DOI: 10.5061/dryad.rjdfn2zcx
title: Climate variability and aridity modulate the role of leaf shelters for arthropods: a global experiment

description: Current climate change is disrupting biotic interactions and eroding
 biodiversity worldwide. However, species sensitive to aridity, high
 temperatures and climate variability might find shelter in microclimatic
 refuges, such as leaf rolls built by arthropods. To explore how the
 importance of leaf shelters for terrestrial arthropods changes with
 latitude, elevation, and climate, we conducted a distributed experiment
 comparing arthropods in leaf rolls vs. control leaves across 52 sites
 along an 11,790 km latitudinal gradient. We then probed the impact of
 short- versus long-term climatic impacts on roll use, by comparing the
 relative impact of conditions during the experiment versus average,
 baseline conditions at the site. Leaf shelters supported larger organisms
 and higher arthropod biomass and species diversity than non-rolled control
 leaves. However, the magnitude of the leaf rolls’ effect differed between
 long- and short-term climate conditions, metrics (species richness,
 biomass, and body size), and trophic groups (predators vs. herbivores).
 The effect of leaf rolls on predator richness was influenced only by
 baseline climate, increasing in magnitude in regions experiencing
 increased long-term aridity, regardless of latitude, elevation,
 and weather during the experiment. This suggests that shelter use by
 predators may be innate, and thus, driven by natural selection. In
 contrast, the effect of leaf rolls on predator biomass and predator body
 size decreased with increasing temperature, and increased with increasing
 precipitation, respectively, during the experiment. The magnitude of
 shelter usage by herbivores increased with the abundance of predators and
 decreased with increasing temperature during the experiment. Taken
 together, these results highlight that leaf roll use may have both
 proximal and ultimate causes. Projected increases in climate variability
 and aridity are therefore likely to increase the importance of biotic
 refugia in mitigating the effects of climate change on species
 persistence.
description: Experiments at all 52 sites were conducted following a
 standardized protocol. Each experiment followed a randomized block design.
 For each site, we randomly selected 10 to 20 paired trees (hereafter, a
 pair of plants is referred to as a block). Paired plants within a block
 were at least 2 m apart. Blocks were at least 6 m apart from each other.
 In most sites, the experiment was conducted using a single native plant
 species, typically the locally most common species. However, in some
 tropical forests, due to high species diversity and low relative density,
 we used different plant species among blocks, with plants within a pair
 always belonging tothe same species. We used only broadleaf native plant
 species that did not exhibit any obvious type of indirect defence (e.g.,
 domatia, extra-floral nectaries, glandular trichomes).
 One plant per block was randomly selected as a control plant,
 while the second plant was used for the leaf-rolling (shelter addition)
 treatment. On each plant, we selected 5-10 fully expanded leaves without
 obvious damage (hereafter referred to as ‘sample unit’). Prior to the
 experiment, any arthropods present on experimental leaves were removed by
 hand. Then, the leaves of the treatment plant were rolled by hand from the
 adaxial to the abaxial surface across the leaf axis to form a cylinder
 approximately 0.6 cm in diameter to mimic shelters built by caterpillars -
 a phenomenon occurring in at least 17 moth and butterfly families,
 including Hesperiidae, Nymphalidae, Gelechiidae, Oecophoridae,
 Lasiocampidae, Pyralidae, Gracillariidae, Tortricidae, Geometridae,
 Erebidae. The leaf rolls were secured with a metal hairpin. In the control
 plants, 5-10 unrolled leaves were 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) and that species richness within
 leaf rolls reaches saturation within a few days. Moreover, leaf abscission
 was observed in some plant species during the 10-day experiment (e.g.,
 Psychotria, G.Q. Romero, pers. obs.), thus precluding experiments of
 longer duration. 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 by replicates and treatments; both control and rolled leaves were
 quickly placed into a zip-lock plastic bag and sealed. 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. Mites could not be considered
 in the analysis, because identifying both mite taxa and feeding guilds is
 extremely difficult, with very few specialists worldwide. We identified
 the invertebrates to the lowest taxonomic level possible and classified
 them into morphospecies and feeding guilds (i.e., predator, parasitoid,
 herbivore, detritivore, omnivore). However, the sample size was only
 sufficiently large for separate analyses of predators and herbivores.
 Individual body size (dry body mass) was estimated from the dry mass
 (dried at 70&lt;spanstyle="font-family:cambria;
 font-size:8pt;=""
 vertical-align:super"=""&gt;oC for 24h) or by measuring
 total body length and then calculating the dry mass using published
 taxon-specific allometric equations (Hódar, 1996). Three dependent
 variables were used for the analyses: species richness, community biomass,
 and mean individual body size. As expected, total abundance explained much
 of the species richness (R2 = 0.78, F1,50 = 185.5, P&lt;0.001).
 Because of the small subsamples (5 to 10 leaves per plant), which would
 lead to highly stochastic results, we were unable to determine rarefied
 species richness. Therefore, we focus our analyses on the number of
 species (richness) and assume that this is a product of increased overall
 abundance. Arthropod biomass represents the sum of all individual body
 masses. Arthropod richness and biomass were weighted by the number of
 leaves sampled per plant (e.g., number of species per leaf), and average
 individual body size was calculated for each sampling unit by dividing the
 total biomass for each feeding guild (predator or herbivore) by the number
 of arthropods within that feeding guild - thus, the sample unit for body
 mass is the average mass per arthropod upon leaves.