Effects of livestock traffic on rock fragment movement on hillsides in a semiarid patchy rangeland

The roles of livestock traffic and surface cover types in controlling the movement of rock fragments was studied on north-and south-facing hillsides in Israel's northern Negev. Experimental plots were mapped in terms of three surface cover types: shrub patch, flock trampling route and elsewhere in the intershrub. Numbered rock fragments in three size categories were distributed uniformly across the plots, half of which were fenced off as controls, and half were traversed by a flock of goats during two grazing seasons. The coordinates of the numbered rock fragments were recorded at the beginning, middle and end of each season. Coordinates were converted to patch type by using the plot map, and transition probabilities and movement distances were computed. There was considerable movement of rock fragments in the open plots, but none in the control plots. The probability of a rock fragment in the open plots being moved was 86, 60 and 5 per cent, respectively, for initial placement on cover types: trampling route, intershrub and shrub. The mean distance travelled by all rock fragments or only those that moved was greater for those placed on a trampling route than for those placed elsewhere in the intershrub. There was evidence that livestock traffic clears the trampling routes of rock fragments and causes their accumulation on shrub patches. We would expect disturbance of the patch structure to lead to a reduction in surface roughness that would intensify rock fragment movement and increase soil erosion.