Experimental studies on the dispersal of plants and animals on sheep in calcareous grasslands.
In June to September 1993 a transhumant flock of sheep on the Schwäbische Alb was examined with respect to which plants and animals are transported on and in the wool of sheep within calcareous grasslands. Several factors possibly influencing the attachment and detachment of diaspores on fleeces were studied, as well as the distribution of diaspores on the various body-parts of a sheep. The effects of different modes of sheep locomotion on dispersal were examined with the help of a sheep dummy. In order to assess the retention time of diaspores and animals on sheep, some experiments with marked diaspores and animals were carried out on two tamed sheep. In 16 intensive examinations of the fleece of a single sheep, over 8500 diaspores of 85 vascular plants species were found. The highest numbers of diaspores were attached at the breast and neck of the sheep. Height of diaspore presentation, surface structure of diaspores and sheep locomotion were found to be the decisive factors for the reception and transport of diaspores in the wool. In addition, plant frequency and the length of the disseminating period are of importance. Marked diaspores with both adhesive and smooth surfaces remained on the sheepskin for up to seven months, and can consequently be dispersed over the entire roaming area of the sheep. Amongst the animals transported by sheep, only grasshoppers (13 species) were observed frequently on the flock of sheep. The period of time marked grasshoppers stayed on sheep ranged from 1 to 69 min, with an average of 14 min. During this period sheep can cover distances of over 100 m when grazing and well over 500 m when roaming. It is suggested that the importance of the dispersal of diaspores, and especially animals, by animals has so far been largely underestimated. This is mostly due to the methods previously used to examine dispersal mechanisms. Conservation management of rare and endangered species should consider the importance of sheep for maintaining the species richness of calcareous grasslands. It is likely that transhumant sheep farming is irreplaceable in the restoration of grasslands threatened by fallow and woody successional stages. Moreover, traditional shepherding facilitates the exchange of individuals of both plants and animals between isolated patches. That is, sheep are able to maintain dynamic processes even in a greatly fragmented landscape; this is probably essential to long-term population viability of many species.