Tory peer Lord Goldsmith is believed to be staying on in his role as minister of state for the Pacific and environment at the Foreign Office.
In the letter announcing he was leaving to staff Goldsmith appeared to fire a warning to Truss.
“We have so much more to do to turn the tide here,” he wrote in the letter seen by The Guardian.
“The UK is, after all, one of the world’s most nature-depleted countries. But if Defra [the Department for Environment, Food and Rural Affairs] continues to get the backing you need and deserve across government, you can and you will turn the tide.”
His sacking prompted fears that a trophy hunting ban he championed could be halted as well as plans to drop the kept animals bill.
One MP told The Guardian: “Liz might abandon it. She had no interest in animal welfare while a minister in Defra.”
Activists fear that there will be a drop in animal welfare standards with reports Goldsmith clashed with new environment minister and farmer Mark Spencer.
“Thank you, Lord Goldsmith; we look forward to continuing to work with you to influence animal welfare through other channels.”
As minister for Pacific and international environment, Lord Goldsmith has previously said: “The world’s ocean is in crisis and we have reached a tipping point. This week world leaders came together to redouble their efforts to protect the marine environment.
“But it is absolutely critical these commitments are translated into action and that our ocean remains firmly on the global agenda.”
The sacking comes after Downing Street said the reshuffle had been halted during the period of mourning following the Queen’s death.