Scale-dependency
We predicted that wild bees are affected on smaller scales compared to
hoverflies (H3). Landscape heterogeneity and arable field cover affected
wild bees at spatial scales that correspond to maximal foraging
distances from the nest of small bees (140m – 350m, Wright et al.
2015). In contrast, hoverflies that ‘migrate’ through the landscape were
affected at much larger spatial scales (>750m, with the
exception of the positive effect of landscape heterogeneity on hoverfly
abundance, see below). Taking together, these results support our
hypothesis and indicate that the scale of effect, i.e. at which spatial
scale has a landscape parameter the largest effect on a response
variable, depends on the foraging behavior of pollinators. However, we
detected other scales of effect than previous studies. For example,
Meyer et al. (2009) found the strongest (positive) effect of landscape
heterogeneity on hoverfly richness in calcareous grasslands on 250m,
while Földesi et al. 2015 observed that landscape heterogeneity
positively affected hoverfly species richness at smaller spatial scales
(300m) compared to wild bees (500m). These deviations demonstrate that
the scale of effect may be primarily driven by the landscape context
rather than the ecological traits of the species (Galán-Acedo et al.
2018), making it tricky to deduce the scale of effect for other
landscapes. Moreover, our study shows that even the direction of
landscape effects may change with spatial scale. As outlined above, we
assume that the negative effect of landscape heterogeneity on hoverfly
abundance arises due to a negative correlation with arable field cover.
Therefore, it seems likely that the scale of effect and even the
direction of landscape parameter effects is driven by correlations
between landscape parameters that are associated with different
processes and change with scale. Therefore, we see a strong need to
thoroughly analyze and report correlations of possible confounding
landscape predictor across scales, in order to better understand the
underlying mechanisms of how the scale of effect and direction of
landscape effects arise.