Is vs. are

I know this has been discussed previously, but I cannot find a satisfactory explanation. Here is the sentence in question:

"Prior experience with experimental field ecological research, expertise in the use and application of computer-controlled systems for process management, and practical experience in collection and analysis of environmental and biological measurements are essential."

I think the bolded word should be is. Please help me explain why I am correct. All I can say is "exprience is essential" is correct, but cannot explain why.

Thanks for any help.

Doug