Hemlock forests across the eastern US are succumbing to the invasive insect, Hemlock Woolly Adelgid (HWA; Adelges tsugae). HWA sucks sap from the trees, causing hemlock needle loss and eventual tree death. This results in complete stand mortality, and there is currently no way to effectively protect an entire forest.
In the Appalachian Mountains, many headwater streams run through these hemlock forests. Headwater streams are closely linked to their riparian zones through exchanges of carbon, organic matter, and prey (e.g. energy). We seek to understand how disturbances in the riparian zone (in this case, the complete death and eventual replacement of hemlock as a foundational plant species) alter stream biodiversity and ecosystem functions. We are particularly interested in how HWA invasion affects stream-riparian trophic linkages, including terrestrial and in-stream primary producers, aquatic macroinvertebrates, and riparian consumers such as orb-weaving spiders. We are also measuring water chemistry and nutrients as well as stream physical properties (e.g., substrate, large wood, etc.). We anticipate that this research will contribute to both basic science (invasion ecology) as well as management of headwater stream ecosystems.
Some key findings of our work include:
Basal resources
Basal resources are the foundation upon which food webs are built. Streams get their trophic energy from two main sources: aquatic primary production (e.g., algae and aquatic plants) and terrestrial detritus (e.g., leaves and other organic material that falls into the stream). Because the riparian vegetation can influence the quality and quantity of both sources, we investigated the influence of HWA and hemlock decline on periphyton and terrestrial detritus.
While the amount of plant detritus in streams did not change, the composition of that detritus did, and uninvaded sites had a higher proportion of hemlock than invaded sites (see graphic below, and click image for caption). We also observed periphyton ash-free dry mass to be greatest at reference sites.
Average herbivore makeup of all functional feeding groups by hemlock decline category (HDC 1 = reference, 2 = moderate decline, and 3 = severe decline). Different letters represent significant differences between group means.
Macroinvertebrates
Macroinvertebrates make up the intermediate nodes in the food chain. Aquatic macroinvertebrates are often described by their functional feeding group, which refers to their primary food resource and how that food is obtained. Because changes to quality and quantity of basal resources can affect consumers, we wanted to see how the macroinvertebrate communities would respond to HWA and hemlock decline.
Herbivorous invertebrates are the scrapers, piercers, and shredders of aquatic primary production like periphyton and aquatic plants. Of the different functional feeding groups, we found that the relative abundance of herbivores increased from 4% at uninvaded sites to 23% at sites severely impacted by invasion (see graphic below). Across all our sites, proportion herbivore was best determined by physical streambed variables (including substrate size and proportion bedrock). One type of herbivorous mayfly, Epeorus sp., was present in high numbers at sites in severe decline and drove this pattern.
Riparian spiders
Spiders are at the top of our invertebrate food chains. For this project, we surveyed three families of spiders: Pisauridae (fishing spiders, that hunt on the water’s surface), Araneidae (orb-weaving spiders), and Tetragnathidae (another orb-weaving spider that is known to build webs horizontally over water surfaces to capitalize on emerging aquatic insects).
We confirmed a trophic linkage between riparian spiders (families Araneidae and Pisauridae) and emergent aquatic insects as a prey resource via positively related naturally abundant stable isotope signatures (δ15N).
Interestingly, we observed different patterns between densities of the two orb-weavers (see below). While araneid density was not influenced by hemlock decline, tetragnathid densities diverged and were highest at sites in severe decline. We hypothesize that the tetragnathids were limited by a lack of web-building substrate at lower levels of hemlock decline.
Publications
Diesburg, K.M., S.M.P. Sullivan, and David W.P. Manning. Consequences of a terrestrial insect invader on stream-riparian food webs of the central Appalachians, USA. Submitted to Biological Invasions
Diesburg, K.M., S.M.P. Sullivan, and David W.P. Manning. 2019. Changes in benthic invertebrate communities of central Appalachian streams attributed to hemlock woolly adelgid invasion. Aquatic Sciences 81:11 DOI: 10.1007/s00027-018-0607-y
Costigan, K.H., P.J. Soltesz, and K.L. Jaeger. 2015. Large wood in central Appalachian headwater streams: controls on and potential changes to wood loads from infestation of hemlock woolly adelgid. Earth Surface Processes and Landforms, 40(13): 1746-1763 DOI: 10.1002/esp.3751
Presentations
Diesburg, K.M., S.M.P. Sullivan, and David W.P. Manning. Mechanisms and more: riparian biological invasions alter community structure and ecosystem function in stream-riparian ecosystems. Poster. Society for Freshwater Science Annual Meeting, June 2020, Online Conference
Diesburg, K.M., S.M.P. Sullivan, and David W.P. Manning. Stream-riparian trophic linkage response to a terrestrial invader. Poster. Society for Freshwater Science Annual Meeting, May 2019, Salt Lake City, UT
Diesburg, K.M., S.M.P. Sullivan, and David W.P. Manning. Stream benthic macroinvertebrate communities respond to hemlock decline. Poster. Society for Freshwater Science Annual Meeting, May 2018, Detroit, MI
Diesburg, K.M., David W.P. Manning, and S.M.P. Sullivan. How does hemlock woolly adelgid invasion affect riparian consumer trophic dynamics? Oral. Society for Freshwater Science Annual Meeting, June 2017, Raleigh, NC
Diesburg, K.M. and S.M.P. Sullivan. Ecological linkages between streams and their adjacent riparian zones following Hemlock Woolly Adelgid invasion. Oral. Society for Freshwater Science Annual Meeting, May 2016. Sacramento, CA
Diesburg, K.M. and S.M.P. Sullivan. Stream ecosystem responses to the terrestrial insect invader, Hemlock Woolly Adelgid. Oral. Society for Freshwater Science Annual Meeting, May 2015. Milwaukee, WI.