Please help me with an explanation of the poem "The Poplar Field" by William Cowper.

"The Poplar Field" by William Cowper is a critique of man's willful destruction of the environment. The speaker is in a nostalgic mood, thinking back to the time twelve years previously when he first saw the beautiful field where all his beloved poplar trees once proudly stood. But the trees are no more; they've all been cut down. Where once the speaker sat beneath the shade of a poplar, now he sits gloomily upon a tree stump, an indication of the environmental destruction that's since taken place:

Twelve years have elapsed since I first took a viewOf my favourite field, and the bank where they grew,And now in the grass behold they are laid, And the tree is my seat that once lent me a shade.

The cutting down of the poplars has had a negative impact, not just on the speaker, but also on the animals that used to live among the trees:

The blackbird has fled to another retreat Where the hazels afford him a screen from the heat;And the scene where his melody charmed me beforeResounds with his sweet-flowing ditty no more.

The blackbird has been forced to fly away to find another home. The speaker clearly misses him and his joyful, lovely song. The poplar field just isn't the same without him. But the destruction of this natural habitat has at least made the speaker realize something important: that the pleasures of this world are as temporary as the poplar trees themselves. Life is short, and this encourages us to pursue these fleeting pleasures at whatever cost, whether it's to other people or to the environment. As such, we become selfish, and it's this selfishness that leads to so much environmental destruction:

'Tis a sight to engage me, if anything can, To muse on the perishing pleasures of man;Short-lived as we are, our enjoyments, I see,Have a still shorter date, and die sooner than we.
https://www.poemhunter.com/poem/the-poplar-field/