GRAZING IMPACT OF, AND INDIRECT INTERACTIONS BETWEEN MESOGRAZERS ASSOCIATED WITH KELP

GRAZING IMPACT OF, AND INDIRECT INTERACTIONS BETWEEN MESOGRAZERS ASSOCIATED WITH KELP

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GRAZING IMPACT OF, AND INDIRECT INTERACTIONS BETWEEN MESOGRAZERS ASSOCIATED WITH KELP ( LAMINARIA DIGITATA ) 1 Markus Molis 2 Biologische Anstalt Helgoland, Section Functional Ecology, Alfred-Wegener Institute for Marine and Polar Research, Kurpromenade 201, 27498 Helgoland, Germany Annekatrin Enge and Ulf Karsten University of Rostock, Albert-Einstein-Straße 3, 18059 Rostock, Germany Little is known about the indirect effects of nonle- thal grazing impacts in mesograzer–seaweed interac- tions. Using laboratory experiments, the effect of grazing by the seasonally abundant kelp-associated gastropod Lacuna vincta on subsequent kelp con- sumption by one kelp-associated ( Idotea granulosa ) and one nonassociated species of isopod ( I. emargi- nata ) was determined. Measurements of the tough- ness and elemental composition of different parts of the sporophyte of Laminaria digitata (Huds.) J. V. Lamour., as well as grazer-induced changes in the palatability of the blade, were conducted to explore possible mechanisms of indirect effects. In situ grazing pressure was the highest between July and September, with the blade being the preferred part of the kelp sporophyte, despite missing differ- ences in the elemental composition among kelp parts. The laboratory experiments supported our hypotheses in that kelp consumption by both species of isopods was lower on intact than on L. vincta damaged areas of the blade. This pattern was not caused by grazing-induced changes in blade palat- ability. Instead, the observed increase in isopod con- sumption following grazing by L. vincta resulted more likely from the combined effects of a reduc- tion in the toughness of L. vincta –damaged kelp blades and some unknown gastropod cue(s). These results suggest that kelp-associated and nonassociat- ed mesograzers may beneFt from the nonlethal graz- ing impact of L. vincta due to changes in physical traits of the seaweed. Thus, the nonlethal grazing impact by one species of mesograzer can positively modify the trophic interactions between kelp and other potential competitors, suggesting that the interactions among mesograzers might be more complex than previously assumed. Key index words: facilitation; indirect interaction; interaction modiFcation; Lacuna vincta ; Laminaria digitata ; seasonal patterns; seaweed; trophic inter- action Abbreviations: AC, actual consumption; EU, exper- imental unit Herbivores can strongly alter the ±tness and per- formance of algae and plants. There are numerous studies documenting, for instance, the impact of grazers on plant biomass and community structure (Cyr and Pace 1993, McQuaid 1996, Korpinen et al. 2008), feeding preferences of herbivores (Toth and Pavia 2002), or plant responses to herbivory (Cronin 2001). However, most studies assessed the magni- tude of lethal effects between one species of grazer and plant. Studies on pair-wise plant–animal interac- tions reveal useful information on, for example, the feeding biology of grazers. However, they lack real- ism as they are decoupled from the underlying com-
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GRAZING IMPACT OF, AND INDIRECT INTERACTIONS BETWEEN MESOGRAZERS ASSOCIATED WITH KELP

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