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

Elena L Zvereva,Mikhail V Kozlov,Lucas N Paolucci

doi:10.1007/s00442-020-04659-z

Elena L Zvereva, Mikhail V Kozlov + Show 1 more

Open Access

https://doi.org/10.1007/s00442-020-04659-z

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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.

Highlights

Plant ontogeny is one of major factors influencing plant–herbivore interactions at both ecological and evolutionary time scales (Boege and Marquis 2005)
The area consumed from a damaged leaf and the proportion of damaged leaves were both higher in saplings than in mature trees (Table 1, Fig. 1b, c), but our sites differed in terms of the relative importance of these two components of overall leaf loss
We found that herbivorous insects experience a higher predation pressure when located on mature trees than on conspecific saplings, and we concluded that this difference could have contributed to the observed ontogenetic changes in herbivore damage

Summary

Introduction

Plant ontogeny is one of major factors influencing plant–herbivore interactions at both ecological and evolutionary time scales (Boege and Marquis 2005). Ontogenetic patterns in plant defensive traits were found to be inconsistent with ontogenetic patterns in herbivory for woody plants (Barton and Koricheva 2010). This inconsistency suggests that factors other than chemical and physical plant defences substantially contribute to the ontogenetic changes in foliar damage imposed by insects on plants. The effects of the third trophic level are only rarely accounted for in explanations of the differences in herbivory between plant ontogenetic stages (but see Boege and Marquis 2006)

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