Only the most common species of wild bees pollinate most crops, according to new research. Scientists argue that there are ecological and ethical reasons for targeting conservation methods to the other rare species of wild bees. (Photo: <a href="http://www.shutterstock.com/da/pic-116899945/stock-photo-bumble-bee-pollinating-a-flower-lit-by-the-sun.html" target="_blank">Shutterstock</a>).