Original text:

Snakes did not acquire their poison for use against man but for use against prey such as rats and mice, and the effects on these of viperine poison is almost immediate.

有点迷惑effects后面的 is.

例如：

Economic effects of such high speed change are also unpredictable and somewhat chilling …— Genevieve Stuttaford, Publisher's Weekly, 29 July 1996

effects 以复数的形式出现，到底后面接单数，还是复数的系动词？

谢谢！