Columbia University’s endowment dropped 15 percent in the second half of 2008, Columbia President Lee Bollinger announced Saturday. Endowment funds available to the school will drop 8 percent, but Columbia depends on its endowment for only 13 percent of its income, compared to 44 percent at Yale, so the drop will not affect Columbia as severely as it has Yale. Bollinger did not unveil any specific measures that Columbia is taking in reaction to the financial difficulties at this time.