This is basic biology. If a species of bird eats a particular species of plant seeds, then the bird proceeds to disperse or drop the seeds in new places through its fecal matter, the plant will continue to grow in new places and thrive.
If the species of bird is eaten or killed off by say a disease, there are no birds to spread the plant seed, therefore resulting in the death of the plant species
National Grange of the Order of Patrons of Husbandry
