Semi-desert steppes on all continents are dominated by perennial shrubs, often with semi-fleshy or velvety foliage, and tussock grasses interspersed with bare ground. Low variable rainfall and extreme temperatures favour flora and fauna with drought and stress-tolerance adaptations, such as deep roots and nomadism. Growth and reproduction of shrubs and grasses is varies with rainfall, the cover of grass diminishing to near zero in extended droughts. Grass cover also depends on soil fertility and grazing animals, which may also limit shrub recruitment and growth. These steppes are among the most productive of desert ecosystems, with relatively abundant small and large herbivorous and seed-eating mammals supporting bird and mammal predators and scavengers.