Intraspecific competition facilitates the evolution of tolerance to insect damage in the perennial plant Solanum carolinense

Abstract

Tolerance to herbivory (the degree to which plants maintain fitness after damage) is a key component of plant defense, so understanding how natural selection and evolutionary constraints act on tolerance traits is important to general theories of plant-herbivore interactions. These factors may be affected by plant competition, which often interacts with damage to influence trait expression and fitness. However, few studies have manipulated competitor density to examine the evolutionary effects of competition on tolerance. In this study, we tested whether intraspecific competition affects four aspects of the evolution of tolerance to herbivory in the perennial plant Solanum carolinense: phenotypic expression, expression of genetic variation, the adaptive value of tolerance, and costs of tolerance. We manipulated insect damage and intraspecific competition for clonal lines of S. carolinense in a greenhouse experiment, and measured tolerance in terms of sexual and asexual fitness components. Compared to plants growing at low density, plants growing at high density had greater expression of and genetic variation in tolerance, and experienced greater fitness benefits from tolerance when damaged. Tolerance was not costly for plants growing at either density, and only plants growing at low density benefited from tolerance when undamaged, perhaps due to greater intrinsic growth rates of more tolerant genotypes. These results suggest that competition is likely to facilitate the evolution of tolerance in S. carolinense, and perhaps in other plants that regularly experience competition, while spatio-temporal variation in density may maintain genetic variation in tolerance.

Plant damage and herbivore performance change with latitude for two old‐field plant species, but rarely as predicted

“…The degree to which plants in a particular location express tolerance versus resistance could influence herbivore abundances (Stinchcombe 2002, Fornoni 2011). Both Solanum and Solidago are known to tolerate damage by herbivores and tolerance has been shown to vary with environmental context such as successional stage (Hakes and Cronin 2012) or intraspecific density (McNutt et al 2012). To my knowledge, no study has examined latitudinal variation in tolerance, which is likely to be an important missing component in latitudinal gradient studies (Johnson and Rasmann 2011).…”

Parental environments and interactions with conspecifics alter salinity tolerance of offspring in the annual Medicago truncatula

“…While direct tests of the stress-gradient hypothesis at the intraspecific level exist, they are limiting; results from studies suggest the importance of interactions between density, environmental stress and genotype (e.g. Stanton, Thiede & Roy 2004;McNutt et al 2012). For example, McNutt et al (2012) demonstrated facilitative interactions of Solanum carolinense when exposed to herbivory and genetic variation for herbivory tolerance.…”

“…Stanton, Thiede & Roy 2004;McNutt et al 2012). For example, McNutt et al (2012) demonstrated facilitative interactions of Solanum carolinense when exposed to herbivory and genetic variation for herbivory tolerance.…”

The dynamic regeneration niche of a forest following a rare disturbance event

“…However, increases in initial seedling densities can buffer plant populations against herbivory (Clark et al, 2012) and drought (Yates & Ladd, 2005). High regeneration densities might also increase fitness through both adaptive (Bailey et al, 2013) and plastic (McNutt et al, 2012) responses to intraspecific competition. For example, intraspecific competition can drive niche partitioning, making overall resource use more efficient (Boyden et al, 2008) and can increase tolerance to herbivory (McNutt et al, 2012).…”

“…High regeneration densities might also increase fitness through both adaptive (Bailey et al, 2013) and plastic (McNutt et al, 2012) responses to intraspecific competition. For example, intraspecific competition can drive niche partitioning, making overall resource use more efficient (Boyden et al, 2008) and can increase tolerance to herbivory (McNutt et al, 2012). The positive effects of cool, wet environmental conditions on seedling density could therefore have positive effects on the shortand long-term persistence of E. regnans.…”