Study area and sampling
design
Sampling was conducted at six holm oak (Quercus ilex L.) dehesa
farms located in the province of Cáceres, western Spain. Three of them
were used for free-range livestock rearing (farms 1, 2 and 3); in the
other three, livestock had been absent long-term, for at least ten years
before the beginning of the study (farms 4, 5 and 6). At farms 1-3, we
randomly chose eight pairs of oaks per farm in February 2016. Within
each pair, one tree was randomly chosen for experimental short-term
livestock exclusion and the other one was left as control. To sum up,
there were eight trees x three farms without livestock for ten years or
more, and eight trees x two treatments x three farms with livestock.
Therefore, 72 Q. ilex trees were monitored (N (control) =
24; N (one-year exclusion) = 24; and N (ten-year
exclusion) = 24) (Figure 1, Tables S1-2).
In March 2016 (early spring), experimental trees of farms 1-3 were fully
enclosed by a fence to avoid livestock grazing beneath their canopies.
In April 2017 (spring), we sampled all study trees following the same
methodology. We vacuumed 2 m2 of the floor (2900 W
vacuum machine) beneath the canopy of each tree: a 1
m2 plot at the northern half of the canopy and 1
m2 at the southern half. Each plot was vacuumed for 30
seconds to sample all the arthropods on the soil surface and on the
vegetation. We pooled both samples of each tree and stored them in
cardboard boxes that were taken to the laboratory and placed in the
fridge at 4 ºC until sorting. By doing this, we preserved the specimens
and kept them inactive, thus preventing them flying off during sample
cleaning and classification. The content of each box was emptied on a
tray and carefully inspected. Plant debris and soil were discarded, and
all the arthropods taken and placed in a bottle filled with 96%
ethanol. After 5 minutes without finding any new specimen, we considered
sample revision as completed.
As livestock effects on arthropods can be mediated by changes in
vegetation structure, we measured vegetation density and height beneath
the tree canopy while sampling arthropods (raw data available in Table
S3). At each tree, we annotated vegetation height at 8 random points
with a calibrated rod that we placed vertically on the ground. At each
point we registered whether vegetation touched the rod at: ground level,
0-10 cm, 11-25 cm, 26-50 cm, and taller than 50 cm; we also measured the
maximum vegetation height (see Muñoz et al., 2009 for a similar sampling
procedure).