 Well, why are oaks supporting over the country 950 species of caterpillars? There's a number of hypotheses. It's an old genus, so they've been around a long time. A lot of things have adapted to them. It's a large genus. There's 91 species of oaks in North America. It's a very persistent plant. So a single oak tree can live hundreds of years compared to a spring beauty, a little flower, which only lives two weeks.