2.1 Study areas
For this study, water bodies surrounded with different proportions of
forest versus agricultural land were selected: different landscapes to
cover the whole gradient from fully forested to mainly agricultural were
chosen. Landscape might affect not only the lake trophic status, but
also the nest predator community, as stated by Holopainen et al.
(2020a). Studies were conducted at lakes and ponds at two areas in
Finland, Evo and Maaninka (Figure 1). Both areas have permanent lakes,
which carry water through the summer. Lake shorelines may be affected by
spring floods, but otherwise the water level is rather constant. The
trophic level among these lakes vary from oligotrophic to eutrophic (see
Holopainen & Lehikoinen, 2021). In addition to permanent lakes, in both
areas the study included other wetlands, which were shallow and had
temporally varying shorelines: seasonal ponds, beaver ponds and man-made
ponds (hereafter ponds). Hereafter all lakes and ponds are called water
bodies. The same mammalian predators and all common corvid species occur
in both study areas (Lindén et al., 1996; Valkama et al., 2011).
Evo in southern Finland (61°12’N, 25°07’E) represents a typical boreal
forest landscape in Finland. Human settlements are scarce, with few
agricultural fields (hereafter fields) inside the study area and larger
agricultural lands south from the studied water bodies. The lands within
1 km of the Evo water bodies consists on average 1 % of fields (range
0–6%).
In Evo duck surveys were undertaken at 45 water bodies (27 permanent
lakes, 10 beaver ponds and 8 seasonal ponds) within a c.
39-km2 area. Water body size varied between 0.04–37
ha. The median shoreline length for permanent lakes was 1.0 km and for
ponds 0.5 km. From the 45 study water bodies, six seasonal, six beaver
ponds and 12 permanent lakes were chosen for nest predation experiments
and invertebrate surveys (for more information, see Supplementary
Appendix S1).
Maaninka in eastern Finland (63˚15’N, 27˚30’E) is a mosaic of
agricultural land and forests with some internationally important
bird-lakes (Natura 2000 and IBA -lakes; Leivo et al., 2002). The
area represents typical agricultural landscape of Finnish lake district.
Duck surveys were made at 18 lakes and seasonal ponds across c. 47
km2: 17 water bodies were used for the experiments,
among them the important bird-lakes. All seasonal ponds were included
that occurred during the study years and to which permission was granted
from the landowners. The lands within 1 km of the water bodies consists
on average 59% of fields (range 24–75%), the remainder mainly private
forest. Water body sizes were 1–149 ha. The median shoreline length for
permanent lakes was 3.0 km and for ponds 1.1 km. In total there were
seven permanent lakes, two man-made ponds and eight seasonal ponds for
nest experiments and invertebrate surveys.
To estimate the landscape structure (i.e. the field percentage) near the
study water bodies, QGIS 2.18.7 (QGIS Development Team, 2017) and
topographic vector map (National Land Survey of Finland, 03/2019) were
used. The 1 km radius zones around the water bodies were measured from
the shoreline. We used the 1km radius, as in the comparisons of nest
predation patterns between 500 m and 250 m radius zones, the 250 m zone
has shown to have habitat specific effects, while the effects disappear
with a larger zone (Uusihakala, 2021). Thus we believe that 1 km radius
zone describes the landscape scale foraging range of mammals. Waters
from the zones were excluded in order to count the field percentage of
surrounding land areas.