Collembolan trophic preferences determined using fatty acid distributions and compound-specific stable carbon isotope values

The trophic preferences of soil invertebrates such as Collembola are often determined by the analysis of gut contents, or through visual observations of the location of individuals. As an alternative approach, two species of Collembola, Folsomia candida and Proisotoma minuta, were offered a choice of the soil fungus Cladosporium cladosporioides or the bacterial feeding nematode Panagrellus redivivus; each exhibited distinct fatty acid profiles and stable carbon isotopic compositions. Over 21 days, the fatty acids i15:0, i17:0, 18:1(n-7) and 18:2(n-6) all increased in abundance in both collembolan species consistent with direct routing from the nematode dietary choice which contained a high concentration of these components. Collembolan fatty acid δ13C values increased by between 5.7 and 21.6‰ over 21 days reflecting those of the nematode diet. Therefore, both fatty acid profiles and δ13C values were consistent with a strong feeding preference of F. candida and P. minuta for the nematodes over the offered fungi. In fact, neither collembolan species consumed any detectable amount of C. cladosporioides. Comparison of the δ13C values of the 16:0 and 18:0 fatty acids (which are biosynthesised by the Collembola as well as directly incorporated from the diet) and the 16:1(n-7) and 18:2(n-6) components (which are not biosynthesised by the Collembola) demonstrated that the input of distinct pools of C can lead to large shifts in δ13C values between diet and consumer. The fatty acids that were not biosynthesised by Collembola better reflected the δ13C values of the diet helping to differentiate between biosynthesised and directly incorporated compounds; an important prerequisite in the interpretation of compound-specific δ13C values in trophic behaviour tests. The combination of fatty acid distributions and δ13C values is a significant improvement on traditional methods of examining feeding preferences, since it determines directly the assimilated dietary carbon rather than relying on indirect observations, such as the proximity of individuals to a defined food source.