INTRODUCTION
Climate warming, extreme precipitation events and temperature
variability are all intensifying at unprecedented rates (IPCC Climate
Change, 2014; Donat et al. , 2016; Bathiany et al ., 2018).
Such changes cause species–environment mismatches, and have already led
to shifts in the geographic ranges and elevational or vertical
distributions of species as they track their climatic niche (Vázquezet al ., 2017; Essl et al ., 2019; Trisos et al .,
2020). As other means to avoid adverse conditions, terrestrial species
may seek microclimatic refuges (Scheffers et al ., 2014; Suggittet al ., 2018) and even manipulate their own microclimate
(Pincebourde and Casas, 2019). Microclimatic refuges provided by a
variety of habitat structural components – including plant
architectural complexity, tree bark, downed woody debris, leaf litter,
and leaf shelters – reduce the exposure of their inhabitants to climate
variability and extremes (Scheffers et al ., 2014; Suggittet al ., 2018; Pincebourde and Casas, 2019; Pinsky et al .,
2019). Such refuges can be particularly important to ectotherms (Pinskyet al ., 2019), such as arthropods. Ectotherms depend on external
energy to thermoregulate and are amongst the taxa most threatened by
global change (García-Robledo et al ., 2016; Warren et al.,2018; Wagner, 2020; Wagner et al ., 2021). The availability of
these refuges, and their impact on insect use, may be one factor
mitigating the various negative climatic factors leading to insect
decline.
Protection against harsh environmental conditions is typically provided
by ecosystem engineers, which are organisms that modify their habitats
by creating physical structures that can be used as structural refuges
by other organisms (Jones et al. Hastings et al ., 2007; Romeroet al ., 2015). In terrestrial ecosystems, bark beetles and stem
borers make stem and trunk galleries, gallers manipulate plant
physiology to produce galls, and various arthropods (e.g., caterpillars,
aphids, mites and thrips) build leaf shelters, such as leaf rolls and
ties, thus providing refuge to many other plant-dwelling organisms (Wanget al., 2012; Vieira and Romero, 2013; Cornelissen et al .,
2015; Priest et al ., 2021). The role of leaf-rolling engineers as
biodiversity amplifiers can be even stronger in dry seasons, and extend
to the whole plant, thus influencing arthropod assemblages at temporal
and microspatial scales (Vieira and Romero, 2013). After such shelters
are abandoned by their creators, they become available for other
arthropods for a long period of time (Vieira and Romero, 2013). Similar
to many other biotic interactions, which are stronger at lower latitudes
(Schemske et al., 2009), the facilitative effects of vertebrate
and invertebrate ecosystem engineers seem particularly pronounced in the
wet tropics, but also in arid regions (Romero et al ., 2015).
Therefore, these latitudinal, regional (Romero et al ., 2015) and
temporal patterns (Vieira and Romero, 2013) suggest that climatic
conditions may be a common driver underlying such facilitative
interactions.
Large-scale patterns in climatic signatures have been recently
investigated using standardized replicated experiments, which can
differentiate the direct effects of climate from the indirect effects of
latitude or elevation on biotic interactions (LaManna et al .,
2017; Roslin et al ., 2017; Romero et al ., 2018). Most of
these macroecological studies have focused on antagonistic interactions,
such as predation and competition (LaManna et al ., 2017; Roslinet al ., 2017; Romero et al ., 2018). Although the
importance of facilitative interactions among organisms is expected to
increase under stressful conditions (He et al ., 2013; Romeroet al ., 2015), no empirical studies to date have examined the
impacts of current climate and climate variability, or predicted the
impacts of future climates, on facilitative interactions at global
scale.
Here, we test the effects of current and future climatic scenarios on
facilitation mechanisms provided by leaf-rolling ecosystem engineers. We
compare how current and projected climate affects the usage of leaf
rolls by terrestrial arthropods, and how shelter usage is affected by
body size and trophic position. We hypothesize that abundance, species
richness and biomass of arthropods within leaf rolls will increase with
increasing climate variability and drought. We also expect that species’
responses to climate will vary with their trophic position and body
size, both of which are known to influence the metabolic requirements of
ectotherms (Daufresne et al., 2009) and their capacity to
dissipate heat (Rubalcaba et al., 2019). Competition at higher
trophic levels (predators) selects for larger body size. Larger-bodied
organisms typically require more favourable climatic conditions than
their prey (Petchey et al., 1999; Voigt et al., 2003;
Brose et al., 2012). Therefore, we expect larger predators to be
dominant in refuges especially in arid and climatically variable regions
(Fig. 1). Finally, with continuing climate change, we expect current
climatic dependencies to reflect future changes in geographic patterns
of refuge use by arthropods.
To test the above predictions, we conducted a global, coordinated
experiment at 52 sites across an 11,790 km latitudinal gradient (from
45.8° S to 60.2° N, Fig. 1) and elevation spanning from 5 to 2900 m
above sea level. To measure the beneficial effects of leaf rolls, we
recorded abundance and diversity of arthropods colonizing manually
rolled and control (unrolled) leaves, and calculated the standardized
mean difference (Hedges’ d) between arthropod abundance, species
richness, biomass and body size in rolled and control leaves for each
site. To test how climate influences shelter colonization, we
investigated the effects of different categories of moderators
(predictor variables) on the Hedges’ d effect size, including both
geographic distance (absolute latitude, elevation) and direct measures
of climate (temperature and precipitation). Finally, we provide
geographical interpretations of climate change scenarios on the patterns
of refuge use and predict future changes in refuge use by projecting
effect sizes to future (2070) climatic conditions at the study
locations.