J.J. de Leeuw

Vegetation succession and herbivory in a salt marsh: changes induced by sea level rise and silt deposition along an elevational gradient

1 The relationships between soil development, vertical vegetation zonation, vegetation succession and herbivory by Brent geese, Branta bernicla, were studied in a coastal salt marsh. We were able to analyse up to 100 years of salt marsh development by comparing sites where vegetation succession had progressed for-various periods of time. These data were related to a continuous daily record of high water levels measured since 1824. 2 Most elevational variation in edaphic conditions (and therefore vertical vegetation zonation) could be attributed to variation in height of the sandy subsoil, as rapid dune formation occurs on the beaches early in succession. In the intermediate part of this elevational gradient, the maximum annual increase of 1.2 mm of silt corresponded to an annual increase of 5.6 g N m(-2) in the topsoil (0-50 cm). The average sea level rise in this area over the last 170 years was 0.63 mm year(-1). A sedimentation model suggests that this has had strong effects on sedimentation and the annual inundation frequency in the mid-part of the elevational gradient, thus affecting vegetation zonation on the salt marsh. For the major part of the investigated transects, sea level rise has probably speeded up succession due to an increased rate of sedimentation. 3 The occurrence and dominance of all plant species were recorded in 3927 plots, and and for the 11 most common species response surfaces were calculated for their dependence on elevation and transect age. Most plant species were clearly separated along these axes. Most halophytic species, which were preferred by the geese, occurred early in succession and low on the gradient, where we observed the highest densities ge quality of Festuca I rubra increased towards the lower salt marsh. Other preferred forage species (Puccinellia maritima and Plantago maritima) were gradually displaced during succession by the tall grass Elymus athericus, especially in the mid-and upper salt marsh. Few geese grazed in areas where Elymus was dominant. 4 Herbivores first increased in numbers but then declined along a gradient of primary productivity. We propose that declining forage quality (due to changing vegetation composition during succession) is a better explanation for this pattern than the classic explanation of predator control of herbivores at high levels of primary productivity. This quality threshold hypothesis, as an alternative explanation of the exploitation ecosystem hypothesis, is expected to hold especially where smaller (quality-sensitive) herbivores such as geese are present. 5 Grazing by cattle in a 200-year-old part of the salt marsh led to the disappearance of Elymus athericus, to establishment of early successional halophytes and a return of Brent geese. Crazing by a larger herbivore therefore facilitated conditions for smaller herbivores by preventing the dominance of plant species that were good light competitors, and thus improved forage quality. Populations of these small herbivores could then become regulated by predators, although none was present at our site.

MICRO-PATTERNS IN GRASSLAND VEGETATION CREATED AND SUSTAINED BY SHEEP-GRAZING

An initially uniform Holcus lanatus-dominated sward came partly under hay-making and partly under sheep-grazing. Preferential grazing by sheep resulted in grazing at different intensities giving rise to a macro-pattern of various plant communities. Besides this macro-pattern a micro-pattern developed in the grazed area, which was absent under hay-making. In the micro-pattern short, heavily grazed areas alternated with taller, lightly grazed patches, both having the same species composition. The heavily grazed area was characterized by equal amounts of monocots and dicots. The lightly grazed patches were dominated by Agrostis tenuis, and had a large amount of litter which probably causes the absence of mosses. The protein percentage of green material is higher in the heavily grazed areas than in the lightly grazed patches. Sequential charting indicated that the micro-pattern was more or less stable. An interaction between the vegetation micro-pattern and grazing patterns is suggested. Heavy grazing results in forage with a high protein content and hence attracts animals. Light grazing results in forage with a relatively low protein content, animals avoid the area and litter accumulates.

Body cooling and its energetic implications for feeding and diving of tufted ducks

Wintering in a temperate climate with low water temperatures is energetically expensive for diving ducks. The energy costs associated with body cooling due to diving and ingesting large amounts of cold food were measured in tufted ducks (Aythya fuligula) feeding on zebra mussels (Dreissena polymorpha), using implanted heart rate and body temperature transmitters. The effects of diving depth and food ingestion were measured in two sets of experiments: we measured body cooling and energy costs of six tufted ducks diving to different depths in a 6‐m‐deep indoor tank; the costs for food ingestion and crushing mussel shells were assessed under seminatural winter conditions with the same ducks feeding on mussels in a 1.5‐m‐deep outdoor pond. Body temperature dropped during feeding bouts and increased gradually during intermittent resting periods. The temperature drop increased linearly with dive duration. The rate of body cooling increased with feeding depth, but it was lower again at depths below 4 m. Half of the increment in energy costs of diving can be attributed to thermoregulatory heat production, of which approximately 50% is generated after diving to warm up the body. The excess costs for ducks feeding on large‐sized mussels could be entirely explained by the estimated energy cost necessary to compensate the heat loss following food ingestion, suggesting that the heat production from shell crushing substituted for thermoregulation. Recovery from heat loss is probably a major component of the activity budget of wintering diving ducks.

Wintering Tufted Ducks Aythya fuligula diving for zebra mussels Dreissena polymorpha balance feeding costs within narrow margins of their energy budget

Diving ducks face the strongly cooling properties of aquatic environments. In experiments with Tufted Ducks Aythya fuligula feeding on zebra mussels Dreissena polymorpha in outdoor cages, we measured changes in food consumption and diving behaviour in relation to water temperature (3-22 degrees C). Water efflux and daily energy expenditure (DEE) were measured using doubly-labelled water. Tufted Ducks swallow mussels whole and crush the shells in the gizzard. Due to the low flesh content of mussels, daily mussel consumption was extremely high in winter (up to 3 times the duck body mass of 600 g). Water efflux was 6.5 to 13 limes higher than allometrically predicted, with a maximum of 1.1 1 d(-1). Because of high water turnover rates, DEE estimates from doubly-labelled water measurements were variable, but on average agreed with DEE estimates obtained from food consumption and time-energy budgets (ca. 5% higher values). DEE increased with lower temperatures from 2.5 (summer, 20 degrees C) to more than 4 times the basal metabolic rate (winter, 3 degrees C). Costs of thermoregulation and heating up ingested mussels (as estimated from the theoretical caloric heat) primarily explained the high DEE in winter. At lower temperatures, dive duration was shorter and less time was spent selecting small mussels at the bottom, while more larger mussels were brought up and were ingested at the water surface. Thereby, the energy costs of crushing shells probably increased in favour of reducing diving costs. The narrow margins of the energy budget, wherein feeding costs must be balanced, imply that food availability and water temperature are important attributes to winter distributions of diving ducks.

Net Aerial Primary Production NAPP of the marsh macrophyte 'Scirpus maritimus' estimated by a combination of destructive and non - destructive sampling methods

Net aerial primary production (NAPP) of marsh macrophytes is usually estimated either by destructive sampling techniques or by phenometric techniques. Destructive methods, however, are thought to be inaccurate while phenometric techniques are very labour intensive. In this study a new method is presented which allows an accurate and more efficient estimation of NAPP. The method combines destructive sampling to determine end-of-season biomass and phenometric techniques to estimate the mortality of biomass before the end of the season. NAPP is derived through summation of these two estimates. Techniques needed to calculate the precision of the NAPP estimate are provided. The so called hybrid technique was used to estimate NAPP ofScirpus maritimus L. in a brackish marsh along the Westerschelde estuary, the Netherlands. Estimated NAPP was 1372 g m-2. End-of-season biomass accounted for 1106 g m-2, while mortality contributed 266 g m-2. Precision of the end-of-season biomass and the mortality estimates, expressed as coefficient of variation, was 18.2 and 26.0% respectively. The precision of the resultant, NAPP, was higher: 17.2%. These results indicate that NAPP could be estimated with a higher precision than end-of-season biomass. This contradicts the view that the accuracy of NAPP estimates can only be improved at the expense of its precision.