Light environment and the impacts of foliage quality on herbivorous insect attack and bird predation

Abstract

Theory predicts that variation in plant traits will modify both the direct interactions between plants and herbivores and the indirect impacts of predators of those herbivores. Light has strong effects on leaf quality, so the impacts of herbivores and predators may differ between plants grown in sun and shade. However, past experiments have often been unable to separate the effects of light environment on plant traits and herbivory from direct effects on herbivores and predators. We first manipulated light availability in an open habitat using a shade cloth pre-treatment to produce oak saplings with different leaf qualities. Leaves on plants exposed to high light were thicker and tougher and had lower nitrogen and water contents, and higher carbon and phenolic contents than leaves on plants under a shade cloth. Then, in the main experiment, we moved all plants to a common shade environment where bird predators were excluded in a factorial design. We measured insect herbivore abundance and leaf damage. Herbivores were significantly more abundant and caused greater leaf damage on sun trees, although these leaf characteristics are usually associated with low-quality food. Bird exclusion did not change herbivore abundance but did increase leaf damage. Contrary to our predictions, the effects of birds did not differ between trees grown in sun and shade conditions. Thus, differences in effects of predators on herbivores and plants between light habitats, when observed, might be due to variation in predator abundance and not bottom-up effects of host plant quality.

The role of leaf traits and bird-mediated insect predation on patterns of herbivory in a semiarid environment in central Mexico

“…In sunny environments the increase in carbon content in plant tissues and the consequent changes in the carbon to nitrogen ratio correlates negatively with the rate of acquisition of nutrients (nitrogen) by herbivores; more leaf tissue has to be consumed to assimilate enough nutrients due to the nitrogen dilution effect (Mattson Jr. 1980). In agreement with our findings it has been reported that herbivory correlates positively with sunlight exposition (Louda & Rodman 1996, Dicke 2000 although the opposite pattern (i.e low herbivory in sunny microhabitats) has also been documented in some systems (Guerra et al 2010;Barber & Marquis 2011). Explanations for the negative relationship between herbivory and sunlight invoke three main issues: 1) Predation rates on herbivores is high in open sites and low in shaded sites.…”

“…This is also concordant with our observations since we found less herbivory, despite a high abundance of herbivores in the site where leaves had higher content of nutrients. Then, our evidence agree with three of the most likely process that should lead to low levels of herbivory in sunny and warm habitats (Guerra et al 2010, Barber & Marquis 2011. However, it was in the open canopy site where we found higher herbivory as compared with the more shaded site.…”

Shade trees decrease pest abundances on brassica crops in Kenya

“…Furthermore, the decreased temperature and increased humidity under shaded conditions can have an impact on plant quality and palatability, which may influence pest abundances. Shade can decrease the rate of photosynthesis (Sleeman and Dudley 2001), alter plant biomass, plant height, leaf area and protein content, and alter plant chemical composition (Olsson et al 1998;Sleeman and Dudley 2001;Barber and Marquis 2011). Chemical changes induced by shade include the decrease in phenolic compounds, known to be important for plant defence (Barber and Marquis 2011).…”

“…Shade can decrease the rate of photosynthesis (Sleeman and Dudley 2001), alter plant biomass, plant height, leaf area and protein content, and alter plant chemical composition (Olsson et al 1998;Sleeman and Dudley 2001;Barber and Marquis 2011). Chemical changes induced by shade include the decrease in phenolic compounds, known to be important for plant defence (Barber and Marquis 2011). Shade trees may also modify a crop's nutritional quality by either depleting soil nutrients following competition between crops and trees or by increasing fertilization by nitrogen-fixing trees (Rao et al 2000).…”

“…Shade trees may also modify a crop's nutritional quality by either depleting soil nutrients following competition between crops and trees or by increasing fertilization by nitrogen-fixing trees (Rao et al 2000). Plant nutritional quality can also be altered by decreased water stress induced by shading (Barber and Marquis 2011). Low water stress can decrease the soluble nitrogen content in brassica crops, while nitrogen is often a limiting factor for population growth of many insect pests (Burgess et al 1996).…”