Kīpuka demonstrate how fragmentation affects forest communities
From our results, we find that kīpuka are, in fact, generally more sensitive to biological invasions than continuous forest. Incursions of non-native species into native forests is notoriously high on islands (Vitousek 1988), which may be associated with low phylogenetic and functional diversity (e.g. Sayol et al. 2021, Baiser et al. 2018). This effect is accentuated by fragmentation that, besides carving up native landscape, creates edge habitat within the forest patches, with both the matrix and edges associated with an increase in non-native species (As 1999). Such disturbance facilitates invasions of exotics by creating open ecological space (Meyer et al. 2021, Hobbs 2000). Previous work has suggested that the kīpuka system, being at high elevation and being contained within an otherwise in a relatively undisturbed forested area, is characterized by an arthropod fauna that is almost entirely native, with non-natives mainly affecting edge and lava areas (Vandergast & Gillespie 2004). However, we also found considerable proportions of non-native species in kīpuka cores.
In accordance with our second hypothesis, we observed a strong decline in diversity towards kīpuka edges in the present study. Many forest arthropods have narrow climatic niches (Lim et al. 2022), which appears to have limited them to climatically stable forest cores. Many forest taxa clearly avoided the open forest at kīpuka edges, which was instead occupied by higher abundances of the few edge-tolerant species. Additionally, we found significantly higher proportions of non-native taxa in edge than in core habitats, making kīpuka edge and core communities entirely distinct from one another.
In line with our third hypothesis, we found significantly more non-native taxa in the cores of smaller kīpuka. In fact, we found that small kīpuka below a size of about 5,000 m2 were profoundly impacted by non-native species and had very low species richness, even in their cores. This finding may either mean that the smaller a kīpuka was, the less resilient its core forest community appeared against incursion of non-natives, or alternatively that small kīpuka behave entirely as edge where non-natives adapted to disturbed conditions are better able to colonize and persist.