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Effects of Food Availability, Nutritional Value, and Alkaloids on Food Choice in the Generalist Herbivore Arianta arbustorum (Gastropoda: Helicidae)

Bernhard Speiser and Martine Rowell-Rahier
Oikos
Vol. 62, No. 3 (Dec., 1991), pp. 306-318
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3545495
Stable URL: http://www.jstor.org/stable/3545495
Page Count: 13
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Effects of Food Availability, Nutritional Value, and Alkaloids on Food Choice in the Generalist Herbivore Arianta arbustorum (Gastropoda: Helicidae)
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Abstract

The plant Adenostyles alliariae (Asteraceae) produces pyrrolizidine alkaloids (PA). These are highly toxic to mammals, but the effects on invertebrates are poorly known. Since the production of secondary compounds imposes costs on plants, they should only be produced if they provide increased protection from herbivores. To test this hypothesis, we investigated the relationship between PA and feeding preference by a naturally occurring generalist herbivore, the snail Arianta arbustorum. By examining microscopically the faeces of field-collected snails, we determined what they had recently consumed. The snails' diet is very broad and it includes arthropods, wilted flowers, fresh and decayed plant material, leaf litter, and soil. We identified the species of herbaceous plants in A. arbustorum's diet. The most important food items were Adenostyles alliariae, Rubus sp., leaf litter, Festuca altissima and Stachys silvatica. The proportion in the snail faeces of some of these food items changes drastically during the season, with A. alliariae increasing and S. silvatica and leaf litter decreasing with time. We tested whether the snail consumption of the herbaceous plant species was related to their availibility per unit area in the field or their nutritional value (water and nitrogen content), relative to that of other plant species. Availability of the different food items is significantly correlated with the amounts eaten. Water and nitrogen content are correlated with each other, but not with the amounts of a plant eaten by the snails. For the most frequently eaten plant species, the seasonal changes in availability and nutritional value within each species were analyzed; they were not consistent with the feeding preferences of the snails. The PA content of Adenostyles alliariae leaves decreases strongly over the season, while consumption of A. alliariae by the snails increases. Herbivore damage to the leaves of A. alliariae in the field increases similarly over the season. In feeding experiments, the consumption of A. alliariae relative to the consumption of lettuce was also lower in May than in September. We present evidence that A. arbustorum eats A. alliariae regularly. The negative correlation between snail feeding and PA content of A. alliariae strongly suggests that Arianta is deterred by plant PA. This snail, along with other herbivores, could provide a selection pressure on the plant favouring the production of PA.

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