Studies on the natural population regulation in chamois (Rupicapra rupicapra L.) in the Swiss National Park

Summary

The chamois (Rupicapra rupicapra) is the only ungulate species that populates the Swiss National Park (SNP) since its establishment at a constantly high density. This is in spite of the fact that large short-term variations in population size indicate high instability of local populations. The mechanisms behind this dynamic stability are currently under investigation in a project called "Population Biology of the Chamois Rupicapra rup. rupicapra in the Swiss National Park" which was initiated by the SNP management office. The data presented here were collected in summer of 1999 as part of this project.

In this study I developed a general model for the natural population regulation of chamois from empirical hypotheses about mutual relationships between population regulating factors among ungulates by different authors. The basic concept of this model explains the regulation of the population size through the social environment of the individuals: The higher the population density, the more critical the food resources are. The behaviour of individual animals also becomes more aggressive. Therefore, as a consequence of food shortage, more subordinate animals are forced to move into a habitat of lower value. The lowered average reproduction rate of those animals lowers the reproduction rate of the whole population. The number of individuals starts declining and the pressure on the food resources is reduced. This self-regulating loop, however, doesn't run within a closed system, but rather is susceptible to the physical environment. The model suggested was tested on the two populations "Val dal Botsch" and "Trupchun" - living under different conditions of competition and food - to assure that no contradictory evidence was found using observations at different levels.

Concerning the levels of population development and population structure, both populations showed a sex ratio that was noticeably shifted towards the females. It was determined that the already higher mortality rate of the males increased additionally during winters that had a high snowfall, even when the individuals were in good condition. The females on the other hand were rarely, or in lower numbers, pushed towards their subsistence limits due to scarce food resources. Not only for males, but also for yearlings and kids, the winter was clearly shown to be the critical time of the year. An early start of winter at the end of September following a cold and wet summer caused many deaths among the kids. In the barren dolomite landscape of the Val dal Botsch the youngest generation was hardest hit. The females, who at the same time forced the weaning of the kids, possibly made things worse.

Concerning spatial distribution the weather again proved to be an influential factor. Current precipitation quality explained habitat use fairly well and peak daytime temperature almost flawlessly. Habitat use along with the time of the year for its part determined the size of the mixed female groups. With increasing openness of the terrain and further into the summer, the size of the groups started to increase. The distribution of the animals on a small scale within the groups changed over time and was strongly influenced by the social classes involved. Females accompanied by kids usually stayed closer to other individuals than those without. This influence of the presence of kids on the distance of their mothers to other individuals showed that the kids were clearly considered as individuals in their own right by other conspecifics. In general the average distances within groups showed an indirect proportional ratio to the size of the groups. From July onwards the individual distances decreased. This most likely reflected the yearly process of finding the new social organisation in the summer home range. As the structure of neighbouring animals showed, the females separate themselves during parturition and thereafter they first form smaller groups with females who also have newborn kids. Other females and yearlings continually join these groups during the summer. The results showing that 1) yearlings during June significantly often had conflicts with females, and 2) agonistic behavioural elements in mixed groups were most often encountered during July, again indicate this time pattern of social reorganisation.

With respect to social behaviour the results show that the social gradient of middle-aged and middle-ranked females is quite flat. In contrast a clear hierarchy was found amongst the higher and lower ranks - the very old and the very young animals. The repeatedly observed build-up of temporarily stable subgroups was interpreted as a strategy to reduce potential conflicts.

The synthesis of all results led me to believe that the populations of the Swiss National Park are primarily limited by density independent, extrinsic factors (weather) and secondarily - if at all - by density dependent, intrinsic factors. The intrinsic factor of social organisation may even have a regeneration effect. The social system of the chamois remained on a rather low evolutionary level and, as such, guarantees a quick response to increased mortality rates because it leaves room for physiological and ecological flexibility.