Summary of Part 2:
A demographic model for the Common Crane Grus grus in Europe
by J.C. Alonso, J.A. Alonso and L.M. Bautista
 

Simulation modeling has been increasingly applied during the last years to wildlife ecology and management (e.g. Miller & Botkin 1974, Miller 1978, Johnson 1982, Verner et al. 1986, Rykiel 1989). This technique may be specially useful to help understand population dynamics of 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 crane species  seemingly declining and some of them even  endangered (see Johnsgard 1983, Archibald & Pasquier 1987, Archibald & Harris 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) present describe some basic demographic parameters 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) (see Miller et 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 years 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 et al. 1986a, Alonso et 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 et al. 1989) first breeding was assumed at the age of 4 years. Longevity and mortality rates for each age©class were taken from Binkley & Miller (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.

We run the model 9000 times, each with a different combination of the parameters (5400 with the more density©dependent version and 3600 with the less density-dependent one), and selected those resulting in :  (a) a stabilized population size between 65000 and 70000 birds, and  (b) a percentage of juveniles of 12.75 ± 5%. Values  (a) and (b) were also used to adjust the densityªindependent version of the model. The 26 combinations given in Table 2 represent a selected sample of the series of plausible combinatins that are consistent with conditons (a) and (b). For the simulations, we selected combinations no. a₅  and no. b₁₄ , respectively for the more and less density-dependent alternatives. The simulated scenarios include the effects of various hunting rates, changes in breeding success, survival and age of first breeding, and a catastrophic decrease in the population size. All the experimental situations start on the 35th year of a simulated series of 75 years (see Figs. 3.1 - 3.13). The simple model developed should be considered as a first attempt to describe the Common Crane population dynamics. Its mathematical simplicity is determined by the lack of better field data on the demography of the species, and its deterministic nature, by the absence of demonstrated causal relationships between population characteristics and environmental variables. Its predictive utility depends on the reliability of the parameter values used. However, the main objective was to provide a basic model that may be used to identify: parameters to which it is more sensitive, lacks in our knowledge of the species, and recommended management actions.

Simulated hunting showed a relatively high sensitivity of the population to low hunting rates (1-5%, i.e. 600-3000 birds), with

a result varying between extinction and significant decreases in the number of birds. Simulated situations affecting the breeding success of the population, like reductions of the breeding habitat or deteriorations of the feeding conditions at breeding or wintering areas, or reducing survival, like the mentioned poor feeding conditions or an epizooty, could probably be more dangerous than hunting. Annual recruitment rates lower than 10% determined the extinction of the population for every combination of the other parameters involved. Interestingly, the average annual recruitment rates measured for various crane species stay close over that limit (see Buller 1979, Johnsgard 1983, Prange 1989 and present study). Additional risks could be those determined by the stochastic nature of the environment, which could enhance the negative effects predicted by the deterministic model. Current research needs include:  study of mortality rates and age of first breeding; density-dependent effect on breeding success, mortality and age of first breeding; effect of environmental variables on breeding success; response of the population to artificial increse of the breeding habitat; degree of renewal of disappeared breeders by non©brreders or floaters in the population. Such objectives will need field studies based on individual marking, and the continuation of censuses and recruitment samplings at breeding, stopover and wintering areas.