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Endangered and Threatened Species of the Platte River 6 PIPING PLOVER AND INTERIOR LEAST TERN Antithetical though it may seem from their vernacular name, shorebirds are common in North America’s vast interior. Sandpipers, willets, curlews, and their many relatives are found across the Great Plains and central Canada north to the high Arctic. Why piping plovers and interior least terns, now recognized as endangered and threatened species, reside in Nebraska, so far from ocean and sea, is immediately apparent to the astute observer. Great Plains rivers—with their braided channels, gravel bars, and sandy shores—offer conditions for rearing young that are remarkably similar to the conditions that support plovers and terns on distant coastal shores. But those conditions, which have been sustained for millennia by dynamic fluvial geomorphic processes, are changing rapidly. Impoundment and exports of Platte River Basin waters have altered the river’s hydrograph and caused the disappearance of landscape features that sustain the two species. As their habitat has been lost, populations of piping plovers and interior least terns have declined dramatically along the central Platte River. The following chapter explores three specific questions posed to the committee: Do the present habitat conditions on the Platte River affect the likelihood of survival and recovery of the piping plover? Do they affect the likelihood of survival and recovery of the interior least tern? Is the currently designated critical habitat on the central Platte River for the piping plover supported by existing science? Observations in this chapter also support conclusions related to a more general question concerning gaps in knowledge regarding the two species.
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Endangered and Threatened Species of the Platte River PIPING PLOVER The piping plover (Charadrius melodus) is a small shorebird that has threatened or endangered status throughout its range (USFWS 1988; Haig 1992; Thompson et al. 1997). The species is distinguished from other, smaller plovers by a single black neck band that is present during the breeding season and a short, stout bill (orange during breeding), pale gray back and wings, white belly, and orange legs (Figure 1-4). The name “piping” refers to the bird’s distinctive flute-like vocalizations. Distribution The U.S. Fish and Wildlife Service (USFWS) and Nebraska Game and Parks Commission (NGPC) collect and maintain data on plover nesting. Methods used to monitor plovers in the Platte River are the same as or similar to those used by biologists elsewhere in the range of this species in North America. Protocols for monitoring breeding pairs, estimating productivity, and reporting results have been formalized.1 Since the Northern Great Plains (NGP) population was listed, Platte River nesting records have been consistently collected and maintained. The general method used to obtain estimates of breeding pairs throughout the piping plover range is to walk toward potential nest habitat or approach it by boat (Plissner and Haig 2000). Potential habitat is identified on the basis of historical records (plovers are very site-faithful if habitat is suitable) and knowledge of habitat characteristics. In the central Platte, USFWS and NGPC biologists obtain estimates by using an airboat (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003); they do not go on the land, because some of it is private property. At sandpits and Lake McConaughy, monitors approach plover habitat on foot (Mark Peyton, CNPPIR, pers. comm., September 2003; Jim Jenniges, Nebraska Public Power District, pers. comm., May-August 2003). USFWS’s surveys by river use two observers and are conducted in May and early June, depending on river conditions. At potential habitat sites, the monitors stop and check the landscape with a telescope and record the presence or absence of plovers. Monitors also record the Global Positioning System (GPS) location of the site. If plovers are present, the number of individual birds is counted, and their behavior in riverine habitat is recorded. Plovers typically vocalize and are usually visible to monitors because of their flight or rapid movements on the ground. If monitors are close to a nest, plovers may perform “broken-wing 1 Governance Committee, Executive Director’s Office, Tern and Plover Monitoring Protocol Implementation Report, unpublished material, Feb.12, 2001.
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Endangered and Threatened Species of the Platte River behavior” to distract the observer. Monitors record the presence or absence of birds at a particular site and note behavior that indicates nesting. If there is evidence of nesting or if the team believes that the birds will nest, they return daily or several times per week to determine whether chicks have hatched. Visits are made until adults leave or it is decided that no chicks are present, typically June-July. At the end of each season, data on pair estimates and nesting success are sent to NGPC, where the Nebraska piping plover database is maintained. Since records were first recorded for piping plovers, the methods have become more formalized, particularly after the methods section of the Cooperative Agreement Tern and Plover Monitoring Protocol was prepared. The piping plover breeds only in North America. Its total population was estimated at nearly 6,000 adults in 2001 (Susan Haig, USGS, pers. comm., July-August 2003). The Great Lakes population is recognized as endangered under the Endangered Species Act (ESA), and the NGP and Atlantic Coast populations are recognized as threatened. Birds nesting on the Platte River are part of the NGP population. Piping plovers breed on open beaches along the Atlantic Ocean and Great Lakes, on alkali flats, on islands in broad prairie rivers, and along reservoir shorelines in the NGP. They winter along the Atlantic Coast from Virginia to southern Florida, in the Caribbean, and along the Gulf Coast from Texas and Mexico. Historical distribution and nesting records exist for piping plovers in Nebraska back at least to the 1800s (EA Engineering, Science and Technology, Inc. 1988; Lutey 2002). Before listing, the species was reported from 32 of 97 counties in the state. Information on population status is more recent: statewide estimates were not conducted until after the “population” was listed in 1986. The presettlement plover breeding-population size in Nebraska is unknown. The breeding range of the NGP population includes southern Alberta, southern Saskatchewan, and southern Manitoba; extends south to eastern Montana, North Dakota, South Dakota, southeastern Colorado, Iowa, and Nebraska; and extends east to Lake of the Woods in north central Minnesota. Most of the breeding pairs in the U.S. portion of the population’s range are in North Dakota, South Dakota, Montana, and Nebraska. USFWS conducted international winter and breeding censuses for piping plovers in 1991, 1996, and 2001. Trend data indicate that the NGP piping plover population declined by 15% from 1991 to 2001. Plovers breed in Nebraska on sandbars, along reservoir shorelines, in commercial sand mines, and at other artificially created sites along three major rivers (Figure 6-1, 6-2, and 6-3). In the northeastern corner of the state, along the border with South Dakota, nesting occurs along about 64 km of the Upper Missouri River and 153 km of the lower Niobrara River
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Endangered and Threatened Species of the Platte River FIGURE 6-1 Sandy low bars along central Platte River serve as nesting areas for piping plovers and interior least terns. Source: Photograph by W.L. Graf, May 2003. (Figure 6-4). Farther south, plovers are found along about 390 km of the central and lower Platte River, from the Missouri River west to Lexington. Breeding also occurs at Lake McConaughy in western Nebraska and on the Middle Loup and Loup rivers in central and eastern Nebraska (L.C. Wemmer, USFWS, unpublished material, February 11, 2001). Census efforts during the breeding season estimated 398, 366, and 300 pairs of piping plovers in Nebraska in 1991, 1996, and 2001, respectively (Figure 6-5) (Susan Haig, USGS, pers. comm., July-August, 2003). Those estimates do not include plovers on the portion of the Missouri River that is shared with South Dakota. Current estimates indicate that nesting pairs of piping plovers in Nebraska make up 10-12% of the NGP population, and about 9% of Nebraska’s breeding piping plovers nest on the central Platte River (Dinan, NGPC, pers. comm., May-September 2003); about 1% of the NGP population of piping plovers nest on the central Platte River. Lutey (2002) reported that the numbers of piping plovers observed on the Platte River declined from 1987 to 1998, a period during which the number of breeding pairs averaged just one for the South Platte, 35 for the North Platte (at Lake McConaughy), four on the upper Platte, 29 on the
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Endangered and Threatened Species of the Platte River FIGURE 6-2 Sand mines along margin of central Platte River serve as nesting areas for piping plovers and interior least terns, but are not as suitable as sand masses in river. Source: Photograph by W.L. Graf, May 2003. central Platte, and 50 on the lower Platte. John Dinan (NGPC, pers. comm., May-September 2003) reported a 61% decrease in piping plovers nesting on the central Platte River from 1991 to 2001, so piping plovers appear to have declined over the last decade at regional, state, and local levels. Declines at all levels are attributed primarily to human activities, including direct and inadvertent harassment of birds and nests by people, domesticated animals, and vehicles; destruction of shoreline habitat as a conse-quence of development projects; increased predation due to human presence in less-visited beach areas; and water-level regulation policies that result in changes in nesting habitat (Haig and Elliott-Smith, in press; Haig 1992). Causes of the declines recorded in the NGP population include predation of eggs and chicks, habitat destruction and degradation that result from channelization of rivers and modification of river flows, disturbance by humans and pets, contaminants, and inadequate regulatory mechanisms (L.C. Wemmer, USFWS, unpublished material, February 11, 2001). To understand habitat use by plovers in the Platte River, it is important to establish whether birds are sedentary—that is, use one or a few sites during their lifetime—or disperse extensively. That is important because it
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Endangered and Threatened Species of the Platte River FIGURE 6-3 Sandy shore of Lake McConaughy provides nesting areas for piping plovers and interior least terns when the reservoir is low enough to expose beaches. Source: Photograph by W.L. Graf, May 2003. is a key to understanding local population dynamics, population persistence, and the spatial scale on which management of the Platte River will affect regional populations. Movements of plovers that hatched along the Platte River or were first banded as adults nesting along the Platte have been studied by Lingle (1993c). He reports resightings of 329 plovers banded from 1985 to 1989 between Lexington and Grand Island and observes that in the year after banding, 43% of the plovers banded as adults returned to the same site, but only 18% of the birds banded as chicks returned to the site where they hatched. Those observations suggest that missing birds either died or moved to other locations. An unknown number of birds were probably present in the study area but not detected. Without human observers present to look for banded birds, it is not possible to infer where the surviving banded birds eventually nested. Lingle did, however, obtain several dispersal records worth noting: seven birds were observed at other nest locations in the Platte River Valley, including an adult banded near Kearney that nested 155 mi downstream 2 years after banding and a banded bird captured at Lake McConaughy, 300 mi from its banding site on the lower Platte River. Lingle’s observations indicate that plovers from the Platte River are capable of moving far between breeding seasons and show the difficulty in making inferences
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Endangered and Threatened Species of the Platte River FIGURE 6-4 Piping plovers and interior least terns distribution in Niobrara, Loup, and Platte Rivers, Nebraska. Source: Dinan 2003. about local populations, movements of individuals, and patterns of habitat use without information on the behavior of marked birds. They also suggest strongly that piping plovers in Nebraska are part of a single demographic unit of interacting individuals. Records on timing of breeding-season activities are maintained by the Wildlife Division of the NGPC (Dinan, NGPC, pers. comm., May-September 2003). Piping plovers typically arrive at the Platte River in middle to late April and depart by late August. During that interval, they locate a mate, FIGURE 6-5 Estimated piping plover (PP) and interior least tern (LT) population numbers represented in pairs during 1991, 1996, and 2001 breeding season for Nebraska, excluding Missouri River. Source: Dinan 2003.
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Endangered and Threatened Species of the Platte River construct a shallow nest on sand or gravel substrate, and, between early May and early July, lay eggs and incubate them for about 28 days. First hatching typically occurs in late May or early June but, depending on spring water flow, may extend into late June and July. Renesting can occur if the entire clutch is lost. Chicks require more than 21 days of growth and development after hatching before they are able to fly. After young birds fledge, they continue to feed and mature along the Platte River until they begin autumn migration in August. Influence of Current Central Platte Conditions on Survival of Piping Plovers on Platte River Plovers, like all species, have specific requirements for survival. USFWS identifies among the requirements space for population growth and normal behavior; nutritional or physiological requirements (such as food, water, air, light, and minerals); cover or shelter; sites for breeding, reproduction, and rearing of offspring which are protected from disturbance; and habitats representative of the historical distribution of the species (Fed. Regist. 67 (176): 57638 ). Those survival requirements have been used by USFWS to identify the primary constituent elements (PCEs) of critical habitat in accordance with Section 3(5)(A)(i) of the ESA. PCEs for the NGP population of piping plovers are habitat components (physical and biological) essential for the biological needs of courtship, nesting, sheltering, broodrearing, foraging, roosting, intraspecific communication, and migration (Fed. Regist. 67 (176): 57638 ). According to the NGP population recovery plan (USFWS 1988) and critical habitat final rule (Fed. Regist. 67 (176): 57638 ), PCEs on the Platte River include sparsely vegetated channel sandbars, sand and gravel beaches on islands, temporary pools on sandbars and islands, and interface zones with the Platte River (Figure 6-6). Those habitat features are directly influenced by dynamic precipitation cycles and longer-term climate patterns. Habitat area, abundance and availability of insect foods, brood and nesting cover, and lack of vegetation are all linked to weather and climate. Variability in flow can cause high rates of turnover of naturally variable sandbars. Flowing water creates diverse habitats for feeding, nesting, and brooding. Habitat variables were quantified by Ziewitz et al. (1992) during the 1988 breeding season and used to prepare USFWS’s target species suitable habitat document (USFWS, unpublished material, June 2000). Table 6-1 summarizes habitat characteristics important to the species. Studies on the habitat requirements of the piping plover across its breeding range have been few. However, the study by Ziewitz et al. (1992) quantified a number of variables (such as channel width, sandbar area, mean nest elevation, and maximal nest elevation) present at the piping
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Endangered and Threatened Species of the Platte River FIGURE 6-6 Area of central Platte River channel near Shelton with many primary constituent elements for the piping plover. Channel sandbars lack woodland cover, temporary pools on bare sand are abundant, and interface zones are plentiful. Source: Photograph by W.L. Graf, May 2003. plover nesting sites along the Platte River. The effort was an important contribution to understanding of the piping plover habitat requirements along the central Platte River. Using an airboat, Ziewitz’s research team traveled the reach of the Platte River from Lexington to its confluence with the Missouri River in the middle of June. They located as many nest sites as possible and (to minimize disturbance to breeding birds) marked the nests for future study. Dates of hatching and egg laying were estimated by floating eggs in a container of water. Age is determined by the position of the egg as it floats in the water; older eggs float higher in the water column (Westerskov 1950). Ziewitz et al. (1992) used aerial videography to record the Platte River over the summer. They analyzed scenes at the 26 sites occupied by plovers and univariately compared physical characteristics of occupied and unoccupied sites. The study showed that more plovers nested on the lower Platte River than the central Platte, birds nested on river segments that were wider and had greater areas of sparsely vegetated sandbars, channels were wider and sandbars higher along the lower Platte River than on the central Platte, nests on the lower Platte had greater clearance
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Endangered and Threatened Species of the Platte River above the river’s water line when initiated than did nests on the central Platte, and the average elevation of nests was greater on the lower Platte, which suggested that nest habitat was limited on the central Platte. Several other studies (e.g., Ducey 1988; Faanes 1983) attempted to quantify vegetation at piping plover nest sites, documenting that occupied sites tended to be sparsely vegetated or to lack vegetation early in the nesting season. No studies are available that quantify plover habitat characteristics during periods of especially low water flow. Current information suggests that no suitable piping plover habitat now exists along the central Platte River (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003). Flow has been so low that no new sandbars are being created. Sandbars that once were suitable are now unsuitable because no recent scouring has occurred to prevent establishment of vegetation, and many river reaches have been nearly dry in recent years. In 1988, about 25 pairs nested on the central Platte River and in sandpits; Ziewitz et al. (1992) concluded that reproduction by piping plovers was being compromised by low flows that were punctuated by sudden water peaks in July, which led to nest flooding. Lingle (1993a) attributed the July peaks to a combination of heavy local precipitation and releases of water to the river via the Johnson-2 return during local thundershowers. He reported that nest success was higher on the lower Platte River, where sandbar heights are greater than on the central Platte. Plover nesting in 1988-1996 varied from 20 to 42 pairs on the central Platte (1-15 pairs on the river, 13-30 pairs in sandpits). However, since 1996, fewer than five pairs per year have nested on the central Platte. Chick productivity at river sites along the central Platte declined to about 1.3 chicks per pair in 1997 and 1998; since 1999, there has been no successful reproduction. In addition to riverine habitat, piping plovers nest in commercial sandpits on the central Platte River. Another important adjacent source of alternative habitat is Lake McConaughy to the west. Large numbers of plovers nest along the shoreline of Lake McConaughy when water levels are low. For example, high water elevation, 3,262 ft above mean sea level (msl), along the lake in 1986 resulted in narrow beaches (<25 m) and restricted habitat availability. In May 1991, in contrast, lake elevation was extremely low, 3,245 ft msl, and resulted in beach widths of 400-800 m and abundant nesting habitat (Wingfield 1993). It is important to consider potential habitat at Lake McConaughy because plovers produced at this reservoir may nest along the central Platte River, central Platte plovers may nest at Lake McConaughy (Wingfield 1993), and relationships between nesting along the river and the lake are probably related to water levels and habitat conditions at both locations. Finally, it is important to emphasize that lack of knowledge about population fidelity (tendency to return to same place to nest) and movement on all spatial scales (including both the
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Endangered and Threatened Species of the Platte River TABLE 6-1 Habitat Characteristics Important to the Piping Plover Piping Plover Habitat Observed Measurements of Habitat Parameters (OMHPs) Preliminary Goals for Habitat Management References Riverine habitat Channel or sandbar characteristics Channel width 975-1,554 ft ≥ 900 ft, initially Ziewitz et al. 1992 Kirsch 1996 Sandbar area (early June, at nest initiation) 0.03-3.58 acres Variable Ziewitz et al. 1992 Kirsch 1996 Mean elevation above 400-cfs stage (early June, at nest initiation)a 0.2-2.0 ft (mean, 0.4 ft) Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season Ziewitz et al. 1992 Maximal Elevation above 400-cfs stage (early June, at nest initiation)a 0.4-4.4 ft (mean, 2.7 ft) Low, ephemeral sandbars; high enough to provide dry, bare sand during nesting season Ziewitz et al. 1992 Vegetation Cover: at nest site <5-20% (early in nesting season) Sparsely vegetated or unvegetated Schwalbach 1988 Ducey 1983 On sandbar <10-25% Faanes 1983 Height (at nest site) <2 ft (early in nesting season) Same as OMHP Schwalbach 1988 Prindiville-Gaines and Ryan 1988
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Endangered and Threatened Species of the Platte River by plovers and terns and compared physical characteristics of occupied and vacant sites (every 3 mi along a river transect). The study showed that more terns nested on the lower Platte River than on the central Platte and that birds nested on river segments that were wider and had larger areas of unvegetated or sparsely vegetated sandbars than on vacant sites. Channels were wider and larger and sandbars higher on the lower Platte River than on the central Platte, and nests on the lower Platte had greater clearance above the river water line when initiated than did those on the central Platte. The average elevation of nests was higher than that of vacant sites available on the lower Platte, but lower than that of vacant sites available on the central Platte River. On the basis of those results, Ziewitz and colleagues concluded that habitat availability may be limiting populations on the central Platte River. Current reports describe riverine habitat as nonexistent along the central Platte (Erika Wilson, USFWS, pers. comm., July-September 2003; John Dinan, NGPC, pers. comm., May-September 2003). Recent flows are so low that new sandbars are not being created and old sandbars have become unsuitable because no scouring has occurred to prevent vegetation from colonizing. In 1988, about 60-65 pairs of interior least terns nested on the central Platte River; Ziewitz et al. (1992) contended that later reproductive success and nesting activity were compromised by low flows, which were punctuated by sudden water peaks in July that flooded nests. Tern nesting efforts between 1988 and 1996 varied between about 60 and 90 pairs per year on the central Platte River. From 1997 to 2001, the number of pairs attempting to nest ranged downward from 45 to 15. From 2001 to 2003, the number of pairs on the river was below 12. Since 1991, interior least tern productivity on the river has been no greater than two chicks fledged per pair; no chicks fledged in six of those years, and none during the last five. Reproductive success of interior least terns appears to be influenced primarily by water-flow regimes. The birds require sandbars or small islands that are relatively free of vegetation and are at sufficient elevation for eggs and chicks not to be flooded. Sandbars and islands also provide isolation from mammalian predators. The relationship between water levels and site occupancy is well documented by numerous authors throughout the breeding range of least terns in North America. In a study by Lingle (1993b), predation and flooding accounted for 74% of nest failures in Platte River habitat. Predation was the greatest source of nest failures in sandpits, followed by human disturbance and weather. USFWS and NGPC collect and maintain data on interior least tern nesting. Methods used to monitor terns in the river are the same as or similar to those used by biologists elsewhere in the range of the species in North America. Protocols for monitoring breeding pairs, estimating productivity, and reporting results have been formalized. Since the interior
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Endangered and Threatened Species of the Platte River population of the least tern was listed, nesting records for the Platte River have been among the most consistently collected and maintained. Kirsch and Sidle (1999) reviewed the status of the interior population and stated that “methods, level of effort, and degree of coordination that biologists use on the upper Missouri, the Platte, and lower Mississippi rivers are needed throughout the interior breeding range”; that supports the idea that data used to recommend conservation and management strategies for interior least terns on the Platte River have been drawn from appropriate scientific approaches. Because piping plovers and interior least terns typically co-occur on the Platte River, the species are surveyed at the same time, and similar methods are used, as described earlier in this chapter. Our findings are essentially the same as those reached for the piping plover. According to discussions with USFWS and NGPC biologists (Erika Wilson and John Dinan, respectively), who manage the interior least tern database and survey for nesting birds, only small numbers of terns have attempted to nest along the central Platte River over the last 5 years. All have failed to produce young. Failure is attributed to low water flow and lack of dynamic ecosystem processes appropriately timed to create new island and sandbar habitat without flooding nesting pairs. Therefore, current conditions on the central Platte River appear to be compromising the continued existence (survival) of the interior population of least terns. That conclusion was based on the following observations: few terns have attempted to nest on the central Platte River in the last 5 years, no eggs have hatched or chicks fledged along the river during the last 5 years, water levels deemed necessary to produce nesting habitat have not been reached on the central Platte in the last 5 years, and loss of habitat along the river is forcing birds to use alternative sites that tend not to offer adequate protection from predation and flooding and do not provide habitat conditions needed for survival of adults and chicks. In addition, loss of central Platte River habitats from the geographic range of the interior population of least terns reduces breeding options for an already vulnerable regional population. Influence of Current Central Platte Conditions on Recovery of Interior Population of Least Terns For many threatened and endangered species for which a recovery plan has been prepared, a population goal is set that must be reached before the species or population can be removed from the federal list. Additional criteria are usually stipulated for recovery. For the interior population of the least tern, the recovery plan (USFWS 1990) states that “in order to be considered for removal from the endangered species list, 1) interior least tern essential habitat will be properly protected and managed and
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Endangered and Threatened Species of the Platte River 2) populations will have increased to 7,000 birds.” The population goal is broken down into five river systems—Missouri River, Mississippi and Ohio Rivers, Arkansas River, Red River, and Rio Grande. The Platte River is included in the Missouri River system, for which specific recovery guidelines have been promulgated: the number of birds in the Missouri River system must increase to 2,100 adults; essential breeding habitat will be protected, enhanced, or restored; and breeding pairs will be maintained in a specific distribution (listed by state and river) for 10 years (assuming that a minimum of four censuses have been conducted). The recovery goal is 1,520 adults in Nebraska, including 750 adults along the Platte River. Lutey (2002) reported the average numbers of interior least tern pairs at sites along the Plate River from 1987 to 1998—South Platte River (one), North Platte (Lake McConaughy) (six), upper Platte (13), central Platte (74), and lower Platte (187)—for a total of 550 individuals, not counting six pairs at Lake McConaughy. From 1991 to 2001, interior least tern nesting on the central Platte River decreased by 47% (John Dinan, NGPC, pers. comm., May-September 2003). Like recovery of the piping plover, least tern recovery will require more than the return of a more favorable Platte River hydrograph. Human activities not directly associated with habitat loss, including harassment of adults and young, and destruction of nests also contributed to reducing local populations. Losses of least terns, their offspring, and nests to predation from natural and exotic sources will need to be evaluated and most likely mitigated in some form. Also, as in the case of plovers on the Platte River, the role of contaminants in compromising breeding success needs study to assess its effects on least tern populations. Population Viability Analysis for Least Terns Two PVAs have been completed for least terns: one for birds in the Platte River (Boyce et al. 2002), and one for the California least tern (Akçakaya et al. 2003). The models are completely different in structure and sensitivity to altered vital rates. The model by Boyce et al. (2002) determines extinction probability from time-series data on population sizes, so the only parameters for the model are variance in population growth rate (as determined by changes in population size across years), initial population size, and carrying capacity. In contrast, the model by Akçakaya et al. (2003) is a matrix-based, stochastic simulation model of a metapopulation that uses RAMAS and includes age-specific vital rates. The Boyce et al. model predicted a high probability of persistence of least terns on the central and lower Platte River—an almost 100% probability of persisting in at least one location for 100 years. In addition, it found that increased connectivity between the populations increased persis-
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Endangered and Threatened Species of the Platte River tence probability. At least two problems, however, plague this type of PVA approach. First, sensitivity to altered vital rates cannot be evaluated, because data entering the model come solely from population size. How much immigration contributes to persistence, for example, cannot be assessed, even though it could be essential to population persistence. Second, there is some concern that this type of model is not able to predict persistence very far into the future. Work by Fieberg and Ellner (2000) and Ludwig (1999) suggests that even with perfect knowledge of population size, estimates of extinction probabilities based on this approach are accurate only to 20% of the length of the time series. That means that the 11 years of least tern data used by Boyce et al. might validly predict only 2 years into the future. The authors’ recognition that the central Platte River birds contribute substantially to the joint persistence of the populations is correct in the context of the model, but the model makes implicit assumptions about each of the local populations, for example, that Platte River birds are self-replacing (the population is not maintained in part by immigration), which probably is not valid. Consequently, it is difficult to determine what this model offers for predicting the contribution of Platte River birds to the persistence of their own population, or that of the regional population. The model by Akçakaya et al. (2003) is designed for terns in California but is more useful in a Platte River application. The model treats clusters of colonies or individuals within 5 km (3.1 mi) of one another as elements of the same population. It includes age structure, year-to-year changes in survival and fecundity, regional catastrophes (such as strong El Niño-Southern Oscillation events), and local catastrophes (reproductive failure due to predation). The period used to assess risk of extinction and substantial population declines is 50 years. Lacking local data, they used data on dispersal from a Massachusetts population (Atwood 1999) and included spatial and temporal autocorrelation. Although Akçakaya et al. had no evidence of density dependence in reproductive success, they assumed that reproductive success declined when populations were at high and low population densities. The high-density effect is programmed to occur near carrying capacity, and an Allee effect occurs when a colony has five or fewer females. Sensitivity analysis was carried out by rerunning the model using low, medium, and high values for parameters and quantifying the effects on population viability. The model predicted that during normal (noncatastrophe) years, the finite rate of increase (λ) is ~0.994, which converts to a slow annual decline (-0.6%). In a revised model, λ = 1.07, so slow average population growth was predicted. Using medium values of all parameters, the model predicts zero risk of extinction or substantial decline in the next 50 years. Using lower vital rates, the model predicts a higher risk of decline but a low risk of least tern extinction in California over the next 50 years. The model was sensitive to survival and fecundity rates and moderately sensitive to carrying
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Endangered and Threatened Species of the Platte River capacity. It appears that reproductive success, philopatry (tendency to return to hatching site to nest), and site fidelity are all greater for California birds than for Platte River birds. A PVA commissioned by the committee sheds some light on several points that are important for the recovery of Platte River interior least terns. Analyses suggest that the least tern interior population is likely to persist for 200 years and that the Platte and Loup River birds contribute minimally to its persistence. Reproductive success is zero or very low on the Platte, so birds breeding on the Platte are probably coming from outside Nebraska. Therefore, Platte River habitat is serving as a “sink”; the population there is not replacing itself with local reproduction and could be drawing breeding birds into habitat not suitable for reproduction. Those conclusions, however, are based on a hypothetical spatial structure of least tern populations; before population persistence and the importance of the Platte River birds to the larger interior population can be accurately evaluated, the movements and habitat use of terns in the interior population must be understood. The committee’s conclusions about effects on survival of interior least terns and piping plovers were similar. As in the case of the PVA for the piping plover, the committee used available data to ask several basic questions about the persistence of the Platte River interior least tern population. Again, it is important to note that data are not sufficient to describe in terms of parameters for a PVA with robust predictive capacity. Nonetheless, the committee built a simple three-population model for the interior United States: Niobrara River (eventually adding the Missouri River birds to this population would be reasonable), Platte and Loup Rivers, and all birds outside Nebraska. The model allowed the committee to consider the fate of the interior population after excluding Platte River birds, assuming a completely isolated population of 920 adult birds. The committee also considered the fate of Platte River terns with and without immigration and assuming increased reproductive success. For the least tern PVA, the committee again used VORTEX (9.3). There is no information about the population structure of interior least terns in Nebraska (or, for that matter, the interior United States). Although breeding sites are dynamic—they are lost and created—it had to be assumed that on the average the number of sites and dispersal among sites were consistent. As data become available to evaluate that assumption, a more sophisticated PVA can be developed. Consequently, the PVA is exploratory, so the model and its results should be viewed as hypotheses to be tested. The model used initial population sizes (N) and carrying capacity (K) from the recovery plan (Table 6-6). Survival data came from Thompson et al. (1997) or from California populations (Table 6-7). All populations are
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Endangered and Threatened Species of the Platte River TABLE 6-6 Summary Input for Interior Least Tern PVA Initial Model Interior United States Excluding Platte and Loup Rivers and Niobrara River Populations Platte and Loup Rivers Niobrara River Initial population size (K) 7,000 920 200 Initial carrying capacity (N) 5,900 700 200 Source: Reed 2003. assumed to have the same life-history parameters: age at first breeding, 2 years; longevity, 20 years; maximal clutch size, three eggs; adult survival, 85%; survival from fledging to age 1 year, and from age 1 year to age 2 years, 56% (Akçakaya et al. 2003); environmental variance for reproduction and survival, 20% of the parameter value; no inbreeding depression; no catastrophic environmental events; no change in carrying capacity over time; and no cost to dispersal. It appears that the percentage of nests that fail each year on the Platte River is increasing, so the base model was run with all populations experiencing a 20% failure rate, then with the Platte TABLE 6-7 Some Parameter Values for California Least Tern PVA (Akçakaya et al. 2003) and Parameter Values from Platte River Data Model Parameter California PVA Platte River Data Reproduction (fledglings/female per year) 0.6964 0.47 (Kirsch 1996 as cited in Thompson et al. 1997) Survival, hatching to fledging (normal years) 0.6237 0.504 (Smith and Renken 1993) Survival, hatching to first breeding (normal years) 0.16 Unknown Survival, from 1 to 2 years 0.5627 Unknown Survival, from 2 to 3 years, and 3 to 4 years 0.81 Unknown Survival after age 4 years 0.92 Unknown Philopatry >50% 26% (Lingle 1993c, in Thompson et al. 1997) Site fidelity 70% 28% (Lingle 1993c, in Thompson et al. 1997) Source: Reed 2003
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Endangered and Threatened Species of the Platte River and Loup River population experiencing increasing failure rates. Fledglings per female surviving per year is lower than in California (Table 6-7) at 0.47 (presumably, this includes nest failures) (Kirsch 1996 as cited in Thompson et al. 1997). The fledgling survival per female as reported by Kirsch is very low; to duplicate this low estimate in the model output required use of high nest failure rates in the model. It was assumed that of the birds whose reproductive efforts did not fail, 70% had one offspring, 28% had two, and 2% had three. Those assumptions generated predicted rates of reproductive success higher than observed (just over one fledgling per nest), so the model was run with increased failure rates. In addition, the base model was run with the Platte and Loup population and carrying capacity removed from the model. Dispersal rates are unknown, but philopatry and site fidelity appear to be lower in Nebraska than in Massachusetts or California (Akçakaya et al. 2003), so it was assumed that dispersal in Nebraska is higher than in Massachusetts or California. Lower site fidelity is expected in Nebraska because riverine sandbars are less stable than coastal beaches. High immigration from the greater interior metapopulation would inundate model results, so it was held to 1%. Migration between Nebraska populations was set at 30%; it was set at 10% between Nebraska and other interior sites. The model was run with a variety of dispersal values and with the nest failure rate set at 20%. The current recovery plan has a goal of 920 adults in the Platte River and Loup River combined. A PVA assuming a completely isolated population of this size was run with the base-model parameters, first in a version with immigration and then in a version of the model that assumed increased reproductive success. To model immigration, the population was supplemented with 10 birds (five adult males and five adult females) each year. In a nonimmigration model, nest failure rate was decreased to 10% (from 20%); in another scenario, failure rate was sustained at 20%, but it was assumed that of the birds that did not fail, 50% had one offspring, 42% had two, and 8% had three. Finally, a last scenario used variances in reproductive success and survival that decreased to 10% (from 20%). Most scenarios run with the model showed poor least tern population persistence probability (Table 6-8); all were characterized by multiple extinction and recolonization events among smaller populations. The Platte and Loup River population contributed substantially to the persistence of the greater interior metapopulation as it was modeled. That was demonstrated by the increased extinction probability associated with increasing nest failure in the Platte and Loup population and after removing the population and its carrying capacity entirely (Table 6-8). Model results indicate that the interior metapopulation is more adversely affected by
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Endangered and Threatened Species of the Platte River TABLE 6-8 Results of the Platte and Loup Habitat Removed Scenario PVAs, Varying Nest Failure Rate on Platte and Loup Rivers, and Removing Birds of Platte and Loup Rivers and Carrying Capacity (Fixed Dispersal Rate of Interior United States into Nebraska of 0.0005 and Between Nebraska Populations of 0.01) Scenario % Nest Failure in Platte and Loup Rivers Metapopulation Platte and Loup Rivers Niobrara River P(ext)a Sizeb Mean Timec P(ext)a Sizeb Mean Timec P(ext)a Sizeb Mean Timec (1) Base 20 0.729 216 114 0.757 57 108 0.745 51 107 40 0.788 232 110 0.808 56 103 0.811 52 103 60 0.840 184 104 0.861 45 95 0.855 41 95 (2) Platte and Loup Rivers gone 20 (overall failure) 0.761 280 112 — — — 0.803 52 100 (3) Base, 20% failure Dispersald 1 0.729 216 114 0.757 57 108 0.745 51 107 3 0.658 111 127 0.665 59 123 0.669 55 123 5 0.342 96 125 0.287 68 125 0.414 64 128 aProbability of extinction. bMean N at 200 years for those which lasted. cMean extinction time (years) for those which went extinct. dImmigration rate from interior United States into Nebraska; fixed rates out of Nebraska (10%), and between Nebraska populations (30%). Source: Reed 2003.
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Endangered and Threatened Species of the Platte River Platte and Loup habitat serving as a sink than by an outright loss of Platte and Loup River habitat. That is reinforced when immigration from outside Nebraska is increased, causing a drain from the larger regional population to the smaller, sink population. The same pattern was found by Plissner and Haig (2000) for piping plovers. Under current conditions, an isolated population of 920 adults in the Platte River and Loup River combined would have a poor probability of persisting (Table 6-9). Increasing reproductive success substantially did not have much effect on persistence, but decreasing variance in reproductive success and survival improved persistence (Table 6-9). Under the last scenario, populations almost always persisted, and population sizes tended toward 80% of carrying capacity, on the average, after 200 years. Adding immigrants equivalent to an influx of 10 adults, or 1% of the local carrying capacity, improved persistence probabilities almost to zero probability of extinction (Table 6-9). Under the simulated model structure and conditions, the least tern metapopulation is unlikely to persist for 200 years. The Platte and Loup River population contributes to the persistence of the metapopulation but is a negative factor if reproductive success is very low and immigration into Nebraska is high. Clearly the model is sensitive to variance in reproductive success and survival, so it is important to determine the best estimates for those parameters. Immigration into the population also had a considerable effect in increasing population persistence. That supports the need to understand the spatial structure of the tern populations in this area. TABLE 6-9 PVA Results for Single Platte and Loup River Population with Different Amounts of Immigration Scenario P(ext)a Sizeb Mean timec Isolated, base model 0.973 107 78 10 adult immigrants/year 0.000 781 — Decrease nest failure to 10% 0.921 193 84 Reduced partial brood lossd 0.854 235 88 Last two combined 0.795 363 91 Same, but variance in reproduction and survival decreased to 10% 0.003 786 123 aProbability of extinction. bMean N at 200 years for those which lasted. cMean extinction time (years) for those which went extinct. d50% had one offspring, 42% had two, and 8% had three. Source: Reed 2003.
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Endangered and Threatened Species of the Platte River SUMMARY AND CONCLUSIONS This chapter has reviewed the necessary background to address three questions regarding the piping plover and interior least tern populations of the central and lower Platte River. First, the committee found that habitat conditions on the central and lower Platte River affect the likelihood of survival and recovery of the NGP population of piping plovers. The decline in the river’s plover population has been coincidental with the loss of its preferred habitat, especially in the central Platte River, where suppressed variability in flow has led to reductions in sandbars and beaches and indirectly to increased woodland and reduced open sandy areas. The piping plover population along the Platte River has consistently declined since 1996, and improvement in its numbers is likely to be closely tied to habitat conditions on the river that respond to hydrological adjustments. Breeding along the central Platte River is mostly in artificially created habitats that are not sustainable on a multidecade basis. Recovery requires a reversal of present trends by rejuvenation of a more natural regime of river flows, sediment processes, vegetation, and channel morphololgy. Second, the committee found that, for the same reasons, the current habitat conditions on the Platte River are likely to affect the likelihood of survival and recovery of the interior least tern. The interior least tern has habitat preferences that are highly similar to those of the piping plover, so what affects one bird population affects the other. Because terns seek sandy, beach-like habitats and because these habitats are becoming less common along the Platte River as a result of flow, sediment, vegetation, and morphology changes, the tern population associated with the river is far below recovery goals. That sufficient suitable habitat stimulates a healthy tern population is demonstrated by components of the least tern population in other subunits of its range. Third, the committee found that the USFWS designation of critical habitat for the piping plover is based on the best available scientific knowledge at the time of designation (2002). The agency followed reasonable and established procedures in the designation process, drawing on information from peer-reviewed journals and unpublished information, including agency reports and surveys. It consulted with practicing biologists who had specific useful information. USFWS adopted a prudent strategy in deciding on a general designation from Lexington to the confluence with the Missouri River in the absence of substantial time and support to refine the designation to a finer, parcel-by-parcel scale. As in investigations related to the whooping crane, research into issues related to piping plovers and interior least terns revealed some gaps in data and knowledge. The integrated habitat use by plovers and terns shows the need for knowledge about how multiple species use the same habitat space
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Endangered and Threatened Species of the Platte River and how they interact with other listed and nonlisted species. There is also a need to integrate the Platte River populations of plovers and particularly of terns with their larger populations. Platte River terns, for example, appear to range widely (up to at least 170 mi away from the river), so understanding of the Platte population cannot focus solely on the river. Finally—as is the case with conclusions related to time trends in climatic data, hydrological data, and crane population numbers—conclusions related to trends in the populations of plovers and terns must take into account the longest periods possible rather than focusing on a few years of record.
Representative terms from entire chapter: