For Lack of a Predator
A species may eat a particular bacterium, phytoplankton, smaller fish, or plant in an area. Lacking a predator, these species/populations will overgrow and alter the area’s biology, overwhelming and driving to extinction dozens or hundreds or thousands of other local species.~ Thom Hartmann (1951-05-07 age:66) Threshold: The Progressive Plan to Pull America Back from the Brink page 24
The more species, the greater the biodiversity, the more stable an ecosystem is. When you start killing off half the species, the ecosystem becomes much more fragile and more susceptible to complete collapse from even minor disturbances.