Plant Animal Interactions

Oecologia

, Volume 143, Issue 2, pp 241-250

Do multitrophic interactions override N fertilization effects on Operophtera larvae?

  • Joachim StrengbomAffiliated withDepartment of Ecology and Environmental Science, Umeå UniversityEvolutionary Biology Centre, Department of Plant Ecology, Uppsala University Email author 
  • , Johanna WitzellAffiliated withUmeå Plant Science Center, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences
  • , Annika NordinAffiliated withUmeå Plant Science Center, Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences
  • , Lars EricsonAffiliated withDepartment of Ecology and Environmental Science, Umeå University

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Abstract

We examined how performance of Operophtera brumata (Lepidoptera) larvae was affected by nitrogen (N) fertilization of boreal forest understorey vegetation. We monitored larval densities on Vaccinium myrtillus plants for a period of 7 years in a field experiment. Preliminary results indicated that the N effect on larval densities was weak. To examine if this was due to indirect interactions with a plant pathogen, Valdensia heterodoxa, that share the same host plant, or due to top-down effects of predation, we performed both a laboratory feeding experiment (individual level) and a bird exclusion experiment (population level) in the field. At the individual level, altered food plant quality (changes in plant concentration of carbon, N, phenolics, or condensed tannins) due to repeated infection by the pathogen had no effect on larval performance, but both survival to the adult stage and adult weight were positively affected by N fertilization. Exclusion of insectivorous birds increased the frequency of larval damage on V. myrtillus shoots, indicating higher larval densities. This effect was stronger in fertilized than in unfertilized plots, indicating higher bird predation in fertilized plots. Predation may thus explain the lack of fertilization effect on larval densities in the field experiment. Our results suggest that top-down effects are more important for larval densities than bottom-up effects, and that bird predation may play an important role in population regulation of O. brumata in boreal forests.

Keywords

Bird predation Boreal forests Bottom-up effects Plant–pathogen interactions Plant phenolics