Animals are decreasing in number.
The "number" is singular,therefore "is" is appropriate to use,although it can be confusing.I prefer to phrase it the way I've just translated it.
说动物的数量越来越少,怎么说?
Animals are decreasing in number.
The "number" is singular,therefore "is" is appropriate to use,although it can be confusing.I prefer to phrase it the way I've just translated it.