Distinguishing signal from noise: Long-term studies of vegetation in Makhtesh Ramon erosion cirque, Negev desert, Israel

We consider three case studies of long-term plant population dynamics in Makhtesh Ramon erosion cirque in the central Negev desert of Israel. We show that rainfall is the major driving variable in this system, and that it creates large temporal and spatial variation in plant species diversity and vegetation community composition. This variability makes it extremely difficult to distinguish 'signal' (= pattern in vegetation) from 'noise' (random spatial and temporal variance). Our long-term vegetation studies in permanent plots arranged along the length of the cirque, initiated in 1990 and continuing, show that there is high spatial and temporal variance in plant species' incidences and abundances. This is particularly true of annual plant species. However, using pairs of fenced and unfenced plots arranged along the major abiotic gradient, altitude (which mirrors changes in rainfall), we were able to tease apart the effects of variance in rainfall and herbivory. We found significant negative effects of herbivory by the re-introduced Asiatic wild ass (Equus hemionus) on plant cover and on vegetation community composition. In a study of the effects of herbivory by dorcas gazelles (Gazella dorcas) on the lily Pancratium sickenbergeri, we found that there was little inter-annual fluctuation in lily population size in two sand dunes in Makhtesh Ramon in spite of the high percentage of lilies that is removed by the gazelles and the almost complete herbivory of flowers by these herbivores. This result indicates that the dune lily populations may be maintained by dispersal of seed from other lily populations elsewhere in Makhtesh Ramon where gazelles are rare or absent.