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.