Effect of food availability and habitat on the distribution of platypus (Ornithorhynchus anatinus) foraging activity

Abstract

While the diet of the platypus (Ornithorhynchus anatinus) is known to include a wide range of benthic macro-invertebrates, little is known about how the animals distribute their feeding behaviour along a waterway. This radio-tracking study aimed to determine whether the spatial distribution of platypus foraging activity varied in relation to habitat attributes and/or the amount of food present along a southern Victorian stream. Five O. anatinus were tracked nocturnally for a total of nearly 95 h in March and April 2000. Attributes of the channel and bank were described both in the area used by the animals and adjoining unused sections of stream, and macro-invertebrates were sampled quantitatively in six substrates that varied in abundance in the used and unused areas. Eleven habitat variables showed a significant positive relationship with the occurrence of platypus activity (including the number of medium and large Eucalyptus, Acacia and Populus trees growing along the bank; presence of gravel, pebbles, cobbles, large rocks and coarse particulate organic matter in the channel substrate; amount of riffle habitat; amount of large woody debris in the channel; and amount of undercut banks). Six habitat variables showed a significant negative relationship with platypus activity (including the number of medium and large Salix trees growing along the bank; the presence of silt, solid clay and Salix roots in the channel substrate; the amount of pool habitat; and the maximum channel depth). Substrates that were relatively abundant in the area used by O. anatinus (gravel/pebbles, Eucalyptus litter, Populus litter) did not consistently support more macro-invertebrates than substrates that dominated the unused area (silt, Salix roots, Salix litter). This may reflect the fact that the platypus population was not food-limited (and hence not under pressure to optimise foraging) at the time of the study, that platypus feeding patterns were influenced by habitat features that were not considered in relation to invertebrate abundance, or that the choice of foraging areas by O. anatinus may be influenced by factors other than the total abundance of food items, e.g. the nutritional quality and/or accessibility of prey.