Comparison of two techniques for assessing possum (Trichosurus vulpecula) diet from stomach contents

Two techniques for assessing possum (Trichosurus vulpecula Kerr) diet from stomach contents ("point-sampling" and "layer-separation") are described an d compared. Point-sampling involves sieving stomach contents, systematicall y selecting fragments from the retained material then, identifying and weig hing these. Layer-separation involves separation, identification, and weigh ing of the discrete layers apparent in most possum stomach contents. In 41 of 43 stomachs examined, we were able to separate discrete layers that near ly always comprised a single food item. To compare the two techniques both were applied to these 41 stomachs, with the point-sampling technique applie d as two separate treatments using 1.4-mm and 2.0-mm sieves. There were maj or differences in diet composition estimates between layer-separation and p oint-sampling but with few differences between the two point-sampling treat ments. Relative to layer-separation, point-sampling underestimated the prop ortions of food groups with small average fragment size and overestimated t hose with large fragment size. However, both techniques gave similar freque ncies of occurrence for 8 of 10 food groups tested, although the apparent i mportance of foods based on ranking by frequency of occurrence did not accu rately match the ranking based on percent composition data. Identification of material was usually easier and more complete with layer-separation than with point-sampling (i.e., there were virtually no unidentifiable stems an d fibre after layer-separation). Layer-separation therefore appears likely to provide a simple technique for diet assessment in possums. Although the technique requires formal validation the existence of layers shows that the re can have been little mixing (or digestion) of stomach contents, and ther efore, that the layer-separation estimates cannot differ greatly from what was eaten. Techniques that involve sieving possum stomach contents appear t o have serious limitations, but may be useful as a last resort when layers contain a mixture of foods, or for stomachs in which the layers are not dis tinguishable.