The good news...
Only the most degraded environments lose all their native plants. A lone wattle can support insects, birds, fungi and even mammals. In places that appear to be a waste of privet and lantana a few native trees and stunted seedlings of native plants may be waiting to be rescued. By removing the weeds in their root zone, they can take on new vigour. They will sow their seeds and grow new plants in the spaces from where the weeds were removed. The progress is incremental, and birds and butterflies that depend on the natives will progressively come back. It's well worth the effort to control weeds!
See this before and after weeding comparison
Not so good...
Plants introduced from other continents are rarely food for native wildlife. Few native insects feed on introduced plants. Unless we control them, such plants often become environmental weeds.
If the weeds take over, native plants and therefore wildlife disappears in proportion. The 50 or so local butterflies need specific native plants to feed on. Without them, the butterflies and many other specialised insects just disappear.
.
Some of the weeds that we control are:
privet(s) & camphor laurel (introduced from China)
non-native grasses (from the Americas, Europe & Africa)
blackberry (from Europe)
Brazilian fireweed (from S. America)
lantana (from S. America)
thistles (from Europe)
If you need to identify a weed, try this useful site
tradescantia (from S. America)