Plant and insect herbivore community variation across the Paleocene–Eocene boundary in the Hanna Basin, southeastern Wyoming

Abstract

Ecosystem function and stability are highly affected by internal and external stressors. Utilizing paleobotanical data gives insight into the evolutionary processes an ecosystem undergoes across long periods of time, allowing for a more complete understanding of how plant and insect herbivore communities are affected by ecosystem imbalance. To study how plant and insect herbivore communities change during times of disturbance, we quantified community turnover across the Paleocene­–Eocene boundary in the Hanna Basin, southeastern Wyoming. This particular location is unlike other nearby Laramide basins because it has an abundance of late Paleocene and Eocene coal and carbonaceous shales and paucity of well-developed paleosols, suggesting perpetually high water availability. We sampled approximately 800 semi-intact dicot leaves from five stratigraphic levels, one of which occurs late in the Paleocene–Eocene thermal maximum (PETM). Field collections were supplemented with specimens at the Denver Museum of Nature & Science. Fossil leaves were classified into morphospecies and herbivore damage was documented for each leaf. We tested for changes in plant and insect herbivore damage diversity using rarefaction and community composition using non-metric multidimensional scaling ordinations. We also documented changes in depositional environment at each stratigraphic level to better contextualize the environment of the basin. Plant diversity was highest during the mid-late Paleocene and decreased into the Eocene, whereas damage diversity was highest at the sites with low plant diversity. Plant communities significantly changed during the late PETM and do not return to pre-PETM composition. Insect herbivore communities also changed during the PETM, but, unlike plant communities, rebound to their pre-PETM structure. These results suggest that insect herbivore communities responded more strongly to plant community composition than to the diversity of species present.

Ecology and Evolution of Gall-Inducing Arthropods: The Pattern From the Terrestrial Fossil Record

“…In a separate area of south-central Wyoming, the Hanna Basin, a second series of pre-PETM, PETM, and post-PETM localities were examined for plant diversity, insect damage (DT) richness, and overall ecosystem structure (Schmidt et al, 2019). The results of this study indicate that plant diversity was greatest in pre-PETM floras but do not rebound to these values in younger PETM and post-PETM floras.…”

“…By contrast, damage richness was greatest in those sites with low plant diversity and increased their richness in post-PETM floras. These data include five gall DTs (Appendix 3) and suggest that herbivore communities respond more easily to plant community structure than to plant species diversity in those communities (Schmidt et al, 2019).…”

Local differences in paleohydrology have stronger influence on plant biomarkers than regional climate change across two Paleogene Laramide Basins, Wyoming, USA

“…As the best preserved and studied terrestrial PETM section, the BHB forms the basis for current understanding of terrestrial ecosystem function during abrupt climate change (Abels et al 2016). The HB recorded wetter local environmental conditions during the same time interval (Azevedo-Schmidt, et al 2019;Dechesne, et al 2020) and so comparing records from the two basins will enhance our ability to conceptualize ecosystem response to climate change.…”

“…Leaf compression and wood fossils from BHB (e.g., Currano, et al 2010;Wing and Currano 2013) and HB (Azevedo-Schmidt, et al 2019;Dechesne, et al 2020) show a mix of conifers and angiosperms during the Paleocene-Eocene, with conifers dominating carbonaceous facies (swamps) and angiosperms abundant in siltier and sandier facies (e.g., abandoned channels, floodplains, deltas; Davies-Vollum et al 1998). BHB is dominated by legumes with a complete loss of conifer compression fossils during the PETM (Currano et al 2016;Wing and Currano 2013).…”

“…Plant biomarkers and isotopes have been used to extensively study the PETM in the BHB (e.g., Diefendorf et al 2014); however, these records are likely susceptible to local abiotic and biotic effects. To broaden our understanding of local vs. regional climate and vegetation change during the PETM, the Hanna Basin (HB) serves as a useful companion study site due to its geographical proximity to the BHB and comparable leaf fossils (Azevedo-Schmidt et al 2019). The leaf compression fossils are known from these two basins, allowing us to compare the biomarkers with known vegetation characteristics (Wing et al 1995;Wing and Currano 2013).…”