Just as different species of pollinators differ in the degree to which their diversity and populations have declined, the causes that underlie decline vary widely. Some mortality is particularly important in a narrow range of pollinators; in managed pollination systems, there is clear evidence of reductions in pollinator numbers caused by introduced parasites and pathogens. The evidence indicates that these agents of mortality also could operate in wild pollinator declines. Other causes of mortality affect a cross-section of pollinators (albeit to different extents); habitat degradation and habitat loss, in their many manifestations, have contributed to declines in many vertebrate and invertebrate pollinators.