane roosting habitat.
Purple loosestrife and common reed are continuing to encroach on riverbanks; therefore combined control methods should continue to be used throughout the Platte River Basin to decrease their abundance on the central Platte River. The best control method observed is a combination of disking and applying herbicide in the same summer season; a burn-herbicide method at this location would be beneficial because it will increase the plant diversity and decrease the presence of invasive plant species. As native plant diversity increases and invasive plants decrease, more sandhill crane roosting habitat will be available at Rowe Sanctuary.