Declines in habitat structural complexity have marked ecological outcomes, as currently observed in many of the world's ecosystems. Coral reefs have provided a model for such changes in marine ecosystems, but our understanding has been centred on corals and fishes at broad spatial scales when metazoan diversity on coral reefs is dominated by small cryptic taxa (herein: ‘cryptofauna’). Given the paucity of studies and high taxonomic complexity of the cryptofauna, both of which limit a priori hypotheses, we asked whether hierarchical structuring theory provides a compelling framework to impose order and quantify pattern. In general terms, we explored whether cryptic communities are sufficiently described by broad seascape parameters or limited by a set of processes operating at their distinctly nested microhabitat scale. To address this theory and gaps in knowledge for the cryptofauna, we characterised community structure in coral rubble, an eroded coral condition where biodiversity proliferates. Rubble was sampled along a depth and exposure gradient at Heron Island on the Great Barrier Reef, Australia, to parameterise environmental and morphological indicators of sessile taxa and motile cryptofauna communities. We employed a hierarchical study framework from microhabitat to seascape scales, which were evaluated using non-structured multivariate analyses and Bayesian structural equation modelling. While the non-structured analyses showed the effects of seascape on the cryptobenthos and its community, this approach overlooked the finer hierarchical patterns in rubble ecology revealed only in the structured model. Seascape parameters (exposure and depth) influenced microhabitat complexity (i.e., rubble branchiness), which determined the cover of sessile organisms on rubble pieces, which shaped the motile cryptofauna community. Rubble is likely to be increasingly prevalent on coral reefs in the Anthropocene and is typically associated with low seascape-level complexity and reduced macrofaunal richness. Parallel with hierarchical structuring theory, we show a similar response operating at the microhabitat scale whereby low rubble complexity (i.e., branchiness) reduces cryptobenthic structure, diversity and size spectra. In a future ocean, we expect there may be an initial increase in biodiversity and trophodynamic processes derived from branching rubble, but a delay in ecosystem-scale outcomes if coral, and thus rubble, generation and complexity is not sustained. 

Hierarchical drivers of cryptic biodiversity on coral reefs

Coral reefs underpin the environmental, social, and economic fabrics of much of the world's tropical coast. Yet, the fine-scale distribution and composition of coral reefs have never been reported consistently across the planet. Here, we present new area estimates enabled by global geomorphic zone and benthic substrate maps at 5 m pixel resolution. We revise global coral reef estimates to 348,361 km2 of shallow coral reefs and 80,213 km2 (46,237–106,319 km2, 95% confidence interval) of coral habitat. The mapping used more than 1.5 million training samples supported by 480+ data contributions to deploy a coral reef classification of over 100 trillion pixels from the Sentinel-2 satellites and the Planet Dove CubeSat constellation. The publicly available maps are accessible via the Allen Coral Atlas and Google Earth Engine and are already being used by thousands of people to improve the conservation, management, and research of coral reef ecosystems. 

New global area estimates for coral reefs from high-resolution mapping

Coral carbonate production is fundamental to reef accretion and, consequently, the preservation of essential reef ecosystem services, such as wave attenuation and sustained reef biodiversity. However, the unprecedented loss of coral reefs from anthropogenic impacts has put these valuable ecosystem services at risk. To counteract this loss, active rehabilitation of degraded reef sites has accelerated globally. A variety of restoration practices exist, tailored to local site needs and reef types. For sites where there is a significant unconsolidated substrate, Mars Assisted Reef Restoration System (MARRS, or “Reef Stars”) has been utilised to contribute toward rubble stabilisation and reef accretion. However, the effect of the Reef Stars on the local carbonate budgets and structural complexity has not been assessed. For that purpose, we assess coral cover and reef complexity through a census-based approach to identify the contribution of carbonate producers and eroders alongside studying coral skeletal properties to estimate current carbonate budgets on a rehabilitated site compared to natural unrehabilitated reef and rubble patches on the mid-Great Barrier Reef. Our research identified positive ecological processes and ecological functions such as increased carbonate budget, coral cover and structural complexity at the restored site compared to the non-intervened reef and rubble patches. In general, no impacts on skeletal rigour relative to this active reef restoration were found for two key coral species and the Acropora rubble for most of the skeletal traits. However, Pocillopora damicornis hardness seemed to decrease on the restored site compared to the other sites, demonstrating different performances of coral species during restoration activities that should be considered to maximise return-on-effort of restoration activities. Overall, our data demonstrate that consideration of carbonate budgets is important for measuring success of coral restoration initiatives and that coral restoration can be a relevant tool to recover lost local carbonate budgets.

Carbonate budgets induced by coral restoration of a Great Barrier Reef site following cyclone damage

The juveniles of predatory sea stars can remain in their recruitment-nursery habitat for some time before their ontogenetic shift to the adult habitat and diet. These small juveniles are vulnerable to a range of factors with their sensitivity amplified by climate change-driven ocean warming. We investigate the thermal tolerance of the waiting stage herbivorous juveniles of the keystone coral predator, the crown-of-thorns sea star (COTS, Acanthaster sp.), in context with the degree heating weeks (DHW) model that predicts coral bleaching and mass mortality. In temperature treatments ranging from +1 to 3°C in prolonged heatwave acclimation conditions, the juveniles exhibited ~100% survival in DHW scenarios that trigger coral bleaching (4 DHW), resulting in mass mortality of corals (8 DHW) and extreme conditions well beyond those that kill corals (12 DHW). This indicates that herbivorous juvenile COTS are far more resistant to heatwave conditions than the coral prey of the adults. The juveniles exhibited higher activity (righting) and metabolic rate after weeks in increased temperature. In separate acute temperature experiments, the upper thermal limit of the juveniles was 34-36°C. In a warming world, juvenile COTS residing in their coral rubble nursery habitat will benefit from an increase in the extent of this habitat due to coral mortality. The juveniles have potential for long-term persistence as herbivores as they wait for live coral to recover before becoming coral predators, thereby serving as a proximate source of COTS outbreaks on reefs already in a tenuous state due to climate change.

Juvenile waiting stage crown-of-thorns sea stars are resilient in heatwave conditions that bleach and kill corals.

Abstract. The proportional cover of rubble on reefs is predicted to increase as disturbances increase in intensity and frequency. Unstable rubble can kill coral recruits and impair binding processes that transform rubble into a stable substrate for coral recruitment. A clearer understanding of the mechanisms of inhibited coral recovery on rubble requires characterisation of the hydrodynamic conditions that trigger rubble mobilisation. Here, we investigated rubble mobilisation under regular wave conditions in a wave flume and irregular wave conditions in situ on a coral reef in the Maldives. We examined how changes in near-bed wave orbital velocity influenced the likelihood of rubble motion (e.g. rocking) and transport (by walking, sliding or flipping). Rubble mobilisation was considered as a function of rubble length, branchiness (branched vs. unbranched) and underlying substrate (rubble vs. sand). The effect of near-bed wave orbital velocity on rubble mobilisation was comparable between flume and reef observations. As near-bed wave orbital velocity increased, rubble was more likely to rock, be transported and travel greater distances. Averaged across length, branchiness and substrate, loose rubble had a 50 % chance of transport when near-bed wave orbital velocities reached 0.30 m s−1 in both the wave flume and on the reef. However, small and/or unbranched rubble pieces were generally mobilised more and at lower velocities than larger, branched rubble. Rubble also travelled further distances per day (∼2 cm) on substrates composed of sand than rubble. Importantly, if rubble was interlocked, it was very unlikely to move (&lt; 7 % chance) even at the highest velocity tested (0.4 m s−1). Furthermore, the probability of rubble transport declined over 3 d deployments in the field, suggesting rubble had snagged or settled into more hydrodynamically stable positions within the first days of deployment. We expect that snagged or settled rubble is transported more commonly in locations with higher-energy events and more variable wave environments. At our field site in the Maldives, we expect recovery windows for binding (when rubble is stable) to predominantly occur during the calmer north-eastern monsoon when wave energy impacting the atoll is less and wave heights are smaller. Our results show that rubble beds comprised of small rubble pieces and/or pieces with fewer branches are more likely to have shorter windows of recovery (stability) between mobilisation events, and thus be good candidates for rubble stabilisation interventions to enhance coral recruitment and binding. 

Mobilisation thresholds for coral rubble and consequences for windows of reef recovery

With rubble predicted to increase on coral reefs worldwide, we review the physical, biological, and ecological dynamics of rubble beds, with a focus on how rubble generation, mobilization, binding, and coral recruitment is expected to change on future reefs. Major disturbances, including storms and coral bleaching, are predicted to increase in intensity and frequency, and—like localized impacts including blast fishing and ship groundings—generate large quantities of coral rubble. Reefs will have increasingly smaller recovery windows between successive disturbances, leading to persistence of unstable rubble beds on reefs. With more severe storms and increased bioerosion on future reefs, rubble mobilization thresholds will be met more often as smaller, less complex rubble pieces are generated. If rubble remains stable for adequate time, it can be bound by organisms including sponges and coralline algae, and eventually be cemented. However, increasing rubble mobilization frequencies will reduce the time available for binding, while changing ocean chemistry could reduce the efficacy of calcifying binders. Ultimately, increased rubble cover will negatively impact coral recruitment into rubble beds. Rubble mobilization abrades and smothers corals, and rubble beds typically experience altered environmental and ecological conditions to the coral frameworks that precede them. Several knowledge gaps exist in relation to improved rubble mobilization thresholds, binding rates and strengths, and coral survival in varying rubble bed types and hydrodynamic regimes. Addressing these knowledge gaps will improve our ability to predict the recovery trajectory of rubble beds and assess the need for stabilization interventions.

Coral rubble dynamics in the Anthropocene and implications for reef recovery

Antecedent topography and active tectonic controls on Holocene reef geomorphology in the Great Barrier Reef

Plant parasitism and soil microbes: A tripartite interaction driving plant community structure Jordan C. Argrett1, Nina Wurzburger1, Y.A. Chung2 (1) Odum School of Ecology, University of Georgia (2) Department of Plant Biology, University of Georgia Hemiparasitic plants are common in many natural ecosystems and are known to influence community structure and ecosystem function. They connect to host plants through unique root structures called haustoria to establish a direct connection to its vascular system and form a living physiological bridge by which some nutrients, carbon, and water are extracted. In nutrient-limited ecosystems, hemiparasitic plants influence ecosystem processes and increase productivity through two primary pathways. (1) Hemiparasitic plants produce nutrient-rich and labile litter, stimulating microbial activity and increasing local decomposition rates (litter pathway). (2) Hemiparasitic plants reduce the abundance of dominant species via parasitism, thus facilitating the increase and coexistence of diverse co-occurring species (parasitism pathway). While past work on hemiparasites has focused on the direct impact on aboveground responses like host species and plant diversity, little is known about how these plants affect belowground processes. We aim to investigate the effects of hemiparasitic plants on neighboring plant population and community dynamics, nutrient cycling, and soil microbial communities by assessing their direct and litter-mediated effects. This work will provide a comparison of the two pathways, ultimately contributing to a greater understanding of how hemiparasites impact surrounding plant communities and ecosystem cycling. The gut microbial diversity of a Chagas disease vector varies across single and coinfection status throughout central Panama Kaylee M.H. Arnold1,2, Christina Varian1,2,3, Troy Kieran2,4, Azael Saldaña5, Franklin Samudio5, Jose Calazada5, Nicole Gottdenker1,2,3 (1) Odum School of Ecology, University of Georgia (2) Center for the Ecology of Infectious Diseases, University of Georgia (3) College of Veterinary Medicine, University of Georgia (4) Environmental Health Sciences, University of Georgia (5) Gorgas Memorial Research Institute, Panama City, Panama Chagas disease is caused by the parasite Trypanosoma cruzi that is carried in the guts of bloodfeeding triatomine (kissing bug) vectors. Triatomines are often coinfected with the parasite T. rangeli, which is non-pathogenic to mammals but can reduce the fitness of their triatomine hosts. This study examined the gut microbial diversity of T. cruzi single-infected, coinfected, and uninfected triatomines (n = 288) throughout central Panama. We hypothesized that single and coinfected triatomines would have greater gut microbial diversity than uninfected individuals due to pathogen-microbe interactions within the gut, which may facilitate the proliferation of less dominant bacterial taxa. Coinfections were found in 13% of individuals (40/288), and there was significantly greater alpha diversity in coinfected individuals when compared to both single and uninfected samples (Dunn's test of multiple comparisons, p < 0.001). Furthermore, single T. cruzi infections were found in 32% of sampled individuals (91/288) and also displayed significantly greater alpha diversity when compared to uninfected individuals (Kruskal-Wallis H test, p < 0.001). Finally, the beta diversity of single and coinfected samples was significantly different to uninfected samples (PERMANOVA: p = 0.001). These results highlight triatomine gut microbial diversity patterns that may be influenced by infection status and will be important to consider when developing vector control strategies. Fish habitat associations in the Conasauga River and implications of climate change effects on species conservation Anna Y. Baynes1,2, Seth J. Wenger1,2, Mary C. Freeman1,2, Kyle S. McKay1,3 (1) Odum School of Ecology, University of Georgia (2) River Basin Center, University of Georgia (3) U.S. Army Corps of Engineers Flow is a key determinant of the ecological structure of a riverine ecosystem, but climate change could alter flows to which fishes have evolved. In the Southeastern US, a global hotspot of biodiversity, changing flows are a potential threat to fish communities that have already experienced declines in abundance and richness due to other anthropogenic factors. Our objective was to understand fish relationships with flowrelated microhabitat variables such as depth, velocity, Reynolds number and Froude’s number, while accounting for the effects of other habitat features such as vegetation, to guide fish conservation in a changing climate. We used a long-term data set of annual fish samples in the highly biodiverse Conasauga River, GA, to estimate occurrence and counts of 21 fish species as a function of habitat variables using generalized linear multiple regression models. Results showed that microhabitat models explained a substantial amount of the variation in counts for some species, although other species were poorly predicted. Associations with depth and velocity varied greatly among species, indicating different flow niches. We found some species had associations with specific habitat features, such as aquatic plants and woody debris, but not all our hypotheses about these associations were supported. The characterization of depths and velocities used by shoal-dwelling fish will allow us to understand the effects of future changes, such as increased frequency and severity of droughts, on populations. A wide variety of habitats need to be maintained in a changing climate to support the large fish diversity in the Conasauga River. Life stage specific effects of environmental stressors on epigenetic aging Emily M. Bertucci1,2, Marilyn W. Mason2, Junsoo Bae2, Benjamin B. Parrott1,2 (1) Odum School of Ecology, University of Georgia (2) Savannah River Ecology Laboratory, University of Georgia Age-associated patterning of DNA methylation (“epigenetic aging”) is a strong predictor of biological age as it is correlated with chronological age, the onset of age-related disease, and all-cause mortality. Using measures of epigenetic age to explore the discrepancies between chronological and biological aging offers insight into the molecular determinants of variable life history trajectories. While previous work demonstrates that adverse environmental conditions are associated with accelerated biological aging, it is not clear if some life stages are more sensitive than others to environmental conditions. Given that age-associated methylation patterns are most dynamic during early life, we hypothesized that early life exposures to environmental stress would have disproportionate effects on epigenetic aging relative to adult exposures. We tested this hypothesis by investigating the relative contributions of environmental stressors applied during different life stages of medaka (Oryzias latipes). At two distinct life stages (early life: 7-weeks post hatch and adult life: 4-months post hatch) fish were exposed for 21 days to either methylmercury or exogenous cortisol. After exposure, all fish were reared to a common age of 6-months and assessed for biological age acceleration via several measures epigenetic age estimates, telomere length, and the degree of epigenetic drift. Our results shed light on life stage sensitivity of epigenetic aging and the potential mechanism by which the epigenome translates environmental stressors into accelerated biological aging trajectories. The adaptive significance of temperature-dependent sex determination in the American alligator Samantha Bock1,2, Yeraldi Loera3, Thomas Rainwater4, Benjamin B. Parrott1,2 (1) Odum School of Ecology, University of Georgia (2) Savannah River Ecology Laboratory, University of Georgia (3) Department of Ecology & Evolutionary Biology, Princeton University (4) Belle W. Baruch Institute of Coastal Ecology & Forest Science, Clemson University Many ectotherms, including all crocodilians, rely on temperature cues experienced during development to determine offspring sex. Ever since the first descriptions of temperature-dependent sex determination (TSD) were made over 50 years ago, an understanding of its adaptive significance has remained elusive, especially in long-lived taxa. One novel hypothesis predicts that TSD should be evolutionarily favored when two criteria are met (1) incubation temperature influences juvenile survival and (2) sexes mature at different ages. Under these conditions, a sex-dependent effect of temperature on fitness arises through differences in age at sexual maturity, with the sex that matures later benefiting disproportionately from incubation temperatures that promote juvenile survival. Here, we test this hypothesis in the American alligator, a TSD species displaying a stark sex difference in age at first reproduction. By implementing a combination of artificial incubation experiments and mark-recapture methods, we disentangle the effects of incubation temperature and sex on annual survival in alligator hatchlings at two geographically distinct sites. Hatchlings incubated at male-promoting temperatures consistently exhibited higher survival compared to those incubated at female-promoting temperatures. Interestingly, this pattern appears independent of hatchling sex, as females incubated under male-promoting temperatures exhibited similar survival to their male counterparts. These findings support the hypothesis that TSD represents an adaptive sex-allocation strategy in this species. Ongoing work aims to understand the role of temperature-dependent hatchling phenotypes in mediating observed differences in early-life survival. Multivariate analyses of native and invasive plant community and the effects of deer exclusion in maritime forests Dessa L. Dunn1, Elizabeth G. King1,2, Nate P. Nibbelink2 (1) Odum School of Ecology, University of Georgia (2) Warnell School of Fore

History of the Graduate Student Symposium Year GSS Organizer ( s ) Keynote Speaker Affiliation at the time 1995

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Coral rubble dynamics in the Anthropocene and implications for reef recovery

Antecedent topography and active tectonic controls on Holocene reef geomorphology in the Great Barrier Reef

History of the Graduate Student Symposium Year GSS Organizer ( s ) Keynote Speaker Affiliation at the time 1995