Intertidal habitat loss and wildfowl numbers: applications of a spatial depletion model

1. A spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation at Lindisfarne National Nature reserve, north-east England, was used to investigate the capacity of the site's beds of Zostera and other intertidal vegetation to support brent geese Branta bernicla hrota and wigeon Anas penelope .
2. Recent total winter counts of brent geese and wigeon were both only 40% of the maximum that the food supply at the site could theoretically support. Other factors must have been restricting their numbers. Earlier arrival of brent geese at the site could increase the number of brent goose-days which could be supported, but would have only a slight negative effect on the wigeon-days.
3. The model was used to examine three conservation issues: encroachment of Spartina anglica , sea level rise and loss of food plants from the whole site (which could result from increased autumn storms or plant disease). Loss from the top of the shore through encroachment by Spartina anglica had the greatest effect on the site's capacity to sustain geese and wigeon. Loss from the bottom of the shore, as would occur through sea level rise, had less impact. Increased loss of vegetation over the whole site would have an intermediate effect.
4. This work has important implications for the management of the site. Factors such as hunting, that may be restricting current numbers below those that could be supported by the food supply, require urgent investigation. Model predictions indicate that encroachment of Spartina is likely to depress local populations of brent geese and wigeon under current conditions only if it results in the complete loss of Zostera from the top 500 m of the shore.     

Intra-guild predation relaxes natural enemy impacts on herbivore populations

Abstract. 1. To investigate the role of intra-guild predation in mediating the impact of the natural enemy complex on herbivore populations, a manipulative field experiment was conducted using uncaged plots (islets of Spartina cordgrass) on a North American salt marsh. The densities (moderate or low) of two invertebrate predators, the generalist wolf spider Pardosa littoralis and the specialist mirid bug Tytthus vagus , were manipulated in a 2 × 2 factorial design, and the resulting treatment effects on the population growth of their herbivorous prey, Prokelisia planthoppers, were assessed.
2. The abundance of wolf spiders on experimental islets was unaffected by the presence of mirid bugs, however the density of mirid bugs was influenced very negatively by the presence of the wolf spider.
3. The negative effect of the wolf spider on mirid bugs most probably resulted from the intra-guild predation of mirids by spiders because planthopper limitation by the wolf spider alone was significantly greater than when both predators were present.
4. As a result of intra-guild predation, planthopper population growth was positive in the presence of both predators, despite the fact that each predator alone promoted a decrease in planthopper population growth.
5. Notably, the occurrence of intra-guild predation diminished top-down impacts on planthopper populations in a relatively simple food web where strong top-down effects were expected. This result, however, was limited to habitats on the marsh with simply structured vegetation lacking leaf litter.     

天津沿海滩涂互花米草种群生殖分株数量特征及生殖分配研究

通过对天津沿海滩涂单优群落的取样调查,研究了互花米草种群生殖分株的数量特征和生殖分配规律。结果表明,在籽实成熟期,互花米草种群单个生殖分株重为15.62±9.26 g,穗和种子重分别为2.68±2.08和1.39±1.12 g,小穗和种子数分别为537.7±362.2和490.2±376.3个,生殖分配Ⅰ和生殖分配Ⅱ分别为15.02±5.83和7.62±3.8%,结实率为81.9 ±28.6%。穗和种子的形成分别需要分株积累3.2和3.6 g以上的生物量。穗长、穗重、种子数、小穗数、生殖分配Ⅰ和生殖分配Ⅱ分别与分株高度呈极显著(p<0.01)的正相关关系。穗重、小穗数、种子重、种子数、生殖分配Ⅰ和生殖分配Ⅱ分别与分株重呈极显著(p<0.01)的线性正相关关系。种子数、小穗数、生殖分配Ⅰ和生殖分配Ⅱ分别与茎和叶鞘生物量分配呈极显著(p<0.01)的负相关关系。穗各构件在空间上的分布也存在较强的规律性。     

Effects of saltmarsh invasion by Spartina alterniflora on arthropod community structure and diets

Invasive plants strongly affect physical and biotic environments of native ecosystems. Insects and other arthropods as one of the major components of many ecosystems are very sensitive to subtle changes in abiotic and biotic environments. We examined the effects of exotic Spartina alterniflora invasion on community structure and diets of arthropods in a saltmarsh previously dominated by native Phragmites australis in Yangtze River estuary through net sweeping and plant harvesting methods and stable isotope analysis. Our results showed that diversity indices were not significantly different between exotic and native plant communities, but the total abundance of insects estimated through plant harvesting method was found to be lower in Spartina monoculture than that in Phragmites monoculture. Community structure of insects in Spartina monoculture was dissimilar to that in Phragmites monoculture and Phragmites–Spartina mixture. Moreover, stable carbon isotope patterns of arthropods were significantly different between Phragmites and Spartina monocultures. Although some native arthropods (perhaps generalists) shifted their diets, many native taxa did prefer Phragmites to Spartina even in Spartina monoculture. Spartina invasions resulted in reduced abundances of some arthropds, and increased dominance of others feeding preferably on Spartina. This study provides evidence that invasive plants can change the community structure and diets of native arthropods, which will eventually alter the arthropod food web, and affect the integrity and functioning of native ecosystems within a nature reserve that has been set aside for conserving the native biodiversity and maintaining the ecosystem integrity. In this sense, Spartina invasions in the Yangtze River estuary need to be managed appropriately.     

冬季火烧与收割对互花米草地上部分生长与繁殖的影响

自1995年首次在崇明东滩发现互花米草(Spartinaalterniflora)以来,该外来种已在东滩扩张成为最具优势的植物之一。作为东滩常见的两种人为干扰,冬季火烧和收割可能会对互花米草的生长和繁殖有一定的促进作用,并可能进一步促进互花米草的入侵。因此,作者选择崇明东滩为实验地点,探讨冬季一次性火烧和收割以及相对高程对互花米草生长与繁殖的影响。结果表明:冬季火烧与收割能显著提高互花米草的植株密度和结穗率,显著降低其植株基部直径,即对其生长与繁殖有一定的促进作用,原因可能在于这两种干扰使其生境改变,光照条件得到改善;从Wilks’值的大小可看出,与相对高程相比,这两种干扰对互花米草生长繁殖的影响相对较小,而且这两种干扰对互花米草的影响差异不显著。上述结果表明,冬季火烧与收割将可能促进互花米草在东滩的入侵,有必要采取措施加以管理。     

The significance of spatial and temporal patterns of algal mat deposition in structuring salt marsh vegetation

Question: Are there hot spots of algal mat deposition in space and time at the marsh scale and, if so, how does this affect the coexistence of a dominant (Spartina anglica) and gap dependent (Salicornia europaea) species? Location: The Rattekaai salt marsh in the Scheldt estuary in the southwestern Netherlands (NW Europe). Methods: Mat cover and the abundance of the gap dependent species Salicornia europaea were monitored at the scale of a marsh. The effects of mat cover on the vegetation structure were studied by applying three mat removal treatments over three growing seasons. Results: The low marsh border was found to be a hot spot of algal mat deposition during the growing season, which had a correlated spatial pattern between two successive years at a 20 m X 20 m scale. The combination of duration, timing and repetition of mat cover determined growth inhibition of the competitive dominant Spartina anglica, and thereby the abundance of subordinates such as Salicornia europaea. Mat cover reduced the storage of carbon reserves in Spartina and our results imply that repetition of non‐lethal mat cover can lead to ‘gap creation’. Gaps gave only temporary habitat to less dominant species since Spartina quickly re‐invaded them. The gap dependent annual Salicornia was most abundant at intermediate levels of disturbance measured as a function of both space and time. Conclusions In addition to disturbance level, the spatial and temporal distribution of disturbance are important in creating and maintaining habitat for gap dependent species. Relatively small disturbances will have a large effect on diversity if the spatial and temporal distribution of the disturbances leads to ‘disturbance hot spots’.     

Response of two oligohaline marsh communities to lethal and nonlethal disturbance

Severity is recognized as an important attribute of disturbance in many plant communities. However, the effects of disturbances of different severity on patterns of regeneration in oligohaline marsh vegetation have not been experimentally examined. In these communities, a critical difference in the effects of disturbance severity may be whether the vegetation dies as a result of the disturbance or is merely damaged and hence capable of resprouting. We described the regeneration of vegetation in two Louisiana marsh community types, one dominated by Sagittaria lancifolia L. and the other by Spartina patens (Ait.) Muhl., following three levels of disturbance: no disturbance, a nonlethal disturbance, and a lethal disturbance. In the nonlethal disturbance, aboveground vegetation was clipped to simulate common disturbances such as fire and herbivory that remove aboveground vegetation but leave rhizomes intact. In the lethal disturbance vegetation was killed using herbicide to simulate disturbances causing plant mortality such as wrack deposition, sedimentation, scouring, and flooding following fire or herbivory. Regeneration was assessed over a 2-year period by measuring plant species richness, relative abundance, relative dominance, cover, and final biomass. To elucidate mechanisms for observed responses of vegetation, the species composition of the seed bank, light penetration, water level, salinity, and soil redox potential were evaluated. Despite differences in the structure of undisturbed vegetation in the two community types, they exhibited the same overall pattern of regeneration. Following nonlethal disturbance, the dominant species resprouted and quickly reestablished the structure of the vegetation. In contrast, recolonization following lethal disturbance occurred primarily via seedling recruitment, which resulted in marked shifts in community structure that persisted throughout the study. While the two communities responded similarly overall to disturbance, the response of individual species was not uniform; abundance, dominance, biomass, or cover increased for some species but decreased for others in response to disturbance. Seed bank species occurred in the vegetation following lethal disturbance in the Spartina community and in both disturbed and undisturbed plots in the Sagittaria community, indicating that the seed bank is a source of propagules for regeneration and maintenance of oligohaline marshes. Of the environmental variables measured, light level was most closely related to the effect of disturbance severity on community structure. Our results suggest that lethal and nonlethal disturbances have differential effects on regeneration of vegetation that can create pattern in oligohaline marshes communities. Received: 29 September 1997 / Accepted: 12 May 1998     

The evolution of Spartina anglica G. E. Hubbard (Gramineae): genetic variation and status of the parental species in Britain

The perennial salt marsh grass Spartina anglica is one of the classic examples of allopolyploid speciation. It originated on the south coast of England at the end of the nineteenth century following chromosome doubling in S. × townsmdii , a hybrid between the native British S. marilima and a species introduced from the United States, S. alterniflora. The nature of the origin of S. anglica is beyond doubt; however, it is not known whether it had a single or multiple origin. In order to address this problem we undertook a survey of the genetic variation in the parental species of S. anglica using isozyme electrophoresis. The results show that S. alterniflora has no detectable variation and that S. maritima has extremely low levels of variation. These results, unfortunately, prevent the question of a single or multiple origin from being answered. Possible reasons for the low levels of variation and its influence on the future of the species are discussed. Another problem concerning the parental species is the rapid decline of S. maritima in Britain. It is often assumed that the major factor in this regression is the invasion of its habitats by S. anglica. We have examined the status of S. marilima throughout its range in Britain and have found that S. anglica rarely co-occurs with S. maritima. We propose that the decline of S. maritima is largely due to the physical erosion of its habitats and that this erosion may produce suitable habitats for colonization by S. anglica.     

PRIMARY PRODUCTION OF EDAPHIC ALGAL COMMUNITIES IN A MISSISSIPPI SALT MARSH1

Primary production rates of edaphic algae associated with the sediments beneath four monospecific canopies of vascular plants were determined over an annual cycle in a Mississippi salt marsh. The edaphic algal flora was dominated by small, motile pennate diatoms. Algal production (as measured by ¹⁴C uptake) was generally highest in spring-early summer and lowest in fall. Hourly rates ranged from a low of 1.4 mg C/m² in Juncus roemerianus Scheele to a high of 163 mg C/m² beneath the Scirpus olneyi gray canopy. Stepwise multiple regressions identified a soil moisture index and chlorophyll a as the best environmental predictors of hourly production; light energy reaching the marsh surface and sediment and air temperature proved of little value. Adding the relative abundances of 33 diatom taxa to the set of independent variables only slightly increased R²; however, virtually all variables selected were diatom taxa. R² was only 0.38 for the Spartina alterniflora Loisel. habitat but ranged from 0.70 to 0.87 for the remaining three vascular plant zones. Annual rates of algal production (g C/m²) were estimated as follows: Juncus (28), Spartina (57), Distichlis spicata (L.) Greene (88), and Scirpus (151). The ratio of annual edaphic algal production to vascular plant net aerial production (EAP / VPP) was 10–12% for the first three habitats and 61% for Scirpus. Chlorophyll a concentrations, annual algal production rates, and EAP / VPP values were comparable to those determined in Texas, Delaware, and Massachusetts salt marshes but lower than those reported for Georgia and particularly California marshes.     

Backfilling canals to restore wetlands: empirical results in coastal Louisiana

Wetland restoration is largely a developing science and engineering enterprise. Analyses of results are too few and constrained to observations over a few years. We report here on the effectiveness of one restoration technique used sparsely in coastal Louisiana for several decades. Canals have been dredged in coastal Louisiana wetlands since 1938 for oil and gas exploration and extraction. These canals are typically dredged to 2.5 m depth and are 20 to 40 m wide. Canal lengths vary from 100 m to several 1000s m in the case of outer continental shelf pipeline canals that cross the wetlands.Today, thousands of miles of canals crisscross these wetlands. Studies have linked dredged canals to a number of undesirable effects on the wetland environment including alterations in salinity, flooding and drainage patterns, direct loss of marsh by convention to open water, and increases in marsh erosion rates. These effects have led state and federal agencies charged with managing the wetland resource to look for methods of mitigating canal impacts. One possible method of managing spoil banks after the abandonment of a drilling site is to return spoil material from the spoil banks to the canal with the hope that marsh vegetation will be reestablished on the old spoil banks and in the canal. The movement of former spoil bank material back into the canal is referred to as backfilling.The purpose of this study was to (1) examine how backfilled canals changed over 10 years, (2) examine factors influencing success with multiple regression statistical models, and, (3) compare costs of backfilling with other Louisiana marsh restoartion projects. We examined the sites to document and interpret changes occurring since 1983/4 and to statistically model the combined data derived from these new and previous analyses. Specifically, we wanted to determine the recovery rates of vegetation, water depth, and soils in backfilled canals, restored spoil banks, and in nearby marshes, and to quantify the influence of plugging canals on these rates.The major factors determining backfilling restoration success are the depth of the canal, soil type, canal dimensions, locale, dredge operator skill, and permitting conditions. Plugging the canal has no apparent effect on water depth or vegetation cover, with the exception that submerged aquatic vegetation may be more frequently observed behind backfilled canals with plugs than in backfilled canals without plugs. Canal age, soil organic matter content, and whether restoration was done as mitigation on-site or off-site were the most important predictors of final canal depth. Canal length and percentage of spoil returned (+) had the greatest effect on the restoration of vegetation cover. Backfilled canals were shallower if they were older, in soils lower in organic matter, and backfilled off-site. Backfilling the canal restores wetlands at a cost of $1,200 to $3,400/ha, which compares very favorably with planned restoration projects in south Louisiana.Corresponding Editor: Anonumous