Mon, Aug 15, 2022: 4:00 PM-4:15 PM
516E
Background/Question/MethodsIn 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/ConclusionsWe 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.
Results/ConclusionsWe 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.