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Endangered and Threatened Species of the Platte River 5 WHOOPING CRANE The whooping crane is one of the world’s most imperiled species. About 200 individuals exist in the single natural population, which migrates over a large portion of North America, from overwintering grounds on the gulf coast of Texas to breeding grounds in central Canada. During two annual migration passes through the American Midwest, individuals stop over along the central Platte River of Nebraska for periods of a day to several weeks. In support of the federal listing of the whooping crane as endangered, the U.S. Fish and Wildlife Service (USFWS) designated a portion of the central Platte River as critical habitat for the species. This chapter is the response of the National Research Council Committee on Threatened and Endangered Species and the Platte River Basin to questions about habitat for the whooping crane: Do current central Platte habitat conditions affect the likelihood of survival of the whooping crane? Do they affect its recovery? Is the current designation of central Platte River habitat as critical habitat for the whooping crane supported by existing science? In addressing those questions, this chapter reviews relevant databases, examines the use of the central Platte habitat by whooping cranes, and evaluates the influence of the habitat on population parameters, including mortality, natality, and distribution. HISTORICAL AND ECOLOGICAL CONTEXT The whooping crane is native to a diverse array of ecosystems. Until its well-documented decline, it nested predominantly in the northern tall grass
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Endangered and Threatened Species of the Platte River prairie of the upper Midwest and the eastern aspen parklands of Canada (Allen 1952). It also nested in such disparate regions as the taiga and sub-Arctic regions of the Northwest Territories, where the last remaining population has persisted to present times, and in the coastal marshes of Louisiana (Figure 5-1), where a nonmigratory population bred until 1939 and was extirpated from the wild in 1950 (Gomez 1992; Lewis 1995). Wetland ecosystems historically used by wintering whooping cranes were diverse as FIGURE 5-1 Historical distribution of whooping cranes in North America. Source: Allen 1952. Reprinted with permission; copyright 1952, National Audubon Society.
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Endangered and Threatened Species of the Platte River well, ranging from the high plateau wetlands of central Mexico to the varied coastal wetlands that extended from Texas to South Carolina (Nesbitt 1982; Allen 1952). Moreover, migratory populations of whooping cranes followed a variety of flyways between breeding and wintering areas (Figure 5-1). Although whooping cranes used and continue to use a wide range of environs, they primarily have depended on highly productive wetland ecosystems for nesting, overwintering, and migratory stopover (Hayes and Barzen 2003). Whooping cranes from two breeding-wintering populations are thought to have migrated through Nebraska (Allen 1952). Few historical records of whooping cranes summering in Nebraska exist and no records of breeding are available, so it is assumed that the state has been used predominantly by migrants. A whooping crane population that bred in the upper Midwest (and perhaps Manitoba) was thought to have wintered in Louisiana and to have migrated through eastern Nebraska (Figure 5-2a). The disappearance of cranes from eastern Nebraska coincided with the extirpation of the upper Midwest-Louisiana migratory population. No historical whooping crane records (either observations or specimens) are reported for the Platte River downstream of Chapman. A population that bred further west of Manitoba and wintered in Texas (whose remnant is the extant, migrating population) is thought to have passed through the region of the central Platte River and other portions of central Nebraska (Allen 1952). The last remaining wild population of whooping cranes now breeds in and near Wood Buffalo National Park (WBNP, Canada), migrates through the Great Plains (including Nebraska), and winters along the Texas coast (Figure 5-3). In 1860 or 1870, the population may have been as large as 1,300 or 1,400 or as small as 500 or 700 individuals (Allen 1952; Lewis 1995). By 1912, the Aransas-Wood Buffalo migratory population (AWP), wintering in what is now Aransas National Wildlife Refuge (ANWR), was only 36 birds; and by 1941, the number had declined to 15 (Allen 1952). Causes of the decline in the population were primarily overhunting and loss of natural habitat to agriculture (Allen 1952). Subsequent population recovery has been attributed to enforcement of the Migratory Bird Act of 1916 (which made it illegal to shoot whooping cranes) and the creation of WBNP and ANWR. WBNP was established in 1922 as the primary summer area, and ANWR in 1937 as the primary wintering area (Lewis 1995). Federal protection for the whooping crane was conferred in 1967, and critical habitat was designated in 1978. USFWS established a species recovery team in 1975 for the United States, and Canada followed suit in 1985. A recovery plan for the whooping crane was published in 1980 (Olsen et al. 1980; Smith 1986; Lewis et al. 1994) and was last revised in 2003 (CWS and USFWS, unpublished material, May 2, 2003). A memorandum of understanding signed between the Canadian Wildlife Service and USFWS in
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Endangered and Threatened Species of the Platte River FIGURE 5-2 Distribution of whooping crane sightings in Nebraska. (a) 1820-1941, (b) 1943-1999. Sources: (a) Modified from Allen 1952. Reprinted with permission; copyright 1952, National Audubon Society. (b) Austin and Richert 2001.
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Endangered and Threatened Species of the Platte River FIGURE 5-3 Map showing present (2003) home range of migrating whooping cranes and central position of Platte River, Nebraska. Source: CWS and USFWS, unpublished material, 2003.
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Endangered and Threatened Species of the Platte River 1985 further assisted coordination of monitoring and conservation efforts (Smith 1986). Apart from the work of Allen, little was known about whooping cranes at the time of their listing in 1967. The AWP consisted of 43 birds; by the time critical habitat was designated in 1978, it had increased to 75 (CWS and USFWS, unpublished material, May 2, 2003). Since 1938, investigators at ANWR have counted birds in the AWP each winter (Lewis 1995). Surveys of breeding birds on the nesting grounds in WBNP began soon after the breeding site was discovered in 1954. In 1977, researchers at WBNP began a banding program to capture and tag flightless young with colored leg bands (Kuyt 1979). In migration areas, a coordinated effort to collect, verify, and maintain records of migrating whooping cranes began with the fall migration of 1975 (interview with USFWS personnel, Aug. 13, 2003) and was expanded in 1977 (Olsen et al. 1980). Using newly acquired data and the few papers published since Allen’s work, Johnson and Temple (1980) completed the first comprehensive analysis of migration habitat for the whooping crane. When USFWS designated critical habitat for the species, surprisingly few data existed beyond the historical data (reviewed below) and winter estimates of the population size (Johnson and Temple 1980). USFWS identified the central Platte River as critical to protect “so that it would not be lost to development before its overall significance to whooping cranes could be determined” (Johnson 1982, p. 34). The goal of the recovery plan was to ensure protection of key stopover points along the flyway that had the highest historical and current use by cranes. That string of protected areas along the north-south migration route would assist the species by decreasing the distance between stopover locations. The central Platte River became part of a series of stopovers protected as critical habitat that included Quivira National Wildlife Refuge, Salt Plains National Wildlife Refuge, and Cheyenne Bottoms State Wildlife Area (Figure 5-3). DIFFICULTIES IN DEFINING CRITICAL HABITAT FOR THE WHOOPING CRANE Endangered species with low population numbers are especially problematic for researchers. First, researchers cannot determine the importance of various habitat types for the species by simply investigating present use patterns. Inferences about the importance of various habitat types depend on assessments of what habitats are available (Manly et al. 2002), random selections of habitat for evaluation (Pereira and Itami 1991), and evaluations of nonused habitats (Faanes et al. 1992). The species is defined as showing preference or avoidance when it uses a particular habitat type disproportionately or when habitat use differs substantially from random
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Endangered and Threatened Species of the Platte River selection. When an otherwise preferred habitat is not fully occupied, absence of animals from it does not necessarily mean that the habitat is of poor quality or not desired. Habitats of high quality may be nonused simply because there are not enough animals in the population to occupy all available habitats. Traditional habitat-use studies are constrained when used for low population numbers. For example, the AWP presently (2002) numbers only 185 (CWS and USFWS, unpublished material, May 2, 2003), and this small population cannot be assumed to be at carrying capacity or to have saturated any habitat that it uses during its annual cycle (Lewis 1995). That caution is relevant to both the historical analysis provided by Allen (1952) and more current analyses (Austin and Richert 2001). Habitat use by whooping cranes varies during different parts of their annual cycle of activities: summer breeding period, nonbreeding period in summer, fall staging, fall stopover, winter, and spring stopover (Lewis 1995; Kuyt 1979, 1992; Howe 1989). Migrating birds use staging habitats for several days or weeks to acquire and store energy or nutrients that are required for future stages in the annual cycle (Melvin and Temple 1982). In contrast, cranes use stopover habitats to meet immediate needs for energy and nutrient provisioning (such as water and protein) for up to several days while they wait for appropriate weather conditions to continue migration (Melvin and Temple 1982). Habitat limitations during any part of the annual cycle can limit a population independently (although, given present data, this does not appear to be the case for whooping cranes). For example, if breeding habitats are limiting, populations can grow more slowly or decline even if all other habitats used during the annual cycle are abundant. Allen (1952) and Lewis (1995) argued that protecting breeding habitats (WBNP) and winter habitats (ANWR) has been critical to survival and recovery of the whooping crane. Others (EA Engineering, Science and Technology, Inc. 1985; Lutey 2002; R. Brown, Colorado Water Conservation Board, unpublished material, August 11, 2003) have also suggested that inasmuch as the AWP continues to increase, stopover habitats used by whooping cranes during migration cannot be limiting the species. No data indicate that any specific habitat related to a particular stage of the annual cycle is generally limiting the AWP (CWS and USFWS, unpublished material, May 2, 2003). Some data suggest that changing winter habitat conditions may become limiting in the future or are limiting now in some years (Chavez-Ramirez 1996; Stehn 2001). Improved understanding of the roles of the various habitats used by the crane population requires long-term, multiyear assessments of the whooping crane population that compare historical and modern data. Long-term studies would also illuminate time lags that affect the AWP. Natality, for example, occurs only during spring, although factors that
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Endangered and Threatened Species of the Platte River influence natality may occur on habitats outside the breeding area (Chavez-Ramirez 1996). Adult mortality occurs throughout the year but primarily during migration (Lewis et al. 1992). Changes in population numbers thus take a full year to become measurable. The lifespan of whooping cranes is 30-50 years (Mirande et al. 1991, 1997; Brook et al. 1999), so population dynamics can take many years to become apparent. Given current population growth rates and assuming no substantial change in population measures, the AWP will take 20 years (Reed 2003) to reach the minimal AWP goal of 1,000 birds and 175 pairs—numbers that would have to be maintained for an additional 10 years to achieve downlisting. Because of those long response times for the crane population, conservation strategies must take into account planning horizons of 30 years or more. Critical habitat for the species must not only serve the needs of the present small population but be adequate to accommodate the needs of the larger population anticipated for meeting the delisting requirements. Extrapolation of future needs will have to take into account the possible influence of a variety of factors, including adjustments in agricultural policies and land-use patterns. Changes in agricultural land use, for example, have already affected sandhill cranes (Krapu 2003), and such changes may become more relevant for whooping cranes as the AWP changes. CURRENT DATABASES Data on whooping cranes made available to the committee varied in years covered, variables included, and geographic region covered. The committee carefully reviewed them for errors and requested updates from those who created or maintained the databases to provide the most accurate, complete, and current information for this report. Historical Sightings by Allen (1952) In the 1940s, when it was clear that whooping cranes were close to extinction, Robert Allen documented the distribution, abundance, and ecology of the species. His work, published by the Audubon Society in 1952, provided the first thorough investigation of historical records of whooping cranes. Allen interviewed many people who had resided in the whooping crane home range during the late 1800s, and he captured valuable information that would otherwise have been lost. It is not possible to derive quantifiable conclusions from those interviews, but they allow some informed conclusions about the ecological relationships between the species and its habitat. Allen concluded that Nebraska was important for migrating whooping cranes and that the central Platte River (which he referred to as the Big
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Endangered and Threatened Species of the Platte River Bend region) was an important habitat for migrating whooping cranes. He based his conclusions on three lines of evidence: hunting data, observational data, and qualitative information from interviews. Nebraska had more reported whooping crane hunting kills than any other region in the United States and Canada (45 of 333, Allen 1952, p. 76, Table J); within the state, more kills were in the central Platte than anywhere else (Figure 5-4). Cranes vulnerable to hunting are those not flying at altitudes typical of migrating birds; migrating birds would have been too high to be killed with a shotgun (Kuyt 1992). The distribution of cranes shot on the central Platte River reflects historical use of local habitats, much of it on the river itself (Allen 1952, p. 102). Hunting records are particularly reliable because mistakes in species identification are infrequent in contrast with sightings. Allen also reported historical observations from the central Platte River (Figure 5-2a). Some of the historical observations, however, may have been erroneous. Inaccurate observations from Swenk (1933) and Brooking (1943) led some scientists to conclude that Allen’s compilation of observations was FIGURE 5-4 Available kill record locations in United States. Each black dot equals one hunter-killed whooping crane. Whooping cranes killed in Canada are not mapped. Source: Allen 1952. Reprinted with permission; copyright 1952, National Audubon Society.
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Endangered and Threatened Species of the Platte River “burdened with remaining uncertainty” (EA Engineering, Science and Technology, Inc. 1985). To evaluate the historical record, the committee remapped Allen’s original records and distinguished the observations of Swenk (1933) and Brooking (1943) from other records (Figure 5-2a). Whooping crane locations reported from other observers or from museum and hunter-shot specimens (where identity is not in question) did not vary greatly from the records in Swenk (1933) or Brooking (1943). There is little evidence to suggest, therefore, that whooping crane use in Nebraska as summarized by Allen (1952) was inaccurate. Allen interviewed residents who lived along the Platte River during the late 1800s and early 1900s. The veracity of interviews and their qualitative data can be determined only through recommendations of the interviewer or through assessment of the completeness (and believability) of the descriptions. The following interview is representative (Allen 1952, p. 79): While in Nebraska a few years ago I talked with Loren Bunney, State Conservation Agent at Ogallala. He told me that he had lived as a boy in Harlan County, south of Holdredge, between the Republican and Platte Rivers. He started hunting as a boy of ten, ‘some fifty years ago,’ and, even at that time (about 1897), whooping cranes were a rare sight, although the area in which he lived was in the main pathway of their migration. When flocks of sandhill cranes went through he always ‘looked them over for the big white ones.’ Now and then a few were seen. That interview, coupled with other similar reports, suggests that whooping cranes did not frequently stop south of the Platte and crossed the region during migration. Residents’ observations corroborate the lack of hunter-killed whooping cranes from the area (Figure 5-4) and suggest that cranes may not actually have stopped as frequently south of the Platte River on migration. Confirmed Sightings in 1976-2002 A coordinated effort to collect, verify, and maintain records of migrating whooping cranes on a consistent basis was initiated in fall 1975 (Wally Jobman, USFWS, pers. comm., August 13, 2003) and expanded in 1977 (Olsen et al. 1980). Before then, records of whooping crane sightings were collected haphazardly. USFWS has closely monitored migration information since 1975 to identify appropriate survey periods for whooping cranes on the Platte. In spring, for example, Grand Island USFWS staff coordinate their survey efforts with ANWR staff that monitor when whooping cranes initiate migration. Observers at each stopover site also communicate as the birds continue moving north. Providing further continuity, Wally Jobman has coordinated whooping crane sightings for the Platte River region and
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Endangered and Threatened Species of the Platte River elsewhere since 1978. On the central Platte River, ground surveys rely on reports from the public and from government staff and coordinated surveys. About two-thirds of all whooping crane sightings originate with the general public, and agency or other qualified biologists attempt to verify all sightings (Wally Jobman, USFWS, pers. comm., August 13, 2003). Large numbers of tourists visit the central Platte River each spring to observe migration, and many hunters visit the region each fall. In both seasons, potential observers are numerous and well distributed along the central Platte River. Nongovernment observers have contributed observations at the Rowe Sanctuary since 1975, as have the Whooping Crane Trust staff since the early 1980s. Aerial surveys in the 1980s found only one group of three whooping cranes that ground surveyors had not already located. Aerial surveys were re-established in 2001 and have been helpful in confirming sightings but not in discovering new birds. The Nebraska Wildlife Federation initiated its Whooper Watch Program in 2002. Most birds (be-sides those seen at the Rowe Sanctuary) are first reported in upland areas and then watched to determine where they roost at night and monitored for as long as they stay in the area of the central Platte River. Overall, because most sightings of whooping cranes in the central Platte originate in observations of the general public (ubiquitously spread along the central Platte in spring and fall), the search effort for whooping cranes has probably varied little among years since the 1980s. Whooping crane sightings that originate with the public undoubtedly underestimate stopovers on the Platte River because many stopovers are of short duration and because birds are not always detected when they first arrive (Kuyt 1992). Fall detection of cranes is also unreliable because hunters can be reluctant to report whooping crane sighting for fear that their hunting areas will be closed until the birds depart. A federal waterfowl production area (the Funk Basin, a wetland complex in the nearby Rainwater Basin) was temporarily closed in 1993 while whooping cranes stayed there (Wally Jobman, USFWS, pers. comm., August 13, 2003). In a larger-scale effort, Austin and Richert (2001) summarized whooping crane sighting data collected throughout the United States from 1943 to 1999, analyzing their contribution to whooping crane migration ecology, and evaluated their usefulness. They identified data-comparability concerns due to unequal effort at locations throughout the Northern Plains. For example, many observers watch for whooping cranes in the Platte River Basin, especially during spring, when thousands of tourists come to the Platte River to see migrating sandhill cranes, but comparatively few people watch for them in the Sandhills to the north. Because of such variability in search effort among regions, it is not possible to compare frequencies of sightings among regions. Howe (1989) demonstrated that effect with cranes
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Endangered and Threatened Species of the Platte River cranes have roosted off-channel; when doing so, most have used wetlands associated with the Platte River (Howe 1989). Riverine roost sites are important whooping crane habitats throughout Nebraska (Austin and Richert 2001), not just on the central Platte. In Nebraska, and elsewhere in the flyway, wetlands not associated with the river also serve as important roosting habitats for whooping cranes in the AWP (Austin and Richert 2001). Night roosts used by whooping cranes tend to be isolated from human disturbance and are within 1 mi of foraging sites. The relative availability of nonriparian habitats is not known. In general, wetlands are important to migrating whooping cranes for both foraging and roosting. In the central Platte, use of riverine habitats by whooping cranes is well documented and appears important to their survival. Preference studies have not been done on this scale and, although useful, will be limited by the same problem of small population size as noted above. When the scale of analysis was expanded to include the entire flyway for the AWP, one-third of the habitats used by roosting whooping cranes were riverine, a different distribution of use from Nebraska, where 59% of the roosting sites are riverine (Austin and Richert 2001). Telemetry studies did not find riverine habitats important to roosting whooping cranes, however, because the few tracked birds primarily used palustrine wetlands (Howe 1989). Bias occurred in both studies. Austin and Richert (2001) identified the limitations to analysis of habitat-use evaluations for migratory whooping cranes. Comparisons among sites are difficult because efforts to observe whooping cranes along the entire 4,000 km migration route are uneven. Some regions are subject to underreporting (Howe 1989). Still, the high incidence of use of riverine habitat by whooping cranes in Nebraska, compared with other states, is striking. Migration stopover sites in Nebraska include the central Platte River, Rainwater Basin, central Table Playas, and northern Sandhills (Richert 1999; USFWS, unpublished material, June 16, 2000; Austin and Richert 2001). The quality of both palustrine and riverine habitats in Nebraska is high (Stahlecker 1997) and, accordingly, these wetlands are used extensively by whooping cranes (Austin and Richert 2001). With the opportunistic and nontraditional patterns of habitat selection exhibited by whooping cranes, it is difficult to predict habitat preferences on the scale of home ranges. Although nonhabitat variables (such as weather events) can influence habitat selection greatly (Kuyt 1992), the overall pattern of habitat use in Nebraska suggests that the general region of the central Platte (including the Rainwater Basin) is important to migrating whooping cranes. On large time scales, as wet and dry periods cycle through the region, the interaction among the sites will probably be important (Richert 1999). Whooping cranes will need stopover habitat during droughts when wetlands in the Rainwater Basin are dry and during wet years when the Platte River is above flood stage. Maintaining a complex of
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Endangered and Threatened Species of the Platte River wetlands over a large geographic area will help to mitigate the effects of climatic variations. Few data relevant to use of habitats by migrating whooping cranes on the largest geographic scale exist. However, telemetry data reported by Howe (1989) suggest differences in use of stopover areas between fall and spring migrations. Whooping cranes appear to spend more time in Nebraska in spring than they do in fall (Howe 1989, Table 2, p. 6). In fall, many whooping cranes tend to stop in a Saskatchewan staging area for several weeks before continuing their migration, presumably to acquire fat reserves for migration. It takes cranes about two daily flights to reach that region in Saskatchewan after leaving their breeding grounds in WBNP (Kuyt 1992). After leaving Saskatchewan, the birds migrate rapidly and reach Nebraska in 2-3 days and Texas in 5-7 days more (Kuyt 1992). During fall migration from 1981 to 1984, several radio-tracked whooping cranes flew over Nebraska without stopping in the state at all (Kuyt 1992). In spring, however, cranes do not use a staging area (Kuyt 1992). For spring migration, Nebraska is about the same distance from ANWR as Saskatchewan is from WBNP. Longer periods spent in Nebraska in spring may therefore be a function of proximity to ANWR as a starting point. Collectively examining habitat selection for whooping cranes at all four geographic scales suggests that Nebraska, and the central Platte in particular, provide critical habitat for this species. Climatic variations and development pressures likely over the next 30 years in which whooping cranes will need to recover (assuming that current population trends continue) will further emphasize the importance of providing stopover habitats for whooping cranes on the central Platte. WHAT DO WHOOPING CRANES GAIN FROM CENTRAL PLATTE HABITATS? Two basic ecological needs of migratory bird species are met by migration (stopover or staging) habitats: food and a safe resting place. Energy and nutrients are acquired and stored for use during future phases of the annual cycle (see Alisauskas and Ankney 1992). The reserves can be used to provision future flight, allow birds to persist through periods of food shortage, or help to meet reproductive needs when exogenous food resources are insufficient. Because energy and nutrient reserves are acquired in one stage of the annual cycle and used in another, mortality and natality can be influenced by events that occur in other stages of the annual cycle, producing a cross-seasonal effect (Weller and Batt 1988; Fretwell 1972). Birds require safe environs at each habitat they visit. Protection from natural predators and relative isolation from human activities contribute to the utility of stopover sites. The central Platte River has historically filled both needs for whooping cranes during their migrations.
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Endangered and Threatened Species of the Platte River Energy and Nutrient Storage When whooping cranes leave breeding areas at WBNP, they often fly for 2 or more days to reach a staging area in Saskatchewan, where they remain for a few days to a month (Kuyt 1992); 2-3 weeks is the average period of occupancy in this fall staging area. Once whooping cranes leave the Saskatchewan staging area, they typically move rapidly to ANWR, stopping for just short periods at individual stopover sites (Kuyt 1992). Although no direct data exist, the pattern of migration suggests that birds leave their breeding area with varied body conditions (such as amount of stored fat), stop in Saskatchewan and feed intensively on grains to build fat reserves, and then continue with migration once sufficient fat is stored. A similar pattern of fat storage (more directly measured) occurs in migrating sandhill cranes in fall (Krapu and Johnson 1990). Presumably, whooping cranes then arrive at ANWR lean, having used substantial amounts of fat reserves during fall migration. These measures are also indirect and are inferred from similar patterns of migration physiology of sandhill cranes. Sandhill cranes complete long migrations with substantially reduced fat reserves (Krapu et al. 1985). Chavez-Ramirez (1996) used behavioral descriptions and energetic models to conclude that whooping cranes regain fat reserves in winter areas before beginning their spring migration. They migrate back to WBNP relatively quickly, using stopover habitats en route but no staging areas. According to data from radio-tracked whooping cranes, not all delays at stopover sites were related to weather delays (Kuyt 1992, pp. 33-35). Presumably, energy and nutrient reserves acquired in winter are used to migrate quickly from Texas to Canada. Once in breeding areas, whooping cranes initiate nesting soon after arrival. Little food is available when whooping cranes arrive at WBNP, because ice and snow cover typically does not disappear until after nests have been initiated. Stored fat reserves, in addition to their importance for migration, are probably used for egg formation and incubation. Similar patterns occur in waterfowl, in which links between stored reserves and reproduction have been studied better (Alisauskas and Ankney 1992). As with sandhill cranes, cross-season effects probably are important for whooping cranes. Chavez-Ramirez (1996) argued that variations in winter habitat conditions are related to reproductive effort in the following spring and that winter body condition could affect winter survivorship. If winter habitat conditions influence mortality or reproduction, it may be that migration-habitat conditions affect whooping cranes in a similar way. Stehn (2001) has suggested that habitat quality at ANWR may be declining. The amount of fresh water entering ANWR is important ecologically because it influences overall productivity in the estuary, especially for
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Endangered and Threatened Species of the Platte River such species as blue crabs, which form the primary food of whooping cranes during winter. Declining fresh-water flows have been linked to the declining abundance of blue crabs (Stehn 2001). Fresh water for the estuarine refuge comes from the Guadalupe River. Today, 29% of the river’s flow is appropriated, with additional diversions likely to be necessary to support growing urbanization in southeast Texas (Stehn 2001). Whooping cranes that leave ANWR with insufficient stored fat will either migrate to WBNP, facing the potential for added mortality and reduced productivity (Chavez-Ramirez 1996), or increase stopover times on migration to acquire fat reserves. Responses by individual birds are likely to vary. For example, nonbreeding subadults may be able to stop longer on migration than can breeding adults because they do not need to initiate nests. The potential for cross-seasonal effects means that if deteriorating habitat conditions in ANWR are severe and long-lasting, whooping cranes may not be able to acquire sufficient fat reserves and will have to acquire them in other habitats, such as the central Platte River. Few data are available for testing the hypothesis that spring migration habitats influence energy reserves, reproduction, or mortality in whooping cranes. Some birds stay longer on spring stopover areas than do others (Kuyt 1992; Howe 1989), and more whooping cranes stop at the Platte River for long periods than did 25 years ago (Figure 5-6b). If fall staging areas are important for whooping cranes, as argued by Pitts (EA Engineering, Science and Technology, Inc. 1985; Howe 1989), the situation may be reversed in spring as birds move from winter to spring migration habitats. Other species of waterfowl acquire energy and nutrient reserves on spring staging areas before arrival on breeding grounds in the manner suggested for whooping cranes (Barzen and Serie 1990). Although relevant data are sparse, the hypothesis warrants further investigation. Documented migration patterns may not predict future patterns of movement in that habitat conditions change over time at the numerous stopover and staging areas. The changes might be related to the crane population, to climatic adjustments, or to changes in altered agricultural practices. It is possible that spring migration areas could be used in new ways by whooping cranes. This possibility cautions against underestimating the ecological value of stopover habitats to whooping cranes, especially stopover habitats on the central Platte River. Satellite telemetry studies over multiple years would help greatly in testing these ideas as they have done with sandhill cranes (Krapu 2003). Safe Stopping Areas Population monitoring at ANWR since winter 1938-1939 is useful for assessing the dynamics of the AWP (CWS and USFWS, unpublished material, May 2, 2003). Accurate yearly counts of the birds arriving on the
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Endangered and Threatened Species of the Platte River winter grounds coupled with low summer adult mortality and counts of birds at the end of winter provide estimates of mortality at several stages of the annual cycle. As much as 80% of whooping crane deaths appear to occur during migration, even though this period constitutes less than 20% of the birds’ annual cycle (Lewis et al. 1992). Although the total mortality is reasonably well known, causes of death are more problematic. Of 133 adults and subadults that disappeared away from winter areas between April and November (mortality on breeding areas is assumed to be low) from 1950-1987, the cause of death is known in 13 cases, five of which were due to collisions with power lines (Lewis et al. 1992). Because most deaths of adult whooping cranes occur during migration, mortality may be linked to the quality or quantity of stopover habitats. A significant portion of the whooping crane population stops at the Platte River during migration (Figure 5-6a,b). If the Platte River were no longer available, whooping cranes would probably shift their use to other habitats in Nebraska. Would these shifts alter mortality? Migrating whooping cranes use wetlands near the central Platte River in the Rainwater Basin south of the river or in the central Table Playa to the north (Richert 1999). Those off-stream wetlands apparently serve as good migration habitats for whooping cranes, except in droughts or during waterfowl disease outbreaks to which these wetlands are prone (Friend 1981). The Platte River ecosystem is not prone to disease outbreaks, because, in contrast with off-stream wetlands, water is flowing rather than stagnant. The Platte River also does not dry out as frequently as do surrounding wetlands; therefore, the Platte River provides safer conditions than do surrounding wetlands at least in some years (Johnson 1982). Crowding of waterfowl and cranes and short-term use of unusual habitats may also affect crane mortality. An experimentally released pair of whooping cranes unexpectedly migrated from their release area in Florida (where they were part of a nonmigratory flock) and spent the summer in Michigan (M.A. Hayes, International Crane Foundation, Baraboo, WI, unpublished material, 2002). The pair began fall migration normally, but the male disappeared (and presumably died) during a snowstorm on the first day of migration. The female migrated successfully to Florida in 11 days, during which she used many stopover habitats similar to those described by Howe (1989). Some stopover habitats this whooping crane used, however, were unusual. For example, she used an abandoned coal mine pond in Ohio and a forested area (probably including a stream) in a mountainous area of Virginia. Even though this bird survived the migration, did the probability of mortality increase when poor quality habitats were used? An incremental increase in mortality due to poor conditions on stopover habitats would be difficult to document with a small population. Higher mortality due to nocturnal predators has resulted from use of inappropriate habitat with an
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Endangered and Threatened Species of the Platte River experimentally reintroduced flock of whooping cranes in Florida. Once crane behavior was modified, and better roost habitat was used, mortality decreased (Nesbitt et al. 1997). Mortality and Population Viability Analysis The impact that variations in mortality have on the AWP can be examined through population viability analysis (PVA). The sensitivity of a target population to changes in mortality can be assessed with simulations that use measured population characteristics. The biology of whooping cranes, however, makes the species a challenging subject for PVA. In the wild, only one functional population of whooping cranes exists, and its members use the central Platte River only to stop during migration (Lewis 1995). Individual birds typically stay over for just a few days, but on occasion a bird stays for as long as several weeks. About 7% of the migratory population now uses the Platte River each year, but many more, if not all members of the species, use the Platte River at some time during their lives (Lutey 2002). Furthermore, specific habitat use by birds during migration and selective use of habitats by particular age groups (subadult or adult), reproductive status (breeding or nonbreeding), and sex are poorly known. In this context, relating the fitness of individual birds or the likely persistence of the population to central Platte River habitat conditions is tenuous. Nonetheless, three PVAs for whooping cranes are available and warrant consideration in conservation planning for the species. All use the same database for the Platte River population but use different years of data. Mirande et al. (1991, 1997) published PVAs that were done with VORTEX, and Brook et al. (1999) published PVAs that compared the output of different modeling packages, including VORTEX, RAMAS (Age, Stage, Metapop), INMAT, and GAPPS. All are age-based stochastic simulations. According to Brook et al. (1999), analysis of census data yields no evidence of density dependence; even if it did, current populations are probably well below carrying capacity, so density dependence would be unimportant. The Mirande et al. (1991) PVA concluded that the population is likely to grow at a low, steady rate (r = 0.046) and has less than a 1% chance of going extinct in the next 100 years. The model included density-dependent effects. Some simulations included catastrophes that affected reproduction and survival. In sensitivity analysis, increasing recent mortality by 50% (an increase in the annual mortality rate from 9.4% to 14.1%) would cause the population to go extinct, but an increase of 25% in mortality would not. The Brook et al. (1999) analysis provided similar results: all model applications predicted population growth. However, model predictions differed in details, and differences were caused by a variety of features, such as inclu-
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Endangered and Threatened Species of the Platte River sion of an inbreeding depression, stochastic variation in monogamy, and different projections for the same feature (Brook et al. 1999). Because there is no full-time resident or breeding population of whooping cranes in the Platte River Basin, conclusions about the role of the central Platte River in the persistence of the population must be based on some stringent assumptions, including the assumption that the loss of Platte River habitat will result in some increase in mortality or other contribution to decreased fitness of the population. Accepting that assumption, the PVA of Brook et al. (1999) with VORTEX is a reasonable model to address several issues pertinent to the committee’s charge, including lengthening the time frame of the analysis, mortality vs persistence, lifespan reduction, and nutrition. First, does lengthening the goal for population persistence from 50 years (used by Brook et al. 1999), which is too short for conservation planning, to 200 years substantially alter optimistic predictions of the model? The modified Brook et al. model was run with an initial population size of 185. No other modifications were made except lengthening the time frame for viability to 200 years. The population parameter values used in the model were gathered over decades from a population that was growing. If the same parameters are used, starting the model with a population size that is closer to the current size, the prediction is for population viability under current circumstances—a 99% probability that the population will persist for 200 years. Second, how much would adult mortality have to increase for the likelihood of population persistence to decrease significantly? Model inputs were adjusted to increase mortality by 1% increments up to 5%, reflecting loss of or further decline in quality of Platte River habitats. The increments were added to the existing 9.4% mortality for all whooping crane age classes. At its current population size the whooping crane is fairly well buffered against environmental and demographic variation that could cause it to go extinct. If, however, there is a 3% increase in mortality, the model predicts that the population would become nonviable, and the probability of its persistence for 200 years would decline to 86% (Figure 5-7). Third, given that the whooping crane lifespan is not known, what is the effect on population persistence of shortened lifespans that could result from losses of key habitats? Loss of habitat or decreases in its availability on the central Platte River could result in a decrease in the vigor of individual birds, shortening lifespans. However, model runs with lifespans reduced from 50 years to 40 and to 30 years showed that reduction in lifespan had no statistically significant effect on population persistence (Table 5-1). The PVAs suggest relatively stable populations well into the future under current conditions, but the high sensitivity of population persistence
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Endangered and Threatened Species of the Platte River FIGURE 5-7 Results of population viability analysis scenario 2. Viable population numbers are those above 0.95 probability of persistence for 200 years. Source: Reed 2003. to rather minor changes in mortality suggests a note of caution. Reductions in foraging or resting habitat that manifest in decreased vigor of individual cranes could have a substantial effect on the fate of the entire population. Without central Platte River stopover habitats, alternative habitats elsewhere in the migratory pathway will be necessary to provide resources for dispersing whooping cranes. TABLE 5-1 Results of Scenario 3 Simulations Initial Size of Population Extinction Probability Of Populations that Do Mean Go Extinct, Mean Population Size at Target Time Of Populations that Not Go Extinct, Time to Extinction, years Baseline: maximal age, 50 years 18 0.098 1,305 51 185 0.005 1,360 133 Maximal age, 40 years 18 0.113 1,267 55 185 0.008 1,345 117 Maximal age, 30 years 18 0.126 1,253 60 185 0.006 1,344 80 Source: Reed 2003.
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Endangered and Threatened Species of the Platte River If mortality increased only enough to slow growth, rather than to cause a population decline, adverse effects on the AWP could still occur. A net loss of alleles will continue to occur in the AWP through genetic drift (a loss of genetic material from a population due to small population size) until the population becomes large enough for the rate of genetic mutations (which produce new alleles) to roughly equal the rate of drift (which loses alleles). The longer it takes for the AWP to reach a size at which drift and mutation rates are balanced, the more genetic material will be lost from the population. When the AWP went through a bottleneck in 1941, reaching a low of 15 birds, an estimated 66% of the mitochondrial DNA diversity in the population was lost (Glenn et al. 1999). Further loss of genetic diversity because of a slowed recovery could be problematic. SUMMARY AND CONCLUSIONS This chapter began with three questions related to whooping cranes: Do current Platte River habitat conditions affect the likelihood of survival of the species? Do they affect recovery of the species? Is the current designation of central Platte River habitat as critical habitat for whooping cranes supported by science? The chapter has reviewed information collected by the committee from the literature and from unpublished DOI documents concerning the species, from testimony given by agency and other specialists, from a visit to DOI facilities in Denver and Grand Island, and from field examinations. The committee concluded that current habitat conditions along the central Platte River adversely affect the likelihood of survival and recovery of the whooping crane population. Geographically, the central Platte River occupies a critical position along the migration route of the species, between the wintering grounds in coastal Texas and the summer breeding grounds in north central Canada. The river and its closely associated lands provide useful roosting areas for birds in the midst of migrations of thousands of miles. The portions of the river that are not heavily wooded provide open areas separated from the banks by channels—an arrangement that provides security for the roosting birds. Nearby wetlands and agricultural areas provide important forage for the birds and allow them to obtain needed nutrition to support their continued migration. The committee concluded that there are no apparently suitable alternatives to replace the central Platte River in its function as habitat for migrating whooping cranes. The Rainwater Basin, south of the river, includes numerous small wetland basins and patches, but these areas periodically dry completely, whereas the Platte River flows relatively continuously. Because the Platte draws its water from distant mountain watersheds, its flows are less susceptible to periodic drought on the plains than are the
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Endangered and Threatened Species of the Platte River ephemeral wetlands of the Rainwater Basin. Nearby rivers also lack the consistency of the Platte, and the Niobrara and Loup Rivers and smaller streams do not offer alternative habitat with the same qualities as the Platte. On the average, about 7% of the whooping cranes currently use the central Platte River as a stopover during migration, but there is substantial fluctuation in this fraction from year to year. The loss of the Platte River habitat would have potentially serious consequences for the species. If mortality increases by only 3%—a likely scenario if the Platte River habitats become unavailable, because most crane deaths occur during migration—the entire migrating population of less than 200 is likely to become unstable. The general total population of migrating whooping cranes is slowly increasing from its low of only 15 in 1941, and the proportion of the population that uses the central Platte River as a stopover each year is also gradually increasing. The river therefore directly affects species recovery. The committee also concluded that the current designation of central Platte River habitat as critical habitat for whooping cranes was supported by the science of the time of the designation and that it is supported by present scientific understanding. When DOI agencies designated the critical habitat in 1978, they relied on information that was available then. Agency personnel used information that was available in the refereed scientific literature, agency reports, and additional observations made by agency personnel. Agency personnel made the designation by using procedures that the biological community commonly recognized, and internal peer review strengthened confidence in the designation. There have been no developments in scientific knowledge that invalidate the 1978 decisions. Given the small numbers of migrating whooping cranes, some parts of the designated habitat inevitably will not be used in some years. Because both the population and the percentage that stops on the Platte River are slowly increasing, it is likely that the occasionally nonused portions of the critical habitat will be increasingly important. The general utility of the central Platte River for roosting whooping cranes also changes from one year to the next, depending on the hydrological conditions of the river, which are subject to some change. The committee also acknowledges that conservation actions at locations outside the central Platte River, particularly in overwintering and breeding habitats, will have important effects on continued use of Platte River habitats, and that a wide variety of management responses—including captive rearing, reintroductions, and translocations—will continue to play critical roles in the conservation of the species as long as populations continue to persist at low levels. Finally, in addition to addressing the three questions related to whooping cranes, the committee identified gaps in knowledge and data that should be addressed. The committee strongly urges managers of the central Platte River to adopt a multispecies approach to their research and decisions.
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Endangered and Threatened Species of the Platte River Solving problems related to whooping cranes without reference to other species will probably lead to additional problems. Knowledge and data on how the various species may interact with each other through habitat manipulation are especially important. When managers clear woodland for whooping crane habitat, for example, there appear to be favorable outcomes for the crane population, but there is little knowledge about the effects of such actions on other species, so continued monitoring and measurement efforts are required. The committee also recognizes the importance of long-term records and data in reaching conclusions about whooping crane use of the central Platte River. Because of the annual fluctuations in the river processes and in crane use, trends of only a few years are not likely to be informative regarding longer-term, decade-long trends, so analysis and prediction require datasets on birds and environmental conditions that exceed a few years.
Representative terms from entire chapter: