Top-down factors contribute to differences in insect herbivory between saplings and mature trees in boreal and tropical forests

Abstract

Ontogenetic changes in herbivory are generally not consistent with ontogenetic changes in defensive traits of woody plants. This inconsistency suggests that other factors may affect ontogenetic trajectories in herbivory. We tested the hypothesis that top-down factors contribute to differences in foliar losses to insects between juvenile and mature trees in tropical and boreal forests. We used artificial caterpillars made of modelling clay to compare predation rates between saplings and mature trees of two common forest species, Siparuna guianensis in Brazil (tropical site) and Betula pubescens in Finland (boreal site). Leaf area losses to chewing insects in saplings were 2.5-fold higher than in mature trees in both species. Physical plant defences (measured as specific leaf area, SLA) did not differ between saplings and mature trees in the boreal forest, whereas in the tropical forest, SLA was greater in saplings than in mature trees. Attack rates on the model prey by birds were higher in the boreal forest, whereas attack rates by arthropod predators were higher in the tropical forest. Overall, predation rates on model prey were consistently higher on mature trees than on saplings at both sites, but in the boreal site, this pattern was primarily driven by birds, whereas in the tropical site, it was primarily driven by arthropod predators. We conclude that the effect of predation on herbivorous insects may considerably contribute to ontogenetic differences in herbivory, but the relative roles of different predatory groups and of top-down and bottom-up factors may vary between environments.

Assessing invertebrate herbivory in human‐modified tropical forest canopies

“…Second, studies only measuring the leaf area affected may miss key energetic costs for the host plant from gall-forming or mining herbivores (Giron et al, 2016;Tooker & Giron, 2020). Third, overlooking miners and gall formers can limit our understanding of the relative importance of top-down and bottom-up controls (Vidal & Murphy, 2018), as they differ ecologically and are regulated by different top-down processes (Ohgushi et al, 2012;Zvereva et al, 2020). For example, external feeding habits (e.g., chewers) have increased predation risk due to the exposure and vulnerability at the leaf surface (Kaplan et al, 2014;Schmitz et al, 1997), while miners are less susceptible to predation than external feeders (Hawkins et al, 1997).…”

Predation on artificial caterpillars following understorey fires in human‐modified Amazonian forests

“…Mammals predated 5.5% of the caterpillars, a figure slightly higher than that previously reported—that is, <1% to 4% (Schwab et al, 2021; Seifert et al, 2015). Predation of caterpillars by reptiles is generally rare (Murray et al, 2020; Schwab et al, 2021), with some studies not registering any (Roels et al, 2018; Roslin et al, 2017; Zvereva et al, 2020). We found only two caterpillars predated by reptiles, out of the 2,858 which presented predation marks.…”

“…Studies across the globe, including tropical forests, have found a high incidence of arthropod predation on artificial caterpillars (Fáveri et al, 2008; Loiselle & Farji‐Brener, 2002; Roslin et al, 2017; Witwicka et al, 2019; Zvereva et al, 2020). Arthropod predation can account for up to 98% of the total predation attempts on artificial caterpillars in undisturbed rainforests after 48 h of exposure (Seifert et al, 2016), a figure similar to ours (91% on average, after 14 days).…”

“…Furthermore, the color and shape of our caterpillars are likely biased towards soliciting interactions from generalist predators (Boesing et al, 2017). Therefore, the study design may fail to record specialized interactions (Zvereva et al, 2019(Zvereva et al, , 2020. Moreover, natural pray abundance can also be an important driver of predation incidence-for example, higher prey abundances lead to predator satiation, which may affect the attack incidence on artificial caterpillars.…”

“…Top‐down control by predatory species such as vertebrates, other arthropods, and parasitoids may help regulate the abundance of folivorous arthropods (Hairston et al, 1960; Mäntylä et al, 2011). In tropical forests, other arthropods (Lemessa et al, 2015; Zvereva et al, 2020) and birds (Mäntylä et al, 2011; Van Bael et al, 2003) are the most important predators of folivorous arthropods, exerting significant herbivory control (Vidal & Murphy, 2018). However, the relative importance of predator groups, such as arthropods and birds, is not static—that is, it can be altered by changes in forest condition (Dodonov et al, 2016).…”