Sampling
We sampled the arthropod community in continuous and fragmented wet
forest communities at different locations across the slopes of the Mauna
Loa volcano on the Big Island of Hawaiʻi (Fig. 1). Sites were at
approximately the same elevation, with similar precipitation, and all
were dominated by the canopy tree Metrosideros polymorpha .
Undergrowth was dominated by Cybotium glaucum fern and other
native plants, with minimal impact of invasive plant species. We took
GPS coordinates from each site and acquired data for geographic
distances between sites. Kīpuka areas were measured in GIS by
calculating the area of polygons manually drawn around kīpuka margins in
satellite images of the study landscape.
The two continuous forest areas were sampled in August 2019. On the
windward side of the mountain, we sampled 12 sites in the Waiakea Forest
Reserve, hereafter referred to as Stainback, from 876 – 1,494 m a.s.l..
The Stainback sites receive 2,767 – 6,126 mm of average annual
precipitation, and their annual temperatures range from 13.45 – 16.84
°C. The sites were situated 333–11,001 m from one another, with an
average distance of 5,437 m. On the leeward side of Moana Loa, we
sampled 10 sites from 1,058 – 1,455 m a.s.l. in the Kona Hema Forest
Reserve. Kona Hema sites receive 803-1,005 mm of average annual
precipitation, with annual temperature ranging from 14.21 – 16.74 °C.
These sites were 349 – 3,256 m away from one another, with their
average distance being 1,652 m.
In addition, we sampled 13 kīpuka on Mauna Loa, which resulted from a
volcanic eruption in 1855. The kīpuka system is a mid-elevation area in
a relatively remote area of the island, in which the forest is almost
entirely native (Wagner 1999). Forest fragments, previously connected by
continuous forest (Lockwood et al. 1988), were isolated from one another
by an 1855 lava flow from Mauna Loa Volcano (Fig. 1; map coordinates: N
19°37’40” and W 155°21’15”). Elevation at this area ranges from 1,524
– 1,646 m, annual temperature from 12.00 – 13.25 °C, and annual
precipitation from 2,279 – 2,765 mm. The position of the kīpuka in a
very similar climate belt controls for the effects of climate and makes
this an ideal system to study the effect of geographic isolation on
communities. The forested area of the sampled kīpuka ranged from 555 –
100,081 m2, and the average size was 23,447
m2. The distance between kīpuka ranged from 49 m –
4,744 m, and the average distance was 1,766 m.
To compare arthropod communities in the dense forest in the kīpuka
center with the more open edges, we sampled them separately. At each
kīpuka, an edge sample was collected approximately 10 m into the
forested area. The kīpuka are surrounded by mostly continuous lava,
which is very hot and dry compared to the forest vegetation in the
kīpuka. To explore which arthropods can easily cross this hostile
habitat, we collected at four sites in the lava matrix surrounding the
sampled kīpuka, in which vegetation of stunted and isolatedMetrosideros polymorpha trees was found.
Arthropods were collected from each site using beat sampling, according
to the approach described in Lim et al. 2022. Beat sampling of all
undergrowth vegetation was performed in a 10 m radius around the
centroid of the forest plot for two person hours. All arthropod
specimens were collected from the beat sheets, stored in 99 % ethanol,
and transported back to University of Trier in Germany.