PhD Defense by Madison Willert

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In partial fulfillment of the requirements for the degree of


Doctor of Philosophy in Biology

In the

School of Biological Sciences


Madison Willert


Will defend her dissertation


Anthropogenic-Mediated Simplification of Marine Food Webs


28 November 2022

12:30 PM



Meeting ID: 645 866 6440, Passcode: 274690


 Thesis Advisor:

Mark Hay, Ph.D.

School of Biological Sciences

Georgia Institute of Technology


Committee Members:

Joseph Montoya, Ph.D.

School of Biological Sciences

Georgia Institute of Technology


Lin Jiang, Ph.D.

School of Biological Sciences

Georgia Institute of Technology


Carole Baldwin, Ph.D.

National Museum of Natural History

Smithsonian Institution


Christine France, Ph.D.

Museum Conservation Institute

Smithsonian Institution


ABSTRACT: Anthropogenic-mediated stressors such as overexploitation, habitat destruction, climate change, and species introductions are changing food webs in marine ecosystems. In this dissertation, I first evaluate how these stressors are shifting trophic interactions via increased dietary overlap and interspecific competition within trophic levels, truncation of nutrient flow between trophic levels and ecosystems, and simplification and compression of entire food webs. Stable isotope analysis is a powerful tool to measure species’ trophic positions and thus, food web shifts over time and space. I show that δ15N values and δ13C values from formalin-preserved seaweeds are generally reliable, validating stable isotope analysis of herbarium specimens. Seaweeds are useful as nutrient baselines for trophic ecology studies, as well as for assessing nutrient runoff and pollution; this finding shows that preserved herbarium specimens can be used in these types of studies to reconstruct food webs of the past. I then use nitrogen stable isotope analysis of both herbarium specimens and museum fish specimens from New England, USA to show that the common piscivore Centropristis striata (black sea bass) and the common benthivore Stenotomus chrysops (scup) have experienced significant declines in trophic position in this area since pre-1950. C. striata declined almost a full trophic level and S. chrysops declined half a trophic level, and these species are now converging on similar trophic positions coincident with the increase in destructive bottom fishing in New England. Next, I used nitrogen stable isotope analysis of >1000 museum fish specimens from coral reefs worldwide to assess dietary changes of common coral reef mesopredators since 1850 in regions of both the tropical Atlantic and the Indo-Pacific. I found that trophic instability has been common in the tropical Atlantic during the 20th century, with the trophic position of most Atlantic species decreasing further going into the 21st century. Unlike in the Atlantic, historically unstable species in the Indo-Pacific are now increasing in their trophic positions; this suggests that relatively higher levels of overfishing and coral loss in the tropical Atlantic are reflected in greater mesopredator trophic instability. Finally, I used nitrogen and carbon stable isotope analysis of vertebrae from Sphyrna mokarran (great hammerhead) and Sphyrna lewini (scalloped hammerhead) sharks to evaluate ontogenetic shifts in these two species in the U.S. South Atlantic and the eastern Gulf of Mexico. S. lewini occupies a high trophic position throughout its life, reaching peak predator status as a subadult and occupying more offshore pelagic habitats. Despite its larger body size, S. mokarran occupies a lower trophic position and relies more on benthic and inshore habitats, especially in the juvenile stage. I elucidated the nuances of these predators’ trophic ecology and found no evidence of within- species differences in sex or location with regards to dietary habits. A better understanding of individual species’ trophic ecology, as well as historic human impacts on marine food webs, is crucial to maintaining and promoting healthy ecosystems into the future.


  • Workflow Status:Published
  • Created By:Tatianna Richardson
  • Created:11/11/2022
  • Modified By:Tatianna Richardson
  • Modified:11/11/2022