Antagonistic interactions between competition and insect herbivory on plant growth

Abstract

Summary1 Although substantial theoretical work suggests that competition and herbivory should exhibit a wide variety of interactions in their effects on plant growth, empirical studies have shown that the predominant interactions are simply multiplicative. 2 To determine both the relative strengths of, and interactions between, competition and herbivory, we conducted a field experimental study in a native grassland using four focal species: Koeleria macrantha , Coreopsis tinctoria , Linum lewisii and Helianthus petiolaris . 3 The effects of competition and herbivory on plant growth, biomass allocation and survival varied among species. When effects were present, neighbouring vegetation reduced plant growth and survival, and insect herbivores decreased relative biomass allocation to roots and increased plant mortality. 4 Although reduced herbivory caused by insecticide application had little direct effects on plant biomass, it did interact with competition to affect growth for three of the four species. Herbivory reduced the strength of competition experienced by Coreopsis , but increased it in Linum . For both Coreopsis and Helianthus , the combined effects of herbivory and competition on plant growth were less than expected from a simple multiplicative response. 5 We suggest that interactions were found because we used an experimental design that modified insect densities on both neighbours and the focal plant, allowing for determination of both direct and indirect effects. 6 We suggest that the antagonistic interactions between competition and herbivory may have occurred because herbivores altered the competitive environment by harming the neighbouring plants, or because the presence of neighbours facilitated focal plant growth by distributing herbivore loads over the greater amount of plant biomass available. 7 Although as an isolated factor competition is more intense in this system than insect herbivory, herbivory can alter the strength of competitive interactions and thus its importance cannot be determined by measuring intensity alone. This result emphasizes that these two ecological processes are not truly discrete, and further suggests that studies that simultaneously manipulate multiple ecological processes are needed.

Additive effects of aboveground polyphagous herbivores and soil feedback in native and range‐expanding exotic plants

“…Then, the shoot and root biomass reduction for the soil feedback effect (difference between own and control soil) and the effect of herbivore reduction (difference between herbivory and no-herbivory treatment) were calculated for each plant species, added, and compared with the treatment where plants were exposed to the combination of soil feedback and aboveground herbivory ( Fig. 2; Haag et al 2004). Because plant biomass data are always positive they tend to follow a logarithmic distribution when biomass values approach zero, which would transform the linear additive effect towards a multiplicative effect (Slob 1987).…”

No interaction between competition and herbivory in limiting introduced Cirsium vulgare rosette growth and reproduction

“…Third, the insecticide treatment, by being concentrated only on the focal experimental plant rather than on the entire plot, could have increased herbivory on the neighbor plants by shared generalist herbivores, weakening the competitive effect on the focal rosette (Haag et al 2004). We did not measure herbivory on the neighboring plants in our plots; however, we found that neighboring plants did not reduce herbivory on the focal plant (Table 1), suggesting neighbors did not lure herbivores off of our focal experimental plant.…”

“…If the effects of herbivory and competition are antagonistic, then their joint effect on plant performance should be less than the product of their individual effects. Antagonistic interactions could result if neighbors hide a plant from its herbivores, reducing the magnitude of herbivory (also known as associational resistance) (Tahvanainen and Root 1972); or, it could result if herbivores feed preferentially on the neighbor plants reducing their competitive effect on the focal plant (Hamilton et al 1998;Haag et al 2004). Since competition and herbivory are widespread, understanding whether and how they interact to limit plant reproduction is of general importance in predicting population dynamics in basic and applied contexts.…”

“…Competition and herbivory might influence plant fitness: (1) independently (Rees and Brown 1992;Reader and Bonser 1998;Hämback and Beckerman 2003), (2) synergistically (McEvoy et al 1993;Meiners and Handel 2000), or (3) antagonistically (Parmesan 2000;Haag Submitted February 26, 2009;accepted July 28, 2009;published online August 20, 2009. No interaction between competition and herbivory in limiting introduced Cirsium vulgare rosette growth and reproduction Tomomi Suwa,1 Svata M. Louda, 1 and F. Leland Russell 2 et al 2004). If the effects of herbivory and competition are independent, then their combined effect on plant performance is predicted by multiplying their individual effects (See Fowler and Rausher 1985;Rees and Brown 1992).…”

Combined effects of folivory and neighbor plants on Cirsium altissimum (tall thistle) rosette performance

“…Our finding that effects of aboveground parts of neighbor plants and insect folivory acted independently in reducing growth of tall thistle rosettes adds to a short but growing list of field experiments in natural or agroecosystems that demonstrate that independent effects of neighbor plants and insect folivory on host plant growth may be more common than either synergistic or antagonistic interactions (independent effects on growth: Rees and Brown 1992, McEvoy et al 1993, Erneberg 1999, Newingham and Callaway 2006; synergistic effects on growth: Nötzold et al 1998, Agrawal 2004; antagonistic effects on growth: Haag et al 2004). In spite of the independence of their effects on plant growth, we did find evidence that the presence of neighbor plants magnified the impact of insect folivory on the transition rate of rosettes to reproductive life-stages.…”