Jean-Claude Lefeuvre

Comparative biodiversity along a gradient of agricultural landscapes

The aim of this study is to compare biodivemity in contrasted landscape units within a small region. In western France agricultural intensification leads to changes in landscape structure: permanent grasslands are ploughed, fields enlarged and surrounding hedgerows removed or deteriorated, brooks are straightened and cleaned. South of Mont Saint Michel Bay, four landscape units have been identified along an intensi- fication gradient. Several taxonomic groups (small mammals, birds, insects and plants) have been used to evaluate the characteristics of biodiver- sity along this gradient. The hypothesis that intensification of agricultural practices lead to changes in biodiversity has been tested. Biodiversity is measured by the species richness, Shannons diversity index, equitability and similarity indexes. Our results show that intensification of agriculture does not always lead to a decrease in species richness, but to several functional responses according to taxonomic groups, either no modification, or stability by replacement of species, or loss of species. For most of the studied taxo- nomic groups species richness does not vary greatly along the gradient. Depending on the landscape structure and farming systems this gradient is probably truncated and does not allow to show major changes in species richness. An alternative hypothesis is that used indexes are not sensitive enough to reveal changes in biodiversity. Nevertheless, similarity indexes reveal that sensitivity to changes varies, invertebrates being more likely to perceive the dynamics of the landscapes studied than vertebrates or plants. These points have to be taken into consideration when elaborating policies for sustainable agriculture or nature conservation. 0 Elsevier, Paris

In search of a real definition of the biological invasion phenomenon itself

The many qualifying terms attributed to invasive species reveal the lack of precision surrounding the notion of biological invasion itself. In spite of several proposed definitions, some basic disagreements persist concerning characterization of the phenomenon. These primarily arise from the lack of pertinence of both of the main current criteria—the geographic (or biogeographic) criterion and the impact criterion—to what is really intended by “invasion.” Faced with this situation, it seems preferable to adopt an ontological approach allowing a return to the basic principles of the elaboration of a definition. Starting with the nature of the phenomenon itself (i.e., its essence), we try to elucidate the notion of biological invasion and we suggest a general definition compatible with most of the ideas already expressed.

Invasive species can also be native

In a recent review in TREE, Wilson et al. [1] aim to link success of biological invasions and the dispersal pathways of species. The authors emphasize the characteristics of human-mediated extra-range dispersal pathways (i.e. rapidity, high propagule pressure and high genetic diversity of the introduced population) in order to argue that biological invasions are, above all, a biogeographical phenomenon of unprecedented magnitude and, thus, that it would be better to separate biological invasions from ‘considerations of ecological dominance’ [1].

European salt marshes diversity and functioning: The case study of the Mont Saint-Michel bay, France

The macrotidal Mont Saint-Michel bay has beenstudied intensively since 1990. The objectives ofthis study, supported by the European Union, wereto understand various processes underlying thefunctioning of this hydrosystem with a special focuson organic matter and nutrient fluxes betweensaltmarshes and marine waters. This paper presents asynopsis of these studies. The tidalflats are unvegetated and primary production isexclusively due to microphytobenthos communitiesdominated by diatoms. Halophile plant communitiescolonize the top parts of the tidal flats. Theircomposition and production vary according to amaturity gradient and sheep grazing. In ungrazedsaltmashes, production ranged from 1080 gDW m-2·yr-1in the lower marsh to 1990 gDW m-2·yr-1in the upper marsh whereas it was only 200 to500 gDW m-2·yr-1in Salicorniaspp.dominated pioneer zones and sheep grazed areas. Mostof this organic matter (OM) was trapped in situ,processed by fungi and bacteria, and then releasedseaward via tidal fluxes, groundwater and runoff as particulate OMand nutrients: –497 kg N, –1200/–1000 kg P-PO4and –9900/–4200 kg inorganic carbon). A small amount ofOM was exported to the bay as macrodetritus. Fattyacids and stable isotopes, used as markers, showedthat OM produced by the marsh halophytes contributedto the diet of all the tidal flats invertebrates thatwere studied. Transient fish species were shown tocolonize the saltmarshes to forage or graze, exporting about 50 tons POM (DW)·y-1. Therefore,it is assumed that the saltmarsh production enhancesthe production of the whole bay. But the functioningis still poorly known because the nutrient sinks havenot all been identified. Part of the nutrients inputwas provided by precipitation (+327 kg y-1), butthe contribution of the catchments was not quantifieddespite the fact that their influence was shown by thepresence of lindane in all the compartments of thesystem. Dynamics of saltmarshes are mainly influencedby natural sedimentation (1.5 million m3·y-1in the bay), plant community succession, and management (i.e., reclamation and agricultural activities).

Role of fish communities in particulate organic matter fluxes between salt marshes and coastal marine waters in the Mont Saint-Michel Bay

Among the 90 fishspecies censused in the Mont Saint-Michel Bay (France), 23 colonise and forage in the salt marshes during flood. Therefore, this environment may play an important trophic and nursery role for these species. This community is largely dominated by mullets (81% of the biomass), Liza ramada and secondarily L. aurata. But gobies (mainly Pomatoschistus minutus and P. lozanoï) and sea bass (Dicentrarchus labrax) are also present; they represent respectively 11% and 4% of the biomass. During the tide cycles, mullets export from salt marshes about % of their body weight (FW) containing a mixture of sediment (43%), organic matter (24%) and water (33%). Gobies and sea bass mainly feed on dwelling macro-invertebrates, and they export respectively 4.5% and 10% of their body weight during a tide cycle. Thus, we estimated that 50 tonnes year-1 of particulate organic matter (dry weight POM) are exported from the 4000 ha of salt marshes to the marine coastal waters. These fish communities appear to be POM transporters and could play a significant role in the global energy budgets of coastal environments such as Mont Saint-Michel Bay. According to the seasons and the years, the energy exported by fish communities is assumed to range between 0 and 10% of the total POM output.

Feeding ecology of 0-group sea bass, Dicentrarchus labrax, in salt marshes of Mont Saint Michel Bay (France)

Abstract0-group sea bass,Dicentrarchus labrax, colonize intertidal marsh creeks of Mont Saint Michel Bay, France, on spring tides (e.g., 43% of the tides) during flood and return to coastal waters during ebb. Most arrived with empty stomachs (33%), and feed actively during their short stay in the creeks (from 1 to 2 h) where they consumed on average a minimum of 8% of their body weight. During flood tide, diet was dominated by mysids,Neomysis integer, which feed on marsh detritus. During ebb, when young sea bass left tidal marsh creeks, the majority had full stomachs (more than 98%) and diet was dominated by the most abundant marsh (including vegetated tidal flats and associated marsh creeks) resident amphipod,Orchestia gammarellus. Temporal and tidal effects on diet composition were shown to be insignificant. Foraging in vegetated flats occurs very rarely since they are only flooded by about 5% of the tides. It was shown that primary and secondary production of intertidal salt marshes play a fundamental role in the feeding of 0-group sea bass. This suggests that the well known nursery function of estuarine systems, which is usually restricted to subtidal and intertidal flats, ought to be extended to the supratidal, vegetated marshes and mainly to intertidal marsh creeks.

IMPACT OF THE INVASIVE NATIVE SPECIES ELYMUS ATHERICUS ON CARBON POOLS IN A SALT MARSH

The spread of the invasive native clonal grass Elymus athericus is one of the most significant changes that have affected the plant communities of European salt marshes in the last decade. The goal of this study was to investigate the rate of this invasion and its impact on C cycling in a non-grazed salt marsh of the Mont Saint-Michel Bay (France). Aboveground net primary production (ANPP), litter quantity and quality, and decomposition rates were quantified in Elymus athericus and in the original plant communities (“control”). Since 1991, Elymus athericus has been replacing the characteristic plant communities at a rate of 4 ha yr−1. ANPP was significantly higher in Elymus athericus (3011±347 gDW m−2 yr−1 and 1181±123 gC m−2 yr−1) than in the control (2028±239 gDW m−2 yr−1 and 771±76 gC m−2 yr−1) stands. Lignin content of Elymus athericus’ litter was 3.5 times greater than in the litter of the control communities, explaining its accumulation under the vegetation (i.e., litter quantity was from 2 to 10 times greater under Elymus athericus than under the control vegetation). C mineralization in the sediment (expressed as rate of CO2 per mass of sediment) was significantly lower under Elymus athericus than under the control vegetation. Our results suggest that Elymus athericus increased C trapping within the salt marsh and thus limits the potential for C exportation toward coastal waters.

Factors affecting the population dynamics of Suaeda maritima at initial stages of development.

This study describes life history and population dynamics of the annual halophyte Suaeda maritima, at initial stages of seedling development, in salt-marshes of the Mont-Saint-Michel Bay. Effects of salinity on seed germination and seedling development were studied both in the laboratory and in the field. Salinity was measured as sediment electrical conductivity. Experiments in the laboratory showed that Suaeda maritima was relatively tolerant to salinity during the germination stage. In the field, germination occurred when soil salinity was minimal. Almost all seeds germinated in the field; this may explain the absence of a persistent seed bank. Salinity did not seem to affect the growth of seedlings either. Mortality in the field was attributed to physical factors such as anoxia during temporary immersion, burial by sediment and tidal uprooting. The intensity of these effects varied according to the location of Suaeda maritima seedlings: in hollows, along the edge of tidal creeks, in gaps among dense vegetation cover, or in pattern with Puccinellia maritima. Perennial vegetation did not restrict Suaeda maritima germination. In contrast, perennials such as Puccinellia maritima and Halimione portulacoides limited the development of Suaeda maritima seedlings since only a small number of this annual emerged beneath their canopies. The fate of seedlings depended to a great extent on the permeability of the perennial canopy to light. Thus, disturbance resulting in bare patches inside the perennial vegetation seemed essential for the development of this annual species. Suaeda maritima can grow in the presence of Puccinellia maritima so long as the latter present as an open matrix. Suaeda maritima may also benefit from protection against desiccation and tidal action where this occurs.

Impact of sheep grazing on juvenile sea bass, Dicentrarchus labrax L., in tidal salt marshes

The diet of young of the year sea bass, Dicentrarchus labrax L., from sheep grazed and ungrazed tidal salt marshes were compared qualitatively and quantitatively in Mont Saint-Michel Bay. In areas without grazing pressure, the vegetation gradient changes from a pioneer Puccinellia maritima dominated community at the tidal flat boundaries through a Atriplex portulacoides dominated community in the middle of the marsh to a mature Elymus pungens dominated community at the landward edge. The A. portulacoides community is highly productive and provides important quantities of litter which provides a habitat and good supply to substain high densities of the detrivorous amphipod Orchestia gammarellus. In the grazed areas, the vegetation is replaced by P. maritima communities, a low productive grass plant, and food availability and habitat suitability are reduced for O. gammarellus. Juvenile sea bass colonise the salt marsh at flood during 43% of the spring tides which inundate the salt marsh creeks. They forage inside the marsh and feed mainly on O. gammarellus in the ungrazed marshes. In grazed areas, this amphipod is replaced by other species and juvenile sea bass consume less food from the marsh. This illustrates a direct effect of a terrestrial herbivore on a coastal food web, and suggests that management of salt marsh is complex and promotion of one component of their biota could involve reductions in other species.

Do fish communities function as biotic vectors of organic matter between salt marshes and marine coastal waters

The contribution of fish communities to organic matter (OM) fluxes, especially between salt marshes and adjacent marine coastal waters are reviewed. For this a data set from the bay of Mont Saint-Michel and literature is examined and discussed. In a range of macro-tidal coasts of Europe, salt marshes are only flooded at spring tides for a short time. Many animals, including fish, then invade the salt marshes through tidal creeks. They forage there for up to a few hours and swim back to sea at ebb. Meanwhile, organic matter is exported as gut content. In the 4000 ha of salt marshes of the bay of Mont Saint-Michel mullets were responsible for the export of about 8 kg of dry weight OM ha−1 in 1996 and of roughly 12 kg in 1997. Although spatio-temporally variable, the fish communities appear to play a more or less significant role, as ‘biotic vectors’ in the nutrient fluxes between salt marshes and coastal waters.