This is a classical case of ecological succession, that we learned in Class XII.
Friends it is the visible... a gradual and orderly process of changes in an ecosystem brought about by the progressive replacement of one community by another until stability is established.
Here, the variety of food plants planted on landscaped land, formed pioneer community. Pioneer community is the initial community colonizing an modified/uninhabited ecosystem.
But with the kiddies gone 'the garden' was all to be cared for, by mother nature.
The selection of food plants soon got overrun by seral stages of perennial grasses, shrubs and eventually will form a community of woody shrubs and trees...the climax.
So, when the school bells ring during monsoon days of 2021 they are for a surprise...
to see Fire Crow and Yellow-Browed Bulbul when they were expecting the fluttering of bluebottles, swallowtails and tailed jay.