Let's imagine two systems, one with 50 species and one with 4000. Both of them experience a draught and as a result 25 species of the first one die (that's quite likely) and 800 of the second population die out (more would not be very likely). So the first one lost 50% of species and the second one 20% - that means that the second one, with greater biodiversity has a bigger stability:)
