Host- and habitat-related variables
During sampling we recorded the locations of the plants with GPS and
measured the size of the plants (Table 1). We recorded signs of
herbivore damage, and surveyed surrounding vascular plant communities
between 1930 June 2017 by counting the number of vascular plant species
within multiple one-square-meter quadrats in each population. We used
the vegetation data to calculate the Shannon diversity index (Shannon
1948) for each population (Table 1).
We estimated the proportion of agricultural area surrounding each study
population from Corine Land Cover (CLC, version 2020_20u1) with QGIS
(QGIS Development Team 2019) by creating a one-kilometre buffer zone
around each study population following the patch borders and calculating
the proportion of 20 m × 20 m pixels falling under agricultural land use
category within this buffer zone (Table 1). We also estimated the
coverage of P. lanceolata foliage in square meters in each
population. We quantified the connectivity of a host population with
respect to other populations by calculating the Euclidian distances
between populations and calibrating these measures by the species
dispersal capacity (Hanski 1999).
We obtained weather observations for the study populations from the
Finnish Meteorological Institute (Aalto et al. 2016). We
calculated the number of severe winter days during the winter before the
sampling season (2016-2017), and the sum of temperatures of the
effective summer days during previous summer (2016). See Table 1 and
Supplementary Material and Methods for more details, including
biological justification for the selected variables.