The requirement for space in marine hard bottom assemblages is paramount for life. Due to the crowded nature and demand for space in these sessile assemblages, bare space that is freed is quickly occupied. The intense push and pull for space within these assemblages is represented by heavy overgrowth interactions. These competitive interactions often result in the displacement of native species and the formation of competitive hierarchies amongst species, with some individuals settling as epibionts and causing the mortality or stress of those beneath them. Ascidian invasions have become more frequently reported on a global scale, becoming an emerging issue on many coastlines. Colonial ascidians (the key phylogenetic class of this study) and their ability to dominate and occupy vast amounts of space, has landed them their reputation as notorious marine invaders. With urbanization comes the general increase in anthropogenic activity, which in turn has been known to coincide with the increase in the translocations of nonindigenous species. Vectors such as interoceanic trade and travel have contributed heavily to the spread of nonindigenous ascidians, allowing them to overcome geographic barriers. Unmanaged populations of nonindigenous colonial ascidians heavily foul much of the submerged substrate in the Tauranga Harbour, threatening the biodiversity, population structure and function of native communities as well as fouling of marina and port substrates (wharves, pylons, ropes, boat hulls). The framework of this research infers that the competitive abilities of non-indigenous colonial ascidians through their rapid occupation of substrate is a key determinant in their invasion success. Ex-situ manipulative experiments were used to examine potential competition (epibiotic or bare space settlement) recruiting species may have on nonindigenous and native ascidians. Simultaneously, we examined if the status (nonindigenous or native) of the test species impacted the settlement of the nonindigenous and native recruiting species. Few studies have attempted the novel process of rearing ascidian cultures ex-situ, where manipulation and control can be maximized. We attempted to develop a robust ascidian culture system to better study ascidian species. The lines of evidence provided by these experiments revealed that nonindigenous colonial ascidians are often opportunistic settlers and can largely determine the dynamics of native sessile communities, as they settled the most as both epibionts and bare space recruits on the experimental plates in this study. Experimental results found most recruits to prefer settling on bare substrate than on the surfaces of other organisms, supportive of the concept that surface microtopography and secondary metabolite release may play a role in recruit settlement. Findings bring focus to the unrivalled ability of nonindigenous species to exert settlement pressures on existing sessile communities, illustrated by their competitive power. Introduced species’ ability to settle heavily as both basibionts and epibionts allows them to litter submerged substrates with larvae, illustrative of high propagule pressures found in this study. Experimental results offer managers to better utilise biosecurity management resources when dealing with nonindigenous incursions in the future.