Responses of mammal dispersers to fruit availability: Rowan (Sorbus aucuparia) and carnivores in mountain habitats of northern Spain

Despite the well known fact that carnivore mammals are important fruit consumers and legitimate seed dispersers in temperate habitats, little is known about their quantitative responses to fruit availability. Here we show the results of two studies conducted at two different temporal and spatial scales, that were intended to assess the response of pine martens (Martes martes) and red foxes (Vulpes vulpes) to variations in the supply of rowan (Sorbus aucuparia) fruits in the Cantabrian Range (northern Iberia). First, we studied the association between fruit availability and the importance of rowan fruit in the diet of carnivores during a period of 11 consecutive years. This was accomplished by comparing fruit-crop size in 54 trees and the analysis of faecal contents in a sample of 863 faeces. Secondly, we assessed the consumption of fruits by these two species underneath the canopy of 20 rowan trees along 10 consecutive days. In the first study, the diet of martens and foxes consistently tracked interannual variations in rowan fruit availability, despite large fluctuations in fruit yield that included three mast years of heavy rowanberry crops and three non-fruiting years. For both carnivores total crop size was correlated with the frequency of occurrence and the proportion of rowan by volume in faeces. The second study suggested that carnivores feeding on fallen fruit tended to visit the trees that exhibited a higher density of fruits under the canopy. Thus, carnivores apparently choose to feed on high-density patches of fruit, which in turn were located underneath the canopy of the trees that produced the larger crops. Our results stress the need to pay proper attention to the role of carnivores as seed dispersers, in order to disentangle the evolutionary and ecological outcomes of plant-animal interactions in mixed-dispersed plants.