Production and nutrient dynamics of plant communities on a sub-Antarctic Island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in a fjaeldmark and two fernbrake communities on Marion Island (46°54S, 37°45E). These communities are an important component of the island's vegetation on rocky plateaux and slopes. For most species the aboveground accumulations of N, P and K early in the season were more rapid than increases in the aerial biomass. Rates of Ca, Mg or Na accrual were either similar to, or lower than, rates of aboveground growth. Nutrient (N+P+K+Ca+Mg+Na) standing stocks at the three communities were high; 71 g m^-2 at fjaeldmark, 116 g m^-2 at open fernbrake and 154 g m^-2 at closed fernbrake. The aboveground component accounted for 47% to 65% of these values. N was the most abundant element in the vegetation, followed by K (closed fernbrake) or Ca (open fernbrake and fjaeldmark). Nutrient standing stocks at the two fernbrakes were mostly higher than for most sub-Arctic and alpine dwarf-shrub tundras. Nutrient pool sizes (i.e. the total quantities of nutrients contained in the soil/plant system to a depth of 25 cm) were lower than those reported for arctic tundra meadows but were similar to, and often greater than, those found at heath communities, sub-Arctic dry meadows and dwarf-shrub tundras and some boreal forests. Annual net primary productions of the fernbrake vegetations were high and substantial quantities of nutrients are aquired annually from the soils by the vegetations. Depending on plant species, either N or K was the element taken up in the largest quantity, whereas P was mostly taken up in the lowest amount. A large proportion (mostly all) of the Ca and Mg and a substantial proportion of the N taken up aboveground was lost in the litterfall but little of K taken up was lost in this way.     

Production and nutrient dynamics of plant communities on a sub-Antarctic island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in two mire-grasslands at Marion Island (46°54S, 37°45E). Mire-grasslands are an important component of the island's vegetation, occurring on very wet peats and dominated by graminoids and bryophytes. Peak aboveground standing stocks of N, P and K in the vascular plant species of the mire-grasslands mostly occurred earlier in the season than did peak aboveground biomass, implying that aboveground accumulation rates of these nutrients were greater than the rate of biomass accumulation. Maximum Ca standing stocks coincided in the season with peak shoot biomass. Depending on the plant species, peak Mg and Na standing stocks occurred either before, or later than, peak shoot biomass. Total (above-plus belowground) standing stocks of nutrients (N+P+K+Ca+Mg+Na) at the time of peak aboveground biomass were 51 g m^-2 at study mire 1 and 44 g m^-2 at study mire 2. The most abundant element in the vegetation was N, followed by K. The net quantities of most nutrients translocated into the aboveground growth were mostly greater than the seasonal mean standing stocks in the aerial biomass. Except for Ca, nutrient standing stocks in the vegetation of the mire-grasslands are in the upper part of the range reported for sub-Arctic and Arctic graminoid communities. They are more similar to standing stocks at oceanic moorlands, montane grasslands and heath communities. Low Ca concentrations occur in the plants so that Ca standing stocks are lower than in most comparable northern hemisphere communities. Pool sizes (i.e. total quantities contained in the plant/soil system to a depth of 25 cm) of N, P, K and Ca are in the lower part of the range reported for wet, graminoid-dominated tundra and tundra-like communities of the northern hemisphere.     

Production and nutrient dynamics of plant communities on a sub-antarctic island

Summary Investigations of the seasonal changes in vegetation standing crop have enabled an assessment of annual net primary production (ANP) at a fjaeldmark, open fernbrake and closed fernbrake at Marion Island (46°54S, 37°45E). These communities represent a successional sequence on relatively dry ridges and slopes on the island. Together, they are representative of a large proportion of the island's lowland (c. 300 m above sea level) vegetation. Aboveground ANP's were 728 g m^-2 y^-1 at closed fernbrake, 502 g m^-2 y^-1 at open fernbrake and 226 g m^-2 y^-1 at fjaeldmark. Total (above-plus below) ANP's were 1958 g m^-2 y^-1, 1578 g m^-2 y^-1 and 685 g m^-2 y^-1, respectively. These values are greater than those found for most tundra and tundra-like shrub and dwarf shrub-dominated communities of the northern hemisphere. The island's oceanic climate ensures a long growing season (c. 300 days for vascular plants, 365 days for bryophytes) and aboveground productivities for the island communities (including two mire-grasslands reported on previously), based on the length of the growing season, were 0.9 to 2.9 g m^-2 d^-1, lower than for most comparable shrub and dwarf shrub sub-Arctic or alpine communities and more similar to low Artic and low alpine sedge-moss and grass-herb communities. Production efficiencies (0.7% to 2.1% of photosynthetically active radiation) were in the range reported for northern hemisphere subpolar vegetations.     

Moss-inhabiting diatom communities from Heard Island,sub-Antarctic

In this paper, we list 192 diatom taxa, collected from bryophyte samples from Heard Island (52°05S, 73°30E). The Heard Island diatom flora shows a marked similarity to those of the Crozet and Kerguelen archipelagos, and is quite dissimilar to the moss-dwelling diatom flora of Macquarie and Amsterdam Island. Based on species composition, three main groups of samples could be discerned. Each group represents a different habitat. Samples dominated by Diadesmis ingeae, Pinnularia borealis and several Psammothidium species seem to prefer the driest habitats. A special form of this community was found in shaded areas. Samples from wet habitats such as flush areas and pools were dominated by species such as Adlafia bryophila, Sellaphora tumida, Fragilaria capucina and Planothidium lanceolatum. An intermediate group was observed in more acid, but terrestrial, conditions, characterised by Eunotia paludosa and Chamaepinnularia soehrensis var. muscicola. These community patterns are similar to those found on other sub-Antarctic islands, and we suggest that a group of typical sub-Antarctic species and communities exists.     

The distribution and composition of vascular plant communities on Heard Island

Summary Six major higher plant communities are defined for sub-Antarctic Heard Island: tussock grassland, meadow, herbfield, pool complex, cushion-carpet, fellfield. The communities were mapped at a scale of 1:50000, using colour aerial photographs and field observations. The floristic composition of twelve vegetation transects and of thirty sample quadrats along these is used to describe the communities and identify dominant or indicator species. The vegetation is compared with that of other sub-Antarctic islands.     

The invertebrates of sub-Antarctic Bishop Island

We report on the first comprehensive collection of invertebrates made on sub-Antarctic Bishop Island, which lies 33 km south of Macquarie Island. A total of 15 species were collected, of which 14 also occur amongst the 168 species recorded from Macquarie Island. We hypothesise that the greater species richness of Macquarie Island relative to Bishop Island is largely caused by the greater habitat diversity on Macquarie Island but that other factors, including accidentally introduced species, may also be important. Received: 3 July 1996 / Accepted: 2 October 1996     

Influence of root herbivory on plant communities in heterogeneous nutrient environments

While plant species respond differently to nutrient patches, the forces that drive this variability have not been extensively examined. In particular, the role of herbivory in modifying plant-resource interactions has been largely overlooked. We conducted a glasshouse study in which nutrient heterogeneity and root herbivory were manipulated, and used differences in foraging among plant species to predict the influence of root herbivores on these species in competition. We also tracked the influence of neighborhood composition, heterogeneity, and herbivory on whole-pot plant biomass. When herbivores were added to mixed-species neighborhoods, Eupatorium compositifolium, the most precise forager, was the only plant species to display a reduction in shoot biomass. Neighborhood composition had the greatest influence on whole-pot biomass, followed by nutrient heterogeneity; root herbivory had the smallest influence. These results suggest that root herbivory is a potential cost of morphological foraging in roots. Root herbivores reduced standing biomass and influenced the relative growth of species in mixed communities, but their effect was not strong enough at the density examined to overwhelm the bottom-up effects of resource distribution.     

Macquarie Island House mice: A genetical isolate on a sub-Antarctic island

Macquarie Island (54°37'S; 158°54'E) lies half-way between Australia and the Antarctic Continent, 400 miles from the nearest land. Its climate is wet, windy, cool and cloudy. The mean annual temperature is 4.7°C, and ranges only from 3.1°C in the coldest month to 6.7°C in the warmest. Despite these inclement conditions, House mice ( Mux museulus ) live over most of the island (up to at least 1000 feet), and apparently thrive. They breed all the year round. Their diet seems to be largely the seeds of the Macquarie cabbage ( Stilocarpa polaris ), supplemented by insects.
The object of the study was to determine the effect of life in an equable but stressful environment for the mice. The animals were not remarkable in any morphological traits, although they were fairly large and had relatively short tails. However six of 17 gene-loci investigated electrophoretically were polymorphic. The average heterozygosity was 6.8%, similar to mice living under much more fluctuating conditions in the Northern Hemisphere. The frequency of the Hbb^d allele decreased from 71 ± 6 % in mice under three months of age to 54±5% in older ones. This change was most marked in animals caught well away from human habitation.     

The microarthropods of sub-Antarctic Prince Edward Island: a quantitative assessment

The biodiversity in the sub-Antarctic region is threatened by climatic change and biological invasions, which makes the understanding of distributions of biotas on sub-Antarctic islands essential. Although the distribution patterns of vascular plants and insects on sub-Antarctic islands are well documented, this is not always the case for microarthropods. This study provides the first quantitative assessment of the distribution and abundance of microarthropods on Prince Edward Island (PEI), one of two islands in the Prince Edward Island group. Microarthropod community structure differed significantly between PEI and nearby Marion Island, with only two invasive alien species found on PEI compared with Marion Island. Furthermore, species richness, abundance and community structure differed significantly between habitat types on both islands. This study emphasizes the importance of quarantine measures when visiting PEI to maintain its status as one of the more pristine islands in the sub-Antarctic region.     

Restricted plant species on sub-Antarctic Macquarie and Heard Islands

The recent distributional history of two Macquarie Island vascular plant species, Carex trifida, Poa litorosa, and the Heard Island vascular plant, Ranunculus crassipes is examined. C. trifida is known from only one small population on the north west coast of Macquarie Island. Four populations of P. litorosa were first recorded in the 1980s; we believe however, that it was first observed, but misidentified in the 1950s. R. crassipes was first discovered on Heard Island in the late 1980s. We argue that all three species are indigenous and arrived on their respective islands within the last 200 years by natural processes, most likely from warmer neighbouring islands, where these species have more extensive distributions. There have been small-scale changes in distribution of all species, mainly expansion. Further expansion of all three species is expected as a response to warming climate. Feral rabbit grazing is having a confounding negative influence on populations of P. litorosa.     

Effects of delayed hay removal on the nutrient balance of roadside plant communities

1. Mass losses and nutrient losses from fresh roadside cuttings were studied in the field during a 6-week period. Large amounts (over 50%) appear to be lost from the cuttings. The losses were positively related to initial nutrient concentrations during this short-term study. Mass and nitrogen losses were best explained by the initial C:N ratio, phosphorus and potassium losses by the initial phosphorus concentration.
2. For potassium the losses were particularly large (up to 90%). For this element only, the observed relationship between loss rate and initial chemical composition could not be established significantly. It is concluded that potassium is mainly lost by leaching whereas the major nitrogen, phosphorus and mass losses are most probably caused by rapid microbial decomposition of readily soluble substances.
3. Using existing data on chemical composition of other roadside cuttings, nutrient losses after different hay removal delay times were modelled for different plant communities. All or most of the losses were assumed to return to the soil system.
4. When soil impoverishment is aimed for, cuttings should be removed within 1 or 2 weeks in most plant communities. If removal is delayed longer, the amounts of nutrients removed will often fall below the annual atmospheric input. In plant communities where annual above-ground production of nitrogen and phosphorus are lower than the annual atmospheric deposition already, rapid removal of the cuttings may be the only way to maintain at least potassium at a limiting level.
5. The main effect of hay-making on the soil nutrient status most likely consists of a reduction of the potassium availability, at least on sandy soils with a low cation exchange capacity and provided there is little delay in hay removal.     

Food, reproduction and survival in mice on sub-Antarctic Marion Island

The house mouse Mus musculus is the most widespread introduced mammal on sub-Antarctic islands, where it may alter ecosystem function. Ambient temperature and food availability affect reproduction and survival for mice. It is unclear how these factors influence mouse demography in the sub-Antarctic, and we tested the influence of food experimentally on Marion Island. Using food supplementation trials, we did not alter reproduction or overwinter survival. Alternatively, we argue ongoing climatic change on Marion could increase mouse densities through summer, while increased winter survival may reduce population growth rates the following summer through density dependence. The overall influence of these apposing forces depends on their relative strengths but may limit changes in mouse numbers with ongoing changes in climate in the sub-Antarctic.     

Population dynamics of plant and pollinator communities: stability reconsidered

Plant-pollinator networks are systems of outstanding ecological and economic importance. A particularly intriguing aspect of these systems is their high diversity. However, earlier studies have concluded that the specific mechanisms of plant-pollinator interactions are destabilizing and should lead to a loss of diversity. Here we present a mechanistic model of plant and pollinator population dynamics with the ability to represent a broad spectrum of interaction structures. Using this model, we examined the influence of pollinators on the stability of a plant community and the relationship between pollinator specialization and stability. In accordance with earlier work, our results show that plant-pollinator interactions may severely destabilize plant coexistence, regardless of the degree of pollinator specialization. However, if plant niche differentiation, a classical stabilizing mechanism, is sufficiently strong to overcome the minority disadvantage with respect to pollination, interactions with pollinators may even increase the stability of a plant community. In addition to plant niche differentiation, the relationship between specialization and stability depends on a number of parameters that affect pollinator growth rates. Our results highlight the complex effects of this particular type of mutualism on community stability and call for further investigations of the mechanisms of diversity maintenance in plant-pollinator systems.     

Effects of a short-term climate change experiment on a sub-Antarctic keystone plant species

The cushion plant Azorella selago is widespread across the sub‐Antarctic, and is considered a keystone species in the dominant fellfield vegetation. However, the impact of current changes in climate in the region (increasing temperature and declining rainfall) on this species is unknown. Here, the response of A. selago to reduced rainfall (a direct effect of climate change) and increased shading (a predicted indirect effect of increasing temperatures, via enhanced growth and wider distribution of more responsive competitors and epiphytes) was experimentally determined. Reduced rainfall increased stem mortality and accelerated autumnal senescence. Furthermore, under this treatment senescence was unequally distributed across individual plants, hypothesized to be a consequence of an interactive effect between rainfall and wind patterns. Shaded stems grew more, and carried larger leaves with lower trichome densities, than their exposed equivalents. As a result, shaded plants were less compact and their surface integrity reduced. The species' response to combined drying and shading was generally similar to its response to shading alone, suggesting that, at least over the short term, the indirect effects of climate change could be more severe than the direct effects. Thus, despite the species' slow growth rate and the short duration of the experiment, persistent direct and indirect effects were observed, both with potential longer‐term consequences for A. selago populations. Climate change is, therefore, likely to impact negatively on this long‐lived keystone species, with significant implications for the structure and functioning of fellfield systems.     

The microenvironment of house mice on Marion Island (sub-Antarctic)

On Marion Island, house mice ( Mus musculus) establish burrow systems that range from unbranched corridors 0.5 m long with a single chamber (in some instances without a chamber) to complexly branched systems extending over an area of up to 4 m² and containing up to four chambers. Total underground area occupied by burrow systems (chambers plus corridors) was from 5 to 23 m² ha^-1, corresponding to burrow-system volumes of 250-1,300 dm³ ha^-1. In autumn, about three-quarters of chambers contained small food caches. Most (87%) entrances to burrow systems faced away from prevailing winds, especially winds that bring snow, hail or rain. Seasonal and diurnal temperature variations in burrows are considerably dampened (daily minimum in burrows seldom drops below 2°C), compared with the air just above the vegetation canopy. Over the whole year, total night-time warmth in a burrow (heat sum, 24,883 degree hours) was 53% greater than at the top of the canopy (16,317 degree hours). Burrows' entrances are generally connected above ground by runways (paths and tunnels through the vegetation). Runways also represent a warmer environment than the air above the canopy during the breeding season at night (13,466 degree hours at the runway surface compared with 11,900 degree hours at the top of the canopy). House mice, which are living close to their physiological limits, temperature-wise, on Marion Island thus evade the worst extremes of the island's climate by constructing burrows and above-ground runways and this is an important factor in their survival.     

Community dynamics and heterogeneities in mixed bacterial communities subjected to nutrient periodicities

Sixteen replicate microcosms were inoculated with a mixed assemblage of heterotrophic bacteria and provided with discrete pulses of protein as carbon and energy source. The dynamics of community structure were monitored by 16S rRNA gene polymerase chain reaction denaturant gradient gel electrophoresis (PCR-DGGE). The results were consistent with a strong role for biological interactions in maintaining diversity. Replicate microcosms developed different microbial communities. For systems exposed to nutrient pulses every 7 days, the number of DGGE bands averaged 13 +/- 4 (mean +/- SD) and the Dice similarity coefficient between pairs ranged from 0.08 to 0.67. In each of 16 systems provided protein once each day, there were dynamic changes over the first 30 days but community composition was stable over the next 20 days. However, most systems differed from each other; two-thirds of the pairwise comparisons had similarity coefficients in the range of 0.35-0.63. These 16 systems contained 10 +/- 2 phylotypes (mean +/- SD) and in aggregate 34 phylotypes were found in the 16 systems. Most phylotypes were found in < 25% of the systems, and there were not strong networks of association among phylotypes.     

崇明岛上沙地区蜘蛛种类及其动态分析

于2011年4月初到2013年10月底,在崇明岛上沙地区选择6个代表性区域,采用捕捉、扣管、扫网和陷阱法对蜘蛛种类进行系统调查和定点采集,共采集蜘蛛标本700头,初步鉴定出蜘蛛63种,隶属13科38属,另有8种鉴定到属。该地区蜘蛛优势种类是跳蛛科、肖蛸科、园蛛科、狼蛛科。文章介绍了蜘蛛种群年度消长规律,同时探讨了不同环境因子的变化对蜘蛛种群的影响,分析表明：蜘蛛种群密度与生境和农药的使用存在显著差异,蜘蛛适宜生存的温度在15℃～35℃,适易的相对湿度在65％-85％。