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Vertebrate Herbivores as Predators of Insect Herbivores: An Asymmetrical Interaction Mediated by Size Differences

R. Zamora and J. M. Gómez
Oikos
Vol. 66, No. 2 (Mar., 1993), pp. 223-228
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3544808
Stable URL: http://www.jstor.org/stable/3544808
Page Count: 6
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Vertebrate Herbivores as Predators of Insect Herbivores: An Asymmetrical Interaction Mediated by Size Differences
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Abstract

We have analysed the effect of wild goat (Capra pyrenaica) on the reproductive success of a chalcid gall maker (Systasis encyrtoides), both species being herbivores of Hormathophylla spinosa, a stunted mediterranean high-mountain shrub. The gall's position on H. spinosa is very specific, reflected in the fact that the gall maker attaches principally to the basal floral buds of the inflorescence, and in the lower sector of the plant. We labelled inflorescences with and without galls in order to quantify goat herbivory and gall predation. The goat feeds both on plants and infructescences with or without galls, resulting in gall destruction and larval death (phenotypic selection). The browsing behaviour of goat was influenced by the general stunted growth of the plant, as well as by the abundance of spiny stems. This fact allows chalcids to escape goat herbivory on a very fine spatial scale. Gall survival strongly depends on gall position in the inflorescence, being greater in the lower position. Furthermore, galls situated in the lower sector of the plant have a greater survival chance than galls situated in the upper sector. The goat acted as a true predator of the gall maker wasp, becoming a potential selective agent conditioning the reproductive success of Systasis. The relationship between goat and gall-maker is an asymmetrical hierarchy mediated by size differences, the bigger one conditions the smaller one, but the reverse does not occur. We propose that under low herbivore pressure, the gall makers (and similar invertebrate sessile forms) will show a spatial oviposition preference which is less selective that under strong herbivore pressure.

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