Dispersal strategies in communal versus privately-owned rangeland in Namaqualand, South Africa

Abstract

Dispersal and disturbance are significant forces in the ecological and evolutionary dynamics of a population. Dispersal is a functional trait and plays an important role not only in range expansion but also in determining the spatial and genetic structure of populations at local and landscape scale. Livestock is regarded as a disturbance and is major determinant of vegetation dynamics. The study was carried in Paulshoek, one of nine villages in the Leliefontein Communal Reserve in Namaqualand, Northern Cape. The site of the present study is a 1 km2 section on the southern boundary of the Paulshoek commons and includes another observatory, which extends approximately 1 km from the Paulshoek boundary into one of the adjacent privately owned farms, Remhoogte. Livestock grazing has been identified as a major threat to the biodiversity in the Succulent Karoo Biome. The aim of this study is to compare the dispersal strategies in communal with privately-owned rangeland and further focuses on the importance of myxospermy. Based on the concept that vegetation is the most obvious external feature on which to classify ecosystems, a phytosociological classification of the vegetation of the study area was made. The six plant communities found largely reflect differences in land type, although the effect of different grazing intensity on communal and commercial rangeland is not excluded. The dispersal spectra of the two rangelands and the six communities were dominated by ombrohydrochory, autochory, plumed diaspores, dust diaspores and winged diaspores. Grazing, water, and landscape seem to be the major factors influencing the composition of different dispersal types. Despite differences in trait spectra among rangelands and communities, all dispersal modes are represented in nearly all communities in the two rangelands. Mucilaginous diaspores produce a copious amount of mucilage once they come into contact with water. If they dry out, the seeds adhere to the soil. Information on the factors controlling the germination of these seed types will improve our limited knowledge on the survival of these plant species and could aid the management and conservation of arid systems. The optimum conditions for germination differed among species, but in general most species obtained their highest germination percentage at low and intermediate temperatures which in the field represent the autumn and winter seasons. The various ideas that have been suggested about the ecological importance of myxospermy, and its ability to fulfill various functions led to the further investigation of the origin and chemical composition of mucilaginous seeds. Results have suggested three types of mucilage production which can be distinguished as follows: 1) epidermal and subepidermal mucilage production where the release of mucilage is correlated with the rupture of the cuticle of the epidermal cells, 2) species where the wings or the pappus disintegrate and become completely mucilaginous, and 3) species with mucilage excreting hairs. The mucilage of all these diaspores investigated contained both cellulose and pectic substances exclusively or a complex of pectino-cellulosic compounds. Despite the diversity of the mucilage origin and structure, they accomplish more or less the same ecological functions.