Urban domestic gardens (XIII): Composition of the bryophyte and lichen floras, and determinants of species richness

Private, residential gardens form a substantial proportion of the undeveloped land in urban areas. Evaluating their role in supporting biodiversity is crucial to (i) predicting which plant and animal species can persist in towns and cities, (ii) understanding the regional impacts of urbanisation, and (iii) guiding sympathetic garden management by owners. To obtain baseline information on a poorly-studied component of garden biodiversity, we measured the size and composition of the cryptogam assemblages in 61 domestic gardens in the city of Sheffield, UK. A total of 67 bryophyte and 77 lichen taxa were recorded. Bryophytes ranged from 3 to 24 species per garden, with a mean richness of 11.3 species; lichens ranged from 2 to 30, with a mean of 14.9 species. Stone substrates supported the highest lichen richness, although minor substrates contributed unique species. Just over one fifth of bryophyte species were recorded in grass lawns, and these were more widespread than those of other habitats. Most cryptogams were scarce, with around one quarter of both bryophytes and lichens occurring in single gardens, and only 10% were found in more than half of the gardens. Garden area - correlated with substrate richness - and garden altitude were the only two factors explaining variation in cryptogam richness (bryophytes 39.1%, lichens 32.4%). Positive correlations existed among bryophyte, lichen and vascular plant richness, and these were only partially mediated by the effect of garden area. Therefore the opportunity remains for garden owners to support cryptogam richness, the most effective action being to enhance substrate diversity.     

The roles of habitat features, disturbance, and distance from putative source populations in structuring alien plant invasions at the urban/wildland interface on the Cape Peninsula, South Africa

Natural areas are becoming increasingly fragmented and embedded in an urban matrix. Natural and semi-natural areas at the urban/wildland interface are threatened by a variety of ‘edge effects’, and are especially vulnerable to invasion by introduced plants, with suburban gardens acting as significant sources of alien propagules. Urban/wildland interfaces also provide access for humans, leading to various types of disturbance. Alien plant invasions are one of the biggest threats facing remaining natural areas on the Cape Peninsula, South Africa. The area provides an ideal opportunity to study the dynamics of invasions at the urban/wildland interface, since the largest natural area, the Table Mountain National Park (TMNP), is surrounded by the city of Cape Town. We explored invasion patterns in Newlands Forest (a small section of the TMNP) and detailed the roles of habitat features and distance from putative source populations in three main habitat types: natural Afromontane forest, riverine woodland habitats, and plantations of exotic pines (Pinus radiata and P. pinaster). We also examined the role of disturbance in driving invasions in two of these habitat types (Afromontane forest and pine plantations). We hypothesized that alien richness and alien stem density would decrease with distance from the urban/wildland interface, and that alien richness and alien stem density would increase with increasing levels of human disturbance.Distance from putative source populations and levels of anthropogenic disturbance influenced alien richness in Newlands Forest but not alien stem density. Alien richness decreased significantly with distance from presumed sources in the pine habitat, and increased significantly with disturbance in the forest habitat. Percentage overstorey cover and soil pH were important environmental variables associated with alien plant species. A socio-economic approach is discussed as being the most effective approach to the management and prevention of alien plant species in Newlands Forest.     

Extending the rotation period in larch plantations increases canopy heterogeneity and promotes species richness and abundance of native beetles: Implications for the conservation of biodiversity

We studied the effects of extending the rotation period (long-rotation) and related ecological variables on the biodiversity of six beetle families in larch (Larix kaempferi) plantations. Beetle diversity in the forest understory and the canopy of long-rotation and middle-aged plantations was compared. The relationships between the species richness of each beetle family and 24 ecological variables were identified. We found that beetle diversity tended to increase with long-rotation and responded differently to the ecological variables in two woodland strata (understory and canopy); e.g., an increase in phytophagies occurred in the canopy. Among the 24 measured ecological variables, the species richness of naturally regenerated mature trees, their abundance, and the quality and quantity of coarse woody material (CWM) had the strongest influences on beetle diversity. The former two variables increased with long-rotation; however, most variables related to CWM did not increase because thinned trees were left in middle-aged plantations as fallen logs. Therefore, the occurrence of naturally regenerated native trees that had grown sufficiently tall to reach canopy height in long-rotation plantations underpinned the improved biodiversity in larch plantations, contributing to native beetle species richness in these woodlands. Long-rotation makes plantations more heterogeneous, particularly in the canopy, and promotes greater native beetle diversity.     

Removal of an invasive shrub (Chinese privet: Ligustrum sinense Lour) reduces exotic earthworm abundance and promotes recovery of native North American earthworms

This study investigated the possibility of a facilitative relationship between Chinese privet (Ligustrum sinense) and exotic earthworms, in the southeastern region of the USA. Earthworms and selected soil properties were sampled five years after experimental removal of privet from flood plain forests of the Georgia Piedmont region. The earthworm communities and soil properties were compared between sites with privet, privet removal sites, and reference sites where privet had never established. Results showed that introduced European earthworms (Aporrectodea caliginosa, Lumbricus rubellus, and Octolasion tyrtaeum) were more prevalent under privet cover, and privet removal reduced their relative abundance (from >90% to ∼70%) in the community. Conversely, the relative abundance of native species (Diplocardia michaelsenii) increased fourfold with privet removal and was highest in reference sites. Soils under privet were characterized by significantly higher pH relative to reference plots and privet removal facilitated a significant reduction in pH. These results suggest that privet-mediated effects on soil pH may confer a competitive advantage to European lumbricid earthworms. Furthermore, removal of the invasive shrub appears to reverse the changes in soil pH, and may allow for recovery of native earthworm fauna.     

Habitat use and spatial behaviour of the endangered Southern river otter (Lontra provocax) in riparian habitats of Chile: Conservation implications

The spatial behaviour of 10 Southern river otters Lontra provocax was analysed using radio-tracking data collected in the Queule River, Southern Chile. The estimated otter density was 0.25 individuals km⁻¹. Females with cubs and adult otters displaying mating behaviours were the only social group recorded. Studied otters were solitary and observed behaviour patterns suggest an intrasexual territoriality. No home range overlap among males existed while an average of 33% and 87% home range overlapped among females and between sexes, respectively. In the situation of core areas, only one case of overlapping between an adult male and an adult female was observed. Average home range and core area lengths were 11.3 km and 0.9 km, respectively. Otters used rivers more frequently than expected, whereas they avoided small streams. Low intensity livestock farming, without canalization of watercourses and riparian vegetation extraction practices, seems compatible with the presence of Southern river otter populations since the spatial behaviour of the studied otters was not affected under these circumstances. Based on the present official reserve sizes and distribution within the country, and also the increase of variant anthropogenic land uses outside these areas that result with consequent habitat loss and deterioration, we strongly recommend considering the importance of spatial dimension in quantity and quality habitats in further conservation planning for the Southern river otter using an integrated catchment management.     

Spread and attempted eradication of the grey squirrel (Sciurus carolinensis) in Italy, and consequences for the red squirrel (Sciurus vulgaris) in Eurasia

In 1997, the National Wildlife Institute, in co-operation with the University of Turin, produced an action plan to eradicate the American grey squirrel from Italy, as this introduced species replaces the native red squirrel through competitive exclusion and damages trees through de-barking. The first step, a trial eradication of a small population of grey squirrels at Racconigi (Turin) to evaluate the efficiency of the removal techniques, started in May 1997. Preliminary results showed that eradication was feasible, but the project was opposed by radical animal rights groups which took the National Wildlife Institute to court in June 1997. This legal action caused a suspension of the project and led to a lengthy judicial enquiry that ended in July 2000 with the acquittal of the Institute. Nevertheless, the 3-year suspension of all actions led to a significant expansion of the grey squirrel's range and thus eradication is no longer considered practical. Therefore, in the medium to long term, grey squirrels are likely to expand through continental Eurasia. This constitutes a major threat to the survival of the red squirrel over a large portion of its distribution range and will have a significant impact on forests, with economic damage to timber crops.     

The fate of arsenic in soils adjacent to an old mine site (Bustarviejo,Spain): mobility and transfer to native flora

Background, aim, and scope  

The mobility of arsenic in soils and its transfer to other environmental components present significant environmental risks. The management of polluted land is determined by the availability, mobility, and transfer of inorganic pollutants to different ecosystem compartments. In this paper, the fate of arsenic at this mining site has been evaluated to determine future management practises to minimise such risk.     

Landscape,Soil, and Plant Analysis of Japanese Stiltgrass (Microstegium vimineum) Invasion in the Piedmont Region,SC

Japanese stiltgrass (Microstegium vimineum), an exotic invasive plant, is native to Southeast Asia. This study was conducted to determine the chemical composition of Japanese stiltgrass as well as soil and landscape characteristics that correlate with invasion of Japanese stiltgrass around Lake Issaqueena in the upper Piedmont of South Carolina. Geographic Information Systems (GISs) and Light Detection and Ranging (LiDAR) were used to determine the spatial pattern of invasion with respect to the aspect, slope, canopy cover, soils, and distance to roads and trails. Japanese stiltgrass was distributed on both sides of Lake Issaqueena in Pacolet and Madison soil map units (Fine, kaolinitic, thermic Typic Kanhapludults) on the average slopes of 21%, but it was particularly common on the eastern shore of the lake in low-lying wet and shaded areas (mean canopy cover 51%). In addition, invasion by Japanese stiltgrass was correlated with the proximity to roads and trails. Plant tissue analysis revealed many differences in the distribution of macronutrients, macrominerals, and micronutrients in the leaves, stems, and roots of Japanese stiltgrass, although those differences were not always statistically significant. Concentrations of nitrogen (N), phosphorus (P), and calcium (Ca) were the highest in leaves while zinc (Zn) concentrations were the highest in stems and concentrations of magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), and sodium (Na) tended to be higher in roots. Carbon (C), sulfur (S), and potassium (K) concentrations were generally higher in above-ground tissues versus roots. Soil chemical analysis revealed no statistical differences between control and invaded plots. Our findings suggest that watershed areas surrounding lakes may be particularly susceptible to the invasion of Japanese stiltgrass due to their microclimates, low-lying wet pathways for seed distribution and recreational uses.     

Feeding preferences of native terrestrial isopod species (Oniscoidea,Isopoda) for native and introduced leaf litter

Due to current predictions for Central Europe that forecast higher frequencies of hot and dry summers, Mediterranean drought-tolerant oak species are being evaluated as future forest trees for German forest sites that are becoming increasingly damaged by water deficit. As a result of planting foreign tree species, the leaf litter composition and thus the food resources of native saprophagous macroarthropods will change, possibly altering primary decomposition processes. Therefore, experiments concerning the acceptance and palatability of introduced versus native litter for native isopods were undertaken. Consumption rates of four native isopod species (Porcellio scaber, Oniscus asellus, Trachelipus rathkii, Trachelipus ratzeburgii) were investigated in laboratory choice tests with introduced (Quercus pubescens, Quercus frainetto, Quercus ilex) and comparable native (Fagus sylvatica, Quercus robur) leaf litter. Litter was characterized by measurement of C/N-ratios and lignin content. Although species-specific preferences of isopods could be observed in the experiments, Mediterranean oak litter was consumed by all investigated species. Furthermore, two isopod species even preferred the leaf litter of the introduced Q. ilex. Compared to native beech or oak litter, litter from these introduced tree species thus apparently do not negatively influence the consumption rates of terrestrial isopods. Possible reasons for the determined preferences are discussed.     

Recovery of native plant communities after the control of a dominant invasive plant species, Foeniculum vulgare: Implications for management

The control and/or removal of a dominant invasive species is expected to lead to increases in native species richness and diversity. Small pilot studies were performed on Santa Cruz Island (SCI), California, in the early 1990s to test the efficacy of different methods on the control of Foeniculum vulgare (fennel) and management’s effects on native species recovery. We chose a treatment that showed significant native species recovery, applied it at the landscape scale, and followed its effects on fennel infested plant communities. We tested the hypothesis that results from small-scale studies translate to the landscape level. We found that although the control of fennel translated from the small to landscape scale, decreasing from an average of 60% to less than 3% cover, native species recovery did not occur in the landscape study as it did in the pilot studies. Invasive fennel cover was replaced by non-native grass cover over time. Unexpectedly, fennel cover in untreated fennel plots decreased significantly (though not as drastically) from over 60% cover to just under 40% cover while native species richness in untreated areas increased significantly. The correlation between precipitation and changes in native and non-native species richness and abundance in this study imply that changes in species abundances were highly correlated with environmental fluctuations. The lack of a native seedbank and the accumulation of non-native grass litter likely prevented the recovery of native species in treated areas. Greater vertical complexity found in fennel communities, which increased visitation by frugivorous birds and likely increased native seed dispersal, may have been responsible for the increase in native species richness in the untreated areas. These results suggest that successful invasive species control and native species recovery experiments conducted at small scales may not translate to the landscape level, and active restoration should be an organic component of such large-scale projects.     

Spatial patterns, density changes and implications on biodiversity for old trees in the boreal landscape of northern Sweden

Logging and modern forest management reduced the frequency of key features of old-growth forest, especially old, dying and dead trees in the Scandinavian boreal forest during the 20th century. To quantify the decline and spatial differences, we have analysed density changes of old trees between 1926 and 1996 on a regional scale (approximately 50,000 km²) in boreal Sweden. The occurrence of old-conifer trees (+159 years) has dramatically decreased in the studied area and today only one third as many old Scots pines (Pinus sylvestris L.) and Norway spruces (Picea abies (L.) Karst.) can be found as in the 1920s. However, the density of old deciduous trees (+99 years) has not decreased since the early 20th century. Historical baseline data such as these are needed for modern forest management, conservation programmes and ecological restoration projects. Existing old trees are very important for biodiversity, and may also preserve elements of cultural heritage from pre-industrial times.     

Breeding system and inbreeding depression in the clonal plant species Knautia arvensis (Dipsacaceae): implications for survival in abandoned grassland

Larger clonal sizes observed in abandoned than in managed grassland could result in increased geitonogamy in Knautia arvensis. The species was investigated for self-compatibility, seed set without pollinators, and inbreeding depression in a greenhouse experiment. Seed set was equivalent in hand-selfed and hand-outcrossed plants (c. 53%), indicating full self-compatibility. Seed set was low (2.8%) in caged, unmanipulated flower heads. There was no significant difference in seed weight, germination, and survival between selfed and outcrossed progeny. Juvenile biomass was significantly lower in selfed than in outcrossed progeny (on average 45% lower). Cumulative fitness calculations for germination, survival, and juvenile biomass showed that the mean inbreeding depression was 58% in offspring produced by selfing. The results suggest that in K. arvensis, which is protected against self-pollination within flower heads through protandry, the likely mechanism for selfing is via geitonogamy among flower heads. Geitonogamy among ramets within the larger clones observed in abandoned grassland could reduce offspring viability in K. arvensis since the fitness of selfed offspring was low. Inbreeding depression could therefore add to the recruitment problems experienced by plants in abandoned grassland, both in K. arvensis and probably in other clonal grassland species, depending on their growth structure and breeding system.     

The impacts of Phalaris arundinacea (reed canarygrass) invasion on wetland plant richness in the Oregon Coast Range, USA depend on beavers

Invasive plants can threaten diversity and ecosystem function. We examined the relationship between the invasive Phalaris arundinacea (reed canarygrass) and species richness in beaver wetlands in Oregon, USA. Four basins (drainages) were chosen and three sites each of beaver impoundments, unimpounded areas and areas upstream of debris jams were randomly chosen in each basin for further study (n = 36). Analysis of covariance (ANCOVA) showed that the relationship between Phalaris and species richness differed significantly (p = 0.01) by site type. Dam sites (beaver impoundments) exhibited a strong inverse relationship between Phalaris and species richness (b_D = −0.15), with one species lost for each 7% increase in Phalaris cover. In contrast, there was essentially no relationship between Phalaris cover and species richness in jam sites (debris jam impoundments formed by flooding; b_J = +0.01) and unimpounded sites (b_U = −0.03). The cycle of beaver impoundment and abandonment both disrupts the native community and provides an ideal environment for Phalaris, which once established tends to exclude development of herbaceous communities and limits species richness. Because beaver wetlands are a dominant wetland type in the Coast Range, Phalaris invasion presents a real threat to landscape heterogeneity and ecosystem function in the region.     

Effects of land management on the abundance and richness of spiders (Araneae): A meta-analysis

Human land use, through forest management and conversion of natural habitats into agroecosystems, has often resulted in loss of biodiversity. Spiders are important predators in terrestrial ecosystems, biological control agents against pests in agroecosystems and forests, and useful indicators due to their sensibility to changes in habitat structure and microclimate. Effects of land use on spiders have been reviewed several times but existing reviews are narrative and usually focused on one single kind of management. We summarize the impacts of land use on spider abundance and species richness using meta-analysis. The main ecosystems where the effect of management on spiders has been studied were forests, agroecosystems and rangelands (including meadows and grasslands). Although our survey retrieved studies from all parts of the world and climatic zones, a dominance of studies from temperate habitats in Europe and North America was evident. The meta-analysis showed negative effects on spider species richness and abundance for agroecosystems and rangelands, but were less evident for forests. From 10 kinds of land management identified, all but forest plantation showed effects on spider richness or abundance. These land management scenarios affected either species richness (forest fragmentation and logging, farmland abandonment, and miscellaneous management including plowing, cutting and fire), or spider abundance (forest fragmentation, forest fire, conventional farming, insecticides, grazing, and farmland abandonment), but not both. Edge effects had contrasting effects depending on the ecosystem, affecting spider species richness in agroecosystems and abundance in rangelands and forests. Direct destruction of spiders or negative effects on habitat heterogeneity or prey populations were the most likely causes of the negative effects detected.     

Historical influences on the vegetation and soils of the Martha’s Vineyard, Massachusetts coastal sandplain: Implications for conservation and restoration

Both disturbance history and previous land use influence present-day vegetation and soils. These influences can have important implications for conservation of plant communities if former disturbance and land use change species abundances, increase colonization of nonnative plant species or if they alter soil characteristics in ways that make them less suitable for species of conservation interest. We compared the plant species composition, the proportion of native and nonnative plant species, and soil biogeochemical characteristics across seven dominant land use and vegetation cover types on the outwash sandplain of Martha’s Vineyard that differed in previous soil tillage, dominant overstory vegetation and history of recent prescribed fire. The outwash sandplain supports many native plant species adapted to dry, low nutrient conditions and maintenance of native species is a management concern. There was broad overlap in the plant species composition among pine (Pinus resinosa, P. strobus) plantations on untilled soils, pine plantations on formerly tilled soils, scrub oak (Quercus ilicifolia) shrublands, tree oak (Q. velutina, Q. alba) woodlands, burned tree oak woodlands, and sandplain grasslands. All of these land cover categories contained few nonnative species. In contrast, agricultural grasslands had high richness and cover of nonnative plants. Soil characteristics were also similar among all of the woodland, shrubland and grassland land cover categories, but soils in agricultural grasslands had higher pH, extractable Ca²⁺ and Mg²⁺ in mineral soils and higher rates of net nitrification. The similarity of soils and significant overlap in vegetation across pine plantations, scrub oak shrublands, oak woodlands and sandplain grasslands suggests that the history of land use, current vegetation and soil characteristics do not pose a major barrier to management strategies that would involve conversion among any of these vegetation types. The current presence of high cover of nonnative species and nutrient-enriched soils in agricultural grasslands, however, may pose a barrier to expansion of sandplain grasslands or shrublands on these former agricultural lands if native species are not able to outcompete nonnative species in these anthropogenically-enriched sites.     

Distribution of potassium fractions in sweet potato (Ipomoea batatas) garden soils in the Central Highlands of Papua New Guinea and management implications

Previous studies indicated that potassium (K) deficiency is an important soil‐related factor for yield decline of the sweet potato gardens in the Central Highlands of Papua New Guinea, where sweet potato is an important staple food crop. An effort was made to characterize various fractions of K in the diverse soils of this region under sweet potato, to ascertain the probable reasons behind the observed K deficiency and its relationship to decreasing yield trends. Soils from two depths (0–10 cm) and (10–20 cm) in two types of gardens (old and new gardens) were assessed for different fractions of soil potassium in volcanic and non‐volcanic soil groups. Volcanic soils (Hydrandepts and Andaquepts) were significantly lower (P < 0.05) in exchangeable K than the non‐volcanic soils (Dystropepts, Tropoqualfs and Eutropepts). Mean exchangeable K content of the non‐volcanic soils was 95.5 mg/kg, whereas that of volcanic soils was 72.4 mg/kg. Similarly, new gardens had an average exchangeable K content of 94.1 mg/kg, which was significantly greater than 71.6 mg/kg soil of older gardens. Non‐exchangeable K content differed significantly (P < 0.001) between the soil types; mean K content was 85.9 mg/kg for the volcanic soils, whereas in non‐volcanic soils, it was 184.9 mg/kg. Garden types also differed significantly (P < 0.05) with respect to non‐exchangeable K content; new gardens registering higher average values (by almost 20%) than the older gardens. Multiple regression analysis showed that variability in the tuber yield was as a result of variability of water soluble and exchangeable K (up to 22%), non‐exchangeable K (2%), mineral K (4%) and leaf K concentrations (10%). Older gardens, which are in volcanic soil groupings, are more susceptible to the K depletion problem because of continuous sweet potato cultivation, possibly owing to their lower K reserves. Such gardens should be managed either with sufficient fallow periods for regeneration of soil fertility or with suitable application of mineral K fertilizers to enhance productivity.     

Cities and biodiversity: Perspectives and governance challenges for implementing the convention on biological diversity (CBD) at the city level

City governments are fundamental to implement international environmental agreements, such as the convention on biological diversity (CBD). Even though many of them are not directly involved in the negotiation of international agreements, which are signed by national governments, most of those agreements are in fact implemented at the city level. The importance of city governance to tackle the challenges of biodiversity loss has increased as urban population has grown enormously in the last decades, particularly in developing countries. The way cities are designed, planned and governed influences the magnitude of their direct and indirect impacts on biodiversity.This paper analyzes the relationship between cities, local governance and biodiversity. Initially, we examine the relationships between cities and biodiversity by looking at the major influences cities have on biodiversity loss or conservation within and outside the city boundaries, as well as the benefits of biodiversity conservation to cities, such as the provision of ecosystem services. The paper then moves to understand what are the main urban processes and governance mechanisms that can be improved to make cities effective to implement the directives of the CBD.Urbanization creates new challenges for biodiversity conservation. As a large part of the world’s population moves from rural to urban areas, there are changes in the link between human activities and biodiversity, and consequently to the way we should think biodiversity conservation policies. However, scarce attention has been given to understand how to make cities more biodiversity friendly, both within the urban fabric, but particularly in faraway places.     

Effects of conventional and organic (manure) fertilization on soil,plant tissue nutrients and berry yields in vineyards. The use of the original native soil as a control

The effects of conventional and organic (manure) fertilization on soils of vineyards were examined in the area of Nemea in southern Greece. The soil properties of the adjacent natural environment (maquis), which served as a control (considering the land use changes as treatments), were also examined and compared with those of the vineyards. The null hypothesis was that there was no difference among them. The results rejected the null hypothesis. It was found that the maquis top soil had the highest C concentration than both types of vineyards and the organic vineyard had significantly higher C concentration than the conventional one. The organic soil N followed the same pattern. The vine leaves (blades) in the conventional vineyards had significantly higher concentrations of Ca, Mg, Mn, and Zn. The average values of Mg and Zn concentrations in leaves in the organic vineyards were below the suggested ranges for nutrient sufficiency in vines and for this reason a foliar application of Mg and Zn is recommended for the organic vineyards. The average berry yield was significantly higher in the conventional vineyards.     

Ovaries of Petrobius brevistylis (Archaeognatha, Machilidae) and Tricholepidion gertschi (Zygentoma, Lepidotrichidae): Morphology, ultrastructure and phylogenetic implications

The ovaries of Petrobius brevistylis (Archaeognatha) and Tricholepidion gertschi (Zygentoma) are composed of seven metamerically arranged panoistic ovarioles that are covered with the external epithelial sheath (=tunica externa) and the basal lamina (=tunica propria). The germaria are small and enclose oogonial cells and somatic prefollicular cells. The vitellaria contain 10–15 linearly arranged oocytes that are surrounded by a one-layered follicular epithelium. Oocyte nuclei (=germinal vesicles) are large and possess giant, compact (P. brevistylis) or lobate (T. gertschi) nucleoli. In both studied species the majority of yolk is incorporated to the oocyte via endocytosis; however, appearance of numerous Golgi complexes and rough endoplasmic reticulum cisternae at the onset of vitellogenesis suggests the involvement of these organelles in synthesis of yolk precursors. The obtained results are discussed in the phylogenetic context and support the idea that Lepidotrichidae are the most basal zygentoman clade.     

Impact of an invasive plant (Elymus athericus) on the conservation value of tidal salt marshes in western France and implications for management: Responses of spider populations

As a result of Elymus athericus (Poaceae) invasion in the last 10 years, a major change in vegetation cover has occurred in salt marshes of the Mont Saint-Michel bay (France). Such invasions are known to modify biodiversity and consequently ecosystem proprieties and functions as well as the conservation value of invaded areas. In this study, we especially focus (1) on the impact of the invasive species on the conservation value of the invaded ecosystems and (2) on the impact of mowing in the invaded parts of salt marshes. Among the various biological models, spiders were selected for the present study because they are an abundant, diversified, taxonomic group of generalist predators. Furthermore, data are available on this group since 1984, allowing a comparison of species distribution ranges in salt marshes before and after the E. athericus invasion. This diachronic approach showed that the invasion of salt marshes promoted the progression of non-coastal species, web-building spiders and cursorial spiders, but did not interfere with resident species distributions, finally resulting in higher spider densities and species richness in the entire area.The species composition, functional groups, abundances and densities of spider assemblages were also compared using complementary sampling techniques (i.e., pitfall traps, collecting by hand and quadrat technique) between natural and invaded stations in 2002 for synchronic approach. One main result is that some halophilic species decreased in abundances and densities in invaded plots, which is thought to alter the marine character of salt marsh arthropod assemblages. Lastly, mowing, by reducing non-coastal species and enhancing halophilic ones, tends to reduce the negative effects of Elymus on salt marsh conservation value. The relation between the treatments tested (invasive species and mowing) and the spider assemblages by changes in environmental variables confirms the high value of this group as bioindicators.