[Seminar Blog] Studying Biodiversity Changes in Hawaiian Arthropods over Time using DNA Metabarcoding

written by: ​Angela Saenz and Eva Perry
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Islands have been the backdrop of considerable scientific research and advancement for centuries, and not just because they tend to double as a nice vacation spot. What makes many islands so special to science and scientists is their isolation from other land masses, limiting the movement of species to and from them. This isolation provides a rare open-air opportunity to study how evolutionary processes shape ecological communities: think Darwin’s finches and the Anolis lizards of the Caribbean, or, in Dr. Natalie Graham’s case, arthropods on the Hawaiian archipelago. 

Dr. Natalie Graham, an Assistant Professor of Biology at the University of Hawai'i in Hilo, presented her research on the importance of ecological and evolutionary dynamics in understanding how biodiversity accumulates through time at the University of Maryland (UMD) Entomology Department’s weekly Colloquium seminar. She explained that interactions across space and time shape biodiversity, and while models have been developed to infer the dynamic processes shaping the evolution of communities, empirical data for understanding the interplay of ecology and evolution over long temporal scales are almost entirely lacking.

The Hawaiian archipelago provides an excellent backdrop for this research because each island is the result of the Pacific tectonic plate moving over a stationary “hot spot”. This means that each island began as a blank slate and represents a progressively longer amount of time that ecological and evolutionary processes have been able to shape present-day communities: younger islands tend to have communities with simpler networks and more generalized interactions, while older islands have more established, complex, and specialized interactions. 

Studying ecological genetics on islands with different geological development times has the potential to increase our understanding of interactions between species, and how they have changed over time. Plant-arthropod interactions are an excellent model for this type of research because of the tremendous diversity in species and life histories, meaning we can find many more species and more specialized relationships between arthropods and plants (Fig. 1). To read more about ecological networks of arthropods and plants over the Hawaiian Island chronosequence check out Dr. Graham’s published work in Molecular Ecology at DOI: 10.1111/mec.16873.

Figure 1: As communities assemble over time, species will be added through ecological and evolutionary processes. Network size will increase over time and there will be a trend towards greater specialization. Recently introduced species (i.e., non-natives) evolved elsewhere would not have adapted to biotic and abiotic factors, thus limiting their specialization within communities at all stages of development. Modified from Graham et al. (2023).

Genomic data is among the most important methods to measure biodiversity. For many arthropods, relatedness between specimens can be difficult to discern by traditional identification methods. For example, Dr. Graham can make use of high-throughput sequencing and DNA metabarcoding, a method to identify species in bulk collections of specimens using short segments of DNA. This result can be paired with plant morphological or genetic species identifications to find evidence of species biotic interactions.  Using DNA sequencing in this manner can provide important contextual information that can be combined with remote sensing information that can help with assessing the invasiveness or invasion potential of a species. Dr. Graham emphasized that including morphological taxonomy and DNA barcoding of museum vouchers should be used in concert with DNA metabarcoding to verify results and to avoid under- or over-inflated biodiversity predictions from molecular methods alone.
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Dr. Graham used DNA metabarcoding libraries from arthropod-plant communities representing 50 years to 4.15 million years of ecosystem development to examine the patterns of biodiversity over evolutionary time. She found that alpha diversity (diversity describing a single community) increases over time linearly for most arthropod groups (Fig. 2a), and in communities that are more than 300 years old beta diversity (the measure of how dissimilar communities are to one another) increases linearly over time as well. In contrast, in areas of the island that were affected by the 1973 Kilauea volcano eruption the community was experiencing fast and dynamic changes to its composition, and beta diversity was unexpectedly high at sites younger than 300 years (Fig. 2b). Interestingly, Dr. Graham also found that although alpha diversity increases over time there appears to be a limit on how many species can occupy a space through time. She shares the example of Maui, where a peak of biodiversity was reached and then declined on the older islands of Molokai and Kauai for many arthropod groups (Fig. 2a). She explains that the peak diversity might be different for each insect order and that different conditions aside from substrate age, such as these species evolutionary history, could influence it.

Figure 2a: Alpha diversity increases as age of substrate increases for most insect groups. Shown here are three insect orders (Diptera, Hemiptera, and Lepidoptera) sampled from six major locations that follow this trend. Note that for Hemiptera and Lepidoptera, we see a “peak” in alpha diversity on the island of Maui that then decreases for the older islands.

Figure 2b: Beta diversity also increases over time, with two notable exceptions: the two youngest sites, indicated with arrows.

Predicting changes in biodiversity through space and time can help inform us of a community’s resilience. Dr. Graham explains that rapid change can indicate when a system is out of balance. Imbalance happens a lot faster when introduced species accumulate compared to communities with mostly native species that are adapting to their environment. She uses the example of a non-native Ambrosia beetle (Xyleborus ferrugineus), which carries a very destructive tree-colonizing fungus (Roy et al., 2019) that is spreading very rapidly within the Puna district of Hawai'i, and of non-native ants which have a far larger impact on younger islands than on the oldest ones.

Multiple groups in Hawai’i, ranging from native communities to stakeholders and the general public, can benefit from genetic biodiversity research. The Paoakalani Declaration, a document describing the rights of native Hawaiian traditional knowledge holders, recognizes the cultural significance of endemic species to the native peoples of Hawai’i (Lindsey 2011; Hutchins et al., 2023). With this in mind, Dr. Graham’s lab co-develops participatory research with community partnerships to ensure data sovereignty in accordance with the  CARE Principles for Indigenous Data Governance. This is important for creating opportunities to involve communities in the stewardship of their natural systems, which can improve awareness and understanding of vulnerable species/ecosystems without harming community access to scientific knowledge through more historically common extraction-based data collection methods.
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Cited literature:
Hutchins, L., Mc Cartney, A., Graham, N., Gillespie, R., & Guzman, A. (2023). Arthropods are kin: Operationalizing Indigenous data sovereignty to respectfully utilize genomic data from Indigenous lands. Molecular Ecology Resources.
Graham, N.R., Krehenwinkel, H., Lim, J.Y., Staniczenko, P., Callaghan, J., Andersen, J.C., Gruner, D.S. and Gillespie, R.G., (2023). Ecological network structure in response to community assembly processes over evolutionary time. Molecular Ecology.
Lindsey, R.H. (2011). Responsibility with Accountability: The Birth of a Strategy to Protect Kanaka Maoli Traditional Knowledge. Harvard Law Journal, 48(2):2005.
Roy, K., Ewing, C. P., Hughes, M.A., Keith, L., & Bennett, G. M. (2019). Presence and viability of Ceratocystis lukuohia in ambrosia beetle frass from Rapid ʻŌhiʻa Death‐affected Metrosideros polymorpha trees on Hawaiʻi Island. Forest Pathology, 49(1), e12476.
 
Authors:
Angela Saenz is a PhD student in the Gruner lab, and her research focuses on the synchrony and phenology of the Emerald Ash Borer and its introduced natural enemies in forested areas.
Eva Perry is a Masters student in the Burghardt lab, researching the interaction between plant genetics and the environment on insect communities in urban and managed fores

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