Serfdom was hardly efficient; serfs and nobles had little incentive to improve the land. However, it was politically effective. Nobles rarely challenged the tsar for fear of provoking a peasant uprising. Serfs were often given lifelong tenancy on their plots, so they tended to be conservative as well. The serfs took little part in uprisings against the empire as a whole; it was the Cossacks and nomads who rebelled initially and recruited serfs into rebel armies. But many landowners died during serf uprisings against them. The revolutions of 1905 and 1917 happened after serfdom's abolition.