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.