Current Projects
Markwith, S. H. 2009, FAU Environmental Sciences Everglades Fellowship Initiative for project titled, "Hydrochory Analysis in the Greater Everglades Ridge and Slough Ecosystem: Decompartmentalization and Sheet Flow Enhancement’s Potential to Restore Ecological Connectivity and Species Composition." $60,734.
Project Summary
Dissection of the Everglades by canals and levees has compartmentalized and drained the Everglades system, interrupting flow and altering water levels, distribution, and timing. These disturbances have resulted in the replacement of the dominant ridge and slough ecosystem with a more uniform topographical and vegetation environment. Everglades National Park and other agencies and stakeholders involved in the Comprehensive Everglades Restoration Plan (CERP) have a direct interest in restoring natural flow regimes and biological and ecological processes to the ecosystem. The anthropogenic barriers to flow and concomitant increase in sawgrass (Cladium jamaicense) density and dominance may both create barriers to the dispersal and migration of aquatic organisms. The former barrier is being addressed by decompartmentalization, i.e. removal/degradation of levees and canals to restore natural flow. The later barrier created by ecosystem conversion is not directly addressed by CERP projects, but is expected to be overcome by restoration of the historic hydrologic regime and sheet flow. Unfortunately, few studies have examined the interaction of flow with biological processes or its role in sustaining or restoring the prevailing ecosystem (Aumen, 2003).
This research will mark the first known attempt to quantify the efficacy of flow as a dispersal vector for vegetation in the Everglades. The results will help us determine how effective decompartmentalization and the restoration of sheet flow will be in restoring the interaction between flow and ecosystem function and structure. Results may indicate that decompartmentalization alone may not restore the reference vegetation communities, and that additional efforts may be necessary to supplement the seed pool diversity and/or remove dense vegetation from degraded sloughs by mechanical means or prescribed fire. The results will also be useful for incorporation in models concerning vegetation succession/dynamics in ridge and slough, which, to this point, have been used to make predictions in the absence of a complete understanding of dispersal.
The research holds promise for demonstrating a direct interaction of flow with biological processes directly related to the ability of the system to recover once sheet flow is improved. The objectives will be accomplished by field sampling seed pool and vegetation diversity and composition using floating seed traps; experimental seed release; vegetation sampling; and using common statistical methods. The research will answer the following questions:
1. What is the species diversity and quantity of seeds of each species dispersed by flows in the Everglades ridge and slough system?
2. What is the rate and potential distance of seed movement?
3. What is the relationship of hydrochory patterns to flow and vegetation characteristics?
4. Do hydrochory, flow, and vegetation patterns differ between intact and degraded ridge and slough?
Markwith, S. H. and Xie, Z. Grant In Review. National Science Foundation Geography and Regional Sciences Program for project titled, “Modeling Gene Movement in Linear Environments: Spatial Genetic Patterns of Riverine Plants. $264,778.
Project Summary
The proposed research will determine the influence of various gene flow patterns on spatial genetic diversity, isolation-by-distance, and genetic structure of aquatic plant populations in stream networks. In the first phase of research, the genetic patterns resulting from three new simple linear gene flow models will be compared to each other, the expectations of the one-dimensional stepping stone model, and a previously existing hypothesis concerning genetic diversity patterns in streams. In the second phase, characteristics of the models (i.e. inclusion of tributaries and variation in gene flow patterns within the same model) will be independently manipulated to examine the influence of these model characteristics. The final phase of research will result in a modeled real world stream system supporting populations of Sagittaria latifolia, an aquatic plant that will be concurrently analyzed using molecular methods. The specific questions include:
1. How do the linear gene flow models compare in terms of the predicted pattern of genetic diversity and rate of isolation-by-distance?
2. In what manner does the addition of tributary branches and variation of the gene flow patterns within the same model stream network influence genetic diversity, the rate of isolation-by-distance, and partitioning of genetic variation within and among tributaries?
3. Can spatially explicit modeling of gene movement be used to predict genetic patterns among natural populations of the aquatic plant Sagittaria latifolia?
The questions will be addressed with the use of simulation modeling of idealized and spatially explicit stream networks. In addition, genetic patterns predicted by spatial modeling of populations of Sagittaria latifolia will be compared to observed patterns obtained by population genetic analysis using microsatellite markers. The same statistical analyses common to population genetics will be applied to both the model output and the molecular data to facilitate comparison of results. In addition, the natural populations will be analyzed to confirm gene flow patterns and to identify any processes other than gene flow (i.e. non-equilibrium population dynamics) that may mask the influence of gene movement.
Intellectual Merit of Proposed Research
This research will substantially expand understanding of the influence of the stream environment on genetic patterns and microevolution of aquatic plants, and potentially other aquatic organisms in these habitats. This project will mark the first analyses of three recently introduced gene flow models, the Linear Asymmetrical Adjacent Flow model (LAAF), Linear Symmetrical Non-adjacent Flow model (LSNF), and Linear Asymmetrical Non-adjacent Flow model (LANF). It will also be the first study to model the effects of linear gene flow patterns on genetic diversity, which is commonly hypothesized to increase in the downstream direction. The incorporation of tributaries in gene flow models is also a novel experiment to be conducted with this project. These iterative advancements will culminate in the first known gene flow simulation model of natural populations of aquatic plants in a spatially explicit linear network, followed by comparison of the model results to nuclear DNA molecular analysis of the natural populations.
Broader Impacts of Proposed Research
Aquatic plants play pivotal roles in the functioning of freshwater ecosystems, and their decline may have negative effects that degrade ecosystem services. The demands we are placing on freshwater systems for water consumption, waste removal, electricity, and recreation are often undertaken with little consideration of ecological or genetic impacts. The influence of stream flow on genetic patterns in aquatic plants, and other freshwater organisms such as mussels, remains an essential question for further population research if society intends to manage our impacts in freshwater systems appropriately.
Recent Research
Markwith, S.H. and Leigh, D.S. 2008. Subaqueous Hydrochory: Open-Channel Hydraulic Modeling of Non-Buoyant Seed Movement. Freshwater Biology, vol. 53, no. 11, p. 2274-2286.
Abstract
1. Subaqueous transport may be a significant dispersal and migration mechanism of non-buoyant seeds of aquatic and riparian plants, and also secondary transport of seeds once they have lost buoyancy, but the efficiency of this difficult to observe process is largely unexamined. This study uses hydraulic modeling to establish the discharges that move the non-buoyant seeds of Hymenocallis coronaria as bedload or suspended load; uses stream gage data to examine the frequency of effective discharges from late June to late September, the seed maturation and germination period; and the potential transport distance of the seeds.
2. The results show that the majority of non-buoyant seeds of H. coronaria can be transported as bedload through entire modeled stream reaches of lengths 10.8, 18, and 14.4 km with the 0.5 year return interval flow. Bedload apparently has the ability to move seeds over great distances, and may be a substantial factor determining the genetic structure, demography, and dynamics of populations and communities. However, prolonged movement of non-buoyant seeds in suspension appears to be quite rare.
3. Although insect mediated pollination and biochory occur concurrently with bedload transport, bedload transport alone may be sufficient to account for the established gene flow rate of H. coronaria. Many potential seeds transport distances exceed that between populations and migration may occur more frequently than the species’ generation time.
4. This is the first known study to use open-channel hydraulic modeling and sediment transport analysis to determine the effectiveness of non-buoyant seed transport. This method of analysis shows promise for application in other contexts, and especially where flow management is a critical issue for maintenance of rare species.
Markwith, S. H., Davenport, L. J., Shelton, J., Parker, K. C., Scanlon, M. J. 2009. Ichthyochory, the Suwannee Strait, and Population Divergence in Hymenocallis coronaria. Florida Scientist, vol. 72, no. 1, p. 28-36.
Abstract
Closure of the Suwannee Strait is a recognized vicariance event influencing the genetic divergence of populations of freshwater fauna on either side of the Atlantic/Gulf drainage divide. The macrophyte Hymenocallis coronaria also exhibits this divergence pattern, and a molecular clock analysis dates the divergence of populations to 8.6 million years ago (95% confidence interval 9.3 to 7.9 MYA), a divergence date consistent with the closure of the Suwannee Strait. This divergence pattern, along with other local scale genetic patterns, suggests a potential role of ichthyochory in the dispersal and migration of H. coronaria. We also discuss preliminary results from limited controlled ichthyochory experiments involving Ictalurus punctatus.
Dissertation Research
Completed with support from Sigma Xi, the Georgia Museum of Natural History, and National Science Foundation grant #0401799.
Markwith, S. H. and Scanlon, M. J. 2007. Multi-scale Analysis of Hymenocallis coronaria Genetic Diversity, Genetic Structure, and Gene Movement Under the Influence of Unidirectional Stream Flow. American Journal of Botany, vol. 94, no. 2, p. 151-160.
Abstract
Understanding gene movement patterns in unidirectional flow environments and their effect on patterns of genetic diversity and genetic structure is necessary to manage these systems. Hypotheses and models to explain genetic patterns in streams are rare, and the results of macrophyte studies are inconsistent. This study addresses Ritland’s (Canadian Journal of Botany 67: 2017-2024) unidirectional diversity hypothesis, the one-dimensional stepping stone model, and the metapopulation model within- and among-populations. Hymenocallis coronaria, an aquatic macrophyte of rocky river shoals of the SE USA, was sampled in four river basins. Within populations and among populations <16.2 km apart had significant isolation by distance. However, the rate of gene flow decay was not consistent with a one-dimensional stepping stone model, nor was evidence strong or consistent for Ritland’s hypothesis. Some evidence indicates that localized metapopulation processes may be affecting genetic diversity and structure; however, gene flow patterns inconsistent with the assumptions of the linear and unidirectional models are also a possible influence. We discuss three variants on the one-dimensional stepping stone model. Future research in linear environments should examine the expectations of these models. This study is also one of the first efforts to calculate population genetic parameters using a new program, TETRASAT.
Markwith, S. H. and Parker, K. C. 2007. Conservation of Hymenocallis coronaria Genetic Diversity in the Presence of Disturbance and a Disjunct Distribution. Conservation Genetics, vol. 8, p. 949-963.
Abstract
Evidence shows that dam construction has affected faunal species distribution and diversity in the southeastern U.S. However, the effects of this perturbation on aquatic flora of the region are heretofore uninvestigated. The objective for this research is to examine the effects of hydroelectric power generation and habitat characteristics on the size and distribution of populations of Hymenocallis coronaria (J. LeConte) Kunth (Amaryllidaceae), an emergent aquatic plant found in the southeastern United States, and to characterize its genetic variability in an effort to target conservation priorities. Multiple linear and logistic regression analyses show that hydropeaking does not have a significant effect on H. coronaria populations located downstream of dams, while other habitat variables associated with the streams are significantly positively related to population size and distribution. Analysis of 24 populations using seven universal cpDNA markers and six nDNA microsatellite markers indicates that the genetic structure for both genomes correlate with the location of a major gap in the species’ range, showing deep divergences in haplotype and population dendrograms between eastern and western regions. An AMOVA confirms that a substantial amount of genetic variation is found among regions, and very little variation is found within regions. The eastern and western regions have diverged for a significant time period, potentially due to the presence of a gene flow barrier in the middle of the species’ range caused by sub-standard habitat. Individual drainage basins contain unique cpDNA haplotypes and should be managed appropriately.
Markwith, S. H., Stewart, D. J., and Dyer, J. L. 2006. TETRASAT: A Program for the Population Analysis of Allotetraploid Microsatellite Data. Molecular Ecology Notes, vol. 6, p. 586-589.
Abstract
Microsatellite markers are quite popular due to their degree of polymorphism and efficiency, however the utility of such markers for analyzing allotetraploid species is often hampered by an inability to determine allele copy number for partial heterozygotes. TETRASAT is a program that uses an iterative substitution process to account for all probable combinations of allele copy numbers in populations with partial heterozygote samples. The program subsequently calculates allele frequencies, and mean Hardy-Weinberg expected heterozygosity (HE), Shannon-Weiner Diversity Index (H'), and Nei’s measure of population differentiation (GST) are reported for each locus and population. Of equal importance is the calculation of statistical variability generated by the missing data and allele substitution process, which allows for assessment of the strength of conclusions drawn from the statistics.
Markwith, S. H. and Scanlon, M. J. 2006. Characterization of Six Polymorphic Microsatellite Loci Isolated From Hymenocallis coronaria (J. LeConte) Kunth (Amaryllidaceae). Molecular Ecology Notes, vol. 6, no. 1, p. 72-74.
Abstract
Microsatellite loci were isolated from the allotetraploid aquatic plant Hymenocallis coronaria. A repeat enriched genomic library was constructed and primer pairs designed, resulting in six polymorphic loci. A total of 230 individuals were genotyped, and allelic richness per locus ranged from 3 to 11, while observed heterozygosity ranged from 0.017 to 0.570. Some amplified products were excised from agarose gel and sequenced to confirm primer specificity and mutation model. These are the first microsatellite markers developed for any member of this genus, and cross amplification was successful with the only other member of the genus tested, and with a member of the related genus Zephyranthes.
Master's Research
Markwith, S. H. and Parker, K. C. 2003. Regenerative Response of a Southern Appalachian Forest to Surface Wildfire and Canopy Gap Disturbances. Southeastern Geographer, vol. 43, no. 1, p. 54-75.
Abstract
Tree regenerative response after surface wildfire and within canopy gaps was examined in second-growth stands on sub-mesic slopes along the Blue Ridge in northeast Georgia. The understory was sampled in ten burned and ten unburned plots consisting of nested quadrats in gaps and non-gaps. Understory diversity, density, mean size, and species composition differed significantly between burned and unburned plots. Diversity and mean size were lower in burned plots, although density was higher in burned plots relative to unburned plots. Shade-intolerant species, Liriodendron tulipifera L. and Robinia pseudo-acacia L., had much greater abundance in burned plots, although Acer rubrum L. maintained dominance in both the burned and unburned understory. The evidence collected for this research supported neither the gap partitioning or density hypotheses.