Aude Ernoult

Fine-scale spatial patterns in grassland communities depend on species clonal dispersal ability and interactions with neighbours

Summary Non-random spatial patterns are a common feature of plant communities. However, the mechanisms leading to their formation remain unknown. The clonal dispersal ability of a species, that is, the average length of spacers between ramets, is commonly acknowledged to influence spatial patterns in clonal plants, although this relationship remains to be demonstrated. Moreover, the clonal dispersal ability of neighbouring species may influence environmental conditions and trigger modifications in clonal characteristics of a focal species. Thus, not only the clonal dispersal ability of a species, but also that of its competitors may influence the fine-scale spatial pattern of a species. In this article, we compared spatial patterns (in terms of colonization and occupation of space) of species with low (L), intermediate (I) or high (H) clonal dispersal abilities. Twelve species were classified within three groups of clonal dispersal (L, I or H) based on their average spacer lengths, and seven types of experimental assemblages consisting of species from one, two or three dispersal groups were studied. Two questions were addressed: (i) does the species clonal dispersal ability influence their spatial patterns and (ii) are species fine-scale spatial patterns affected by the clonal dispersal of neighbours? Species spatial patterns were recorded for each assemblage and were then analyzed using point pattern analysis. Despite strong species-specific effects, L-species displayed the highest level of local aggregation, which is indicative of limited space colonization, and the lowest level of local co-occurrence with other species, which is indicative of a high level of space occupation. The opposite pattern was observed in H-species, while that of I-species was intermediate. The species spatial patterns were modified by the clonal dispersal ability of competitors. Synthesis. This study emphasizes the importance not only of clonal dispersal but also of biotic interactions and, more precisely, of plant neighbour characteristics, in the spatial patterning of grassland plant communities.

Reconsidering the role of ‘semi-natural habitat’ in agricultural landscape biodiversity: a case study

Semi-natural habitats are considered as the main source of biodiversity in agricultural landscapes. Most landscape ecology research has focused on the amount (relative surface) and spatial organisation of these habitats. However, these two components of landscape heterogeneity, composition and configuration, are often correlated. Also, landscape structure effects on biodiversity were mostly observed locally, while there is a great need for studying landscape-scale gamma diversity. We conducted a mensurative experiment to determine the independent effects of semi-natural habitat amount and configuration on gamma diversity of carabid beetles and vascular plants. The influence of landscape heterogeneity components were tested on species richness, evenness and composition. Local diversity (species richness and composition) was compared across the various cover types to determine their relative contributions. Only carabid species evenness and composition were influenced by semi-natural habitat amount. Carabid and plant species richness and plant species composition remained unaffected by semi-natural habitats. Local diversity analysis showed that three types of habitats can be distinguished in agricultural landscapes: grasslands (temporary and permanent ones), woody habitats (woodlands and hedgerows) and row crops. These results beg for a re-evaluation of the semi-natural covers. Temporary and permanent grasslands are often similar, probably because of comparable farming management. Permanent grasslands and woody habitats are often combined as semi-natural covers, although they support very different communities. The lack of effect of semi-natural habitat amount and configuration on gamma diversity results from a more complex organisation of biodiversity in landscapes and supports the move from semi-natural vs. farmland to habitat mosaic landscape representations.

Landscape level processes driving carabid crop assemblage in dynamic farmlands

Landscape heterogeneity has been shown to be a major factor in the maintenance of biodiversity and associated services in agricultural landscapes. Farmlands are mosaics of fields with various crop types and farming practices. Crop phenology creates asynchrony between fields sown and harvested in different periods (winter vs. spring crops). The present study was conducted to examine the influence of such spatio-temporal heterogeneity on biodiversity, with the hypothesis that it would lead to spatio-temporal redistribution (shifting) of species. Species richness and activity-density of carabid beetles in winter cereal (winter) and maize (spring) crops were compared across 20 landscapes distributed along a double gradient of relative area and spatial configuration of winter and spring crops. Maize fields were sampled in spring and late summer for comparison over time. The response of carabid species richness to landscape heterogeneity was weak in spring, but maize field richness benefited from adjacencies with woody habitat, in late summer. In spring, increased length of interfaces between winter and spring crops lowered carabid activity-density in winter cereal fields, suggesting that maize fields acted as sinks. Interfaces between woody habitats and crops increased activity-density in both crop types. We found no evidence of spatio-temporal complementation, but different species benefited from winter cereals and maize in spring and late summer, increasing overall diversity. These findings confirm the role of adjacencies between woody and cultivated habitats in the conservation of abundant carabid assemblage in winter cereals and maize. We conclude that between-field population movement occurs, and advocate for better consideration of farmland heterogeneity in future research.

Functional connectivity in replicated urban landscapes in the land snail (Cornu aspersum)

Urban areas are highly fragmented and thereby exert strong constraints on individual dispersal. Despite this, some species manage to persist in urban areas, such as the garden snail, Cornu aspersum, which is common in cityscapes despite its low mobility. Using landscape genetic approaches, we combined study area replication and multiscale analysis to determine how landscape composition, configuration and connectivity influence snail dispersal across urban areas. At the overall landscape scale, areas with a high percentage of roads decreased genetic differentiation between populations. At the population scale, genetic differentiation was positively linked with building surface, the proportion of borders where wooded patches and roads appeared side by side and the proportion of borders combining wooded patches and other impervious areas. Analyses based on pairwise genetic distances validated the isolation‐by‐distance and isolation‐by‐resistance models for this land snail, with an equal fit to least‐cost paths and circuit‐theory‐based models. Each of the 12 landscapes analysed separately yielded specific relations to environmental features, whereas analyses integrating all replicates highlighted general common effects. Our results suggest that urban transport infrastructures facilitate passive snail dispersal. At a local scale, corresponding to active dispersal, unfavourable habitats (wooded and impervious areas) isolate populations. This work upholds the use of replicated landscapes to increase the generalizability of landscape genetics results and shows how multiscale analyses provide insight into scale‐dependent processes.

Connectivity drives the functional diversity of plant dispersal traits in agricultural landscapes: the example of ditch metacommunities

ContextStudying communities using a trait-based approach has contributed to major advances in the understanding of community assembly mechanisms, but research has primarily focused on the effect of local biotic and abiotic processes on plant assemblages.ObjectivesAt the landscape level, we expect that the diversity of trait values (i.e. functional diversity) in plant assemblages is not random because there might be strong environmental constraints occurring as a filter of plant species at this level. We expect that fragmentation, through connectivity loss, may reduce functional diversity by filtering plant dispersal traits within the global pool of species toward those that present the trait values that are the most adapted. We tested this hypothesis at the metacommunity level and focused on ditch plant dispersal traits.MethodsWe characterised the functional diversity of 27 ditch bank metacommunities for three seed traits encompassing the whole dispersal process: production, buoyancy and mass. We computed four connectivity variables based on the ditch networks surrounding each metacommunity. Null models were designed to quantify functional convergence (i.e. convergence of the trait values around a mean) or divergence for each trait (i.e. dissimilarity in plant trait values).ResultAt the metacommunity level, we demonstrate that (1) connectivity loss reduces the functional diversity of dispersal traits (seed production and seed buoyancy), leading to the convergence of trait values, whereas more connected landscapes promote randomness, or even divergence (seed mass) in trait values; (2) these changes are due to the modification in the number of occurrence of plant species over the nine local communities sampled, rather than in species identity.ConclusionsOur results indicated that, at the metacommunity level, reduced connectivity acts as a filter on the dispersal traits of plant species, promoting the broad distribution of species with efficient dispersal abilities in such landscapes. Thus, functional diversity helps us to understand the mechanisms underlying the effects of fragmentation on biodiversity.

Landscape potential for pollen provisioning for beneficial insects favours biological control in crop fields

ContextThe importance of landscape complexity for biological control is well-known, but its functional roles are poorly understood.ObjectivesWe evaluated the landscape capacity to provide floral resources for beneficial insects and its consequences for biological control in fields.MethodsThe gut contents of adult hoverflies sampled in 41 cereal fields were analysed to determine which plant species are exploited. The relative value of each habitat in providing adequate pollen resources was evaluated by vegetation survey. Then 15 cereal fields were selected along a gradient of landscape complexity, where the abundance of aphids, hoverfly larvae and aphid parasitism was monitored. The habitat’s proportions in landscape buffers surrounding these fields were used as landscape descriptors and to assess the potential level of pollen resources provision (LP index).ResultsAphid abundance significantly decreased with an increase of the LP index mainly sustained by grassy strips and weeds in fields. However, hoverfly larvae abundance also decreased with the increasing LP index. The enhancement of the aphid parasitism rate with the LP index suggests that aphid parasitoids may benefit from the same floral resources as hoverflies. Their crop habitat specialism may give them a competitive advantage in fields where both aphid and floral resources are abundant.ConclusionsComplex interaction networks involved in biological control may disrupt the expected direct effects of floral resource provisioning for a focal beneficial species. We highlighted fields and grassy strips as habitats provisioning floral resources for which the LP index could be very helpful to optimize agroecological management strategies.

Plant dispersal traits determine hydrochorous species tolerance to connectivity loss at the landscape scale

Aims: Landscape fragmentation has strong negative consequences on biodiversity. In networks of linear elements, connectivity loss results in a decreased length of connected elements and increased potential barriers, directly impacting the ability of plants to disperse. However, species vary in their tolerance to connectivity loss, likely due to differences in dispersal strategies. We investigated whether species tolerance to decreased ditch network connectivity is determined by seed traits. We selected, as a case study, water-dispersed plant species in a ditch network. Location: Ditch network in an intensive agricultural area in northern France. Methods: We selected 27 sites of 500 m 9 500 m, where we calculated connectivity indices based on the length of connected ditches, intersections and culvert number. For each parameter, we calculated plant tolerance levels by analysing species changes in occurrence in response to change in connectivity values. Concurrently, we measured in laboratory conditions five seed traits involved in plant movement and establishment in standing aquatic systems and analysed their explanatory power in plant tolerance to fragmentation. Results: All traits were significantly related to at least one component of ditch network connectivity. We interpreted the following two strategies in plant tolerance to connectivity loss from the results: (1) in networks where the connected network length was short, plants displayed short-distance dispersal with less efficient sexual reproduction, probably in favour of local vegetative multiplication; and (2) in networks with a high density of culverts or intersections, plants displayed seeds with reduced local retention, where seeds had the capacity to overcome long and frequent trapping events. In highly branched networks, plants also exhibited higher germination rates, promoting seed establishment when trapped along the banks. Seed capacity to be dispersed by wind at the water surface was only a marginal factor in plant tolerance to fragmentation. Conclusions: Connectivity loss acted as a filter on species seed traits. The results of our study offer an enhanced understanding of plant dispersal in fragmented standing aquatic networks and emphasize the importance of developing functional approaches in landscape studies.