The role of phenolic compounds and other plant constituents in feeding by Canada geese in a coastal marsh

Summary We have assessed the relative importance of phenolic compounds, other secondary metabolites, and gross nutrient levels as feeding cues to Canada geese. Phenolic content was the most significant constituent influencing feeding selection by geese. Nutrient content had little or no effect on feeding selection.Correlative data showing the negative influence of plant phenolics on food choices by wild geese were supported by feeding preference tests. Extracts of unpalatable plants inhibited feeding by captive geese relative to extracts of palatable plants. In high phenolic plants, the phenolic containing methanol extract was more inhibitory than extractions made with petroleum ether. In a relatively low phenolic, unpalatable plant, an inhibitory factor was extractable in petroleum ether, indicating that for this species, another class of deterrents was involved. Preference tests with individual secondary metabolites showed that tannic acid and quebracho tannin were very effective in inhibiting feeding by geese and phenolic acids were slightly inhibitory, but a sesquiterpene lactone was not deterrent. These results point out the primacy of some secondary metabolites in determining food choices by geese.     

Nitrogen flows in Louisiana Gulf Coast salt marsh: Spatial considerations

Nitrogen flux data was synthesized in developing a nitrogen flow budget for a Louisiana Barataria BasinSpartina alterniflora salt marsh. Results demonstrate the importance of spatial consideration in developing a nitrogen budget for coastal marshes. Using a mass balance approach nitrogen inputs balanced nitrogen sinks or losses from a marsh soil-plant system with a specific rooting depth. However, per unit areas on a local scale, marshes serve as a large sink for nitrogen due to rapid accretion which removes 17.O g N m^–2yr^–1 through subsidence below the root zone. On a larger spatial scale (regional) it is shown that the marshes do not serve as a large nitrogen sink. The rapid marsh deterioration currently occurring in the rapidly subsiding marshes of the Mississippi River deltaic plain account for a net regional loss of 12.5 g N m^–2yr^–1. Thus, regionally the net sink is equivalent to only 5 g N m^–2yr^–1 as compared to 17.0 g N m^–2yr^–1 on a local scale.     

Nitrogen resorption from senescing leaves of three salt marsh plant species

Seasonalvariation in leaf nitrogen of mature green and senescent leaves and nitrogenresorption efficiency in three plants (Spartina maritima, Halimioneportulacoides and Arthrocnemum perenne) of aTagus estuary salt marsh are reported. Total nitrogen concentrations in greenand senescent leaves were higher during winter (December and March). Soilinorganic nitrogen availability showed an opposite pattern with higherconcentrations during summer (June and September) when total leaf biomass washigher. Nitrogen resorption efficiency ranged between 31 and 76% andH. portulacoides was the plant that better minimizednitrogen loss by this process. Nitrogen resorption occurred mainly from thesoluble protein pool, although other fractions must have been broken down duringthe resorption process. No significant seasonal variation in nitrogen resorptionefficiency and no relation to leaf total nitrogen or soil nitrogen availabilitywere found. This suggests that the efficiency of the resorption process is notdetermined by the plant nitrogen status nor by the availability of the nutrientin the soil. Nevertheless, resorption from senescing leaves may play animportant role in the nitrogen dynamics of salt marsh plants and reduce thenitrogen requirements for plant growth.     

Nitrogen effects on an interaction chain in a salt marsh community

Nutrients can structure communities by influencing both plant interactions and plant herbivore interactions, though rarely do studies integrate these processes. In this study we examined how nitrogen fertilization influenced (1) the positive interaction between the marsh elder, Iva frutescens, and the black rush, Juncusgerardi, and (2) the quality of Iva as a host plant for the aphid, Uroleuconambrosiae. Previous studies have shown that by mitigating soil salt accumulation and hypoxia, Juncus is essential to the survival of Iva and its aphid herbivore at mid-marsh elevations. To address the effects of nitrogen on this interaction, we compared fertilized and unfertilized Iva plants subject to Juncus removal and control treatments in the field. Additionally, we measured the monthly population growth rates of aphids transplanted onto these Iva plants. Iva leaf biomass and flower number results indicated that fertilizing Iva eliminated its dependence upon Juncus, such that fertilized plants grown without Juncus were not different from unmanipulated plants. Aphid monthly population growth rates through mid-summer revealed that fertilization also eliminated the indirect dependency of aphids on Juncus, so that aphid growth rates on fertilized Iva without Juncus neighbors were similar to rates on unmanipulated Iva. Results also indicated that fertilizing Iva grown with Juncus increased Iva size, potentially enabling these plants to support larger aphid populations. Our results suggest that only under conditions of nitrogen limitation are the positive effects of Juncus essential to the mid-marsh persistence of Iva and its aphid herbivore. Furthermore, we found that nitrogen effects on aphid populations may arise not only from a direct effect of nutrients on Iva size but also through the indirect effects of nitrogen on the interaction between Juncus and Iva. We argue that studies integrating processes occurring both within and between trophic levels, are important to fully understanding the community-wide effects of nutrients. Received: 14 November 1997 / Accepted: 11 May 1998     

Food, feeding and growth rates of peracarid macro-decomposers in a Ria Formosa salt marsh, southern Portugal

The diet, feeding rates and growth rates of three species of isopod and three species of amphipod from a Ria Formosa salt marsh in southern Portugal are compared to test the hypotheses that the relative success of amphipods as macro-decomposers in salt marshes worldwide can be a) attributed to their utilizing a distinctly different range of potentially available food resources and b) attributed to them using similar food resources but at different rates.The first hypothesis was tested using a combination of gut contents analysis, stable isotope analysis and multiple-choice food preference tests. The results of all three analyses showed that there was a very broad overlap in the resource utilization curves for these species for the most abundant potential foods available in the upper salt marsh. The first hypothesis was therefore rejected.The second hypothesis was tested with palatability experiments in which consumption rates of each of the test animals were compared for each potential food offered alone. The amphipods ate all five of the foods significantly faster, consuming from 3-73× more food per unit mass than the isopods.Analyses of their relative growth rates from when released from the marsupium until first breeding, showed that amphipods have a faster growth rate than isopods in the field which is consistent with other traits in their rapid development-high fecundity life-history strategy. We conclude that these data support the second hypothesis and that their morphological adaptations to a shredding, high ingestion-rate rapid gut turnover digestive strategy enable them to have a more efficient resource acquisition rate than the slower growing, lower fecundity and slower ingestion-rate longer gut throughput time strategy of most isopods.     

Nitrogen dynamics in an Alaskan salt marsh following spring use by geese

Lesser snow geese (Anser caerulescens caerulescens) and Canada geese (Branta canadensis) use several salt marshes in Cook Inlet, Alaska, as stopover areas for brief periods during spring migration. We investigated the effects of geese on nitrogen cycling processes in Susitna Flats, one of the marshes. We compared net nitrogen mineralization, organic nitrogen pools and production in buried bags, nitrogen fixation by cyanobacteria, and soil and litter characteristics on grazed plots versus paired plots that had been exclosed from grazing for 3 years. Grazed areas had higher rates of net nitrogen mineralization in the spring and there was no effect of grazing on organic nitrogen availability. The increased mineralization rates in grazed plots could not be accounted for by alteration of litter quality, litter quantity, microclimate, or root biomass, which were not different between grazed and exclosed plots. In addition, fecal input was very slight in the year that we studied nitrogen cycling. We propose that trampling had two effects that could account for greater nitrogen availability in grazed areas: litter incorporation into soil, resulting in increased rates of decomposition and mineralization of litter material, and greater rates of nitrogen fixation by cyanobacteria on bare, trampled soils. A path analysis indicated that litter incorporation by trampling played a primary role in the nitrogen dynamics of the system, with nitrogen fixation secondary, and that fecal input was of little importance.     

Nitrogen and phosphorus limitation in a coastal barrier salt marsh: the implications for vegetation succession

1 A factorial fertilizer experiment was conducted in a 15-year-old coastal barrier salt marsh with a low soil nitrogen content, and in an older 100-year-old marsh with a higher nitrogen content. Plots were fertilized at high and low marsh elevations in both marshes. Nitrogen and phosphorus were applied at low and high concentrations both separately and in combination in each of 3 successive years.
2 Nitrogen limited above-ground plant growth in both young and old salt marshes in all years. Phosphorus limitation of plant growth was apparent in the first year in the young marsh and in the last year in both marshes. In young marshes with low soil organic matter, phosphorus limitation may occur. In addition, phosphorus limitation occurs at both successional stages when a marsh is saturated with nitrogen.
3 Plant species that are typical of nitrogen-rich habitats and late successional stages significantly increased in biomass after fertilization. Limonium vulgare , a low stature species of early and intermediate successional stages, decreased in biomass, whereas the taller Elymus pycnanthus and Artemisia maritima increased. After 3 years of fertilization, plant species composition in a young marsh was similar to the species composition in an unfertilized older marsh. Fertilization of a 100-year-old marsh, however, still resulted in a change in plant species composition, suggesting that succession was still occurring and that, overall, plants in marshes of different age are similar in their response to fertilization.     

Nitrogen further promotes a dominant salt marsh plant in an increasingly saline environment

Aims Human alterations of the environment are combining in unprecedented ways, making predictions of alterations to natural communities a difficult and pressing challenge. Estuarine systems have been subject to a high degree of modification, including increased nitrogen (N) inputs and altered salinity, factors important in shaping estuarine plant communities. As human populations increase and the climate changes, both N and salinity levels are likely to increase in these coastal marshes. Our objective was to evaluate the interactive effects of N and salinity on US West Coast salt marsh species; in particular, the performance of the dominant species Sarcocornia pacifica (pickleweed) alone and in mixed species assemblages. We expected increased salinity to favor S. pacifica but that N enrichment could help maintain greater species richness through use of N in salinity tolerance mechanisms.Methods We crossed treatments of N (added or not) and salinity (salt added or not) in a field experiment at a salt marsh in the San Francisco Estuary, California, USA, in each of three habitats: (i) monotypic pickleweed on the marsh plain, (ii) monotypic pickleweed along channels and (iii) mixed assemblages along channels. In a greenhouse experiment, we crossed treatments of N (added or not) and salinity (at three levels to simulate brackish to saline conditions) in (i) pots of pickleweed only and (ii) the same species mix as in the field.Important findings N addition doubled S. pacifica biomass and branching in both channel and marsh plain habitats regardless of salinity and greatly increased its dominance over Distichlis spicata and Jaumea carnosa in mixed assemblages along channels. In the greenhouse, S. pacifica biomass increased 6- to 10-fold with N addition over the range of salinities, while D. spicata and J. carnosa biomass increased with N addition only at lower salinity levels. Thus, while localized management could influence outcomes, expected overall increases in both N and salinity with human population growth and climate change are likely to enhance the production of S. pacifica in US West Coast marshes while reducing the diversity of mixed species assemblages. This decline in diversity may have implications for the resilience of marshes already subject to multiple stressors as the climate changes.     

Growth,reproduction, and feeding oF Fundulus heteroclitus (L.) on a North Carolina salt marsh

Fundulus heteroclitus (L.) from a North Carolina Spartina marsh teed largely on small crustaceans (amphipods, tanaids and copepods) and polychaetes. Fish longer than 30 mm standard length also ingested considerable amounts of living plant material. Smaller individuals were distinctly carnivorous. Recognizable particles of Spartina detritus occurred in less than 15% of the guts examined. The relationship of weight to length changed significantly during the year. Although females were larger than males of the same age. males were heavier than females of the same length, except for a brief period at the peak of the spawning season in the early spring. An average second season fish may lay up to 512 eggs from March through August, but first season fish did not reach reproductive size by the end of the spawning season. Growth of first season fish in mid-summer averaged 5% of their total weight per day. These significant seasonal changes in ecological properties of killifish populations are important in any estimates of growth, reproduction, and production.     

The role of organic acids in assisted phytoremediation processes of salt marsh sediments

Some plants have high ability to absorb heavy metals in high concentrations. In this study, Spartina maritima was tested in conjunction with low molecular weight organic acids (LMWOA), in order to evaluate the possible use of this plant in phytoremediation processes in salt marshes. Three different LMWOA (citric acid, malic acid and acetic acid) were applied to contaminated intact cores of S. maritima colonized sediment and several heavy metals (Cd, Zn, Pb, Cu, Cr and Ni) were analyzed in sediment and plant parts. Acetic acid application proved to be the most efficient, enhancing greatly the uptake of all metals analyzed. Citric acid also showed good results, while malic acid proved to be very inefficient in most of the cases. The highest enhancement was observed for Cr with a 10-fold increase of the uptake upon application of acetic acid, while improving the Pb uptake proved to be the most difficult, probably due to its low solubility.     

Effects of regular salt marsh haying on marsh plants, algae, invertebrates and birds at Plum Island Sound, Massachusetts

The haying of salt marshes, a traditional activity since colonial times in New England, still occurs in about 400 ha of marsh in the Plum Island Sound estuary in northeastern Massachusetts. We took advantage of this haying activity to investigate how the periodic large-scale removal of aboveground biomass affects a number of marsh processes. Hayed marshes were no different from adjacent reference marshes in plant species density (species per area) and end-of-year aboveground biomass, but did differ in vegetation composition. Spartina patens was more abundant in hayed marshes than S. alterniflora, and the reverse was true in reference marshes. The differences in relative covers of these plant species were not associated with any differences between hayed and reference marshes in the elevations of the marsh platform. Instead it suggested that S. patens was more tolerant of haying than S. alterniflora. Spartina patens had higher stem densities in hayed marshes than it did in reference marshes, suggesting that periodic cutting stimulated tillering of this species. Although we predicted that haying would stimulate benthic chlorophyll production by opening up the canopy, we found differences to be inconsistent, possibly due to the relatively rapid regrowth of S. patens and to grazing by invertebrates on the algae. The pulmonate snail, Melampus bidendatus was depleted in its δ¹³C content in the hayed marsh compared to the reference, suggesting a diet shift to benthic algae in hayed marshes. The stable isotope ratios of a number of other consumer species were not affected by haying activity. Migratory shorebirds cue in to recently hayed marshes and may contribute to short term declines in some invertebrate species, however, the number of taxa per unit area of marsh surface invertebrates and their overall abundances were unaffected by haying over the long term. Haying had no impact on nutrient concentrations in creeks just downstream from hayed plots, but the sediments of hayed marshes were lower in total N and P compared to references. In sum, haying appeared to affect plant species composition but had only short-term affects on consumer organisms. This contrasts with many grassland ecosystems, where an intermediate level of disturbance, such as by grazing, increases species diversity and may stimulate productivity. From a management perspective, periodic mowing could be a way to maintain S. patens habitats and the suite of species with which they are associated.     

Palatability of salt marsh forbs and grasses to the purple marsh crab (Sesarma reticulatum) and the potential for re-vegetation of herbivory-induced salt marsh dieback areas in cape cod (Massachusetts, USA)

Intense herbivory by a growing population of intertidal burrowing crabs Sesarma reticulatum (purple marsh crabs) has denuded large areas of salt marsh on Cape Cod (Massachusetts, USA). Spartina alterniflora (smooth cordgrass) and, to a lesser extent, S. patens (salt marsh hay) have been the primary taxa affected while halophytic forb populations of Salicornia spp. (pickleweed), Suaeda maritima (sea-blite), and Limonium carolinianum (sea lavender), that normally constitute a relatively low proportion of marsh vegetation, have remained intact. In addition, these forb species appear to be colonizing some of the marsh grass dieback areas. Because the loss of vegetation results in considerable subsidence and erosion, the objective of this study was to (1) confirm whether certain taxa are unpalatable to S. reticulatum and (2) determine whether unpalatable species could be used to re-vegetate dieback areas as an interim measure to control marsh sediment and elevation loss. The results suggest that S. reticulatum prefers Spartina alterniflora over forbs and that one or all of these forb species are good candidates for vegetation restoration in dieback areas.     

Geographic variation in salt marsh structure and function

We examined geographic variation in the structure and function of salt marsh communities along the Atlantic and Gulf coasts of the United States. Focusing on the arthropod community in the dominant salt marsh plant Spartina alterniflora, we tested two hypotheses: first, that marsh community structure varies geographically, and second, that two aspects of marsh function (response to eutrophication and addition of dead plant material) also vary geographically. We worked at eleven sites on the Gulf Coast and eleven sites on the Atlantic Coast, dividing each coast up into two geographic areas. Abiotic conditions (tidal range, soil organic content, and water content, but not soil salinity), plant variables (Spartina nitrogen content, height, cover of dead plant material, but not live Spartina percent cover or light interception), and arthropod variables (proportional abundances of predators, sucking herbivores, stem-boring herbivores, parasitoids, and detritivores, but not total arthropod numbers) varied among the four geographic regions. Latitude and mean tidal range explained much of this geographic variation. Nutrient enrichment increased all arthropod functional groups in the community, consistent with previous experimental results, and had similar effects in all geographic regions, contrary to our hypothesis, suggesting widespread consistency in this aspect of ecosystem function. The addition of dead plant material had surprisingly little effect on the arthropod community. Our results caution against the uncritical extrapolation of work done in one geographic region to another, but indicate that some aspects of marsh function may operate in similar ways in different geographic regions, despite spatial variation in community structure.     

Saturnospora ahearnii,a new salt marsh yeast from Louisiana

Saturnospora ahearnii, a new, nonhyphal saturn-spored yeast, is described. The type strain (NRRL Y-7555, CBS 6121) of this species was isolated from the rhizosphere of marsh grass in Louisiana. Comparisons of nuclear DNA complementarity showedS. ahearnii to be closely related toS. saitoi, a species presently known only from Japan.     

Aphid feeding deterrency of ellagitannins, their phenolic hydrolysis products and related phenolic derivatives

Feeding deterrent activities of ellagic acid, two ellagitannins, gallic acid, pyrogallol, and several gallic acid derivatives towards three species of aphids were determined. The most sensitive species tested was Schizaphis graminum (Rondani), the least sensitive was Acyrthosiphon pisum (Harris). Myzus persicae (Sulzer) was of intermediate sensitivity. Ellagic acid (ED₅₀=15 ppm) and n-decyl gallate (ED₅₀=16 ppm) were particularly potent against S. graminum, while n-octyl gallate was the most active compound tested against A. pisum (ED₅₀=182 ppm) and M. persicae (ED₅₀=56 ppm). The ellagitannins, geraniin and pedunculagin, were active against S. graminum and M. persicae, but not against A. pisum. Methylation of the free hydroxyl groups of gallic acid resulted in a large decrease in activity, while esterification of its carboxyl group with alkyl chains of increasing length resulted in increasing activity against S. graminum. Against A. pisum and M. persicae, ellagic acid, gallic acid and 3,4,5-trimethoxybenzoic acid were inactive, whereas pyrogallol and the gallate esters were at least moderately active as feeding deterrents.

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Résumé L'examen a porté sur l'action répulsive, lors de l'alimentation de trois espèces de pucerons, de l'acide ellgique, de deux ellagitanins, de l'acide gallique, du pyrogallol et de plusieurs dérivés de l'acide gallique. Schizaphis graminum Rondani a été l'espèce la plus sensible, tandis que Acyrthosiphon pisum. Harris a été la moins sensible; la sensibilité de Myzus persicae Sulzer était intermédiaire. L'acide ellagic (ED₅₀=15 ppm) et le n-décyl gallate (ED₅₀=16 ppm) ont été particulièrement actifs contre S. graminum, tandis que le n-octyl gallate a été le produit le plus actif contre A. pisum (ED₅₀=182 ppm) et M. persicae (ED₅₀=56 ppm). Les ellagitanins, géraniine et pédunculagine ont été actifs contre S. graminum et M. persicae, mais pas contre A. pisum. La méthylation des groupes hydroxyl libres de l'acide gallique a réduit fortement l'activité, tandis qu l'estérification de son groupe carboxyl avec des chaînes alkyl de longuers croissantes a augmenté l'activité contre S. graminum. Les acides ellagique, gallique et 3,4,5-triméthoxybenzoïque ont été inactifs contre A. pisum et M. persicae, tandis que le pyrogallol et les esters de gallate on été pour le moins des répulsifs modérément actifs au cours de l'alimentation.

     

The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.     

Self-organization and vegetation collapse in salt marsh ecosystems

Complexity theory predicts that local feedback processes may strongly affect the organization of ecosystems on larger spatial scales. Whether complexity leads to increased resilience and stability or to increased vulnerability and criticality remains one of the dominant questions in ecology. We present a combined theoretical and empirical study of complex dynamics in mineralogenic salt marsh ecosystems that emerge from a positive feedback between clay accumulation and plant growth. Positive feedback induces self-organizing within the ecosystem, which buffers for the strong physical gradient that characterizes the marine-terrestrial boundary, and improves plant growth along the gradient. However, as a consequence of these self-organizing properties, salt marshes approach a critical state as the edge of the salt marsh and the adjacent intertidal flat becomes increasingly steep and vulnerable to wave attack. Disturbance caused, for instance, by a storm may induce a cascade of vegetation collapse and severe erosion on the cliff edge, leading to salt marsh destruction. Our study shows that on short timescales, self-organization improves the functioning of salt marsh ecosystems. On long timescales, however, self-organization may lead to destruction of salt marsh vegetation.     

Nutrients, competition and plant zonation in a New England salt marsh

1 We examined the effects of nutrient availability on the competitive interactions of the New England salt marsh perennials that occupy discrete vegetational zones parallel to the shoreline.
2 Fertilized and unfertilized plots of pair-wise mixtures and monocultures of Spartina alterniflora, S. patens and Juncus gerardi were compared in order to assess the effects of nutrient addition on the competitive dynamics of these species in the field. In addition, we examined competition between some of these species and Distichlis spicata , a species common to disturbed marsh habitats.
3 After two growing seasons, changes in above-ground biomass of the species indicated that in fertilized plots, S. alterniflora outcompeted S. patens, S. patens outcompeted J. gerardi, and D. spicata outcompeted both J. gerardi and S. patens. This was the reverse of the interactions seen under ambient marsh conditions, and suggested that, under conditions of nutrient limitation, competitive dominance may result from efficient competition for nutrients.
4 Using a conceptual model of salt marsh zonation as a function of competition, physical stress and nutrient limitation, we hypothesize that a nutrient-induced reversal in the competitive dynamics among salt marsh perennials may result in modification of the pattern of plant zonation in this and similar marshes.