The colonization of a tropical grassland by Stylosanthes from seed transported in cattle faeces

Abstract

The dynamics of the spread of four Stylosanthes species by grazing cattle from sown areas to nearby native grassland R& studied over ten years in the dry tropics of north Queensland. The first recruitment of Stylosanthes in the grassland occurred during the third growing season. Secondary colonization took place around the primary plants, and by the tenth year, 94% of the 0.25 m2 quadrats sampled in the grassland contained one or more species of Stylosanthes. Large seed numbers were produced by Stylosanthes on the sown areas during primary colonization. Mean yields of above-ground biomass were high (4000 kg ha-1year-1) despite heavy grazing pressures, and 6% of this biomass was channelled into seed production (25 000 seeds kg biomass-1). Seed content in the cattle faeces reached a peak at the end of the wet season and accounted for 8% of seed production. The numbers of seeds in the faeces were highly correlated with the numbers of seeds produced on the sown areas. Only 1.5% of the seed excreted in the grassland grew into established plants despite most seeds being viable. Varying the amount of grass had little effect on establishment. The two short-lived herbaceous species, S. guianensis and S. hamata, were more efficient colonizers (i.e. produced more seedlings per unit biomass) than the more perennial shrubby types, S. scabra and S. viscosa. The former two species produced more seeds per unit biomass and more plants per unit of seed in the faeces.