<p>Foundation species play essential roles in their ecosystems owing to their high abundance and importance as a reliable resource for affiliated organisms. Despite their historical abundance, many foundation species are declining and effectively fragmenting into islands within a landscape. Yet we know little about how fragmentation of previously contiguous foundation species influences the assemblages of affiliated organisms. Here, we aim to understand how size and isolation of fragmented patches of a foundation tree species, mulga <i>(Acacia aneura</i>), influences the populations of affiliated insects. We examined whether the assemblages of insects on mulga: (a) differed from those on associated (non-mulga) plants; and (b) were related to mulga stand attributes (stand size, canopy cover, ground cover and stand isolation). We found that the insect assemblages on mulga differed from those on associated plants, with a much greater proportion of psyllids and much smaller proportion of heteropterans. Species richness and Shannon Diversity increased with increasing stand size and decreased with increasing isolation distance from other stands, consistent with island biogeography predictions. Overall insect abundance increased with mulga stand size or decreased with isolation depending on the major taxonomic group considered. Moreover, insect assemblages varied across stand sizes, with large variation among small stands, indicating that a variety of mulga-affiliated insect species subsist, at least temporarily, within very small stands of trees. However, these patterns were inextricably linked to the increased number of trees sampled in larger stands. Our findings suggest that fragmentation of mulga stands negatively affects the richness and diversity of insect taxa affiliated with mulga, highlighting the importance of the conservation of contiguous stands of mulga trees.</p>