Sure so it has nothing to do what us humans do? You know like how we remove mature trees, hedges and shrubs (that small birds love), ground covers/lawns (the skinks love) and all the runoff from our hard surfaces that contaimate our waterways.. Our use of herbicides, insecticides, chemicals and poisions. Our plastic lawns, concrete and outdoor spaces void of substantial vegetation because the developers have squeezed block sizes down to postage stamps. Yep none of that contributes at all to the lack of biodiversity...
You know what also contributes, owning a cat and to a lesser extent a dog..
If you think it’s 100% the cats fault then I reckon you might be the one with blinkers..
note - every area is different in Melb and as I mentioned several times cats should not be outside in environmentally sensitive areas ie parts of the yarra ranges, nillumbik etc..