Beta diversity and kīpuka size
NMDS plots of zOTUs and OTUs showed a clear separation of the arthropod
community composition in our different sites (Fig. 4A & C). The first
axis separated the continuous forests from kīpuka, while the two
continuous forests in Stainback and Kona Hema were well-separated by the
second axis. The NMDS plots also showed a separation of kīpuka center
and edge habitats and barren lava. The differentiation of center and
edge habitat was particularly pronounced in larger kīpuka. While the
five smallest kīpuka (area 555 – 4,212 m2) grouped
closer to the edge samples, the remaining eight (area 5,458 – 100,081
m2) were clearly distinct and formed a separate
cluster. In comparison to sites in each continuous forest area, kīpuka
were considerably more dispersed in the NMDS plots. Moreover, sites in
kīpuka centers grouped closer to continuous forest sites than to the
edge sites, suggesting that kīpuka center communities are more similar
to continuous forest communities. While zOTU and 3 % radius OTU based
NMDS supported similar patterns of differentiation, the differentiation
between the edges and centers of large kīpuka was more pronounced for
the 3 % radius OTU dataset.
The separation of different areas shown in the NMDS plot was also
supported by patterns of beta diversity for zOTUs and 3 % radius OTUs
(Fig. 4B & D). With a notable exception of sites in barren lava and the
center of kīpuka, which showed the highest dissimilarity, the average
within-area dissimilarity (average zOTU|OTU beta Stainback:
0.57|0.55; Kona Hema: 0.60|0.58; kīpuka center:
0.78|0.76; kīpuka edges: 0.69|0.64; Lava:
0.76|0.72) was significantly lower than between-area
comparisons (average zOTU|OTU beta Stainback-Kona:
0.85|0.79; Stainback-center: 0.86|0.83;
Stainback-edge: 0.85|0.83; Stainback-lava: 0.92|0.91;
Kona-center: 0.92|0.88; Kona-edge: 0.91|0.88;
Kona-lava: 0.95|0.94; center-edge: 0.79|0.76;
center-lava: 0.86|0.84; edge-lava: 0.76|0.70).
Within-area dissimilarity in continuous forest was significantly lower
than in kīpuka habitats. This held true particularly for kīpuka centers,
which showed a higher within-area differentiation than edges. Hence, the
edges of different kīpuka were more connected than their corresponding
centers. These patterns held true for zOTUs and 3 % radius OTUs, but
the difference between kīpuka and continuous forest areas was slightly
higher for zOTUs. The higher within-area differentiation of kīpuka
centers compared to continuous forest and kīpuka edge held up even when
only the eight largest kīpuka, which formed a separate cluster closer to
continuous forest on the NMDS plot, were compared to each other (average
zOTU|OTU beta Stainback: 0.59|0.55; Kona Hema:
0.61|0.58; kīpuka Center: 0.79|0.75; kīpuka Edges:
0.71|0.66; Lava: 0.79|0.72).
Centers and edges of the same kīpuka harbored distinct arthropod
communities, particularly for larger kīpuka (> 5,000
m2; Suppl. Fig. 4). In fact, the dissimilarity between
the center and edge of each kīpuka showed a pronounced association with
the area of that kīpuka. Community dissimilarity between centers and
edges steadily increased with kīpuka size for small kīpuka, but began
saturating for kīpuka larger than 5,000 m2.
Our previous NMDS analysis revealed two separate communities in kīpuka
centers: kīpuka so small they behaved more like kīpuka edges
(< 5,000 m2) and larger kīpuka with core
communities more similar to continuous forest communities
(> 5,000 m2). To explore the association
of beta diversity with distance between sites, we thus generated two
kīpuka datasets. The first dataset included all kīpuka, the second
consisted of only the eight largest kīpuka (5,458 – 100,081
m2), which formed a well-separated cluster from edge
habitats in our NMDS plots, and the third one consisted of the five
smallest kīpuka (555 – 4,212 m2). An analysis of
within-area beta diversity for these datasets showed a significantly
higher community dissimilarity between centers of large kīpuka than
those of small kīpuka. At the same time, no significant difference of
community dissimilarity for edges of large and small kīpuka was found.
We next analyzed patterns of community isolation by distance for
continuous forests, kīpuka centers and edges. The continuous forests
both showed a significant and quite similar linear association of
logarithmic geographic distance and community dissimilarity (Fig. 5;
r2Kona Hema= 0.16;
r2Stainback= 0.74; LM p <
0.05). We also found a positive linear association of beta diversity and
logarithmic geographic distance for the kīpuka sites (Fig. 5;
r2center = 0.12;
r2edge = 0.30; LM p < 0.05).
Compared to continuous forest, the overall dissimilarity was much higher
for kīpuka, even at short geographic distances. This pattern was
particularly strong for kīpuka centers, while kīpuka edge habitats
appeared to be more connected. Community dissimilarity between kīpuka
centers saturated at very low geographic distances. The community
differentiation between kīpuka centers isolated by only a few 100 m of
barren lava was comparable to the differentiation between continuous
forest sites isolated by over 10 km. These patterns also held up when
different arthropod orders were analyzed separately (Suppl. Fig. 5 &
6). Nearly all orders showed a saturated or nearly saturated beta
diversity with distance for kīpuka centers, while an association with
geographic distance was found for edge habitat. Diptera was the only
order for which both center and edge showed similar patterns of
isolation by distance.