Factors influencing occupancy and density of salt marsh songbirds in northeast Florida

Salt marshes and the organisms that depend on them are subject to a variety of anthropogenic threats. In Florida, Worthington’s Marsh Wrens (Cistothorus palustris griseus) and MacGillivray’s Seaside Sparrows (Ammospiza maritima macgillivraii) are species of concern that inhabit a small, narrow range of salt marsh in the northeastern corner of the state, an area of increasing human development. The historic ranges of these subspecies encompassed salt marshes in five counties, but their ranges had contracted to just two counties by the early 2000s and their populations declined. We surveyed the historic ranges of the two subspecies during the breeding seasons of 2014 and 2015 to document their distributions, identify habitat features that influenced occupancy and density, and assess whether any recolonization had occurred in areas previously abandoned. We found that the ranges of both subspecies remained relatively stable compared to the early 2000s, with no signs of either further contraction or recolonization. Both Marsh Wrens and Seaside Sparrows were more likely to occupy areas farther from uplands. Marsh Wren occupancy was positively associated with marshes dominated by smooth cordgrass (Spartina alterniflora) and negatively associated with marshes dominated by black needlerush (Juncus roemerianus). Seaside Sparrows were more likely to occur at sites of moderate elevation. We found greater densities of both subspecies in areas farther from uplands, with moderate elevations, and dense vegetation. Marsh Wren density also increased in smooth cordgrass marshes, whereas sparrow numbers increased in areas of moderate vegetation height. Despite these differences between subspecies, the need for dense vegetation away from uplands highlights the importance of smooth cordgrass marshes in the region.     

Geographic structure, genetic diversity and source tracking of Spartina alterniflora

Aim To examine the distribution and structure of genetic variation among native Spartina alterniflora and to characterize the evolutionary mechanisms underlying the success of non‐native S. alterniflora. Location Intertidal marshes along the Atlantic, Gulf and Pacific coasts of North America. Methods amova , parsimony analysis, haplotype networks of chloroplast DNA (cpDNA) sequences, neighbour‐joining analysis, Bayesian analysis of population structure, and individual assignment testing were used. Results Low levels of gene flow and geographic patterns of genetic variation were found among native S. alterniflora from the Atlantic and Gulf coasts of North America. The distribution of cpDNA haplotypes indicates that Atlantic coast S. alterniflora are subdivided into ‘northern’ and ‘southern’ groups. Variation observed at microsatellite loci further suggests that mid‐Atlantic S. alterniflora are differentiated from S. alterniflora found in southern Atlantic and New England coastal marshes. Comparisons between native populations on the Atlantic and Gulf coasts and non‐native Pacific coast populations substantiate prior studies demonstrating reciprocal interspecific hybridization in San Francisco Bay. Our results corroborate historical evidence that S. alterniflora was introduced into Willapa Bay from multiple source populations. However, we found that some Willapa Bay S. alterniflora are genetically divergent from putative sources, probably as a result of admixture following secondary contact among previously allopatric native populations. We further recovered evidence in support of models suggesting that S. alterniflora has secondarily spread within Washington State, from Willapa Bay to Grays Harbor. Main conclusions Underlying genetic structure has often been cited as a factor contributing to ecological variation of native S. alterniflora. Patterns of genetic structure within native S. alterniflora may be the result of environmental differences among biogeographical provinces, of migration barriers, or of responses to historical conditions. Interactions among these factors, rather than one single factor, may best explain the distribution of genetic variation among native S. alterniflora. Comprehensive genetic comparisons of native and introduced populations can illustrate how biological invasions may result from dramatically different underlying factors – some of which might otherwise go unrecognized. Demonstrating that invasions can result from several independent or interacting mechanisms is important for improving risk assessment and future forecasting. Further research on S. alterniflora not only may clarify what forces structure native populations, but also may improve the management of non‐native populations by enabling post‐introduction genetic changes and the rapid evolution of life‐history traits to be more successfully exploited.     

Native and Introduced Ecosystem Engineers Produce Contrasting Effects on Estuarine Infaunal Communities

Cordgrasses in the genus Spartina are good examples of ecosystem engineers that modify habitat structure in estuaries throughout the world. In San Francisco Bay, California, USA, marshes containing native California cordgrass (Spartina foliosa) are being invaded by a hybrid (S. alterniflora × S. foliosa) formed after introduction of S. alterniflora. This study compared vegetation, sediment structure, and infaunal invertebrates in native and invaded marshes. We hypothesized that differences in the physical structure between S. foliosa and hybrid Spartina would be reflected in differences in density, biomass, diversity, and taxonomic composition of infauna. Hybrid Spartina modifies habitat structure more than S. foliosa by producing taller stems, and greater plant biomass both above- and belowground while occupying a much wider tidal range, thereby transforming open mudflats to a vegetated habitat. In general, S. foliosa areas contained significantly higher densities of benthic infauna than adjacent mudflats, while hybrid Spartina areas never contained greater infaunal densities than mudflats. This is because S. foliosa produces a moderate level of structure that can facilitate benthic invertebrates, whereas hybrid Spartina produces so much structure, particularly belowground, that it actually excludes invertebrates. Therefore, we suggest that these two closely related species both act as ecosystem engineers, but with opposing effects on invertebrate communities.     

Low genetic diversity contrasts with high phenotypic variability in heptaploid Spartina densiflora populations invading the Pacific coast of North America

Species can respond to environmental pressures through genetic and epigenetic changes and through phenotypic plasticity, but few studies have evaluated the relationships between genetic differentiation and phenotypic plasticity of plant species along changing environmental conditions throughout wide latitudinal ranges. We studied inter‐ and intrapopulation genetic diversity (using simple sequence repeats and chloroplast DNA sequencing) and inter‐ and intrapopulation phenotypic variability of 33 plant traits (using field and common‐garden measurements) for five populations of the invasive cordgrass Spartina densiflora Brongn. along the Pacific coast of North America from San Francisco Bay to Vancouver Island. Studied populations showed very low genetic diversity, high levels of phenotypic variability when growing in contrasted environments and high intrapopulation phenotypic variability for many plant traits. This intrapopulation phenotypic variability was especially high, irrespective of environmental conditions, for those traits showing also high phenotypic plasticity. Within‐population variation represented 84% of the total genetic variation coinciding with certain individual plants keeping consistent responses for three plant traits (chlorophyll b and carotenoid contents, and dead shoot biomass) in the field and in common‐garden conditions. These populations have most likely undergone genetic bottleneck since their introduction from South America; multiple introductions are unknown but possible as the population from Vancouver Island was the most recent and one of the most genetically diverse. S. densiflora appears as a species that would not be very affected itself by climate change and sea‐level rise as it can disperse, establish, and acclimate to contrasted environments along wide latitudinal ranges.     

Seasonal and vertical demography of dead portions of shoots of smooth cordgrass in a south-temperate saltmarsh

Knowledge of the crop-size and vertical profile with respect to sea level of dead leaf blades and other shoot parts of saltmarsh grasses is a prerequisite for projections of seawater impact upon saltmarsh grass-shoot decomposers. We measured seasonal changes in quantities of leaf blade and leaf sheath+stem of smooth cordgrass in southern temperate saltmarshes of three types: low- (LD), mid- (MD), and high-drainage (HD) marshes (i.e., with low to high densities of drainage creeks). In each type of marsh, we took samples in the three characteristic subsites, short-shoot, intermediate-height, and tall-shoot. Wholly dead (brown) and living (some green) shoots were counted and their canopy heights measured, and the mass of all shoots was measured in 10-cm height increments (above sediment), with separation of living and dead parts. Detached material was also quantified, separated into that trapped in the canopy, and that prostrate on the sediment. Extent of marsh drainage did not affect the density of wholly dead shoots, which ranged from an average of 51 m⁻² in autumn to 103 m⁻² in spring, and 57 m⁻² in tall subsites to 89–96 m⁻² in short and intermediate subsites. The top of the dead-shoot canopy was about 20 cm taller than the living-shoot canopy in winter/spring, but was below it ( ≈50 cm) in summer/autumn. The mass of standing-dead leaf blades was 4- to 5-fold greater in winter than in summer, and it was about 2.5-fold greater at tall than at short sites. Detached litter trapped among shoots above the sediment averaged 26% of total dead-shoot mass, but litter lying on the sediment averaged only 6% of total dead-shoot mass. Since export of macrodetritus from marshgrass stands is inconsequential, the small prostrate-litter mass implies that breakdown of shoots to small particles takes place within the vegetated marsh, and that standing decay predisposes fallen material to rapid disintegration. Comparison of heights of usual tidal contact on shoots at the short- and tall-shoot subsites showed that projected impacts of seawater upon leaf-blade decay would be greater at short-shoot subsites (high marsh) than at tall-shoot subsites (low marsh). This was because the vertical profile of dead-blade mass resulted in more of the short-shoot dead-blade mass residing within the vertical span most regularly contacted by flooding tides.     

Effect of nitrogen source concentration on curdlan production by Agrobacterium sp. ATCC 31749 grown on prairie cordgrass hydrolysates

The effect of nitrogen source concentration on the production of the polysaccharide curdlan by the bacterium Agrobacterium sp. ATCC 31749 from hydrolysates of prairie cordgrass was examined. The highest curdlan concentrations were produced by ATCC 31749 when grown on a medium containing a solids-only hydrolysate and the nitrogen source ammonium phosphate (2.2 mM) or on a medium containing a complete hydrolysate and 3.3 mM ammonium phosphate. The latter medium sustained a higher level of bacterial curdlan production than the former medium after 144 hr. Biomass production by ATCC 31749 was highest after 144 hr when grown on a medium containing a solids-only hydrolysate and 2.2 or 8.7 mM ammonium phosphate. On the medium containing the complete hydrolysate, biomass production by ATCC 31749 was highest after 144 hr when 3.3 mM ammonium phosphate was present. Bacterial biomass production after 144 hr was greater on the complete hydrolysate medium compared to the solids-only hydrolysate medium. Curdlan yield produced by ATCC 31749 after 144 hr from the complete hydrolysate medium containing 3.3 mM ammonium phosphate was higher than from the solids-only hydrolysate medium containing 2.2 mM ammonium phosphate.     

Effects of a tropical cyclone on salt marsh insect communities and post-cyclone reassembly processes

Concepts regarding effects of recurrent natural disturbances and subsequent responses of communities are central to ecology and conservation biology. Tropical cyclones constitute major disturbances producing direct effects (damage, mortality) in many coastal communities worldwide. Subsequent reassembly involves changes in composition and abundance for which the underlying mechanisms (deterministic and stochastic processes) are still not clear, especially for mobile organisms. We examined tropical cyclone-induced changes in composition and reassembly of entire insect communities in 16 Louisiana coastal salt marshes before and after Hurricane Isaac in 2012 and 2013. We used the Shannon index and multivariate permutational ANOVA to study insect resistance and resilience, β diversity partitioning to evaluate the importance of species replacement, and null models to disentangle the relative roles of different assembly processes over time after the tropical cyclone. The α diversity and species composition, overall and for different trophic levels, decreased immediately after the tropical cyclone; nonetheless, both then increased rapidly and returned to pre-cyclone states within one year. Changes in species abundance, rather than species replacement, was the primary driver, accounting for most temporal dissimilarity among insect communities. Stochastic processes, which drove community composition immediately after the tropical cyclone, decreased in importance over time. Our study indicates that rapid reformation of insect communities involved sequential landscape-level dynamics. Cyclone-resistant life cycle stages apparently survived in some, perhaps random locations within the overall salt marsh landscape. Subsequently, stochastic patterns of immigration of mobile life cycle stages resulted in rapid reformation of local communities. Post-cyclone direct regeneration of salt marsh insect communities resulted from low resistance, coupled with high landscape-level resilience via re-immigration. Our study suggests that the extent of direct regeneration of local salt marsh insect communities might change with the size of larger marsh landscapes within which they are imbedded.     

Variation in self-fertility and the reproductive advantage of self-fertility for an invading plant (Spartina alterniflora)

The factors responsible for the reproductive success or failure of individuals in small, founding populations have received little attention. Previous work on a small population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay, California found that most clones flower prolifically but set little or no seed, while a few clones have high rates of viable seed set, producing most of the seeds in the population. This study first identifies recruitment from seeds as the main source of new smooth cordgrass plants during invasion and then tests the influence of growing conditions and pollination treatment on viable seed set among clones established in San Francisco Bay. Field transplants indicated that a clone's seed set rate was not strongly dependent on its site of establishment. Low and high nutrient greenhouse treatments also had little effect on viable seed set rates within most clones. In contrast, pollination treatment (self-pollination or outcrossing) had a major effect on viable seed set rates. Most clones had high seed set rates after outcross-pollination, but clones varied widely in their selfing capacity. Zero or low viable self-seed set rates were most common; however, a few clones had high viable self-seed set rates, comparable to outcross seed set rates. A clone's selfing capacity was significantly correlated across years (r=0.89, P<0.001), and capacity to set viable self-pollinated seeds in the greenhouse was significantly correlated with the clone's rate of viable seed set in the field . In this growing population where cross-pollination is limited, only the clones with high selfing ability had high viable seed set rates in the field. Among primarily outcrossing plant invaders, variation in self-fertility among individuals may be a common phenomenon, with important implications for genetic differentiation, effective population size and patterns of spatial spread during an invasion.