What is ONE major difference in the problems associated with obtaining measurements on the sizes or densities of animal populations versus plant populations?

I think one major difference with that is that animals can move around, go distances in search for food or mate, and thus make the animal densities per geography vary greatly. Birds migrate regularly, so their population densities tend to vary with seasons. Their mobility also depends on the availability of food, so animals go away if there are no food in the area.

Plants on the other hand don't move around faster (they can migrate by reproduction: it's their seeds moving around). Thus their densities tend to be more constant per season/life cycle.

sulfur dioxide

explanation: acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. these substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

allowed for the prediction of the properties of missing elements.