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Empirical test of specific predictions made from a spatial model of the population dynamics of Maculinea rebeli, a parasitic Butterfly of Red Ant colonies

Auteurs : Elmes (Graham Wakely), Clarke (Ralph T.), Hochberg (Michael E.) et Thomas (Jeremy A.)

Année de publication : 1996
Publication : Acta œcologica. Œcologia applicata
Volume : 17
Fascicule : 1
Pagination : 61-80

Résumé :

We test the ability of a complex spatial model to predict the size of M. rebeli populations in two mountain ranges. The published model was based on nineteen parameters that described the population dynamic interactions between the butterfly, its initial food plant Gentiana cruciata, its subsequent host Myrmica schencki, and other competing non-host species of Myrmica. All original model parameters were measured on one site in the Spanish Pyrenees. In this study we measured seven of these parameters, concerning gentians and their habitat, at 12 new sites in the French Alps and at another in Spain. We show that model predictions using these values explained 86% of the variation in M. rebeli egg numbers, independently measured on the other sites. Three other (non-intuitive) model predictions were also partly confirmed: (i) A. priori, our model (which had been parameterised from a single, atypical site with very high gentian densities) predicted a close, indirect, non-linear relationship between gentian and butterfly densities, with egg-numbers being negatively correlated at high gentian densities and positively correlated over low densities. The prediction would be unusual for phytophagous butterflies and is counter-intuitive for a species which experiences only moderate competition on its food plant before spending most of its life inside ant nests, where it is regulated by strong, density-dependent competition. Although this was not fully tested due to a lack of new sites supporting high gentian densities, the empirical results showed a very close positive relationship between egg numbers and gentian densities on sites supporting lt 1200 gentians per hectare. Furthermore, our original mechanistic model also accurately predicted both the slope and the range of the positive relationship found at low densities. (ii) Annual fluctuations in populations of M. rebeli are exceptionally small