Fig. 2 Comparison of the Community Temperature Index (CTI) of
the paired dataset (Δ CTI, 3,839 pairs) for the different calibrated
models (panels a, b and c). In the paired dataset, co-occurrence plots
were matched with presence-only by subsampling the latter and selecting
the closest available pseudo-plot that had been sampled. X-axis: Mean
annual temperature (MAT) of the average of the five years preceding the
respective assemblage survey of the co-occurrence dataset. Y-axis:
Difference between the estimated CTI for the paired dataset (Δ CTI). A
LOESS regression line is shown in each panel. Panel d: Temporal
thermophilization trend. Thermophilization is estimated using the
projection of the combined calibrated model using the paired dataset.
Blue solid line: co-occurrence dataset, green dashed line: presence-only
dataset.
Ad-hoc analyses of the CTI
deviations
Across all transfer functions, there was a tendency for CTI
overestimation in the presence-only data, particularly at lower
temperatures (Fig. 2 and S7 ). This overestimation remained
within the credibility interval when utilizing the transfer function
based on the combined dataset, but deviations from the credibility
interval were noted when using the transfer functions derived solely
from either co-occurrence data or presence-only data (Fig 2. and
S7 ). Because the areas of colder temperatures tend to be placed in
areas of higher topographical reliefs in Norway, we suspected that these
deviations were due to elevational biases in the presence only data
towards more accessible and warmer valley bottoms compared to the
co-occurrence plots. To evaluate this, we assessed elevation differences
between the co-occurrence plots and their correspondingly paired
presence-only pseudo-plots. Elevations were extracted from DEM (Digital
Elevation Model) with 25 m resolution (Copernicus, 2016) using original
coordinates of the co-occurrence data and coordinates provided for the
presence-only data. Subsequently, we computed the average elevation of
all observations of all species occurring in each co-occurrence plot and
pseudo-plot. When multiple observations existed for a single species
within a pseudo-plot, we randomly chose one record per species. Our
findings indicate a general tendency that observations in the
presence-only pseudo-plots were situated at lower elevations compared to
the co-occurrence plots with a mean difference of -35.3 meters (std.
error of the mean = 1.41, Fig. S8 ). Furthermore, it is evident
that larger disparities in elevations correspond to greater variations
in CTI values. The observed trend was quantified using linear models,
yielding a slope of -1.36 × 10-4 °C/m for the
co-occurrence calibration model (std. error =2 × 10-4,
p = 0.645) and -1.07 × 10-3 °C/m for the presence-only
calibration model (std. error = 4 × 10-4, p = 0.005),
with a slope of -1.07 × 10-3 °C/m (std. error = 3 ×
10-4, p = 0.003) the combined model (Fig.3 ).