This paper (accepted for publication and avialable online – but may have some last minute changes added to it in terms of copyediting) looks at trends in wetland bird species across RSPB nature reserves and compares them with ‘counterfactuals‘. You’ll have to read the paper to find out what counterfactuals are in general, and which counterfactuals were used here (it was a new word for me) to understand this properly.
However, a picture is worth quite a few words and these pictures;
…indicate that RSPB lowland wet grassland nature reserves (the dark (purple) lines) do ‘better’ over a 20+ year period for Curlew, Lapwing, Redshank and Snipe but don’t do anything much at all for Yellow Wagtail when compared with what might have happened.
Take home message: buying land and managing it for threatened species works well – for some species – if done well.
The paper takes an interesting approach which comes up with a reassuring answer.