Claire Lavigne

Mixing of propagules from discrete sources at long distance: comparing a dispersal tail to an exponential

BackgroundRare long distance dispersal events impact the demography and the genetic structure of populations. When dispersal is modelled via a dispersal kernel, one possible characterisation of long-distance dispersal is given by the shape of the tail of the kernel, i.e. its type of decay. This characteristic is known to directly act on the speed and pattern of colonization, and on the spatial structure of genetic diversity during colonization. In particular, colonization waves behave differently depending on whether the kernel decreases faster or slower than an exponential (i.e. is thin-tailed vs. fat-tailed). To interpret and extend published results on the impact of long-distance dispersal on the genetic structure of populations, we examine a classification of dispersal kernels based on the shape of their tails and formally demonstrate qualitative differences among them that can influence the predicted diversity of a propagule pool sampled far from two distinct sources.ResultsWe show that a fat-tailed kernel leads asymptotically to a diverse propagule pool containing a balanced mixing of the propagules from the two sources, whereas a thin-tailed kernel results in all propagules originating from the closest source. We further show that these results hold for biologically relevant distances under certain circumstances, and in particular if the number of propagules is large enough, as would be the case for pollen or seeds.ConclusionTo understand the impact of long-distance dispersal on the structure and dynamics of a metapopulation, it might be less important to precisely estimate an average dispersal distance than to determine if the tail of the dispersal kernel is fatter or thinner than that of an exponential function. Depending solely on this characteristic, a metapopulation will behave similarly to an island model with a diverse immigrant pool or to a stepping-stone model with migrants from closest populations. Our results further help to understand why thin-tailed dispersal kernels lead to a colonization wave of constant speed, whereas fat-tailed dispersal kernels lead to a wave of increasing speed. Our results also suggest that the diversity of the pollen cloud of a mother plant should increase with increasing isolation for fat-tailed kernels, whereas it should decrease for thin-tailed kernels.

Chapter Six – Eco-Evolutionary Dynamics of Agricultural Networks: Implications for Sustainable Management

Community and ecosystem ecology are paying increasing attention to evolutionary dynamics, offering a means of attaining a more comprehensive understanding of ecological networks and more efficient and sustainable agroecosystems. Here, we review how such approaches can be applied, and we provide theoretical models to illustrate how eco-evolutionary dynamics can profoundly change our understanding of agricultural issues. We show that community evolution models can be used in several contexts: (1) to improve the selection of agricultural organisms within the context of their ecological networks; (2) to predict and manage the consequences of agricultural disturbances on the ecology and evolution of ecological networks; and (3) to design agricultural landscapes that benefit from network eco-evolutionary dynamics, but without negative impacts. Manipulation of landscape structure simultaneously affects both community ecological dynamics (e.g., by modifying dispersal and its demographic effects) and co-evolution (e.g., by changing gene flows). Finally, we suggest that future theoretical developments in this field should consider appropriate co-evolutionary models and ecosystem services.

Spatial sensitivity of maize gene-flow to landscape pattern: a simulation approach

Pollen dispersal is a critical process defining connectivity among plant populations. In the context of genetically modified (GM) crops in conventional agricultural systems, strategies based on spatial separation are promoted to reduce functional connectivity between GM and non-GM crop fields. Field experiments as well as simulation studies have stressed the dependence of maize gene flow on distances between source and receptor fields and on their spatial configuration. However, the influence of whole landscape patterns is still poorly understood. Spatially explicit models, such as MAPOD-maize, are thus useful tools to address this question. In this paper we developed a methodological approach to investigate the sensitivity of cross-pollination rates among GM and non-GM maize in a landscape simulated with MAPOD-maize. The influence of landscape pattern on model output was studied at the landscape and field scales, including interactions with other model inputs such as cultivar characteristics and wind conditions. At the landscape scale, maize configuration (proportion of and spatial arrangement in a given field pattern) was shown to be an important factor influencing cross-pollination rate between GM and non-GM maize whereas the effect of the field pattern itself was lower. At the field scale, distance to the nearest GM maize field was confirmed as a predominant factor explaining cross-pollination rate. The metrics describing the pattern of GM maize in the area surrounding selected non-GM maize fields appeared as pertinent complementary variables. In contrast, field geometry and field pattern resulted in little additional information at this scale.

Predation by generalist predators on the codling moth versus a closely-related emerging pest the oriental fruit moth: a molecular analysis

1 Biological control by conservation of native natural enemies can, at its best, reduce the need for pesticides and prevent detrimental effects upon the environment. The present study investigated the role of ground‐active generalist predators as natural enemies of two tortricid pests in apple orchards. 2 Predation rates were compared on the well established codling moth Cydia pomonella and the emerging oriental fruit moth Grapholita molesta, which has recently switched hosts to apples. 3 The present study hypothesized that the ground‐active predators consumed the two tortricid pests in significant numbers without preference, and attacked the pests at different developmental stages. 4 Using diagnostic polymerase chain reaction on the gut contents of field‐caught ground‐active predators, no difference in predation rates was found on these two pests. Spiders were the most efficient predators of emergent adult moths in spring, whereas the carabid beetles, feeding on diapausing larvae, were important in the autumn. 5 The temporal complementarity between spiders and carabid beetles, attacking different stages of the pests at different times of year, highlights the need for diverse predator assemblages to optimize biological control.

Early-season predation on aphids by winter-active spiders in apple orchards revealed by diagnostic PCR

Aphids are major pests in apple orchards, debilitating the crop and spreading disease. We investigated whether early-season predation by canopy spiders may be effectively controlling aphid numbers in three organic orchards. For this purpose, we monitored the aphid population dynamics from the winter eggs to colony stages and compared this to spider abundances and rates of predation on aphids detected by diagnostic polymerase chain reaction. For the latter, we applied existing general aphid primers. We found that spiders ate colony fundatrices and that aphid numbers were negatively related to spider abundance. Spiders were the main active predators within the orchards when the first colony fundatrices were present, indicating their importance in the early control of aphid populations.

Multiple origins of the sodium channel kdr mutations in codling moth populations.

Resistance to insecticides is one interesting example of a rapid current evolutionary change. DNA variability in the voltage-gated sodium channel gene (trans-membrane segments 5 and 6 in domain II) was investigated in order to estimate resistance evolution to pyrethroid in codling moth populations at the World level. DNA variation among 38 sequences revealed a unique kdr mutation (L1014F) involved in pyrethroid resistance in this gene region, which likely resulted from several convergent substitutions. The analysis of codling moth samples from 52 apple orchards in 19 countries using a simple PCR-RFLP confirmed that this kdr mutation is almost worldwide distributed. The proportions of kdr mutation were negatively correlated with the annual temperatures in the sampled regions. Homozygous kdr genotypes in the French apple orchards showed lower P450 cytochrome oxidase activities than other genotypes. The most plausible interpretation of the geographic distribution of kdr in codling moth populations is that it has both multiple independent origins and a spreading limited by low temperature and negative interaction with the presence of alternative resistance mechanisms to pyrethroid in the populations.

Nest-Site and Landscape Characteristics Affect the Distribution of Breeding Pairs of European Rollers Coracias garullus in an Agricultural Area of Southeastern France

Abstract. Understanding factors that drive the choice of breeding habitats for birds is important for species management and conservation. We addressed this question in the case of the European Roller Coracias garrulus, an endangered species listed in Annex I of the European Union directive for bird conservation. For secondary hole-nesting birds, such as rollers, the breeding micro- and macrohabitat selection may rely on the location of the nest hole in a tree and its immediate surrounding environment as well as the larger scale foraging habitat. We used both of these criteria to compare the characteristics of Green Woodpecker Picus viridis holes that either were or were not occupied by breeding pairs of the European Roller. Our study was carried out in an agricultural landscape of southeastern France where the population of European Roller is trending towards an increase. When compared to unoccupied holes, occupied holes were at a lower height (approx. 6 m above ground) and were preferentially located in dead trees with other holes in close vicinity. Occupied holes were also located in landscapes with lower land use intensity (i.e., higher proportions of meadows, fallow lands, and fewer hedgerows and built areas) than non-occupied holes. Finally, fallow lands and meadows harboured more Orthoptera, an important food resource for rollers, than other land cover categories. Thus, our results highlight the importance of these land covers for the species management and indicate that they may be favourable areas for nest-box provisioning.

How to quantify a distance‐dependent landscape effect on a biological response

To quantify the effect of the surrounding landscape context on a biological response at a site, most studies measure landscape variables within discs centred on this biological response (threshold-based method, TBM). This implicitly assumes that the effect of a unit area of the landscape is consistent up to a threshold distance beyond which it drops to zero. However, it seems more likely that the landscape effect declines with increasing distance from the biological response point. Here, we develop a method to quantify landscape context effects by weighting the landscape variables by functions that decrease with distance. We illustrate the method using abundance data on birds and insects, and compare the results to the threshold approach. We defined distance weighting functions by the function family (e.g. negative exponential, Gaussian…) and by the parameters for this function. We developed a method to simultaneously estimate the parameters characterizing the effect of the landscape variables and the parameters of the best weighting functions. For each test dataset, we determined which weighting function (family and parameters) had the most support, by optimizing the model AIC. The distance-weighted method (DWM) improved model support over the TBM in three of four datasets, with the exponential power function selected as the best weighing function in all three cases. The observed differences between estimations of landscape context effects by the distance-weighted and the threshold methods have significant implications for landscape management. For example, the DWM suggests that managing a landscape for 90% of its effect on a focal population requires an area over five times larger than the area estimated by the threshold method, a situation that might apply for priority conservation of few remnant populations of a severely endangered species. In contrast, management for 30% of the landscape effect requires only about half the area estimated using the threshold method, a situation that might apply to a management situation with limited resources or low political/societal support. The DWM is applicable to any species-habitat relationship. More comparisons are needed to determine the situations in which distance-weighted estimation of landscape context effects is warranted over the simpler threshold method.

An Unified Framework to Integrate Biotic, Abiotic Processes and Human Activities in Spatially Explicit Models of Agricultural Landscapes

Recent concern over possible ways to sustain ecosystem services has triggered important research worldwide on ecosystem processes at the landscape scale. Understanding this complexity of landscape functioning calls for coupled and spatially-explicit modelling approaches. However, disciplinary boundaries have limited the number of multi-process studies at the landscape scale, and current progress in coupling processes at this scale often reveals strong imbalance between biotic and abiotic processes, depending on the core discipline of the modellers. We propose a spatially-explicit, unified conceptual framework that allows researchers from different fields to develop a shared view of agricultural landscapes. In particular,we distinguish landscape elements that are mobile in space and represent biotic or abiotic objects (for example water, fauna or flora populations), and elements that are immobile and represent fixed landscape elements with a given geometry (for example ditch section or plot). The shared representation of these elements allows setting common objects and spatio-temporal process boundaries that may otherwise differ between disciplines. We present guidelines and an assessment of the applicability of this framework to a virtual landscape system with realistic properties. This framework allows the complex system to be represented with a limited set of concepts but leaves the possibility to include current modelling strategies specific to biotic or abiotic disciplines. Future operational challenges include model design, space and time discretization, and the availability of both landscape modelling platforms and data.

Impact of pest control strategies on the arthropodofauna living in bird nests built in nestboxes in pear and apple orchards

Pesticide applications have a strong impact on biodiversity in agroecosystems. The present study aimed to assess the impact of pest control strategies on the arthropodofauna of Parus major nests built within nestboxes installed in orchards. Unlike many studied groups, these arthropod communities are not in direct contact with pesticide sprays (on account of their being sheltered by nestboxes) and are also unable to move away from the treated area. In this pilot study, we estimated the prevalence and the taxonomic and ecological diversities of arthropodofauna sampled in the nests and assessed the extent to which the whole and nest-specific arthropodofauna were affected by pest control strategies. Sixteen different insect and arachnid Primary Taxonomic Groups (PTGs, order level or below) were found in nests. The best represented PTGs (≥10% occurrence in years 2007 and 2008) were Psocoptera (Insecta, detritivorous/saprophagous), detritivorous/saprophagous Astigmata (Acari) and hematophagous Mesostigmata (Acari). Pest control strategies had a large impact on the prevalence of arthropods in nests, with higher proportions of nests hosting arthropods in organic orchards than in conventional orchards and with intermediate proportions in nests in Integrated Pest Management orchards. In contrast, pest control strategies had no significant effect on the composition of the arthropod communities when only nests hosting nidicolous arthropods were considered.