Environmental drivers of the decline of the fen orchid Liparis loeselii.

<italic>Liparis loeselii</italic> is a rare and declining orchid species restricted to rich fens in the Northern Hemisphere. Habitat destruction, eutrophication, drainage and scrub encroachment have been suggested as reasons for the decline. However, which factors are most important is not well understood. Based on vegetation and environmental properties from extant, potential and historical <italic>L. loeselii</italic> sites, we 1) developed habitat suitability models from either Ellenberg Indicator Values, which were derived from known habitat preferences of co‐occurring plant species, or field‐measured environmental properties, and 2) identified the primary reasons for the observed decline of <italic>L. loeselii</italic>. We found nutrient status to be the most important predictor for <italic>L. loeselii</italic> occurrence, followed by hydrology proxies (Ellenberg reaction and Ellenberg moisture). Vegetation height and Ellenberg light were of minor importance. Effect partitioning based on sites, from which <italic>L. loeselii</italic> has gone locally extinct, pinpointed eutrophication and drainage to be the most likely primary drivers of the species' demise. Phosphorus limitation induced by discharge of calcium‐rich groundwater seems to be crucial for <italic>L. loeselii</italic> to sustain populations in landscapes dominated by intensive agriculture. [ABSTRACT FROM AUTHOR]