GCE-II Question 5: Organism Distribution
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Research QuestionQ5: What is the relative importance of larval transport versus the conditions of the adult environment in determining community and genetic structure across both the longitudinal and lateral gradients of the estuarine landscape? OverviewWe have documented a variety of distribution patterns of different species across the GCE domain. These include 1) a close correlation with salinity (most plants, marine invertebrates), 2) densities decreasing from the barrier island to the mainland (most marine invertebrates), 3) densities low at mid-estuary sites (grasshoppers), and 4) no systematic spatial variation (beetles). Some of the variation in population density is likely driven by longitudinal and lateral gradients in the estuarine environment (Questions 3 and 4). However, population density may also be affected by transport mechanisms and larval shadows that affect larval delivery, the presence of adjacent upland habitat, habitat suitability for adults, and competition. We are using a combination of recruitment studies, transplant studies, and genetic approaches to begin to explore how various mechanisms might create population structure across the landscape. We are working with a range of species chosen for ecological importance, experimental tractability, and contrasting life histories. Research Components
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Research Question BackgroundThe densities of plants and animals vary across the estuarine landscape. For example, the gastropod Littoraria is more abundant at barrier island sites than at mainland sites (even when these do not differ much in salinity), whereas grasshoppers are abundant at sites adjacent to upland (either barrier island or mainland) and absent at mid-estuary sites. Some of this variation can be explained by differences in the life histories of the organisms involved. Salt marsh plants are highly clonal, and their population distributions will be determined in large part by the performance of established clones. Similarly, populations of animals with direct development will be affected primarily by adult performance and reproduction at each site. In contrast, populations of animals with planktonic larvae are likely to be highly affected by factors mediating the movement of larvae and subsequent recruitment to each site. Sites that are "downstream" of suitable sites may experience a recruitment "shadow" because most competent larvae in the water column have already settled, and sites without an upstream source of larvae will also experience low recruitment. In contrast, currents may have little influence on recruitment of marine invertebrates with direct development, insects or plants (except for those with floating seeds). Moreover, larvae of some marine invertebrates from estuarine habitats behaviorally exploit or defeat current patterns, potentially obscuring simple relationships between currents and larval supply. Once they are established, the performance of plants or animals will be affected by habitat quality. The landscape distribution of different habitat types (upland, intertidal, subtidal) will interact with spatially variable inputs of fresh and sea water to create a mosaic of habitat patches with varying suitability for any particular species. Habitat quality will not necessarily correlate with recruitment or population density. As described above, planktonic larvae may never reach high-quality sites if these sites lack an upstream source of larvae or are downstream from other high-quality sites. Similarly, species requiring upland habitat for some phase of their life cycle will rarely colonize high quality marsh patches far from upland patches. Finally, performance will also depend on interactions with competitors and consumers. High competition or predation may lead to low survival and growth, even if sites are otherwise of high quality. In particular, high recruitment of a species is likely to produce intense intraspecific competition, leading to a negative correlation between recruitment and individual size. Variation in habitat quality and interactions with conspecifics and other species may lead to different patterns of local selection across the landscape, with different genotypes dominating the adult population at different sites, even if the recruit population is well mixed across sites.  We are addressing three major questions:
Our approach involves documenting distribution patterns, measuring recruitment using larval traps, outplanting species with and without competition to measure post-recruitment survival and growth, using molecular tools to identify patterns of genetic structure across sites, and using cellular automata models to explore how various mechanisms might create population structure across the landscape. |
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Distribution of plantsH. Guo (Ph.D. student, UH) and S. Pennings (UH) are conducting a series of transplant experiments to explain patterns of vegetation composition, diversity and productivity (see Question 3) in the GCE domain. Over the years 2007, 2008 and 2009, Guo transplanted three saltmarsh plants, Spartina alterniflora, Batis maritima and Salicornia virginica, three brackish marsh plants, Juncus roemerianus, Spartina cynosuroides, and Schoenoplectus americanus, and three fresh marsh plants, Pontederia cordata, Zizaniopsis milacea and an unidentified species, to replicate fresh, brackish and saline sites, with and without competition. Plants were transplanted in March of each year, and harvested, dried and weighed in October of each year, after 7 months of growth. Sites were categorized into 5 regions of salinity (fresh, brackish, low-salinity salt, medium-salinity salt, and high-salinity salt) based on GCE water column monitoring and porewater salinity data collected by Guo. The strength of competition among transplanted plants and background vegetation was calculated for each salinity zone as RII, Relative Interaction Intensity, or (Biomass with neighbor - Biomass without neighbor)/(Biomass with neighbor + Biomass without neighbor).  Distribution of marine invertebratesLow marsh recruitment: Oysters and barnaclesD. Bishop deployed recruitment samplers at all ten GCE sites on a monthly basis (starting June 2008) to document spatial patterns of larvae (spat) of Crassostrea virginica. Early results indicate lower than normal recruitment for the period of June-July. This has been confirmed by other researchers examining oyster recruitment (Dr. Randal Walker, Marine Extension, pers. com.). One surprising outcome was that the greatest spat recruitment was at GCE 8, which is well within the freshwater influence of the Altamaha River and far from any oyster reefs. This observation is being evaluated in light of transport dynamics and larval retention times. Mid-marsh recruitment: snails and bivalvesB. Silliman (UF) and his students are deploying potted Spartina plants, bundles of oyster shell, and two other recruitment sampler devices in the mid-marsh to document spatial patterns of recruitment of marsh snails (Littoraria), bivalves (Geukensia) and other marsh invertebrates. Silliman has expanded the basic 10-site design of the study to 64 sites in order to look at 1) onshore-offshore patterns caused by recruitment shadows, 2) high- versus low-flow conditions, and 3) high- versus low-stream order conditions. Genetic structure across the GCE domainWares (UGA) is conducting a survey of the genetic structure of organisms in the GCE domain. Extant microsatellite loci were optimized for use in the oyster Crassostrea; novel microsatellite markers were developed for use in fiddler crabs (genus Uca). His initial results indicate no signal of differentiation among sites, suggesting that processes involving local adaptation to environmental gradients are minimal. However, genetic diversity (across 8 species) tends to be highest at sites on the ocean side of the array (GCE 3, 6, 9, 10) relative to genetic diversity at mainland or estuary sites (GCE 1, 4, 7, 2, 5, 8). This may reflect a larval shadow effect, in which the propagule pressure to sites proximal to the ocean is much higher and that translates to a larger number of retained alleles per population regardless of species considered. Distribution of terrestrial invertebratesGrasshoppersAnnual sampling of marsh grasshoppers by Pennings (UH) has documented that densities are highest at barrier island sites, intermediate at mainland sites, and very low at mid-estuary sites. Observations indicated that grasshopper communities were dominated by one species, with a second common at a few sites. A preliminary analysis of spatial variation in grasshopper densities from 2001 through 2008 (data are means and SE averaged over years) found that densities (all species combined) differed more than ten-fold among sites. Several mechanisms might explain these spatial patterns in grasshopper population density. Experiments to address these hypotheses will be conducted by Pennings (UH) and his students in the next 3 years.  ModelingIn coming years, Burd (UGA) will use a variety of modeling approaches to help understand the processes creating different distribution patterns of different taxa across the GCE domain. Additional studiesRelated studies include water column recruitment of blue crabs. |
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Distribution of marine invertebratesHow does predator species evenness affect a trophic cascade? Oyster and barnacle recruitment Snail and bivalve recruitment Distribution of plantsMarsh transplant experiments Photomapping of marsh vegetation Distribution of terrestrial invertebratesGenetic diversity in the pulmonate snail Melampus bidentatus Genetic structure across the GCE domain Grasshopper distribution within the GCE |
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Journal ArticlesDiaz-Ferguson, E., Robinson, J.D., Silliman, B.R. and Wares, J.P. 2010. Comparative Phylogeography of East Coast American Salt Marsh Communities. Estuaries and Coasts. 33:828-839. (DOI: 10.1007/s12237-009-9220-6) Conference Posters and PresentationsGuo, H., Pennings, S.C. and Wieski, K. 2008. Poster: Physical stress, plant productivity, competition, and diversity in Georgia tidal marshes. Coastal Habitats. 93rd Annual Meeting of the Ecological Society of America, August 3-8, 2008, Milwaukee, Wisconsin. |
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Data Sets by LTER Core Area and Site Research TopicCore LTER Data SetsAquatic Invertebrate EcologyPopulation EcologyTerrestrial Insect EcologyLong-term mid-marsh grasshopper abundance and species diversity at eight GCE-LTER sampling sites Abundance of the planthopper Prokelisia at GCE LTER sampling sites in October, 2003-2019 |
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Research Project Principal InvestigatorsThomas Dale Bishop, University of Georgia John Griffin, University of Swansea Steven C. Pennings, University of Houston Brian R. Silliman, Duke University John P. Wares, University of Georgia Other Associated PersonnelHongyu Guo, University of Houston Jesyka Melendez, University of Puerto Rico – Cayey James C. Nifong, Kansas State University Monica Poelchau, University of Georgia John D. Robinson, University of Georgia Brian R. Silliman, Duke University |
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