Mechanisms of Competition Among Insectivorous Mammals

This study investigates the mechanisms of competition between congeneric pairs of insectivorous mammals in two communities in Australia and England. Direct field observations showed that physical interactions between species do not occur, whereas conspecific encounters are frequent. In field enclosures the smaller, subordinate species in each community (Antechinus stuartii: Marsupialia: Sorex minutus: Eutheria) remained alert in the presence of the dominant species (A. swainsonii, S. araneus), and moved quickly away when the latter approached. The rate of prey capture by subordinate individuals also increased immediately after removal of the dominants. Hourly removals of some individuals of the dominant species in each community over 24 h produced hourly increases in the numbers of subordinate individuals that were captured. The rapidity of these responses suggests strongly that the dominant insectivores in each community interfered with the resource use of the subordinate species. Biomass of invertebrates increased inconsistently or slowly within 3 months of removal of the dominant insectivores; hence the rapid responses by subordinate individuals in experiments were not due to simple exploitation or tracking of resource levels. The subordinate insectivores probably detected and avoided contact with dominants instantaneously using auditory or olfactory cues; reciprocal avoidance of congeneric odours was demonstrated using odour-scented traps. Insectivorous mammals may usually compete by interference (or encounter competition, sensu Schoener 1983). For dominant species within communities the cost of interference is minimal and the benefit of gaining exclusive access to resource-rich microhabitats is high. Conversely for subordinate species the benefit of temporarily exploiting the same rich microhabitats may exceed the small costs of vigilance and movement to nearby refugia.