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Predicting population trends of the Black-faced Spoonbill (Platalea minor).

Article Excerpt
Knowledge of how a population changes over time can be used to forecast population size in the future. Population forecasts are important to conservation and management of threatened or endangered species. The Black-faced Spoonbill (Platalea minor) was classified by the IUCN and the Taiwan Government as "critically endangered" in 1992 (Hancock et al. 1992). The cause of the decline is unknown. But events, such as the Korean War (Collar et al. 2001), Vietnam War, Cultural Revolution in China, as well as wetland loss, environmental changes (Yeung et al. 2004), disease (Chyi and Hu 2003, Shiau et al. 2003), and poaching (Liu 2004) may have contributed to the decline of the Black-faced Spoonbill population. More attention has been focused on the species since 1992 (Severinghaus et al. 1995) and the global population of the Black-faced Spoonbill has increased steadily in the past decade. Only 288 birds were recorded in East Asia in 1988 (Kennerley 1990) but the population had increased to 1,206 individuals by winter 2003/2004 (Lee and Liu 2004).

Black-faced Spoonbills breed between April and September on islands along the coast of the Yellow Sea of the Korean Peninsula and China (Won 1966, Chong et al. 1996, Daimon 1997, Ueta et al. 2002, Kim 2004). In winter, Black-faced Spoonbills migrate to wintering sites along the coasts of Japan, Taiwan, the Philippines, Vietnam, Macao, Hong Kong, and China (Hachisuka and Udagawa 1951, Hancock et al. 1992, Lee et al. 1995, Mendoza et al. 2002, Ueta and Higuchi 2002, Ueta et al. 2002). Over 50% of the population in East Asia regularly overwinters in the coastal wetlands of southwestern Taiwan (Ueng and Kuo 1992; Dahmer and Felley 1996, 2000; Liu 2005). Since 1992, hunting of the birds was strictly prohibited and two wildlife reserves, the Chiku Black-faced Spoonbill Reserve and the Shihchu Wildlife Reserve, were established in southwestern Taiwan. The number of wintering birds in Taiwan has increased gradually since then.

We estimated the growth rate of global Black-faced Spoonbills based on existing data. We used the long-term data for Black-faced Spoonbills collected in southwestern Taiwan to analyze the proportion of the wintering subpopulation in Taiwan. We then used a stochastic exponential model (Akcakaya et al. 1999) to predict global population size and to estimate the size of the wintering Taiwan subpopulation in the next decade. We used the survival rate estimated from wintering birds in Taiwan, and the fecundity rate calculated from the above two parameters to predict global population size and the mean and variance of the global growth rates.

METHODS

The Global Population.--The annual global population estimator of Black-faced Spoonbills between 1991/1992 and 2003/2004 was derived from the existing literature. We also estimated the annual global population size between 1991/1992 and 1996/1997 by summing winter count numbers (mainly Dec and Jan) in Japan, China, Hong Kong, Vietnam, and Taiwan (Scott and Rose 1989; Kennerley 1990; Perennou et al. 1990; Perennou and Mundkur 1991; Ueng and Kuo 1992; Dahmer and Felley 1994, 1995; S. Chan, pers. comm.). The annual global population size between 1996/1997 and 2003/2004 was estimated from synchronous counts of each wintering subpopulation in East Asia including South Korea, Japan, mainland China, Vietnam, Hong Kong, and Taiwan (Dahmer and Felley 2000, Lee and Liu 2004, Nguyen 2004).

The Taiwan Subpopulation.--The wintering population in southwestern Taiwan was counted from 1991/1992 through 2003/2004 in our study area on the coastal...