DISCUSSION
Our results indicate that structural refuges provided by leaf rolls
increase the local abundance, diversity, biomass and mean body size of
arthropods from different trophic levels on plants, and they do so on
all four continents on which the experiment was conducted. However, the
pattern of refuge use by herbivores and predators in response to climate
differed: while predators used refuges more with increasing drought and
temperature variability, use of refuges by herbivores showed opposite
trends. This suggests that herbivores might avoid the use of refuges in
sites with increasing drought and temperature variability, where risk of
predation is higher (Wetzel et al., 2016). Such predation risk
might drive herbivores to suboptimal microhabitats in harsh conditions.
Herbivorous insects are often smaller than predatory arthropods (see
Fig. S6), and thus likely are less vulnerable to climatic adversities
(Petchey et al., 1999; Voigt et al., 2003; Brose et
al., 2012). In addition, in contrast to many predators, herbivores can
build their own refuges using plants (Pincebourde and Casas, 2019) and
access a more predictable source of water from food (green leaves).
Predators rely on more stochastic water sources (e.g., prey haemolymph),
which are dependent on prey capture rates. Conversely, larger herbivores
might seek refuges against vertebrate predators under more stable and
favourable climatic conditions (Tvardikova and Novotny, 2012), where
predation pressure on larger herbivores is often high (Romero et
al ., 2018). It is therefore likely that herbivores use leaf shelters
more as a refuge against natural enemies (Baer and Marquis, 2020) than
for protection against harsh climatic conditions.
Climate variability and drought appear
to be the key determinants of refuge use, especially by predatory
arthropods. However, different metrics of predator communities
(abundance, species richness, biomass) were differentially influenced by
components of precipitation and temperature. It is likely that drought
may shape species composition within leaf shelters, as the refuges tend
to be occupied by species sensitive to desiccation. Indeed, predatory
arthropod taxa have varying susceptibility to desiccation (Edney, 2012);
leaf shelters could provide them with humidity via leaf transpiration,
and offer stable microclimatic refuges against long droughts. However,
we found refuge usage to be higher for larger predators. As larger
predators show a smaller surface-to-volume ratio than small ones (Kühselet al ., 2017), they are likely less prone to desiccation. Despite
being less vulnerable to desiccation, larger predators can be
competitively dominant over smaller ones in using available refuges.
Our results also reveal that larger predators were most influenced by
temperature variability. This effect is likely mediated by large
predators being subject to the stronger physiological constraints
imposed by temperature and/or having the strongest competitive ability
under such climatic conditions. In fact, temperature variability imposes
greater risks to arthropods than warming (Vasseur et al ., 2014;
Colinet et al., 2015), especially for species from higher trophic
levels (Stireman et al ., 2005). Refuges might ameliorate
temperature oscillations and heat waves (Caillon et al., 2014),
which are harmful especially to larger organisms, such as predators
(Voigt et al., 2003; Brose et al., 2012; Horne et
al ., 2017). It is known that heat transfer (or dissipation) is reduced
in larger organisms because of their smaller surface area-to-volume
ratios (Horne et al ., 2017), causing a threat in more thermally
variable climates. Due to their higher competitive ability, large
predators in particular may thus rely on refuges against extreme
temperature oscillations, despite being less vulnerable to desiccation
(Kühsel et al., 2017).
The climatic variables that we considered consist of both annual means
and variances, thus characterizing the overall climatic conditions of
each site. We found that increasing variability in climate, and not
changes in the mean of the climate variable, was positively correlated
with predator response to shelter availability. If communities of
predators experience higher climatic variability during at least some
part of the year over time, then shelter use in general may be favoured
by selection. This hypothesis is supported by the fact that the climatic
variable spanning the duration of the experiment itself (average
temperature over 10 days) did not moderate the influence of leaf
shelters on predators. Predators (e.g., spiders) are well known to
actively select sheltered microhabitats in harsh conditions and
fire-prone environments (de Omena and Romero, 2010). Therefore, we
suggest that the behaviour of using leaf shelters might be an
evolutionary response to drought and climatic variability.
Arthropods comprise over two-thirds of all terrestrial species, and are
key elements in intricate food webs. They provide valuable ecosystem
services, such as biological control, nutrient cycling and pollination.
Because arthropods are ectothermic and closely dependent on external
energy, they are also among the most threatened organisms on Earth due
to ongoing global changes (van Klink et al ., 2020; Wagner, 2020,
Wagner et al. 2021). With increase in climate variability and in
frequency of extreme droughts, terrestrial arthropod communities should
be more dependent on suitable microclimate (Pinsky et al ., 2019),
with body size and trophic position having a major influence on
microhabitat use. Understanding how arthropods deal with climate
oscillations and extremes is thus pivotal for improved conservation and
mitigation strategies. Efficient strategies should aim at identifying
and preserving local conditions, such as habitat structural components,
to buffer climatic adversities. Here, we present clear evidence that
refuges provided by ecosystem engineers may mitigate predicted increases
in climate variability and drought. As a direct projection from the
patterns observed, we predict that future changes in climatic conditions
could change the global pattern of refuge use by arthropods. In the
future, ecosystem engineers are expected to increase in importance,
especially for predators and in sites where rainfall is expected to
decrease. Increased future usage of shelters by predators might then
strongly alter predation risk on herbivores, and even strengthen
intraguild predation pressure upon plants.
Our results point to larger predators as the dominant organisms using
refuges. This dominance likely results from higher competitive dominance
and/or higher sensitivity to global changes. The refuges can mitigate
climatic variation, allowing predators to adjust their use of niche
space according to their physiological tolerances. The ability to forage
from such refuges may still have a strong effect on their overall
performance, and on the overall imprint of climate on arthropod
community change. Increasing climatic variability and drought may
decrease the ability of such predators to feed and thus to perform
important ecosystem services (Barton and Schmitz, 2009; Rosenblatt and
Schmitz, 2016) such as biological control. Likewise, increasing refuge
usage by predators with climate change could displace herbivores to
suboptimal microclimates (Barton and Schmitz, 2009). Asymmetric reliance
on refuge use by predator and prey, and mismatches in predator-prey
encounter rates, could cause asymmetric extinctions of larger herbivores
and predators (Clark et al., 2020). While the realism of these
scenarios remains to be seen, current patterns of climatic impacts on
refuge use and their extensions into the future flag a potential
ecosystem transition. Importantly, they identify a minuscule scale in
microclimatic research as an important avenue for future research on
ecosystem functioning and community ecology.