دانلود رایگان ترجمه مقاله زمان و فاصله پراکندگی زایمان برای گراز وحشی Sus scrofa در سوئد (نشریه Bioone 2003)

زمان و فاصله پراکندگی زایمان برای گراز وحشی Sus scrofa در سوئد

Timing and distance of natal dispersal for wild boar Sus scrofa in Sweden

In order to estimate age at natal dispersal and dispersal distance, wild boars Sus scrofa were studied in Sweden by use of mark-recapture and telemetry. To describe the average natal dispersal in the population, we applied a sigmoid regression model to analyse the age specific distance to the natal site. We used the first and second order derivatives of the model to determine at which average age dispersal begins, reaches its maximum rate and ceases. The average dispersal distance was estimated by the maximum value of the model. Using this method, we reached the conclusion that male wild boars begin to disperse at the age of 10 months, reach their maximum dispersal rate at the age of 13 months, and that dispersal ceases at the age of 16 months. At that age, they have dispersed 16.6 km from their natal sites. For females, the corresponding ages were 7,9 and 11 months, with a dispersal distance of 4.5 km. The results follow the prevalent opinion on natal dispersal in polygynous mammals. Dispersal distances are generally short with a negative exponential probability distribution. Dispersal reaches its maximum rate during the age at which sexual maturity is occurring, and males disperse longer distances than females.

Animal dispersal occurs when individuals leave their social group or home range, causing population redistribution that leads to spatial spread (Turchin 1998). Depending on when in life dispersal occurs, it can be divided into natal dispersal if juveniles move from birth sites to breeding sites, and breeding dispersal if adults move between breeding sites (Greenwood 1980). Juveniles generally disperse in higher proportions and roam longer distances than adults, and are assumed to gain advantages by dispersing as they would otherwise be competing with their parents for food, dens and reproductive opportunities (Waser & Jones 1983).

Many species show sex differences in their dispersal patterns (Greenwood 1980, Dobson 1982). Pusey (1987) theorises that the reason for this difference in dispersal patterns may be caused by inbreeding avoidance and sex differences in competition. In polygynous mammals, males disperse in higher proportions and roam longer distances than females. Females are supposed to benefit from the familiarity of an area because of higher parental investment. Males, who are competing for access to mates, should benefit from dispersal (Pusey 1987). In mammals, where individuals of one sex live in social groups, it is common that all members of the opposite sex disperse (Packer 1979, Pusey 1987).

The distance and rate of animal dispersal are of importance in many fields of population biology. Metapopulations, source sink dynamics, invasion, colonisation, gene flow and genetic structure are issues strongly associated with dispersal (Paradis, Baillie, Sutherland & Gregory 1998). The probability distribution of dispersal distances is usually assumed to be negative exponential (f(x) = ae bx; McCallum 2000). It has been suggested that the distributions are shaped this way, because dispersal distance is a function of home range turnover probability, and because individuals settle in the first uncontested home range area they encounter (Murray 1967, Waser 1985). However, many of the described distributions of animal dispersal distances are assumed to be incorrect because of measurement errors (Koenig, Van Vuren & Hooge 1996). If the study area is limited, long distance dispersers may not be detected, and the results may thus become biased towards shorter distances.

The wild boar Sus scrofa is a polygynous mammal with a social organisation characterised by matrilineal groups consisting of females and their offspring and solitary adult males (Martys 1991). Among the characteristics of the species are high rates of reproduction (Mauget 1991) and adaptation to a wide variety of habitats (d’Huart 1991). Wild boars and feral pigs have increased in density and geographic distribution in many countries, primarily due to introductions by humans (Waithman, Sweitzer, Van Vuren, Drew, Brinkhaus & Gardner 1999).

The wild boar was present in Sweden until the sixteenth century, when it became extinct. During the last decades, wild boars have been kept in confined populations maintained for sport hunting or breeding. Escaped or deliberately released individuals from confined populations have established free-living populations, and today, the wild boar is locally and regionally abundant in Sweden. The species is currently expanding geographically, but the expansion rates of the populations are still unknown. Prediction of the rate of spread of a population usually requires data on the dispersal of individuals (McCallum 2000).

The purpose with our study was, on the basis of collected data on the dispersal traits of wild boars in Sweden to consider the described general traits associated with mammalian dispersal to see if they are applicable to wild boars as well. Consequently, we performed a study to describe the dispersal distribution for wild boars. We also intended to estimate the age at dispersal and to test for sex difference in dispersal distance.