Increasing attention has been focused on biodiversity in recent years, but comparatively little attention has been paid to which groups have the greatest impact on maintaining diversity. Hymenoptera is such a group. Not only is it a very large group, but its members are also major plant pollinators, seed dispersers, and parasitoids and predators of other arthropods. This volume tackles how species affect diversity in other organisms, whether the group is prone to extinction, and the consequences of the loss of Hymenoptera to terrestrial ecosystems.