Theories can be dismissed. Long-term data cannot.
In 1974, a fire burned through the vegetation along Dead Man Wash in the Sonoran Desert uplands north of Phoenix, AZ. Giant Saguaro cacti were scattered along the wash. Between them grew diverse native perennials adapted to arid conditions over millions of years.
Fifty years of monitoring revealed what happened next.
After the fire, invasive annual grasses established and spread, creating continuous fuel beds where none existed before. Fire frequency increased throughout the desert and each fire killed fire-sensitive natives. Without the diseases and consumers of their Asian homeland, the invasive grasses soon created a continuous carpet of fine fuel where none had exted before.
A feedback loop emerged: more grass, more fuel, more fire, more native mortality, more open ground, more grass, etc. The system crossed a threshold and reorganized in a new stable state. The Saguaros did not bounce back. They vanished.
Right: The new scene spreading across the deseert.
To a visitor encountering the site today, the weedy landscape appears normal. The memory of the Saguaro forest has been erased.
This pattern repeats across every biome on Earth. The Amazon is savannizing. Coral reefs are shifting to algal dominance. Some changes cannot be undone. Restoration would require immense, unattainable financing and has become an obsolete concept.