Strictly speaking, a speculator is not the same thing as a rentier. A speculator makes money by betting on the direction of price movements. A classic rentier would hold onto the title of his or her bonds until they reached maturity. Today's speculators may only hold a market position for a few minutes, hours or days before selling again on highly volatile, computerized markets. They also profit from their investments by using their financial and political power to maintain high real interest rates and unregulated financial markets.
Bond traders can bid up interest rates through the sale of government bonds.
The power of the financial markets has put into question the sovereignty of the nation state. The money traders have become more powerful than national governments in determining monetary and exchange rate policies. Ultimately, this represents an abdication of power to wealthy investors and their interests. It means that governments must give up any pretense of truly working for the common good. In essence, this means abandoning the most basic principles of democracy.