【正确答案】正确答案：Developing models allowing precise prediction of the effects of crowding·

【答案解析】解析：根据题目中的density-dependent theorists定位到第3段末句。该句开头的one challenge表明该句所述的内容就是the density-dependent theorists仍未解决的问题，即题目中的are…unable to solve对应原文的challenge，故答案为Developing models allowing precise prediction of the effects of crowding。