Verhaltensökologie menschlichen Abwanderungsverhaltens – am Beispiel der historischen Bevölkerung der Krummhörn (Ostfriesland, 18. und 19. Jahrhundert)
Behavioral ecology of human dispersal - using data from the historical Krummhörn population, North-West Germany (18th and 19th century)
Gießen, Justus-Liebig-Universität (2001)
The evolution of natal dispersal is still not well understood. The reasons lie not least in specific empirical obstacles which affect behavioral ecological analyzes of both animal and human dispersal. During the last decades many different models and hypotheses have been proposed in order to explain the evolution and the functional design of dispersal. The aim of this thesis was to examine the relevance of some of the most influential models using data on a historical human population.
The investigation is based on a family reconstitution of the eighteenth- and nineteenth-century Krummhörn population. The entries from the church registers of 19 parishes were assigned to family lineages. In addition local tax lists containing entries on landownership were used to reconstruct the socioeconomic structure of the population. The evaluation of neighboring parishes allowed one to keep track of a substantial number of leavers even after the dispersal event occurred. This is a major strength since in most other studies dispersing individuals leave the observational focus of the investigator which results in serious empirical difficulties.
The results are divided into two parts. In the first part the dispersal pattern of the population was described on the basis of the following records: place of birth, place of marriage, place of children´s births, place of death. Since it turned out that all these records have advantages and disadvantages in indicating both the place of origin and the current residence (after a potential dispersal - both are necessary criteria to decide the status of natal dispersal) a new variable was created (called "Familienschwerpunkt" or "center of family gravity") considering all available information of residences for every individual weighted for their particular significance. In the second part the hypotheses of each explanatory model were tested. To do this the influence of various critical variables on the probability of dispersal were analyzed - in detail they are: sex, resources, number of siblings, probability of marriage functional birth rank, parish size and number of relatives. Further, the analyzes considered aspects of dispersal distance by differentiating between regional dispersal and out-of-area emigration.
The results showed that one model alone is not sufficient to explain the observed dispersal pattern. Rather various mechanisms influenced the dispersal behavior of the individuals though the particular importance differed according to sex and socioeconomic class. Dispersal was female biased thus meeting the expectations proposed for mating systems of a resource defense type. However, although women left their parish of origin more frequently than men this was not true concerning the out-of-area emigration. The dispersal behavior of offspring of wealthy farmers reflected a local resource competition scenario: the opportunity for both sons and daughters to stay at the place of origin and also the opportunity to marry were reduced by having many siblings. For both sexes brothers were in particular crucial. Further sons were more restricted than daughters. Although daughters of laborers were also restricted by the number of siblings they differed from the wealthy daughters since only their sisters had a significant influence and, moreover, it concerned only the probability of regional dispersal rather than emigration - both indicating the existence of a local mate competition scenario. No effects could be found for sons of laborers. It seems that intra-familial competition - if it occurred at all - had no influence on their decision to disperse. Rather it seems that their decisions to leave were initiated mostly by mechanisms described by the "greener pasture syndrome" - that is a triggering of the dispersal event by a given sensitivity of the individual to the possibility of moving to a better home range. Further mechanisms usually suggested to explain the function of dispersal - in particular inbreeding avoidance and bet hedging - may have had an effect but they were consequences of this behavior rather than the cause of dispersal.
In conclusion the observed dispersal behavior can be understood as part of the reproductive strategy both of the individual itself and of its parents - although the particular importance may differ between groups of individuals. Accordingly parental manipulation may have played a significant role. The decision to leave always has to be made by the individual itself but he or she may not always be the person benefiting from this behavior. In order to properly understand the functional design of dispersal it is important to keep this distinction in mind.
This work was funded by the Deutsche Forschungsgemeinschaft (DFG).
Keywords: Germany, migration