Biological Conservation 84 (1998) 35--45 Published by Elsevier Science Ltd Printed in Great Britain PII: S0006-3207(97)00077-3 0006-3207/98 $19.00+0.00 !IELSEVIER LIFE-HISTORY AND VIABILITY ANALYSIS OF THE ENDANGERED HAWAIIAN STILT J. Michael Reed,a* Chris S. Elphickb & Lewis W. OringC °Biological Resources Research Center, University of Nevada, Reno, 1000 Valley Rd, Reno, NV 89512 USA bEcology, Evolution, and Conservation Biology Program, University of Nevada, Reno, 1000 Valley Rd, Reno, NV 89512 USA C Ecology, Evolution, and Conservation Biology Program, and Department ofEnvironmental and Resource Sciences, University ofNevada, Reno, 1000 Valley Rd, Reno, NV 89512 USA (Received 15 July 1996; revised version received 24 May 1997; accepted 29 May 1997) [Hawaiian] stilt, ... .A very fine endemic bird which INTRODUCTION should not be allowed to become extinct or even rare. (Munro, (1946; p.46» Two primary problems in conservation biology are identifying species at risk of extinction and determining what can be done to reduce that risk. Population viabi­ Abstract lity analysis (PVA) is a tool that can be used to address The Hawaiian stilt Himantopus mexicanus knudseni is both problems. Using life-history data and their rela­ an endangered, endemic subspecies of black-necked stilt. tionships with environmental factors, PVA is used to We present life-history data required to perform popula­ estimate persistence probabilities of populations under tion viability analysis (PVA), and the results of a series different conditions (Shaffer, 1981; Salwasser et al., 1984; of PVAs under two scenarios, treating (a) the subspecies Gilpin and Soule, 1986; Marcot et al., 1986; Reed et al., as a single population, and (b) six subpopulations as a 1988; Woodruff, 1989; see Boyce, 1992 for a review). metapopulation. We performed sensitivity analyses on Data inadequacies and simplifying assumptions can model parameters and used results to address various limit confidence in the specific predictions of viability management options. Both basic models predicted that models (Caughley, 1994; Harcourt, 1995; Taylor, 1995). stilts would increase to fill available habitat with no By varying parameter conditions, however, PYA can be chance of a significant decline. Catastrophe, maximum used to explore the consequences of different manage­ age, and density-dependent reproduction had little effect ment schemes on model popUlation dynamics (Walsh, on population projections. Rapid declines in the probabil­ 1995), thereby providing insight useful to managers. In ity of stilt populations persisting occurred when clutch this paper, we use PYA to estimate extinction risk and to failure rate or first-year mortality rate increased above evaluate management options for an endangered shore­ 70%, or when adult mortality rate increased above 30%. bird, the Hawaiian stilt Himantopus mexicanus knudseni. Model predictions of mean population size at 200 years Hawaiian stilts are a subspecies of the black-necked tracked changes in carrying capacity. If current conditions stilt endemic to the Hawaiian islands. They are signifi­ change such that rates of clutch failure or stilt mortality cantly larger than their North American counterpart increase, population declines and eventual extinction (Coleman, 1981), and differ somewhat in plumage becomes more likely. Managers, therefore, should main­ characteristics (Wilson and Evans, 1893). Hawaiian tain predator control, limit water level fluctuations, and stilts are found on all five major islands (Hawai'i, maintain current habitat area. Downlisting is not war­ Kaua'i, Maui, Moloka'i, O'ahu) although their presence ranted because wetland management and predator control on the island of Hawai'i might be due to recent recolo­ are necessary for Hawaiian stilts to persist. Published by nization after several decades of absence (paton et al., Elsevier Science Ltd 1985; Banko, 1988). Stilts also are found in abundance on Ni'ihau, which shares birds seasonally with Kaua'i, Keywords: Hawaiian stilt, Himantopus, shorebird, and since 1989 on Lana'i at a newly available water extinction, PVA, demographic model. source (Engilis and Pratt, 1993). Hawaiian stilts forage in shallow water and nest on adjacent embankments (Coleman, 1981; Engilis and Reid, 1994). Historic and current population sizes have *To whom correspondence should be addressed at: Depart­ depended partly on certain agricultural practices that ment of Biology, Tufts University, Medford, MA 02155, USA, provide breeding and foraging grounds (e.g. taro, sugar Fax: 702-784-4583. cane runoff) (Griffin et al., 1989). Dependence on 35 36 J. M. Reed, C. S. Elphick, L. W.Oring agricultural habitats and practices, coupled with habitat conservation efforts to be focused appropriately. For conversion for housing and business, has resulted in a example, Crouse et at. (1987) found juvenile survival fragmented and reduced wetland landscape, particularly limited adult numbers in loggerhead sea turtles Caretta in coastal wetlands where stilts are confined (Handy and caretta, indicating conservation efforts should focus on Handy, 1972; Shallenberger, 1977; Coleman, 1981; juvenile survival rather than on egg production or hatch Griffin et aI., 1989). success. Because the most sensitive variables require the There are almost no published data on Hawaiian stilt most accurate data, sensitivity analyses also can be used ecology or population biology and only qualitative esti­ to focus researchers' energies on improving estimates of mates of population size before the 1940s. Henshaw, the most important variables (Reed et aI, 1993). We (1902) reported that stilts were common on the island of used results of this analysis to address particular O'ahu in the late 18oos, but by 1900 were very scarce. He management options. attributed the severe decline to overhunting. Although Our final goal (4) was to assess population growth Hawaiian stilt flesh was viewed as being of little value potential. State-wide population size in 1947 was esti­ for food, stilts were hunted even before white settlement mated to be 1000 (Schwartz and Schwartz, 1949). Only (Henshaw, 1902; Handy and Handy, 1972). Hunting six years earlier Munro, (1944) estimated it to be only continued to be legal until 1941 (Schwartz and Schwartz, 200. It has been supposed that Munro's estimate was 1949), and was probably a major factor in keeping low because this rate of growth was viewed as unlikely population sizes low during the late 1930s (Munro, 1938; (e.g., Schwartz and Schwartz, 1949; Fisher, 1951). We Shallenberger, 1977). Munro, (1944) estimated that used our population model to determine whether or not there were approximately 200 individuals in the early growth of this magnitude could have occurred. 1940s. Following cessation of hunting, numbers rose rapidly, and by 1947 there were 1000 individuals (Schwartz and Schwartz, 1949). Since then, Hawaiian METHODS stilt numbers have increased to their current value of approximately 1200 birds (Reed and Oring, 1993). Viability criteria and population structures Little is known of the population structures of We defined a popUlation as safe from extinction if there Hawaiian waterbirds, but evidence suggests that local was less than a 5% probability of its declining signifi­ stilt populations are connected through dispersal (Tel­ cantly in 200 years. Philosophically, we would have fer, 1971, 1972; Pyle, 1978; Telfer and Burr, 1978; preferred having no biologically significant decline as Engilis and Pratt, 1993; Reed et at., 1994). The Hawai­ our criterion, but we believe this cannot be determined a ian stilt population might exist as a metapopulation priori (Reed and Blaustein, 1997). In order to assess (Reed et at., 1994), which adds complexity to popula­ significance of an observed decline, we used a one­ tion processes (e.g. Murphy et at., 1990). Movement tailed, one-sample t-test. Our null hypothesis was that among populations affects demographics, population the population size at the end of 200 years (T = 200) dynamics, and genetics, and is the driving parameter in would be equal to, or greater than, the popUlation size metapopulation models (e.g. Hastings and Wolin, 1989; at the start (T = 0); our alternative hypothesis was that Hansson, 1991; Wu et at., 1993). Little is known about the population size at T = 200 would be significantly Hawaiian stilt dispersal patterns, except that they do less than that at T = O. Target times in population move among wetlands and islands (Munro, 1944; Engi­ projections are arbitrary (cf. Shaffer, 1981), and 200 lis and Pratt, 1993; Reed et al., 1994). years was chosen as a reasonable management time We have several goals in this manuscript: (1) we pre­ frame. Each iteration of a model was treated as a single sent life-history data required for performing popUla­ replicate and 140 iterations were used for each model. tion viability analyses, summarizing data from This sample size gives a statistical test with a power of unpublished studies and supplementing these data with I - {J = 0·80, when a = 0·05 and the desired effect size is our own research; (2) we used VORTEX (Lacy et at., assumed to be small (0·2) (calculated using correction 1995), a stochastic simulation model, to perform popu­ from a two-tailed test; Cohen, 1988 providing a strong, lation viability analyses for the endangered Hawaiian conservative test capable of detecting significant stilt. These analyses were done under two population declines. We ran two basic models using VORTEX structure