Corlatti, L., Bonardi, A., Bragalanti, N. and Pedrotti, L. (2019), Long‐term dynamics of Alpine ungulates suggest interspecific competition. Journal of Zoology, vol. 309, pp. 241-249. https://doi.org/10.1111/jzo.12716
Despite being the most abundant mountain-dwelling ungulate of Europe, some chamois (Rupicapra spp.) populations have recently showed a numeric decline. How is it possible that this goat-antelope, so well adapted for life in the mountains, may suffer local negative trends? It is with this question in mind that my co-authors and I decided to investigate the potential mechanisms behind this bewildering phenomenon. We had the rare opportunity to have a long time-series of count data, eagerly collected by the personnel of the Stelvio National Park, within the central Italian alps, over 34 years between 1984 and 2018. Long-term datasets are essential when investigating the dynamics of animal populations, that is the relationships between demographic parameters such as growth rate and their potential drivers. Identifying these potential drivers, however, is challenging.
Many factors can be at play, and they may act synergistically. The first driver that naturally came to mind was climate change. In the short term, species may cope with climate change in a number of ways. For example, they can shift to or concentrate the time spent foraging at the cooler time of the day, or they can move to more thermally comfortable zones, at higher elevations. These strategies come at a price, as the animals may eventually lose foraging opportunities, either because of time constraints, or because of reduction in available foraging areas. All these mechanisms have been previously suggested to occur in chamois, and may eventually lead to a decline in populations. Nonetheless, the decline in our study population, we though, was unlikely to be explained solely by climate change. After the peak in chamois numbers in the mid-1990s, a consistent negative trend was evident to these days; this change seemed too abrupt to be accounted for by climate change only. Another actor must have been at play.
Recent works in central Italy strongly supported the synergistic role of a competitor, the red deer Cervus elaphus, and climate warming in explaining the decline of the closely related Apennine chamois. Could this be the case also for the Alpine chamois? Indeed, in the early 1980s red deer were a sporadic presence in the park and chamois were abundant. After an initial phase of shared increase, when in the mid-1990s the population of red deer reached conspicuous densities, chamois began their steady decline. To us, this pattern was suggestive of a negative impact of red deer on chamois through some form of interspecific competition. However, proving competition is one of the most challenging endeavours in ecology, as it basically requires proving a cause-effect relationship. The main challenge lies in the need to prove that no other mechanism can be responsible of the observed inverse relationship between the two populations’ trends.
We thus came up with the idea of testing several models that could provide strong support for one of three alternate hypotheses. 1) red deer caused the chamois population decline; 2) the decline of chamois released the red deer population; 3) the dynamics of both populations were independent from each other and could be explained by climatic factors only.
Our analysis revealed unequivocally that the most likely mechanism accounting for the decline in chamois numbers was the increase of the red deer population, therefore strongly supporting the interspecific interaction hypothesis. Climate warming did not have a direct effect on the chamois population. Red deer, and to a lesser extent chamois, were negatively affected by the synergistic effect of density dependence and snow conditions in winter. The fine-scale details of this interspecific competition still remain to be clarified: which segment of the chamois population (males, females, kids) is most affected by the increasing red deer density? Although no chamois species is at risk, several subspecies are threatened and understanding the processes underlying their temporal dynamics will be of key importance for informed management of populations. Competition with other species is a reason for concern that should be carefully evaluated, especially when human interventions such as reintroductions of superior competitors are considered in areas where chamois are present.