COS 33-3 - Conditions for collapse: Nutrient enrichment increases native herbivore density linked to salt marsh dieback

Monday, August 15, 2022

4:00 PM – 4:15 PM EDT

Location: 516E

Presenting Author(s)

JT

Jewel Tomasula

Georgetown University, United States

Coauthor(s)

BM

Billie Maguire

Georgetown University, United States
TR

Tyler M. Rippel, n/a

PhD Candidate
Georgetown University
Washington, District of Columbia, United States
SP

Stephanie Perez

Georgetown University, United States
EL

Eddie Lopez

Georgetown University, United States
AA

Ali Arab, PhD

Professor
Georgetown University, United States
SM

Shannon Murphy

University of Denver, United States
GW

Gina M. Wimp

Georgetown University, United States

Background/Question/Methods

In this time of global change, ecosystems are enduring new stressor patterns, yet little is understood about the conditions that lead to ecological collapse. We examined how different conditions of nutrient enrichment affect densities of a native stem-boring insect larvae that feeds upon a salt marsh foundation plant species, Spartina alterniflora, with subsequent effects on salt marsh dieback. Dieback is characterized by the abrupt death of salt marsh vegetation, leaving barren mudflats. In a natural salt marsh in the mid-Atlantic United States, we set up a full factorial design in which we crossed the amount of nitrogen addition (low - 13.6 g N/m2, medium - 27.2 g N/m2, and high - 54.4 g N/m2) with duration of nutrient addition (one-year pulse or a two-year press) in 10m2 plots. We collected aboveground plant material from one quadrat in each plot and we dissected all of the live S. alterniflora stems to document presence or absence of stem-boring insect larvae. We analyzed percent nitrogen content of S. alterniflora tissue. At the end of the second year, we determined the percent dieback within each plot by visually estimating the percentage of the plot that was barren mudflat.

Results/Conclusions

We found higher percent dieback relative to control only in our high press nutrient treatment. When we compared the effects of nitrogen amount and duration on percent dieback using a two-way ANOVA, we found an effect of nitrogen amount and a marginal effect of press/pulse. Densities of native stem-boring insect larvae were nearly four times greater in high press plots than in control plots. Further, stem-boring insect larvae densities were positively correlated with S. alterniflora percent nitrogen content. By comparing linked and independent Bayesian hierarchical models, we determined both S. alterniflora percent nitrogen and stem-boring larvae densities contributed to salt marsh dieback. Our findings link nutrient enrichment with an increase in native insect herbivory as a cause of salt marsh dieback, whereas previous studies considered nutrients and herbivory as separate factors. S. alterniflora withstands stem-boring larval herbivory during nitrogen pulses, but collapses under conditions of high nutrient presses. Broadly, we provide evidence that high levels of nutrient enrichment that persist for multiple years amplifies native herbivory and is a condition leading to ecological collapse.