Global experiment and arthropod samples
Experiments at all 52 sites were conducted following a standardized
protocol. Each experiment followed a randomized block design; for each
site, we randomly chose 10 to 20 paired trees, shrubs or herbs (blocks).
Paired plants were at least 2 m apart from each other. Each block was at
least 6 m apart from each other. In most sites, the experiment was
conducted using a single native plant species, typically the most common
species locally. However, in some tropical forests, due to the high
species diversity and low relative density, we used different plant
species among blocks, but the plants within each pair always belonged to
the same species. We only used broadleaf native plant species that did
not bear any apparent type of indirect defence (e.g., domatia,
extra-floral nectaries, glandular trichomes) (Romero and Koricheva,
2011).
One plant in each pair was randomly chosen as a control whereas the
second plant was used for the leaf-rolling (shelter addition) treatment.
On each plant, we selected 5-10 fully expanded leaves without apparent
damage. Prior to the experiment, any arthropods present were removed by
hand from leaves. Then, the leaves on the treatment plant were manually
rolled from the adaxial to the abaxial surface transversally to the leaf
axis to form a cylinder of roughly 0.6 cm in diameter (Fig. S1) to mimic
shelters built by caterpillars from at least 17 moth and butterfly
families, including Hesperiidae, Nymphalidae, Gelechiidae, Oecophoridae,
Lasiocampidae, Pyralidae, Gracillariidae, Tortricidae, Geometridae,
Erebidae (Fitzgerald et al ., 1991; Fukui, 2001; Lill and Marquis,
2007). The rolls were secured with a metal hairpin (see Fig. S1 and
Vieira and Romero, 2013). The control plant of the pair had 5-10
unrolled leaves marked with a metal hairpin. Rolled and control leaves
were exposed for 10 days in the field. This was deemed sufficient as
previous bioassays showed that leaf shelters can be colonized very
quickly (within 24 hours) (Vieira and Romero, 2013). Maximum width of
both control and rolled leaves was measured as an estimate of leaf size.
After 10 days of the experiment, we collected rolled and control leaves,
and stored them grouped per replicate and treatment in small zip-lock
plastic bags. The leaves were either frozen for later sorting or
immediately sorted to collect the invertebrates. We collected all the
invertebrates visible to the naked eye (except mites) and stored them in
ethanol. We identified the invertebrates to the lowest taxonomic level
possible and classified them into morphospecies and feeding guild (i.e.,
predator, parasitoid, herbivore, detritivore, omnivore). Individual body
size (dry body mass) was estimated from the dry mass (dried at
70oC for 24h) or by measuring total body length and
then calculating the dry mass through published taxon-specific
allometric equations (Hódar, 1996). Four dependent variables were used
for analyses: arthropod abundance, species richness, biomass and mean
body size. Arthropod biomass represented the sum of all individual body
masses. All variables were weighted by the number of sampled leaves per
plant.