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.