Yannick R. Delettre

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

Differential response of selected taxa to landscape context and agricultural intensification

Understanding the dynamics of biodiversity in changing agricultural landscapes is a goal for nature conservation and agricultural policies. Agriculture operates at several spatio-temporal levels from field to landscape, and induces differential response of communities according to their ecological traits. Reactions of several taxa to landscape and agricultural changes was conducted along landscape gradients in northern Brittany (France) gathering 14 sites. Landscapes ranged from fine grained areas, with a large proportion of permanent grasslands, wood and hedgerows, to coarse grained ones dominated by crops. Response of communities were either loss of species along the gradient (as for Diptera Chironomidae and Empididae), replacement (Coleoptera Carabidae) and no change in species composition (small mammals). In this paper, we present why these taxa react in such different ways according to their life history traits, their mobility patterns and to which parameters of landscape structure or agriculture intensification they are sensitive. Density of hedgerow networks, and permeability of individual hedgerows determine the spatial distribution of adults Diptera according to their flying ability. Mean body size of carabid beetles significantly decreases along the gradient of agricultural intensification, small species adapted to disturbance replacing large ones that are characteristic of stable habitats. The same small mammal species are found all along the two gradients, but their relative abundance is linked to the importance of crops versus more stable habitats in the landscape. We concluded that species survival in those fine grained agricultural landscapes depends on processes operating at the site scale and defining habitat quality, and processes operating at the landscape and/or metapopulation scale such as landscape modifications in connectivity and habitat availability.

Response of the small mammal community to changes in western French agricultural landscapes

We studied the response of the small mammal community (rodents and shrews) to recent changes in agricultural systems of western French landscapes. Work was conducted on twelve sites representative of the diversity of farming systems in this region. The characteristics of small mammal assemblages in each site were assessed using Barn Owl (Tyto alba) pellet analysis. Relationships between small mammal data and landscape descriptors were performed through co-inertia analysis. Richness and specific composition of the small mammal community were not affected by the degree of cultivation but variations in species frequency could be observed. The prevalence of some species allowed us to distinguish three main assemblages which were characteristic of low, medium, and high intensified landscapes. Status and life traits of these species showed that intensification of agriculture has negative effects on density of rare and habitat-specialist species while it favours habitat-generalist species, some of them being known to exhibit fluctuating density. The two main ways of agricultural intensification (maize vs. other crops) did not show any significant relationships with species assemblages. Our results gave us the opportunity to suggest recommendations on agronomical and conservation problems that may arise from these changes of agriculture in western France.

Landscape context and carabid beetles (Coleoptera: Carabidae) communities of hedgerows in western France

Dramatic changes have occurred in agricultural landscapes of western Europe since the 1950s. Their effects on biodiversity have been documented at local scales in terms of agricultural practices and management of uncultivated elements, more than at the landscape level. The highest diversity of agricultural landscape structure found in Brittany (France) is characterised by hedgerows surrounding arable fields and permanent grasslands. Some areas did not change much over a few decades, while others were totally transformed by agricultural intensification, field enlargement and removal of hedges. The present paper aimed at examining if this landscape context affected carabid beetles communities in hedgerows. In 11 sites of 1 km 2 each, land use diversity and heterogeneity, hedgerow connectivity and landscape openness were measured. Carabid beetles sampled in hedgerows were identified to species, and grouped in functional units. Multivariate analyses were used to relate the spatial distribution of insects to different parameters of landscape structure. The results show a significant relationship between landscape structure and carabid communities. Forest species were more abundant in dense hedgerow networks with a relatively high ratio of permanent grasslands. Landscape opening induced a shift in species composition; the relative abundance of large species decreased while small, mobile and more ubiquitous species were favoured. Two trajectories of landscape change were identified, which led to different responses in communities, the increase in maize for milk production opening the landscape far more than cereal crops associated with industrial pig or poultry production. Changes were most important in the latter case.

Space heterogeneity, space use and short-range dispersal in Diptera: A case study

This study investigates the impact of landscape heterogeneity on community structure and population dynamics in two families of Diptera (Empididae and Chironomidae). Adult emergence is compared with aerial flow by means of emergence traps and yellow traps on a transect across four habitats (pond banks, woodland, grassland and heathland) in close proximity to each other. Empids use different space units according to their larval development, sexual behaviour and food requirements. This creates an intermingling of species and individuals originating from different habitats in the lowest part of the transect. Adult chironomids of aquatic species exhibit a preferential use of open habitats while adults with terrestrial larvae disperse largely above the four sites. Habitat fragmentation and heterogeneity lead to opposite patterns in chironomid distribution: some species disperse over the whole set of macrohabitats but others are confined to a single patch, resulting in population isolation. The impact of spatial and temporal landscape patterning is discussed with a view to community structure, life-history tactics and population dynamics.

Diet differentiation between European arvicoline and murine rodents

Small European muroid rodents are generally divided into species which feed on seeds and/or invertebrates and species which feed on green plant material; however, there is considerable plasticity in feeding behavior among species. Here, we analyze diets of 14 low-latitude rodent species from Western Europe based on published studies. The 77 studies were submitted to principal component analysis in order to compare diet plasticity within and between the 14 species. We observed variations in food composition of arvicoline and murine rodents which are associated with differences in morphology and habitat use. Most arvicoline rodents eat mainly green matter of the herbaceous layers of open habitats whereas most murine species are able to use a greater diversity of high energetic plant tissues from denser habitats, where they can exploit the different vegetation layers. Despite its phylogenetic position among arvicoline rodents, the bank vole (Myodes glareolus) shows morpho-physiological and ecological traits which tend to be more similar to murine species. These intermediate evolutionary characters seem consistent with the fact that bank voles are able to exploit a wide spectrum of trophic resources from low energetic lignified tissues to high calorific invertebrate prey. This results in a very diverse diet, which is intermediate between true herbivorous arvicolines and typical seed- and invertebrate-eating murine species. More investigations on genetic affiliation and ecological driving forces will help understand this intermediate position of bank vole diet, and further investigations among other arvicoline species will help determine if bank voles and other Myodes species are unique.

Impact of anthropogenic disturbances on beetle communities of French Mediterranean coastal dunes

In coastal dunes, influenced by anthropogenic activities such as tourism, it is important to determine the relative influence of environmental factors at different spatial scales to evaluate the sensitivity of local communities to disturbances. We analyzed beetle communities of 14 dunes of the French Mediterranean coast: four in the relatively preserved Camargue area, and ten in the Var department, where tourism is intensive. Beetle communities were studied three times in early spring using sand sampling. Species-environment relationships were evaluated at the regional, landscape and local scale using redundancy analysis (RDA) and variability partitioning. About 28 species were identified, of which 15 were sand-specialist species, which accounted for more than 93% of total abundance. The beetle communities of Camargue were significantly different from those of the Var department owing to the pullulation of a Tenebrionid species (Trachyscelis aphodioides Latr.) in the Var, except for one restored dune where the community was very similar to those of Camargue. Our results showed no longitudinal gradient between the two regions. Local factors (dune height, preservation and disturbance index) significantly explained most of the variation in the dominance of T. aphodioides, while some other local factors were important for other psammophilous species. This study also suggests that dune beetle communities are strongly affected on beaches intensively managed for tourism, but beetles are still abundant in much disturbed sites.

Larvae of terrestrial Chironomidae (Diptera) colonize the vegetation layer during the rainy season.

Summary Several qualitative observations show that larvae of terrestrial Chironomidae (Diptera) are able to colonize the vegetation above the soil surface on heathlands in Brittany (France). In winter, during rainy periods, larvae of several species move into the thick layer of lichens on which they feed. This behaviour, previously unknown, expands food resources which can be used by species. Its occurrence only in winter is related to ecophysiological requirements, mainly drought avoidance. It is discussed in a cost-benefit perspective.

A simple technique using wooden stakes to estimate vertical patterns of interstitial oxygenation in the beds of rivers

Silt and fine sediments from anthropogenic activities frequently clog river bed sediments, impairing vertical exchanges between stream and subsurface water. River managers need a simple technique to detect the extent of interstitial clogging and monitor the effectiveness of measures to reduce siltation. We evaluated the use of 30-cm long pine-wood stakes, inserted for 3-6 weeks in the sediments of four French rivers varying in interstitial clogging, to determine the association between changes in the colour of the wood and the adjacent interstitial conditions. There was a general association between depth to interstitial hypoxia and location of the colour change of the wooden stakes from brown to pale grey or black after 3-4 weeks. This change in colour also broadly matched interstitial contents of fine sediment, ammonium, and nitrate although the method could not reliably detect microscale zones of anoxia or short-term changes in dissolved oxygen. Thus, its effectiveness lies in its use as a cheap, simple, and broad-scale indicator for collecting long-term integrated data of interstitial oxygenation in stream sediments with minimal disruption of the gravel bed, and appears an ideal tool for river managers and salmonid fish biologists.

Relating Insect Movements to Farming Systems in Dynamic Landscapes

During the last decade the conservation of biodiversity in agricultural landscapes has been greatly emphasized. Intensification of agricultural production and practices, as well as land abandonment, have been considered as major threats (Solbrig, 1991), by making drastic changes in landscape structure and composition. The agricultural landscape is a shifting mosaic of crops, pastures, fallow lands and woody areas. Landscape elements exhibit their own disturbance regime, which depends on the practices used by farmers and thus interact with insects at several spatio-temporal scales. Changes at regional and long-term scales are the most documented and the most predictive (Baker, 1989; Rackham, 1986; Odum and Turner, 1990; Meeus, 1995; Crumley and Marquardt, 1987). A recent trend in western Europe is a decrease of the area covered by non-cultivated elements such as hedgerows, woodlots, heathlands, within intensive agricultural landscapes (Agger and Brandt, 1988; Bunce and Hallam, 1993; Burel and Baudry, 1990; Morant et al., 1995). In the mean time large tracks of farmland are abandoned. Thus the contrast between different regions increases. The changes within rural landscapes result in an increase in fragmentation of many elements and affect insect populations by reducing available habitats or seasonal refuges for many species. At a finer scale, farmers make decisions on crop succession in their farming system and on management practices for field boundaries and non-productive areas; this creates a changing landscape mosaic. Changes in agricultural landscapes can only be predicted through the knowledge of how farmers will change the land use pattern under different circumstances. Two levels must be considered: 1) changes in the type of production (e.g. from dairy production to cereals) and 2) changes in the techniques of production (e.g. feeding dairy cows with hay or maize silage). If, at a broad regional scale, relationships between the type of farming systems and landscape can be established, it becomes very fuzzy at the landscape scale, relating to, the scales of individual and population dispersal. Deffontaines et al. (1995) provide examples of two farms, in the Pays d’Auge, where the most productive one has more grassland and less annual forage crops. In all cases, the within farm land use diversity is striking, it results from both physical and spatial constraints and the requirements of the system of production (e.g. winter/summer forage). Many factors, out of the agricultural sector, also affect land use and landscape patterns, as in multi-job farms (Laurent et al., 1994). Trajectories of households as well as changes within farm systems are barely related at the individual farm scale. The major consequence from a landscape ecological point of view is that landscape changes cannot be derived from current landscape patterns (Burel and Baudry, 1990). More specifically, abandonment or dereliction of grassland is a stochastic process at the landscape scale, although deterministic at the farm scale.