Comparison of Environmental DNA and Bulk DNA Metabarcoding for Assessing Terrestrial Arthropod Diversity Across Three Habitat Types on Guam

DNA-based methods offer a rapid and cost-effective way for detecting species occurrence and monitoring biodiversity; among them, bulk DNA metabarcoding is well-established, and recently developed environmental DNA (eDNA)-based methods offer a non-lethal alternative. With a goal to develop suitable methods for assessing insect biodiversity for understudied island ecosystems where DNA reference libraries are incomplete, we compared established bulk DNA metabarcoding methods with eDNA across three replicated terrestrial ecosystem types (degraded forest, limestone forest, and grassland) on the island of Guam. Using two mitochondrial COI primer pairs, we performed bulk DNA metabarcoding of standard entomological collection methods (Malaise traps, pan traps, and vegetation beating), and compared the assessment of biodiversity with that from different eDNA sources (flowers, leaves, tree trunks, and spider webs). In our samples, eDNA and bulk DNA metabarcoding both detected a large proportion of overall taxa (OTUs, 86.6% and 60.3%, respectively). Although bulk DNA metabarcoding detected significantly more taxa, eDNA proved to be a reasonable non-lethal alternative. As expected, because of limitations in existing reference databases for understudied systems, species-level identification was achieved for only a few OTUs. Overall, the sampling approach was the dominant driver of arthropod diversity, explaining ~17% of the observed variation, while habitat type accounted for ~4%. Thus, each sampling approach captured some unique diversity and contributed to the complementary effect of maximizing detection. For rapid biodiversity surveys of terrestrial arthropods, we recommend integrating metabarcoding approaches, and in sensitive ecosystems where specimen capture is undesirable, eDNA offers a powerful non-lethal alternative to monitor diversity and community change.