Globally, human modification of land and the human-aided introduction of exotic species are considered to be the main drivers behind species decline and extinction. Human colonisation of New Zealand and subsequent development resulted in the conversion of native forests and shrublands into productive land, which together with the introduction of invasive species has caused the decline of many species, and many more are now threatened with extinction. This issue is particularly prevalent in the lowland eastern areas of the South Island, in which there has been intensive land development. One strongly affected group are species within the genus Olearia, in which several species are threatened with extinction, including nationally declining Olearia lineata. In contrast, other species in this genus, such as non-threatened O. bullata, are seemingly not as strongly impacted. My study investigated the mechanisms behind why O. lineata is nationally declining in comparison to the non-threatened congener O. bullata. To do this, I investigated two main research questions; one question investigated the demographic structure of populations to determine if regeneration is occurring and the vegetation composition of the surrounding community. Sampled populations indicated regeneration failure in O. lineata populations but also in O. bullata populations, despite its non-threatened status. I found that a high proportion communities contained exotic grasses, therefore, the second question was how the presence of the exotic grass- Agrostis capillaris, affected recruitment of O. lineata and O. bullata. Two glasshouse experiments investigated how the presence or absence of grass affected germination and growth of seedlings. Both Olearia species were found to germinate in the presence of grass, but seedlings grew significantly better in the absence of grass. Olearia are also important native Lepidoptera hosts as a suite of native moths feed exclusively on this genus. Therefore, a third research question investigated the abundance of larvae and the community composition of Lepidoptera on O. lineata and O. bullata in comparison to another Lepidoptera host Coprosma propinqua. Moth larvae were collected and some species successfully reared to find there was higher larvae abundance on O. lineata and high species overlap between O. lineata and O. bullata with little species overlap between the two Olearia species and C. propinqua. Overall, my research found that exotic invasive grasses potentially cause regeneration failure in both O. bullata and O. lineata. However, the largest impact in the difference of threat status is due to O. lineata populations occurring in the most developed lowland areas of the eastern South Island, whereas O. bullata populations occur in less human modified, higher altitude areas. The further development of these areas through human activity threatens the future persistence of these Olearia species and the future of the Olearia Lepidoptera specialists.