Impact of plant defense level variability on specialist and generalist herbivores
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ORIGINAL PAPER
Impact of plant defense level variability on specialist and generalist herbivores Tatjana Thiel1
2 · Barbara Drossel1 ¨ · Sarah Gaschler1 · Karsten Mody2 · Nico Bluthgen
Received: 8 September 2019 / Accepted: 29 April 2020 © The Author(s) 2020
Abstract Most organisms are defended against others, and defenses such as secondary metabolites in plants vary across species, individuals, and subindividual organs. Plant leaves show an impressive variability in quantitative defense levels, even within the same individual. Such variation might mirror physiological constraints or represent an evolved trait. One important hypothesis for the prevalence of defense variability is that it reduces herbivory due to non-linear averaging (Jensen’s inequality). In this study, we explore the conditions under which this hypothesis is valid and how it depends on the degree of specialization of the herbivores. We thus distinguish between generalists, non-sequestering specialists, and sequestering specialists that are able to convert consumed plant defense into own defense against predators. We propose a plantherbivore model that takes into account herbivore preference, predation pressure on the herbivores, and the three herbivore specialization strategies we consider. Our computer simulations reveal that defense level variability reduces herbivory by all three populations when nutrient concentration is strongly correlated with defense level. If the nutrient concentration is the same in all leaves, the plant benefits from high defense level variability only when the herbivores are specialists that show a considerable degree of preference for leaves on which they perform best. Keywords Defense level variability · Correlation between a leaf’s nutrient and defense level · Generalist vs. specialist · Herbivore preference · Jensen’s inequality · Plant-herbivore model
Introduction Plants are immobile and not able to physically escape herbivore attacks (Karban and Agrawal 2002; Karban and Baldwin 2007; Gutbrodt et al. 2012). Hence, plants have evolved mechanisms to deal with herbivory, namely (i) tolerance to herbivory, which means that herbivory does not affect plant fitness, for example, because of an increased net photosynthetic rate after herbivore damage (Strauss and Agrawal 1999) and (ii) the reduction of damage by defensive traits that increase resistance to herbivory (Karban et al. 1997; Strauss and Agrawal 1999). Plants express different defensive mechanisms to reduce herbivore
Tatjana Thiel
[email protected] 1
Institut f¨ur Festk¨orperphysik, Technische Universit¨at Darmstadt, Hochschulst. 6, 64289 Darmstadt, Germany
2
Ecological Networks, Technische Universit¨at Darmstadt, Schnittspahnstraße 3, 64287 Darmstadt, Germany
damage. These mechanisms can be categorized into (i) permanent constitutive defenses such as thorns, trichomes (Karban and Baldwin 2007; Lankau 2007), secondary metabolites (van der Meijden 1996; Karban and Baldwin 2007; Dimarco et al. 2012; Gutbrodt et al. 2012), or a higher toughnes
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