Reproductive significance of feeding on saprobic and arbuscular mycorrhizal fungi by the collembolan, Folsomia candida

1. Collembolans have often been credited with negatively affecting arbuscul ar mycorrhizal (AM) symbioses, mainly by grazing and severing the associate d external fungal network from host roots. However, most previous experimen ts were performed using relatively 'clean' systems where other, non-mycorrh izal, fungi were largely excluded. Yet, plant rhizospheres harbour a wide v ariety of highly palatable non-AM fungi, most of which have saprobic lifest yles. 2. In this study we isolated and cultured several rhizosphere fungi, and th e collembolan, Folsomia candida, from the Long-Term Mycorrhiza Research Sit e, University of Guelph, Canada, to test the hypothesis that, given a choic e, collembolans would prefer to feed on saprobic fungi and that such a choi ce is of adaptive significance to the animals. 3. A laboratory food preference experiment revealed that F. candida favours common saprobic fungi over a variety of AM fungi. Coincidentally, fecundit y levels across two Folsomia generations were higher when animals fed exclu sively on the preferred fungus, Alternaria alternata. When fed less palatab le fungi, fecundity was greatly reduced; in fact animals from the F1 genera tion were unable to produce any eggs when placed on an exclusive diet of on e of the following three AM fungi, Acaulospora spinosa, Scutellospora calos pora and Gigaspora gigantea. 4. These results indicate that a strict diet of AM fungi by collembolans ha s reproductive consequences. Therefore, we propose that under natural condi tions these animals spend more time feeding on common saprobic fungi rather than their AM counterparts. This suggests that previous 'clean' studies th at investigated the interactions between collembolans and AM fungi may have reported exaggerated effects of animal grazing. The influence of collembol ans on the functioning of AM symbioses, under more natural conditions, rema ins not well understood.