Rainfall reduction increases insect herbivory in tropical herb communities

Abstract

Questions Climate change is dramatically altering rainfall patterns and species distribution around the world. However, little is known about how rainfall reduction may affect plant–herbivore interactions that are crucial to the of input energy and nutrients into terrestrial ecosystems. Following Coley’s growth rate hypothesis, we assessed whether rainfall reduction in a seasonally dry tropical forest decreases community‐level herbivory owing to water shortage in drier areas. Location Catimbau National Park, Northeast Brazil. Methods We sampled 16 herb communities along a rainfall gradient (564–917 mm). Using digital photographs, we monitored for 3 weeks the frequency, magnitude and type (feeding guild) of insect damage on young, fully expanded, short‐lived leaves (lifespan <1 month). We related herbivory levels to local rainfall and examined potential confounding effects of herb abundance, richness and floristic composition on such relationships. Results We monitored 290 leaves from 202 plants belonging to 28 species. About one‐third of the leaves and six species escaped from herbivory. Leaf‐chewing insects were the most frequent herbivores, attacking 17 species, 44% ± 15% of the leaves and removing the same amount of foliar tissue as all the other insects combined removed (sap‐sucking, rasping, leaf‐mining and ‘unknown’). Contrary to expectations, foliar damage accumulated more in drier areas following the increase in the frequency of leaf‐chewing damage. In addition, frequency and magnitude of herbivory did not vary with herb abundance and richness and were weakly influenced by floristic composition. Conclusions Our findings suggest that rainfall underlies community‐level herbivory by mechanisms not related to plant availability, identity and diversity. Based on recent evidence that aridity favours leaf‐chewing proliferation in the study region, we hypothesize that rainfall reduction may weaken the top‐down control of herbivores, increasing folivory pressure on herbs. If prolonged droughts become more frequent in the coming decades, plant–herbivore interactions may be altered permanently.

Drought‐induced reductions in plant defenses: Insights from extrafloral nectaries in the Caatinga dry forest

“…Perhaps ants are playing an important role in shaping anatomical and morphological EFN traits as rainfall increase in Caatinga dry forest. However, although previous studies focused on herbivory (Andrade et al, 2020) and ant defense (Oliveira et al, 2021) in Catimbau NP, our study only showed the variation in EFN traits along a rainfall gradient. This leaves a gap in the understanding of the sources of variation and function of EFN‐related traits.…”

“…EFN traits are also likely to be a result of water availability direct impact on mutualistic partners, especially between attendant‐ants (Leal & Peixoto, 2017; Nogueira et al, 2012, 2020; Pringle et al, 2013). Although plants in Caatinga dry forest are likely to be susceptible to damage from leaf‐chewing insects in drier environments (Andrade et al, 2020), attendant‐ant species visiting EFN‐bearing plants decrease markedly with water scarcity (Câmara et al, 2018) and vary thought time (Câmara et al, 2019). In fact, P .…”

“…However, previous reports of herbivory and ant distribution patterns along the rainfall gradient in our study region show contrasting pressure on EFN traits. Andrade et al (2020) showed that Caatinga plants might be under higher attack from chewing‐insects in drier sites where leaf damage was twice as great as in wetter sites, which counters RAH intra prediction of higher herbivory under higher resource availability (wetter sites, see Figure 1a). In this scenario, EFN traits would be favored in drier environments.…”

Plant protection services mediated by extrafloral nectaries decline with aridity but are not influenced by chronic anthropogenic disturbance in Brazilian Caatinga

“…Leal, unpubl. data; Andrade, Alvarado, Carlos Santos, & Santos, 2020). Since these studies also include plants without EFNs, our findings suggest that decreased effectiveness of EFNs exacerbates a more general trend of increased herbivory with increased aridity.…”

Dynamic effects of insect herbivory and climate on tundra shrub growth: Roles of browsing and ramet age

“…Abbreviations are as described in the legend to Figure 3 and has been obtained from large-scale geographical comparisons, and the patterns reflect precipitation-dependent variation in plant species composition and anti-herbivore defence (Barrio et al, 2017). In our case, the mechanisms underlying weakening effects of herbivory along with precipitation are uncertain, but it is possible that these include, for example, precipitation-dependent variation in plant tolerance (Kasenene & Roininen, 1999), defensive chemistry (Shure et al, 1998) or changes in bottom-up (Lynn & Fridley, 2019) or top-down control (Andrade et al, 2020). Overall, our evidence for predominantly antagonistic effects of temperature and precipitation on insect herbivory is rather unique and noteworthy, because of substantial increases both in temperature and precipitation projected for high latitudes (Cook et al, 2014;Rinke & Dethloff, 2008).…”