Life Amidst Chaos: Terrestrial Invertebrates' Adaptation to Debris Flow Disasters in Big Creek Reserve

Research Paper for Conservation Field Research Supercourse at UCSC

By Elise Baugh, Department of Ecology and Evolutionary Biology

Abstract: 

Large-scale, natural disturbance events can overwhelm biotic systems causing irreversible changes to composition, richness and abundance. Riparian environments are particularly susceptible to compositional change following high-intensity level debris flows.  The Big Creek riparian environment in Big Sur is home to a number of threatened and endangered endemic species (Steelhead trout, California Condor) and has experienced recent catastrophic wildfires and debris flow, a cause of concern for scientists and conservationists alike.

Our research analyzes invertebrate responses to the debris flow event by measuring richness and abundance within the community. We used the Big Creek riparian zone as our research focus due to its recent debris flow event and the adjacent riparian watershed ‘Devils creek’ as our control which was unaffected. This research is ideally placed 2 years after a catastrophic debris flow. Our focus on the health and resilience of terrestrial invertebrates as a lower trophic level community reveals a healthy response.  Within just two years beyond the event, there is no significant difference in abundance and richness between the Creek experiencing debris flow (Big Creek) and an adjacent reference creek with similar size and geology that was relatively unaffected by the flood. (Devils Creek)   The spatiotemporal documentation of terrestrial invertebrate responses following a major natural disturbance will enable future modeling of richness and abundance allowing for accurate population predictions.  These will be essential management and conservation tools in a future where catastrophic events are occurring more frequently.  

Key Words. Terrestrial Invertebrates, Debris Flow, Landels-Hill Big Creek Natural Reserve, Dolan Fire, Big Sur, riparian zone ecosystem, spatio-temporal response rates, macroinvertebrates, bottom-up trophic effects 

Introduction : 

Terrestrial invertebrates are vital and unique in their ability to serve as a bioenergetic resource to both terrestrial and aquatic-based ecosystems. (Ramey & Richardson, 2017) Their life histories within both aquatic and terrestrial environments enable the unique capability to provide cross-ecosystem resource subsidies and they support that system through their functional roles as pollinators, decomposers, and predators. (Richardson et al., 2005)  They represent a large percent of biological diversity in riparian zones and have many unique adaptations that evolved to equip them with changes in the flow and  water levels that occur in creeks and streams, such as flooding and drought. Our knowledge of how well the invertebrate communities are  equipped to deal with catastrophic level events is limited. On January 27th, 2021, the Big Creek area experienced intensive rain flow which triggered landslides and catastrophic debris flows after a historically significant season of summer wildfires in 2020. While the Big Creek riparian zones experienced heavy debris flows, the adjacent riparian area at Devils creek was not affected and laid the foundation for the ideal natural experiment (Wong, 2021).   Two years later, we have the opportunity to investigate the response rates of the invertebrate community that has responded to the high-intensity disturbance. 

In this study, we surveyed 3 riparian habitats and compared alpha and beta level diversity and we found that there were no significant differences between the riparian zones.  Understanding the lower trophic level responses to catastrophes will provide the necessary  prediction of ecosystem responses and prediction for future catastrophic events which are likely due to changes in climate.  Invertebrates perform vital ecological functions serving as bioenergetic links between both aquatic and terrestrial food webs and the health and diversity of invertebrates in a riparian zone provide a key indicator of the system’s base stability response. Our research reveals a healthy response to high-magnitude debris flow events over a period of two years; the communities show little significant differences in richness or abundance. 

Methods:

Site description:

We selected three adjacent riparian sites within the  Landels-Hill Big Creek Natural Reserve in Monterey County, California. Two adjacent riparian habitats,  Upper Big Creek (UBC) and Devils Creek (DC) adjoin at a confluence point that merges into a single stream. referred to here as Lower Big Creek (LBC.) Lower Big Creek served as our third riparian habitat study area. Upper Big Creek experienced debris flow, Lower Big Creek was unaffected by debris flow and Lower Big Creek was a merging of the two creeks and had mixed levels of sediment flow. 

Survey of Terrestrial Invertebrates:

 We designed a systematic sample method designating 600m of sampling area that began at the confluence point of the three creeks. Samples were collected every 50m along the banks of UBC, LBC, and DC starting from the confluence.  At each 50m mark along the creek bed, we measured +/- 5m from the water. Adjustments to the measured 5m were required if terrain obstacles were present. Each sample consisted of one pitfall trap and one sticky trap per location. 

When collecting and documenting traps, we would transfer the sticky traps and the contents of the pitfall traps into ziplock bags for analysis and data entry at the lab. We created a key for identifying the species morphologically with variations in the characteristics of that morphology that were subcategorized numerically (Example: “Beetle 1,” “Spider 4,” “Fly 2”).  Quantities, morphology codes, and locations were recorded for individual invertebrates   Shannon-Wiener index formula was used to measure the diversity of species within each community. 

Results: 

Morphological categories found in terrestrial invertebrates included: ants, beetles, centipedes and millipedes, cicadoidea, 2 winged flies, moths, spiders, bees and wasps, true bugs, and isopods. Figure 1 Quantities of two-winged flies, cicadoidea, Ants, and Isopods were highest at UBC, while morphological groupings of beetles were highest at DC. Figure 2 When measuring for richness and abundance there were no significant differences found between the three study areas (figure 3)

Figure 1:   Photographic observations by morphologic groupings Top left: two-winged flies, Top right: Centipedes and millipedes, Bottom left: Beetles, Bottom right: Centipedes and millipedes

Figure 2:   Quantities of individuals within their overarching morphologic categories found at LBC, UBC, and DC research areas.

Figure 3:  When analyzing  Values for abundance and richness showed no significant differences were found between UBC, LBC, and DC ( t2= 1.875, p=.170)  Mean values are demonstrated here as percentages of the total value. We constructed error bars using one standard error from the mean.

Discussion 

This research demonstrates a resilient response within the terrestrial invertebrate community two years after a catastrophic debris flow hit the area. The analysis of abundance and richness does not differ significantly from the areas not affected by the disturbance event. The numbers for richness and abundance actually sit higher within the areas impacted by debris flow.  A similar study conducted by Snyder and Johnson, “Macroinvertebrate Assemblage Recovery Following a Catastrophic Flood and Debris Flows in an Appalachian Mountain Stream.” suggested that higher productivity levels can be linked to increases in litter related food resources. They also suggested that macroinvertebrate productivity could rise as a result of a reduction in shade cover which would result in increased primary production. 

Recommendations. Following the debris flow, major shifts and changes occurred in the structural geology of these creeks. It is recommended to take a systematic investigation of the geology of these creeks to understand how the base driving mechanisms within this ecological system have changed over time. This should include the possible confounding factor that lies within the travertines' presence at the Devils creek site.  Further studies should include beta diversity studies to see how diversity gradients occur and uncover what driving mechanisms  create the gradients seen. (Nogueira et al., 2021)

ACKNOWLEDGEMENTS: The faculty from UCSC who helped guide this study are Don Croll, Gage Dayton, Rachel Pausch, and my fellow researchers on the Bio- terrestrial invertebrate research team. 

Literature Cited: 

Nogueira, André A., Antonio D. Brescovit, Gilmar Perbiche-Neves, and Eduardo M. Venticinque. “Beta Diversity along an Elevational Gradient at the Pico Da Neblina (Brazil): Is Spider (Arachnida-Araneae) Community Composition Congruent with the Guayana Region Elevational Zonation?” Diversity 13, no. 12 (November 26, 2021): 620. https://doi.org/10.3390/d13120620.

Ramey, Tonya L., and John S. Richardson. “Terrestrial Invertebrates in the Riparian Zone: Mechanisms Underlying Their Unique Diversity.” BioScience 67, no. 9 (September 2017): 808–19. https://doi.org/10.1093/biosci/bix078.

Richardson, John. “RIPARIAN COMMUNITIES ASSOCIATED WITH PACIFIC NORTHWEST HEADWATER STREAMS: ASSEMBLAGES, PROCESSES, AND UNIQUENESS1.” JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, August 2005, 935–47.

Snyder, C. D., and Z. B. Johnson. “Macroinvertebrate Assemblage Recovery Following a Catastrophic Flood and Debris Flows in an Appalachian Mountain Stream.” Journal of the North American Benthological Society 25, no. 4 (December 2006): 825–40. https://doi.org/10.1899/0887-3593(2006)025[0825:MARFAC]2.0.CO;2.

Wong, Kathleen. “Massive Debris Flow Swamps Big Creek Reserce as Heavy Rains Follow Summer Fires.” National Reserve System, U of C, March 1, 2021. https://ucnrs.org/massive-debris-flow-swamps-big-creek-reserve-as-heavy-rains-follow-summer-wildfire/.