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Modelo demográfico de la Grulla Común Grus grus en Europa Occidental

AutorAlonso López, Javier A.; Alonso López, Juan C. ; Bautista, Luis M.
Fecha de publicación1990
EditorInstituto Nacional para la Conservación de la Naturaleza (España)
CitaciónDistribución y demografía de la Grulla Común (Grus grus) en España, pags. 163-192, (1990)
SerieColección Técnica
ResumenSimulation modeling has been increasingly applied during the last years to wildlife ecology and management (e.g., MILLER & BOTKIN 1974, MILLER 1978, JOHNSON 1982, VERNER el al. 1986, RYKIEL 1989). This technique may be especially useful to help understand population dynamics oí endangered vertebrate species and to identify the key factors relevant for their future conservation. Cranes are a suitable group for such research, with most of their living species seemingly declining and some of them even endangered (see JOHNSGARD 1983, ARCHIBALD & PASQUIER 1987, ARCHIBALD & HARRlS in press), but others enabling the gathering of extensive demographic field data useful to model their, and other family members' population trends. In this paper we: a) describe some basic demographic parameter of the Common Crane (Grus grus) population migrating through the Western European route, b) develop various alternative and updated versions of a simple population model (see ALONSO et al. 1986b) similar to those produced for the Sandhill Crane (Grus canadensis) (MILLER el al 1972, JOHNSON 1979), and c) explore through simulation the responses of the population to various events that could affect the survival and breeding of Common Cranes. Two of the parameters used in the model were obtained during the last eleven year at the species' wintering areas in Spain: the population size, which was estimated around 60000-70000 birds after several censuses carried out during migration and at the wintering sites (ALONSO el al. 1986a, ALONSO el al. 1990), and the percentage of juveniles, which averaged 12.75% for a sample of 173018 birds aged during autumn staging at Laguna de Gallocanta, NE Spain, between 1979 and 1989 (Table 1). The model ignored hatching and fledging success, for which no reliable data are available, and simplified breeding to a single process depending only on the size of the sexually mature population and its annual recruitment to the autumn population. Other demographic data -age of first breeding, longevity and mortality- are still unknown in the Common Crane and were taken from the Sandhill or the Whooping Crane. On the basis of recent literature reviews and captive breeding data (WALKINSHAW 1973, JOHNSGARD 1983, PRANGE 1989) and some field studies (TACHA el al. 1989), first breeding was assumed at the age of 4 year. Longevity and mortality rates for each age-c1ass were taken from BINKLEY & MlLLER (1980), who established 24 age classes, and mortalities of 0.375 for first-year birds and slightly increasing from 0.0183 to 1 between 2-year and 24-year birds, for the Whooping Crane, based on annual censuses and age compositions recorded since 1938. The model was written in FORTRAN-77 and implemented on an IBM.AT PC. Integration interval was 0.20. We developed 3 alternative versions of the model, two of them incorporating different degrees of a density-dependent effect of the number of birds in the population on annual productivity and mortality rates (through inverse logistic functions, see Figs. 1 and 2), and the third completely density-independent. etc...
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