THE UTILIZATION OF NITROGEN FROM INSECT CAPTURE BY DIFFERENT GROWTH FORMS OF DROSERA FROM SOUTHWEST AUSTRALIA

Plants of Dorsera species, neighbouring non-carnivorous plants, and arthropods on or near each Drosera sp. were collected at 11 contrasting habitat locations in SW Australia. At three of the sites clones of the rare glandless mutant form of D. erythrorhiza were collected alongside fully glandular counterparts. The delta-N-15 value (N-15/N-14 natural isotope composition) of insect-free leaf and stem fractions was measured, and the data then used to estimate proportional dependence on insect N (%N(dI)) for the respective species and growth forms of Drosera. The data indicated lower %N(dI) values for rosette than for self-supporting erect or for climbing vine species. The latter two groups showed an average %N(dI) value close to 50%. The %N(dI) increased with length and biomass of climbing but not erect forms of Dorsera. delta-N-15 values of stems were positively correlated with corresponding values for leaves of Drosera. Leaf material was on average significantly more N-15 enriched than stems, possibly due to delayed transport of recent insect-derived N, or to discrimination against N-15 in transfer from leaf to the rest of the plant. The comparison of delta-N-15 values of insects and arthropod prey, glandless and glandular plants of D. erythrorhiza indicated %N(dI) values of 14.3, 12.2 and 32.2 at the respective sites, while matching comparisons based on delta-N-15 of insect, reference plants and glandular plants proved less definitive, with only one site recording a positive %N(dI) (value of 10.4%) despite evidence at all sites of feeding on insects by the glandular plants. The use of the delta-N-15 technique for studying nutrition of carnivorous species and the ecological significance of insect feeding of different growth forms of Drosera growing in a large range of habitats is discussed.