Statistics: Frequency Distributions

Over $20$ years, Liza tracked the population of a rare species of frogs. She looked at the how many thousands of frogs were in the population, making the following frequency table. $$\begin{array}{|c|c|} \hline \text{Year}&\text{Frequency}\\ \hline 00\text{-}04&10\\ 04\text{-}08&12\\ 08\text{-}12&18\\ 12\text{-}16&28\\ 16\text{-}20&42\\ \hline \end{array}$$ What kind of distribution is this?