Defensive effects of extrafloral nectaries in quaking aspen differ with scale |
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Authors: | Brent Mortensen Diane Wagner Patricia Doak |
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Institution: | (1) Department of Biology and Wildlife, Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775-7000, USA;(2) Present address: Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, IA 50011, USA; |
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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. |
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