Probability Density Function Sums to 1?

Hi,

I was looking at the probability density functions (PDF) on Wikipedia for various distributions, such as Uniform, Normal, Pareto, Exponential, and Weibull.

The first two of those distributions have a PDF who integral is one, however the last 3 have PDFs which do not seem to integrate to one. I quickly observed this by quickly looking at the graph of the PDF.

I thought that the area under a PDF must always sum to one. So what has gone wrong here? I was going to query this on the talk pages on Wikipedia , but I figured I would ask here first in case I'm misunderstanding something to do with PDFs.

thanks

Andrew