A counterfactual approach to measure the impact of wet grassland conservation on UK breeding bird populations

Abstract

Wet grassland wader populations in the United Kingdom have experienced severe declines over
the last three decades. To help mitigate these declines, the Royal Society for the Protection of
Birds (RSPB) has restored and managed lowland wet grassland nature reserves to benefit these
and other species. However, the impact that these reserves have on bird population trends has
not been experimentally evaluated, as appropriate control populations do not readily exist. In
this study, we compare population trends from 1994 - 2018 for five bird species of conservation
concern that breed on these nature reserves with counterfactual trends using matched breeding
bird survey observations. Our results showed positive effects of conservation interventions for
all four wader species that these reserves aim to benefit: Lapwing (Vanellus vanellus), Redshank
(Tringa totanus), Curlew (Numenius arquata) and Snipe (Gallinago gallinago). There was no
positive effect of conservation interventions on reserves for the passerine, Yellow Wagtail
(Motacilla flava). We compared reserve trends with three different counterfactuals, based on
different scenarios of how reserve populations could have developed in the absence of
conservation, and found that reserve trends performed better regardless of the counterfactual
used. Our approach using monitoring data to produce valid counterfactual controls is a broadly
applicable method allowing large-scale evaluation of conservation impact.