bioRxiv preprint doi: https://doi.org/10.1101/722140; this version posted August 1, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. 1 1 Herbivore intraspecific variation and evolutionary divergence drive trophic cascade strength 2 3 Arnaud Sentis1,4*, Raphaël Bertram1, Nathalie Dardenne1, Jean-Christophe Simon2, Alexandra Magro1, 4 Benoit Pujol3, Etienne Danchin1‡, and Jean-Louis Hemptinne1‡ 5 1 UMR-5174; EDB (Laboratoire Évolution & Diversité Biologique); CNRS, Université Toulouse III- 6 Paul Sabatier, IRD, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France. 7 2 UMR 1349; IGEPP (Institut de Génétique, Environnement et Protection des Plantes); INRA, 8 Agrocampus Ouest,Université Rennes 1; Domaine de la Motte B.P. 35327, F-35653 Le Rheu cedex, 9 France. 10 3PSL Université Paris: EPHE-UPVD-CNRS, USR 3278 CRIOBE, Université de Perpignan, 52 Avenue 11 Paul Alduy, 66860 Perpignan Cedex, France. 12 4IRSTEA, Aix Marseille Univ., UMR RECOVER, 3275 route Cézanne, 13182 Aix-en-Provence, 13 France. 14 ‡These authors contributed equally to this work. 15 *Corresponding author: Arnaud Sentis. IRSTEA, Aix Marseille Univ., UMR RECOVER, 3275 route 16 Cézanne, 13182 Aix-en-Provence, France. 17 E-mail: [email protected]; phone: +33 4 42 66 99 05. 18 19 Abstract 20 Trophic cascades—the indirect effect of predators on non-adjacent lower trophic levels—are important 21 drivers of the structure and dynamics of ecological communities. However, the influence of intraspecific 22 trait variation on trophic cascade strength remains largely unexplored, which limits our understanding of 23 the mechanisms underlying ecological networks. Here we experimentally investigated how herbivore 24 intraspecific genetic variation and evolutionary divergence related to host-plant specialization influences 25 trophic cascade strength in a terrestrial tritrophic system. We found that the occurrence and strength of 26 the trophic cascade are strongly influenced by herbivores’ intraspecific variation and evolutionary 27 divergence but are not associated with density-dependent effects mediated by herbivore population 28 growth rate. Our findings stress the importance of intraspecific trait diversity and evolutionary 29 adaptations as drivers of trophic cascade strength and underline that intraspecific variation should not be 30 overlooked to decipher the joint influence of evolutionary and ecological factors on the functioning of 31 multi-trophic interactions. 32 bioRxiv preprint doi: https://doi.org/10.1101/722140; this version posted August 1, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. 2 33 Key-words: tri-trophic interactions, herbivory, indirect effects, evo-to-eco, host-plant adaptation, 34 genetic variation 35 36 Introduction 37 Predators strongly influence the structure and function of ecological communities by influencing prey 38 density, distribution, and behavior which, in turn, have cascading effects on lower trophic levels (Sih et 39 al. 1985; Beckerman, Uriarte & Schmitz 1997; Shurin et al. 2002; Schmitz, Krivan & Ovadia 2004; 40 Suraci et al. 2016). This indirect effect of predators on non-adjacent lower trophic levels, the so-called 41 “trophic cascades,” are frequently observed in both aquatic and terrestrial ecosystems (Halaj & Wise 42 2001; Borer et al. 2005; Bruno & O'Connor 2005; Wu et al. 2011; Sanders, Kehoe & van Veen 2015). 43 Trophic cascades are important drivers of the structure and dynamics of populations, communities, and 44 ecosystems (Ripple et al. 2016) and have several implications for theoretical ecology, conservation 45 biology, and ecosystem management (Post et al. 1999; Hulot et al. 2000; Estes et al. 2011). For instance, 46 Delvin et al. (2015) showed that stocking fishes in fishless lakes decreases by a factor 10 the efflux rates 47 of methane—an important greenhouse gas—by reducing zooplankton abundance, which in turn increases 48 the abundance of methanotrophic bacteria. In another study, Schmitz et al. (2017) showed that the 49 composition of the arthropod predator community and associated cascading effects on plant communities 50 explain 41% of the variation in soil carbon retention across a human land-use gradient. Given the 51 importance of trophic cascades, a major issue in ecology and conservation is to determine when and 52 where trophic cascades occur, and what are the factors and mechanisms underpinning their strength. 53 54 Although the existence of trophic cascades has been widely demonstrated (Schmitz 2003; Romero & 55 Koricheva 2011), most studies focused on whether cascades are more likely or stronger in some systems 56 than others (Pace et al. 1999; Shurin et al. 2002), leading to a wealth of predictions about the relative 57 strength of predator effects on plants among ecosystems with a particular focus on aquatic versus 58 terrestrial ecosystems (Shurin et al. 2002; Borer et al. 2005). Although comparing cascade strength 59 among systems is valuable, little is known about what causes variations in the magnitude of cascading 60 effects within or among systems. In particular, the role of intraspecific trait variation for the occurrence 61 and strength of trophic cascades remains largely unexplored (but see(Post et al. 2008; Harmon et al. 62 2009; Jochum et al. 2012; Keiser et al. 2015; Royauté & Pruitt 2015; Start & Gilbert 2017). Most prior 63 studies assumed that species identity or mean trait values adequately represent species interactions and 64 their effects on community dynamics. This assumption is puzzling because it ignores the considerable 65 intraspecific variation of traits (Benesh & Kalbe 2016), thereby overlooking a potentially important bioRxiv preprint doi: https://doi.org/10.1101/722140; this version posted August 1, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC 4.0 International license. 3 66 determinant of species interactions, community structure and dynamics, as well as evolutionary 67 responses to selective pressures (Roff 1997; Bolnick et al. 2011; Violle et al. 2012). Thus, knowledge of 68 how intraspecific variation modulates bi- and tri-trophic interactions is crucial for better understanding 69 and predicting the occurrence and strength of trophic cascades. 70 71 Different hypotheses have been proposed to explain variations in the occurrence and strength of trophic 72 cascades in various types of ecosystems (Hulot et al. 2000; Polis et al. 2000; Borer et al. 2005). 73 Hypotheses linked to habitat spatial heterogeneity, food web linearity or system productivity have 74 received little support (Borer et al. 2005). On the contrary, interspecific variation in plant anti-herbivore 75 defenses (Schmitz et al. 2000; Mooney et al. 2010), predator hunting mode and consumer efficiency 76 (e.g., low metabolic costs, high consumption rate and population growth rate) can significantly affect 77 trophic cascade strength (Romero & Koricheva 2011). In particular, high predator or herbivore efficiency 78 increases cascade strength via high consumption rate of herbivores by predators, and plants by herbivores 79 (Strong 1992; Polis 1999; Borer et al. 2005). Therefore, trophic cascade strength should depend on the 80 herbivore’s population growth rate in the absence of predators and on the predator efficiency in reducing 81 herbivore density (Schmitz 1998; Borer et al. 2005). We thus expect density-mediated effects driven by 82 intraspecific variation in herbivore population growth rate to be an important determinant of trophic 83 cascade strength: the faster they growth, the stronger is the trophic cascade. Deviation from this 84 prediction would stress the importance of considering intraspecific variation in other ecological traits and 85 in evolutionary history of herbivores to better understand and predict cascade strength. 86 87 In this study, we experimentally investigated the effects of herbivore intraspecific trait variation on 88 trophic cascade strength using a broad bean–aphid–ladybeetle system. The pea aphid Acyrthosiphon 89 pisum Harris (Homoptera: Aphididae) feeds on many Fabaceae species and forms host-plant-associated 90 populations (“host races” or “biotypes”) that are genetically differentiated (Via 1999; Hawthorne & Via 91 2001; Peccoud et al. 2009) in a way that affects their performance on different host plants (Via 1999; 92 Via, Bouck & Skillman 2000; Hawthorne & Via 2001). These different features make the pea aphid an 93 interesting biological model to assess the influence of intraspecific trait variation and evolutionary 94 divergence for trophic cascade strength. Here we conducted a full factorial laboratory experiment with 95 six pea aphid clonal lineages (i.e. asexually reproducing aphid genetic lines) specialized either on alfalfa 96 (Alfalfa biotype) or clover (Clover biotype) and exposed or not to a generalist predator. We first tested 97 whether trophic cascade strength varies among aphid clonal lineages, and then investigated whether 98 differences among lineages are best explained by density-mediated