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Report2022Open access

Spatial modelling of habitat suitability for Calypso bulbosa, a protected plant species in boreal moist forest

Stephan, Jörg; Wikberg, Sofie; Sundberg, Sebastian

Abstract

Calypso bulbosa is a rare orchid listed in the Habitats Directive’s annex 2 and 4. Since the species occupies rather common habitats, mesic and moist forest in the true boreal region, a major dark figure of hitherto undetected occurrences of the species is probable. These unknown occurrences are important to discover for species protection and to get a better estimate of the probable population size and distribution of the species in Sweden. Therefore, we modelled and mapped the species’ potential sites of occurrence at the hectare level, based on presence/ pseudoabsence of Calypso and in relation to an initial set of 113 environmental and habitat variables, including e.g. land cover, land use, forest type, climate, soil moisture, soil and bedrock type. We used a forward model selection, using a Bayesian species distribution model, which ultimately resulted in 11 explanatory variables that best explain the presence/absence of the species and had the highest predictive power. Our final model explained typical variation (17%) in the species occurrence given these macro scale environmental variables and could very well discriminate between presences and absences (AUC = 0.9). The resulting habitat suitability map indicates that there may be many undiscovered Calypso sites in spruce forests in the far north, especially in the alpine region in the northwest. The probability map may be used as a guide for finding undiscovered sites/hot spots. After further sampling the accuracy of the model could be tested as the number of false negative and positive would be available. If reliable, the model may also be used to calculate dark figures for the distribution and populations size of Calypso bulbosa.

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Publisher: SLU Swedish Species Information Centre, Swedish University of Agricultural Sciences