Defensive effects of extrafloral nectaries in quaking aspen differ with scale

Abstract

The effects of plant defenses on herbivory can differ among spatial scales. This may be particularly common with indirect defenses, such as extrafloral nectaries (EFNs), that attract predatory arthropods and are dependent on predator distribution, abundance, and behavior. We tested the defensive effects of EFNs in quaking aspen (Populus tremuloides Michx.) against damage by a specialist herbivore, the aspen leaf miner (Phyllocnistis populiella Cham.), at the scale of individual leaves and entire ramets (i.e., stems). Experiments excluding crawling arthropods revealed that the effects of aspen EFNs differed at the leaf and ramet scales. Crawling predators caused similar reductions in the percent leaf area mined on individual leaves with and without EFNs. However, the extent to which crawling predators increased leaf miner mortality and, consequently, reduced mining damage increased with EFN expression at the ramet scale. Thus, aspen EFNs provided a diffuse defense, reducing damage to leaves across a ramet regardless of leaf-scale EFN expression. We detected lower leaf miner damage and survival unassociated with crawling predators on EFN-bearing leaves, suggesting that direct defenses (e.g., chemical defenses) were stronger on leaves with than without EFNs. Greater direct defenses on EFN-bearing leaves may reduce the probability of losing these leaves and thus weakening ramet-scale EFN defense. Aspen growth was not related to EFN expression or the presence of crawling predators over the course of a single season. Different effects of aspen EFNs at the leaf and ramet scales suggest that future studies may benefit from examining indirect defenses simultaneously at multiple scales.

Genotypic variation in plant traits shapes herbivorous insect and ant communities on a foundation tree species

“…The density of extra-floral nectaries (EFNs) had a large negative effect on the incidence of insect species and functional groups. This may be due to increased parasitism and predation, since EFNs attract parasitoids and predators, which has been shown to decrease leaf-mining damage [ 63 ]. EFNs are also thought to attract ants [ 64 ], although in our study, ant populations responded more to the presence of aphids than to EFNs.…”

Mixed effects of ant–aphid mutualism on plants across different spatial scales

“…(Garcia et al 2011;Griffin et al 2013;Miller 2014;Mortensen et al 2011;Vilela, Torezan-Silingardi, & Del-Claro 2014). With a similar experimental design, a previous study found a significant anti-herbivory effect of an ant-treehopper association at both plant and stem scales, but the relative strength of the effect at the two scales had not been explicitly compared (Moreira & Del-Claro 2005).…”

“…But these studies have largely ignored the possible effects of scale because related studies often chose similar-sized branches or plants as experimental units. The possible role of scale in shaping the outcomes of ant-plant interaction has been noted recently (Chamberlain & Holland 2009;Mortensen, Wagner, & Doak 2011). For example, on the host plant Solanum lycocarpum, the effect of ants on the oviposition of a treehopper was negative at the wholeplant scale, but tended to be positive at the stem scale (Moreira & Del-Claro 2005).…”

“…When conducting a meta-analysis on the impact ants have on plants, a study found that, compared with studies at the larger scale (whole plants), experiments at the smaller scale (leaves or branches) may overestimate the positive effects of ants on plants (Chamberlain & Holland 2009). Another experimental study found that the defensive effects of extrafloral nectaries only worked at the stem scale rather than at the leaf scale (Mortensen et al 2011). The role of scale in ant-plant interactions should be explicitly evaluated.…”

Functional Role of Extrafloral Nectar in Boreal Forest Ecosystems under Climate Change

“…However, the role of EFN in boreal ecosystems is still poorly understood [28,34,35] and most experimental field studies in the boreal-subarctic zone have been conducted with Populus tremuloides Michx. in Alaska [36][37][38]. Although EFN together with aphid (homopteran) honeydew is known to have a key role in shaping the structure of canopy-dwelling ant communities in rainforest ecosystems [39], only a study by Wenninger et al [35] has assessed the role of EFN in ant community structures in boreal forest ecosystems.…”