Platte River Recovery Implementation Program
Tern and Plover Habitat Synthesis Chapters
All information contained in this newsletter was prepared by the Executive Director’s Office (EDO) of the Platte River Recovery Implementation Program (Program). The information and results of analyses presented herein were focused solely on informing the use of Program land, water, and fiscal resources to achieve one of the Program’s management objectives: increasing production of interior least tern (Sternula antillarum athalassos; hereafter, tern) and piping plover (Charadrius melodus; hereafter, plover) from the Associated Habitat Reach (AHR) along the central Platte River in Nebraska.
Observations of tern and plover use of the central Platte River in relation to changes in hydrology and channel morphology over historical timeframes has been reviewed in depth by the EDO. The first species observations in Nebraska date to the period of exploration in the early 1800s while observations in the AHR date to the 1940s. By that time, basin hydrology had been altered by irrigation infrastructure and the channel was actively narrowing in response to changing flow, sediment, and disturbance regimes. Given the lack of species observations in the AHR prior to hydrologic alteration, a decline in habitat suitability and use has been inferred from a reduction in unvegetated channel width, a lack of contemporary in-channel nesting, and ongoing species use of the lower segment of the Platte River and other regional river segments. As such, the USFWS has concluded that actions resulting in depletions of flows that could further affect the hydrology of the AHR are likely to jeopardize the continued existence of one or more federally listed threatened or endangered species including tern, plover, whooping crane, and pallid sturgeon. These jeopardy opinions prompted the states of Wyoming, Colorado, and Nebraska and the Department of the Interior to enter into a Cooperative Agreement in 1997 for the purpose of negotiating the Platte River Recovery Implementation Program.
Since 2007, the Program has been implementing an Adaptive Management Plan (AMP) to reduce uncertainties about proposed management strategies and to learn about river and species responses to management actions. Implementation of the Program’s AMP has proceeded with the understanding that management uncertainties, expressed as hypotheses, encompass complex physical and ecological responses to limited treatments that occur within a larger ecosystem that cannot be controlled by the Program. The lack of experimental control and complexity of response precludes the sort of controlled experimental setting necessary to cleanly follow the strong inference path of testing alternative hypotheses by devising crucial experiments. Instead, adaptive management in the Platte River ecosystem has relied on a combination of monitoring physical and biological response to management treatments, predictive modeling, and retrospective analyses. The Executive Director’s Office of the Program pursued all three of these approaches, producing multiple lines of evidence across a range of spatial and temporal scales.
Since 2007, the Program has implemented management actions, collected a large body of physical and species response data, and developed modeling and analysis tools to aid in data interpretation and synthesis. Several lines of evidence now indicate implementation of the Program’s Flow-Sediment-Mechanical (FSM) management strategy may not achieve the stated management objective for terns and plovers. First, the EDO tested the concept that the FSM management strategy will increase sandbar height and produce suitably high sandbars for tern and plover nesting. We evaluated the ability of Short-Duration High Flow (SDHF) releases to create suitably-high nesting habitat for terns and plovers. There was an underlying assumption that sandbars would build to the peak flow stage during high flow events. The Program measured sandbar heights following natural high flow events in 2010, 2011, and 2013. Sandbar height-area relationships following the 2010 high flow event appear to provide the most conservatively high estimate of sandbar height potential. Observed mean sandbar heights following that event were on the order of 1.5 feet below the peak flow stage. At that height, sandbars produced by an SDHF release would typically not meet the Program’s minimum sandbar height suitability criterion for tern and plover nest initiation and would likely be inundated during the nesting season in most years. Secondly, effectiveness monitoring of channel morphology following flow releases and natural high flow events also indicate a decline in the amount of suitable habitat over time as constructed islands eroded. Validation monitoring of in-channel species use indicates a corresponding decline in in-channel nesting incidence. The decline in suitable habitat and nest incidence despite Program management and natural flow events hypothesized to produce suitable habitat were used as lines of evidence that the FSM strategy, as currently conceived, will not improve production of tern and plover from the central Platte River.
Next, we conducted a meta-analysis to examine the relationship between tern and plover nesting colony incidence and hydrology and physical characteristics in several Nebraska river segments used by the species. We found selection of riverine nesting habitat may be similar across river segments located in the lower Platte, Niobrara, and Loup Rivers. The probability of nesting colony incidence increased with increasing channel width so long as channels were not broken by vegetated islands. In channels broken by vegetated islands, probability of nesting colony incidence was low and did not increase with increasing channel width. Approximately 90% of channel widths at lower Platte River and Niobrara River tern and plover nesting colony locations exceeded 1,200 ft, which is much wider than all unmanaged segments of the AHR.
We then examined the concept of a physiological adaptation of terns and plovers to begin nesting concurrent with recession of the spring rise. We evaluated the distributions of central Platte River tern and plover nest initiation dates in relation to the annual hydrograph of the historical central Platte River and contemporary central and lower Platte River. We developed an emergent sandbar habitat model to evaluate the potential for reproductive success given observed hydrology, stage-discharge relationships, and sandbar height distributions. We found no evidence to suggest terns and plovers are physiologically adapted to begin nesting concurrent with the recession of the late-spring rise on the central Platte River. Model results indicate very little potential for plover reproductive success due to the timing and length of the nesting and brood rearing period in relation to the timing of the late-spring rise. Tern nest success potential was slightly higher due to the shorter nesting and brood rearing duration which increases the likelihood of successful renesting following the late-spring rise; however, maintaining a viable population on sandbars within the AHR appears to be unlikely.
Since 2007, the Program has implemented an AMP to explore key uncertainties related to the response of terns and plovers to management actions on the central Platte River. A primary question has been the role in- and off-channel tern and plover nesting habitat play on their productivity within the AHR. The results of substantial investments for in- and off-channel mechanical habitat creation, flow and species monitoring, and related data analysis and synthesis have led the Program to re-examine the benefits of management strategies that place a heavy emphasis on in-channel habitat. The Program has shifted towards species management activities focused primarily on creating and maintaining off-channel nesting habitat while providing a limited amount of in-channel habitat. This shift in management, based on Program learning, represents a successful application of adaptive management, which is unique among riverine restoration programs attempting adaptive management at large scales.
Details summarized here can be found at: https://www.platteriverprogram.org/PubsAndData/ProgramLibrary/PRRIP%202015_Tern%20and%20Plover%20Habitat%20Synthesis%20Chapters.pdf