Extreme droughts, intensified by a warming climate, are increasingly causing ecosystem collapse in many regions worldwide. But models used by scientists to predict the tipping points at which drought stress leads to ecosystem collapse have proven unreliable and too optimistic.A new study by scientists at Duke University and Beijing Normal University may hold the answer why.   The researchers found that these tipping points can happen much sooner than current models predict because of the added pressures placed on drought-weakened plants by grazing animals and fungal pathogens.