Our environment has certainly improved in recent decades, at least in terms of environmental awareness. And while I think this is great, it is sometimes easier to think things are better than they are.
A good example of this is in the Sydney region of Australia, where visitors are often amazed at the sheer quantity of birds in peoples back gardens. The Noisy Miner is a great example of this.
Noisy Miners are are actually a type of honeyeater. Even though they are native the issue is complex. Like most honeyeaters they are very territorial, essentially trying to keep a monopoly on their patch of nectar.
The problem is that they prefer open woodland, which just happens to coincide with typical park management. That is lots of tall trees with virtually no undergrowth. So in a sense their ideal habitat has expanded. Also unlike a lot of honeyeaters they can eat a variety of foods. So they have become more prolific. That means larger "gangs" in ideal surrounds.
The problem isnt really the Noisy Miners, but the way we manage the lands. Especially in light of fire concerns, and in inner suburbs, personal safety concerns. So what would typically be a very mixed habitat, with heath and grasses on the ground, then shrubs and mid sized trees like banksias followed by a typical overstory of large trees has been replaced by effectively lawn and tall trees, normally eucalypt types.
So the typical avian biodiversity of the southeast of Australia has been replaced, with noisy miners, currawongs and similar, sulphur crested cockatoos and corellas. Rainbow lorikeets proliferate on backyard feeding and the planting of ornamental flowering trees. They are all large birds that can easily fend for themselves and fit into our man made habits almost perfectly. Some species such as crested pigeons have increased their range purely due to that effect. Others have simply increased their populations.
So a visitor to Sydney might be amazed at all the parrots and similar abundance of birds, in reality though the biodiversity is slowly being lost. Rather then wrens and australian robins, pardolotes, butcherbirds and all of those things, 200 different species are being replaced by 20 species that suit our own man made habit requirements.
As said the issue is very complex, since we are talking about native birds here it can get emotional. There have been proposals for culling come up at various times, though i dont think anything has come of that. And even if it was done, it would not be solving the real issue anyway.
It also has to be remembered that especially up until recently dead wood etc was removed as part of management strategies. Also in logged forests, nesting sites are virtually non existant so competition for nesting sites is extremely competitive. Another factor is Australia doesnt have birds such as woodpeckers which will make new hollows, much of the work being done by fire and insects like termites.
This is partcularly an issue in National Parks and wilderness areas. Obviously fire control is important, the problem is we don't always understand the complexities of in this case the Australian bushlands. Too little burning is just as bad as too much burning.
These simple factors contribute to a very real loss of biodiversity even in areas which we think are doing well.