(2) when the ship is in traffic density area?

(2) 船舶何时处于交通密度区？

So they manipulated traffic density by constructing bridges of various widths between a colony of Argentine ants and a source of food.

因此，他们通过在阿根廷蚁群和食源地之间建造不同宽度的桥梁来控制交通密度。