1 We studied the establishment of tree seedlings in Mediterranean-type old fields in South Australia in different biotic environments and under different levels of resource availability. Specifically we wanted to: (a) test for a logarithmic relationship between the relative intensity of competition (RCI) and resource availability; (b) assess the potential of confounding resource competition with invertebrate herbivory; and (c) assess whether the architecture of the plant community had any qualitative or quantitative effects upon the relationship between resource availability and RCI. 2 Our glasshouse experiment showed that RCI increased with resource availability at low levels of resources, but not at higher levels, consistent with a logarithmic relationship. 3 The effects of resource competition and invertebrate herbivory were heavily confounded in the field experiment. 4 Plant architecture significantly affected the behaviour and abundance of invertebrates and we therefore conclude that it has the potential to modify the relationship between resource availability and competitive intensity. 5 Although the habitat templet/C-S-R model appears reasonably robust, modification of its general framework may be required because one of its basic underlying assumptions is most accurate when competition is defined phenomenologically. © 2005 British Ecological Society.