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ItemAnthropogenic-Mediated Simplification of Marine Food Webs(Georgia Institute of Technology, 2022-12-08) Willert, Madison ShariAnthropogenic-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. Centropristis striata declined almost a full trophic level and Stenotomus 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. Sphyrna 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, Sphyrna 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.
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ItemClimate change & the physiology, ecology, and behavior of coral reef organisms(Georgia Institute of Technology, 2020-03-16) Johnston, Nicole K.The magnitude of ocean acidification (OA) and warming predicted to occur within the next century could have significant negative effects for organisms that inhabit coral reefs. Our understanding of how these stressors will impact coral reef organisms is complicated by the diverse behavioral and ecological interactions that exist on these reefs. In a series of experiments, I explored interactions between coral reef organisms, evaluated how some of these interactions may be affected by OA and warming, and then studied how environment may shape an organism’s response to a changing climate. First, through a sensory manipulated tank and a twochamber choice flume, I demonstrated that anemonefish respond to both chemical and visual conspecific cues, but they require a combination of these two cues to correctly identify conspecifics. Given that previous research indicates that fish behavioral responses to chemical cues are altered under conditions of future OA, this inability to compensate for the loss of one cue through a second cue could affect their ability to acclimate as climate changes. Second, I found that the common Caribbean mounding coral Porites astreoides, is unaffected by competition with Montastraea cavernosa and Orbicella faveolata under ambient environmental conditions, but exhibits significant reductions in photosynthetic efficiency in areas of direct contact with M. cavernosa and O. faveolata under conditions of elevated CO2 and temperature that are anticipated to occur by the year 2100. These results demonstrated that climate change can interact with competition to alter the rate and severity of coral-coral interactions on reefs of the future. Next, I compared the effects of OA and warming on the physiology of two congeneric coral species (Oculina arbuscula and Oculina diffusa) representing temperate (O. arbuscula) and tropical (O. diffusa) environments and found that, although both corals were negatively impacted by ocean acidification and warming, the temperate coral was slightly more resistant to these stressors. This suggests that temperate species may not be as disadvantaged by climate change as one might expect and may not be easily displaced by more tropical species moving poleward as global oceans warm. Finally, I evaluated the effect of elevated temperature on the well-being of the temperate coral, O. arbuscula when collected from deeper more physically stable environments versus shallower more physically variable environments. I found that corals from both deep and shallow sites were negatively impacted by elevated temperature, but that corals from deeper sites were more strongly impacted. These findings suggest that the physiologies, biotic interactions, and behaviors of reef organisms may all be affected by climate change and that outcomes of these interactions may not be simple to predict as global oceans warm and acidify and as tropical organisms shift poleward and intermix with temperate species to form novel communities.
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ItemEffects of macroalgal versus coral reef dominance on coral survival, chemical defense, and microbiomes(Georgia Institute of Technology, 2018-10-24) Beatty, Deanna S.Coral reefs are among the earth’s most biodiverse and productive ecosystems, but are undergoing precipitous decline due to coral bleaching and disease following thermal stress events, which are increasing in frequency and spatial scale. These effects are exacerbated by local stressors such as overfishing and pollution, collectively causing an increasing number of reefs to shift from coral to macroalgal dominance. These stressors can harm or kill corals through diverse mechanisms, including alterations in how corals interact with microorganisms. By employing a variety of field sampling and field experimental approaches, I investigated consequences of local protection from fishing and coral versus macroalgal dominance of the benthos on coral survival, chemical defense, and microbiomes within paired algal dominated fished areas and coral dominated marine protected areas (MPAs) in Fiji. I demonstrate that i) coral larvae from a macroalgal dominated area exhibited higher pre-settlement mortality and reduced settlement compared to those from a coral dominated area, ii) juveniles planted into a coral dominated MPA survived better than those planted into a macroalgal dominated fished area and differential survival depended on whether macroalgae were immediately adjacent to juvenile coral, iii) corals possess chemical defenses toward the thermally-regulated coral bleaching pathogen Vibrio coralliilyticus, but this defense is compromised by elevated temperature, iv) for a bleaching susceptible but ecologically important acroporid coral, anti-pathogen chemical defense is compromised when coral resides within macroalgal dominated reefs and this effect can be influenced by both the current and historic state of the reef. Effects on coral survival and chemical defense for individuals residing within coral versus macroalgal dominated areas largely coincided with nuanced differences in coral microbiomes (e.g., in microbiome variability and specific indicator bacterial taxa) but not with major shifts in microbiome composition. These findings have implications for reef conservation and for understanding how coral-microbe interactions will respond to the pressures of global change.
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ItemPredation, competition, and facilitation on tropical reefs: implications for corals as reefs degrade(Georgia Institute of Technology, 2017-11-13) Clements, Cody ShaneTropical coral reefs are among the most diverse and productive ecosystems on Earth, but reefs worldwide have experienced dramatic declines in coral and often transitioned from coral- to macroalgal dominance. As local and global threats to corals increase in severity and frequency, there is an urgent need to understand how reef degradation, as well as efforts to manage and restore corals, are reshaping ecological interactions that are critical to the function of coral reef ecosystems. Here, I utilize a range of experimental approaches to investigate how interactions between corals, competing macroalgae, and coral predators (i.e. corallivores) are being altered within mosaics of coral reef habitat characterized by different levels of degradation and local protection in the tropical Pacific. I first demonstrate, via a series of field observations and experiments, the direct negative effects of competition for corals competing with macroalgae that commonly dominate degraded reefs, including the spatial and temporal constraints of these competitive interactions, as well as the indirect positive effects that can arise due to the presence of a common coral predator, the crown-of-thorns sea star (Acanthaster cf. planci). I also provide observational and experimental evidence that protected reefs can help alleviate predation by corallivorous snails (Coralliophila violacea) for some stress-tolerant corals (Porites cylindrica), but that stark habitat contrasts between coral-dominated protected reefs and macroalgal-dominated fished reefs can simultaneously attract and concentrate feeding by other corallivores (Acanthaster cf. planci) – potentially contributing to coral demise and compromising the conservation value of small Marine Protected Areas. Lastly, I use a field-based manipulative experiment to explore the implications of coral species loss for ecosystem function on degraded reefs; demonstrating that greater coral species richness can enhance coral growth and survivorship, and reduced colonization by competing macroalgae. Together, these studies highlight the need to better understand the novel and context-dependent role of ecological interactions – both for fundamental ecology and effective management – in rapidly changing ecosystems subject to increasing disturbances.
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ItemAcclimating across healthy and degraded reefs(Georgia Institute of Technology, 2016-05-31) Dell, Claire Louise AliceAs a result of human activities, many environments are becoming fragmented into areas with different community compositions and selective regimes. The coral reefs of Fiji for example, are divided into ‘fished areas’ (fragments subjected to fishing and trampling) and ‘protected areas’ (fragments with little human pressure) that occur in close proximity and now have differing community compositions and selective regimes. Theory predicts that the species able to survive in such conditions should have highly plastic genotypes allowing them to acclimatise to diverse habitats without the time lag required for local adaptation. Here we use two species -Epinephelus merra (a small grouper) and Sargassum polycystum C. Agardh (a brown macroalga)- which are found in both fished and protected reefs, to investigate this plastic response and understand how these species cope in healthy versus degraded environments. We found that the fish E. merra exhibits plasticity in diet and feeds lower in the food chain in fished reefs than similarly sized conspecifics in protected reefs. The seaweed S. polycystum exhibits plasticity in defensive traits and is able to induce increased defenses in response to being partially consumed. In addition, we found that dense stands of S. polycystum increased the survival and growth of both recruit-sized and mature-sized S. polycystum ramets, suggesting that Sargassum beds protect conspecifics from grazing by herbivorous fishes and construct conditions that facilitate their growth. Implications for management are discussed.
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ItemInteractions between ecosystems and disease in the plankton of freshwater lakes(Georgia Institute of Technology, 2013-11-18) Penczykowski, Rachel M.I investigated effects of environmental change on disease, and effects of disease on ecosystems, using a freshwater zooplankton host and its fungal parasite. This research involved lake surveys, manipulative experiments, and mathematical models. My results indicate that ecosystem characteristics such as habitat structure, nutrient availability, and quality of a host’s resources (here, phytoplankton) can affect the spread of disease. For example, a survey of epidemics in lakes revealed direct and indirect links between habitat structure and epidemic size, where indirect connections were mediated by non-host species. Then, in a mesocosm experiment in a lake, manipulations of habitat structure and nutrient availability interactively affected the spread of disease, and nutrient enrichment increased densities of infected hosts. In a separate laboratory experiment, poor quality resources were shown to decrease parasite transmission rate by altering host foraging behavior. My experimental results also suggest that disease can affect ecosystems through effects on host densities and host traits. In the mesocosm experiment, the parasite indirectly increased abundance of algal resources by decreasing densities of the zooplankton host. Disease in the experimental zooplankton populations also impacted nutrient stoichiometry of algae, which could entail a parasite-mediated shift in food quality for grazers such as the host. Additionally, I showed that infection dramatically reduces host feeding rate, and used a dynamic epidemiological model to illustrate how this parasite-mediated trait change could affect densities of resources and hosts, as well as the spread of disease. I discuss the implications of these ecosystem–disease interactions in light of ongoing changes to habitat and nutrient regimes in freshwater ecosystems.
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ItemChemically mediated competition, herbivory, and the structure of coral reefs(Georgia Institute of Technology, 2012-07-03) Rasher, Douglas B.Corals, the foundation species of tropical reefs, are in rapid global decline as a result of anthropogenic disturbance. On many reefs, losses of coral have coincided with the over-harvesting of reef herbivores, resulting in ecosystem phase-shifts from coral to macroalgal dominance. It is hypothesized that abundant macroalgae inhibit coral recovery and recruitment, thereby generating ecological feedback processes that reinforce phase-shifts to macroalgae and further diminish reef function. Notwithstanding, the extent to which macroalgae directly outcompete coral, the mechanisms involved, and the species-specificity of algal-coral competition remains debated. Moreover the capacity for herbivores to prevent vs. reverse ecosystem phase-shifts to macroalgae and the roles of herbivore diversity in such phenomena remain poorly understood. Here I demonstrate with a series of field experiments in the tropical Pacific and Caribbean Sea that multiple macroalgae common to degraded reefs directly outcompete coral using chemical warfare, that these interactions are mediated by hydrophobic secondary metabolites transferred from algal to coral surfaces by direct contact, and that the outcomes of these allelopathic interactions are highly species-specific. Using field observations and experiments in the tropical Pacific, I also demonstrate that the process of herbivory attenuates the competitive effects of allelopathic algae on corals by controlling succession of algal communities, and that the herbivore species responsible for macroalgal removal possess complementary tolerances to the diversity of chemical defenses deployed among algae, creating an essential role for herbivore diversity in reversing ecosystem phase-shifts to macroalgae. Lastly, I demonstrate with field experiments in the tropical Pacific that algal-coral competition simultaneously induces allelochemicals and suppresses anti-herbivore deterrents in some algae, likely due to trade-offs in the productions of defense metabolites with differing ecological functions. Together, these studies provide strong evidence that chemically mediated competitive and consumer-prey interactions play principal roles in coral reef degradation and recovery, and should provide resource managers with vital information needed for effective management of these ecologically and economically important but threatened ecosystems.
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ItemAquatic plant-herbivore interactions across multiple spatial scales.(Georgia Institute of Technology, 2010-05-21) Morrison, Wendy ElizabethFor decades scientists believed that herbivory had minimal impact on freshwater ecosystems. We now know that herbivory in freshwater systems equals or exceeds herbivory in terrestrial and marine systems. In extreme cases, herbivores can change clear, macrophyte dominated ecosystems into turbid plankton dominated ecosystems. Even though research on plant-herbivore interactions in freshwater systems has increased, there is still much that is unknown. This thesis is comprised of four studies investigating freshwater plant-herbivore interactions across multiple spatial scales. The first study investigated how induced chemical defenses in Cabomba caroliniana suppress herbivore consumption and growth as well as how this herbivore-generated change in plant chemistry affects the growth of plant associated microbes. At the spatial scale of individual ponds or lakes, consumers that induce their host plants may also be indirectly affecting other consumers and microbial pathogens via changes in this shared resource. The second study moves to an ecosystem scale and investigates how exotic versus native apple snails may impact Everglades' habitats. We investigated plant preference, consumption, growth and conversion efficiencies in the singly native apple snail to occur in the U.S. (Pomacea paludosa) versus four introduced species (P. canaliculata, P. insularum, P. haustrum and P. diffusa). We found that even though plant preferences are similar, invasive snails tend to eat more, grow more rapidly, and sometimes more efficiently than natives. This suggests that invasive species could have a large impact on the environment, especially the abundance of submerged plants. The third study investigated how palatability of freshwater plants varies with latitude (i.e. geographic scale). Increased herbivory at lower latitudes is hypothesized to select for increased plant defenses, which has been shown to be true for tropical forests, salt marshes, and seaweeds. When we contrasted eight confamilial plants collected in Indiana versus Southern Florida, three of four herbivores significantly preferred northern plants. When we evaluated a second set of plants collected from Indiana versus Central Florida, only one of three herbivores preferred the northern plants. Overall, our results suggest a preference for northern plants, but the strength of this relationship was variable. We hypothesize that this variability may be driven by 1) local variance in herbivore pressure that creates variance in plant defenses, and/or 2) the effect of winter length on the survival and feeding rate of herbivores. The final study expanded to a world scale, and investigated herbivore preference for native vs exotic plants. We found that both N. American crayfish and S. American snails preferred exotic plants over confamilial natives, despite responding to different plant characteristics. The single species of apple snail that occurs in N. American showed no preference for native or exotic plants from a N. American perspective, but instead exhibited preferences that correlated with its history of evolution in S. America. As the N. American species is a sister species of the S. American snails, feeding by the N. American snail appears more affected by its S. American lineage than its recent history in N. America. This suggests that phylogenetic legacy will affect choices of the herbivore as well as resistance or susceptibility of plants.
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ItemAn Invasive Crab in the South Atlantic Bight: Friend or Foe?(Georgia Institute of Technology, 2006-04-11) Hollebone, Amanda L.The green porcelain crab, Petrolisthes armatus, has recently invaded oyster reefs of the South Atlantic Bight at mean densities of up to several thousand individuals m-². Despite the crab’s tremendous densities and wide-spread occurrence, its population dynamics, the reasons for its success, and its ecological impacts have remained unknown. We used field monitoring in two estuaries of coastal Georgia to assess spatial and temporal patterns of distribution, demographics, reproduction, and effects on native crabs. We used field and mesocosm experiments with constructed oyster reef communities of varying native species richness and adult porcelain crab additions to assess why the invader is successful and how it impacts native species and communities. We found P. armatus distributed throughout the estuaries, primarily in the lower regions and low intertidal. Sex ratios were 1:1 throughout the year. During warmer months mean densities ranged from 1,000-11,000 crabs m-², 20-90% of mature females were gravid, and numerous recruits were present. Despite decreases in density of 64->99% in the winter, populations rebounded in the spring. Maximum mean densities were 37 times the highest densities ever recorded and population fecundity exceeded that of the native range by an order of magnitude, but correlations did not show significant negative effects of P. armatus on native crabs. Field experiments suggested that invasion was successful due to tremendous recruitment overwhelming biotic resistance by native species richness or predation. The crab only needed structure to invade, but the presence of adult conspecifics significantly enhanced recruitment (i.e., intraspecific “invasional meltdown”). We documented several impacts on native biota, including the (1) suppression of oyster growth, benthic algal biomass, native crab recruitment, and native goby densities and the (2) enhancement of bivalve recruitment, macroalgal cover, and survivorship of oyster drills. We did not, though, see an effect on native taxonomic richness. The large direct and indirect effects of P. armatus on growth, survivorship, and recruitment of virtually all of the most common native species on oyster reefs in the short-term (4-12 weeks) and at relatively low experimental densities (750-1500 crabs m-²) imply considerable long-term consequences for a major hard-substrate habitat of the South Atlantic Bight.
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ItemCyanobacteria-Grazer Interactions: Consequences of toxicity, morphology, and genetic diversity(Georgia Institute of Technology, 2006-04-11) Wilson, Alan ElliottInteractions between cyanobacteria and herbivorous grazers play an important role in mediating the responses of freshwater phytoplankton assemblages to nutrient enrichment and top-down manipulation. Negative consequences associated with these interactions include dangerous blooms of harmful blue-green algae that have been implicated in the sickness and death of fishes, livestock, and, in extreme cases, humans. Frequently cited mechanisms influencing the interactions between grazers and cyanobacteria include cyanobacterial toxicity and morphology. To tease apart the importance of these mechanisms, I used meta-analysis to quantitatively synthesize the available literature on this topic. In addition, I conducted several experiments using novel techniques to determine the effect that cyanobacterial secondary metabolites from the bloom-forming cyanobacterium,