Family Dollar Stores Inc., facing a threat from activist Carl Icahn, adopted a shareholder rights plan that will limit investors from acquiring more than 10 percent of the company. The discount chain added the "poison pill" after Icahn said he acquired a new 9.4 percent stake and would seek talks with management. He may push for changes, ask the company to explore strategic alternatives, and potentially seek board seats, according to a filing. Family Dollar has struggled to compete with rival discounters and other retailers.