Predicting spatial variation in heather utilization by sheep and red deer within heather/grass mosaics.
Abstract
Recent work has shown that the dynamics of woody and herbaceous vegetation mosaics under grazing or browsing can be strongly affected by the spatial pattern of the vegetation. A suite of models is described here to predict spatial variation in the utilization of an internationally important woody species, heather Calluna vulgaris, within heather/grass mosaics when grazed by sheep or red deer Cervus elaphus. This type of information is needed for the successful management of free-ranging herbivores grazing these extensive vegetation types. The models predict: (i) heather utilization at the edge of grass patches; (ii) utilization distant from the grass edge; and (iii) the rate of change of utilization with distance from the edge of grass patches as functions of (a) the proportion by area of grass in the mosaic, (b) the length of heather/grass edge per unit area, and (c) the overall heather utilization on a site. Differences between species of grazers and seasons are also taken into account. The models were fitted to data from two series of grazing experiments conducted on natural and artificial heather/grass mosaics in north-east Scotland. Overall heather utilization in the experiments was estimated using a geographical information system (GIS), taking into account higher levels of utilization in proximity to grass patches. A modelling approach for predicting how the heather/grass mosaic pattern will change through time under heavy grazing is proposed. As no field data are yet available on this process, this model was developed by simulation within a GIS. This work demonstrates how incorporation of spatial information can improve the accuracy of utilization predictions, and thus the design of appropriate management practices. As with many other similar systems, the paucity of data on spatial variation in heather utilization by grazing ungulates, and the effects of utilization at the heather/grass edge, particularly under high grazing pressures, is highlighted. This is considered to be a major limitation to the efficacy of currently available decision support tools to aid the management of heterogeneous grazing systems.