Danny A. P. Hooftman

Impact of plant invasions on local arthropod communities: a meta-analysis

Summary 1. Invasive plants can have a major impact on local plant and animal communities. However, effects of plant invasions on arthropod communities and the potential drivers have rarely been studied. 2. We present a meta-analysis of 56 studies on the impact of plant invasions on abundance and richness of local arthropod communities. Moreover, we study the role of five invader and habitat attributes to assess their influence on the direction and magnitude of effect on arthropod communities: the time since introduction; woody vs. herbaceous invaders; presence of native congeners; canopy cover of the invader; and single vs. multiple invaders. 3. We found that overall invaded habitats had a 29% lower arthropod abundance and a 17% lower taxonomic richness compared with non-invaded habitats. Woody invaders had a stronger negative impact on arthropod communities than herbaceous invaders, reducing abundance and richness by as much as 55% and 21%, respectively. 4. Synthesis. Our study demonstrates that arthropod communities are negatively affected by plant invasions, which may have substantial effects on other ecosystem features, such as pollination, food web dynamics, decomposition as well as habitat heterogeneity. Loss of arthropod diversity is generally directly associated with loss of plant species richness. Therefore, the reduction we see could be causally connected to the effect of the invader on the habitat. The physical dominance of woody invaders compared with herbaceous invaders could be a main driver for this effect.

Mapping ecosystem service and biodiversity changes over 70 years in a rural English county

1. Biodiversity and ecosystem services continue to be compromised by land-use change, which is often focussed on enhancing agricultural production. Assessment of losses would be aided by analyses of temporal changes in the extent and spatial pattern of services and biodiversity. To date, no studies have mapped long-term changes in ecosystem services using historical maps. 2. We mapped changes between the 1930s – before the considerable intensification of land use in the UK starting in the 1940s – and 2000 in climate change amelioration services (carbon storage), provisioning services(agriculture and forestry) and plant species richness (biodiversity) for Dorset, a rural English county. 3. We combined land-use maps (1-ha resolution) with multiple proxies of service delivery for the 10 different Broad Habitats in the region. Biodiversity was mapped using plant survey data from the two time periods. We used bootstrapping to include uncertainty due to the different proxies and Gini coefficients to quantify statistical changes in spatial pattern. 4. Overall, we found significant increases in agricultural provisioning and large losses in biodiversity over the period, which reflect widespread conversion and intensification of land use. We found no change in Dorset’s carbon store, because carbon lost through land-use intensification was balanced by increases in woodland over the 20th century. 5. The carbon storage and the delivery of provisioning services both became more unequally distributed, indicating a change from relatively homogeneous delivery of services to concentration into hotspots. The maps from the year 2000 showed spatial dissociation of hotspots for carbon, provisioning and biodiversity, which suggests that, compared to the 1930s, modern,intensive land use creates conflicts in delivery of multiple services and biodiversity. 6. Synthesis and applications. Detailed maps of historical changes in location-specific service delivery and biodiversity provide valuable information for land-use planning, highlight trade-offs and help to identify drivers. Furthermore, historical maps provide an important baseline to indicate the suitability and potential success of suggested actions, such as habitat restoration, and their relevance to traditional land use. Various frameworks could be informed by our approach, including the ecosystem service aims of the EU biodiversity strategy and the newly created UK Nature Improvement Areas.

A synthesis of empirical plant dispersal kernels

1. Dispersal is fundamental to ecological processes at all scales and levels of organisation but progress is limited by a lack of information about the general shape and form of plant dispersal kernels. We addressed this gap by synthesising empirical data describing seed dispersal and fitting general dispersal kernels representing major plant types and dispersal modes. 2. A comprehensive literature search resulted in 107 papers describing 168 dispersal kernels for 144 vascular plant species. The data covered 63 families, all the continents except Antarctica, and the broad vegetation types of forest, grassland, shrubland, and more open habitats (e.g. deserts). We classified kernels in terms of dispersal mode (ant, ballistic, rodent, vertebrates other than rodents, vehicle or wind), plant growth form (climber, graminoid, herb, shrub or tree), seed mass and plant height. 3. We fitted 11 widely-used probability density functions to each of the 168 datasets to provide a statistical description of the dispersal kernel. The Exponential Power (ExP) and Log-sech (LogS) functions performed best. Other 2-parameter functions varied in performance. For example, the Lognormal and Weibull performed poorly, while the 2Dt and Power law performed moderately well. Of the single-parameter functions, the Gaussian performed very poorly, while the Exponential performed better. No function was among the best-fitting for all datasets. 4. For 10 plant growth form/dispersal mode combinations for which we had >3 datasets, we fitted ExP and LogS functions across multiple datasets to provide generalised dispersal kernels. We also fitted these functions to sub-divisions of these growth form/dispersal mode combinations in terms of seed mass (for animal-dispersed seeds) or plant height (wind-dispersed) classes. These functions provided generally good fits to the grouped datasets, despite variation in empirical methods, local conditions, vegetation type and the exact dispersal process. 5. Synthesis. We synthesise the rich empirical information on seed dispersal distances to provide standardised dispersal kernels for 168 case studies and generalised kernels for plant growth form/dispersal mode combinations. Potential uses include: a) choosing appropriate dispersal functions in mathematical models; b) selecting informative dispersal kernels for one’s empirical study system; and c) using representative dispersal kernels in cross-taxon comparative studies.

Locus-dependent selection in crop-wild hybrids of lettuce under field conditions and its implication for GM crop development

Gene escape from crops has gained much attention in the last two decades, as transgenes introgressing into wild populations could affect the latter’s ecological characteristics. However, different genes have different likelihoods of introgression. The mixture of selective forces provided by natural conditions creates an adaptive mosaic of alleles from both parental species. We investigated segregation patterns after hybridization between lettuce (Lactuca sativa) and its wild relative, L. serriola. Three generations of hybrids (S1, BC1, and BC1S1) were grown in habitats mimicking the wild parent’s habitat. As control, we harvested S1 seedlings grown under controlled conditions, providing very limited possibility for selection. We used 89 AFLP loci, as well as more recently developed dominant markers, 115 retrotransposon markers (SSAP), and 28 NBS loci linked to resistance genes. For many loci, allele frequencies were biased in plants exposed to natural field conditions, including over‐representation of crop alleles for various loci. Furthermore, Linkage disequilibrium was locally changed, allegedly by selection caused by the natural field conditions, providing ample opportunity for genetic hitchhiking. Our study indicates that when developing genetically modified crops, a judicious selection of insertion sites, based on knowledge of selective (dis)advantages of the surrounding crop genome under field conditions, could diminish transgene persistence.

The variability of processes involved in transgene dispersal-case studies from Brassica and related genera

Background, aim, and scopeWe strive to predict consequences of genetically modified plants (GMPs) being cultivated openly in the environment, as human and animal health, biodiversity, agricultural practise and farmers’ economy could be affected. Therefore, it is unfortunate that the risk assessment of GMPs is burdened by uncertainty. One of the reasons for the uncertainty is that the GMPs are interacting with the ecosystems at the release site thereby creating variability. This variability, e.g. in gene flow, makes consequence analysis difficult. The review illustrates the great uncertainty of results from gene-flow analysis.Main featuresMany independent experiments were performed on the individual processes in gene flow. The results comprise information both from laboratory, growth chambers and field trials, and they were generated using molecular or phenotypic markers and analysis of fitness parameters. Monitoring of the extent of spontaneous introgression in natural populations was also performed. Modelling was used as an additional tool to identify key parameters in gene flow.ResultsThe GM plant may affect the environment directly or indirectly by dispersal of the transgene. Magnitude of the transgene dispersal will depend on the GM crop, the agricultural practise and the environment of the release site. From case-to-case these three factors provide a variability that is reflected in widely different likelihoods of transgene dispersal and fitness of introgressed plants. In the present review, this is illustrated through a bunch of examples mostly from our own research on oilseed rape, Brassica napus. In the Brassica cases, the variability affected all five main steps in the process of gene dispersal. The modelling performed suggests that in Brassica, differences in fitness among plant genome classes could be a dominant factor in the establishment and survival of introgressed populations.DiscussionUp to now, experimental analyses have mainly focused on studying the many individual processes of gene flow. This can be criticised, as these experiments are normally carried out in widely different environments and with different genotypes, and thus providing bits and pieces difficult to assemble. Only few gene-flow studies have been performed in natural populations and over several plant generations, though this could give a more coherent and holistic view.ConclusionThe variability inherent in the processes of gene flow in Brassica is apparent and remedies are wished for. One possibility is to expose the study species to additional experiments and monitoring, but this is costly and will likely not cover all possible scenarios. Another remedy is modelling gene flow. Modelling is a valuable tool in identifying key factors in the gene-flow process for which more knowledge is needed, and identifying parameters and processes which are relatively insensitive to change and therefore require less attention in future collections of data. But the interdependence between models and experimental data is extensive, as models depend on experimental data for their development or testing.RecommendationsMore and more transgenic varieties are being grown worldwide harbouring genes that might potentially affect the environment (e.g. drought tolerance, salt tolerance, disease tolerance, pharmaceutical genes). This calls for a thorough risk assessment. However, in Brassica, the limited and uncertain knowledge on gene flow is an obstacle to this. Modelling of gene flow should be optimised, and modelling outputs verified in targeted field studies and at the landscape level. Last but not least, it is important to remember that transgene flow in itself is not necessarily a thread, but it is the consequences of gene flow that may jeopardise the ecosystems and the agricultural production. This emphasises the importance of consequence analysis of genetically modified plants.

Genomic regions in crop–wild hybrids of lettuce are affected differently in different environments: implications for crop breeding

Many crops contain domestication genes that are generally considered to lower fitness of crop–wild hybrids in the wild environment. Transgenes placed in close linkage with such genes would be less likely to spread into a wild population. Therefore, for environmental risk assessment of GM crops, it is important to know whether genomic regions with such genes exist, and how they affect fitness. We performed quantitative trait loci (QTL) analyses on fitness(‐related) traits in two different field environments employing recombinant inbred lines from a cross between cultivated Lactuca sativa and its wild relative Lactuca serriola. We identified a region on linkage group 5 where the crop allele consistently conferred a selective advantage (increasing fitness to 212% and 214%), whereas on linkage group 7, a region conferred a selective disadvantage (reducing fitness to 26% and 5%), mainly through delaying flowering. The probability for a putative transgene spreading would therefore depend strongly on the insertion location. Comparison of these field results with greenhouse data from a previous study using the same lines showed considerable differences in QTL patterns. This indicates that care should be taken when extrapolating experiments from the greenhouse, and that the impact of domestication genes has to be assessed under field conditions.

A Bayesian analysis of gene flow from crops to their wild relatives: cultivated (Lactuca sativa L.) and prickly lettuce (L. serriola L.) and the recent expansion of L. serriola in Europe

Interspecific gene flow can lead to the formation of hybrid populations that have a competitive advantage over the parental populations, even for hybrids from a cross between crops and wild relatives. Wild prickly lettuce (Lactuca serriola) has recently expanded in Europe and hybridization with the related crop species (cultivated lettuce, L. sativa) has been hypothesized as one of the mechanisms behind this expansion. In a basically selfing species, such as lettuce, assessing hybridization in natural populations may not be straightforward. Therefore, we analysed a uniquely large data set of plants genotyped with SSR (simple sequence repeat) markers with two programs for Bayesian population genetic analysis, STRUCTURE and NewHybrids. The data set comprised 7738 plants, including a complete genebank collection, which provided a wide coverage of cultivated germplasm and a fair coverage of wild accessions, and a set of wild populations recently sampled across Europe. STRUCTURE analysis inferred the occurrence of hybrids at a level of 7% across Europe. NewHybrids indicated these hybrids to be advanced selfed generations of a hybridization event or of one backcross after such an event, which is according to expectations for a basically selfing species. These advanced selfed generations could not be detected effectively with crop‐specific alleles. In the northern part of Europe, where the expansion of L. serriola took place, the fewest putative hybrids were found. Therefore, we conclude that other mechanisms than crop/wild gene flow, such as an increase in disturbed habitats and/or climate warming, are more likely explanations for this expansion.

Plant functional connectivity – integrating landscape structure and effective dispersal

Summary 1.Dispersal is essential for species to survive the threats of habitat destruction and climate change. Combining descriptions of dispersal ability with those of landscape structure, the concept of functional connectivity has been popular for understanding and predicting species’ spatial responses to environmental change. 2.Following recent advances, the functional connectivity concept is now able to move beyond landscape structure to consider more explicitly how other external factors such as climate and resources affect species movement. We argue that these factors, in addition to a consideration of the complete dispersal process, are critical for an accurate understanding of functional connectivity for plant species in response to environmental change. 3.We use recent advances in dispersal, landscape and molecular ecology to describe how a range of external factors can influence effective dispersal in plant species, and how the resulting functional connectivity can be assessed. 4.Synthesis. We define plant functional connectivity as the effective dispersal of propagules or pollen among habitat patches in a landscape. Plant functional connectivity is determined by a combination of landscape structure, interactions between plant, environment and dispersal vectors, and the successful establishment of individuals. We hope that this consolidation of recent research will help focus future connectivity research and conservation. This article is protected by copyright. All rights reserved.

Crop to wild introgression in lettuce: following the fate of crop genome segments in backcross populations

BackgroundAfter crop-wild hybridization, some of the crop genomic segments may become established in wild populations through selfing of the hybrids or through backcrosses to the wild parent. This constitutes a possible route through which crop (trans)genes could become established in natural populations. The likelihood of introgression of transgenes will not only be determined by fitness effects from the transgene itself but also by the crop genes linked to it. Although lettuce is generally regarded as self-pollinating, outbreeding does occur at a low frequency. Backcrossing to wild lettuce is a likely pathway to introgression along with selfing, due to the high frequency of wild individuals relative to the rarely occurring crop-wild hybrids. To test the effect of backcrossing on the vigour of inter-specific hybrids, Lactuca serriola, the closest wild relative of cultivated lettuce, was crossed with L. sativa and the F1 hybrid was backcrossed to L. serriola to generate BC1 and BC2 populations. Experiments were conducted on progeny from selfed plants of the backcrossing families (BC1S1 and BC2S1). Plant vigour of these two backcrossing populations was determined in the greenhouse under non-stress and abiotic stress conditions (salinity, drought, and nutrient deficiency).ResultsDespite the decreasing contribution of crop genomic blocks in the backcross populations, the BC1S1 and BC2S1 hybrids were characterized by a substantial genetic variation under both non-stress and stress conditions. Hybrids were identified that performed equally or better than the wild genotypes, indicating that two backcrossing events did not eliminate the effect of the crop genomic segments that contributed to the vigour of the BC1 and BC2 hybrids. QTLs for plant vigour under non-stress and the various stress conditions were detected in the two populations with positive as well as negative effects from the crop.ConclusionAs it was shown that the crop contributed QTLs with either a positive or a negative effect on plant vigour, we hypothesize that genomic regions exist where transgenes could preferentially be located in order to mitigate their persistence in natural populations through genetic hitchhiking.

Abiotic stress QTL in lettuce crop-wild hybrids: comparing greenhouse and field experiments.

The development of stress-tolerant crops is an increasingly important goal of current crop breeding. A higher abiotic stress tolerance could increase the probability of introgression of genes from crops to wild relatives. This is particularly relevant to the discussion on the risks of new GM crops that may be engineered to increase abiotic stress resistance. We investigated abiotic stress QTL in greenhouse and field experiments in which we subjected recombinant inbred lines from a cross between cultivated Lactuca sativa cv. Salinas and its wild relative L. serriola to drought, low nutrients, salt stress, and aboveground competition. Aboveground biomass at the end of the rosette stage was used as a proxy for the performance of plants under a particular stress. We detected a mosaic of abiotic stress QTL over the entire genome with little overlap between QTL from different stresses. The two QTL clusters that were identified reflected general growth rather than specific stress responses and colocated with clusters found in earlier studies for leaf shape and flowering time. Genetic correlations across treatments were often higher among different stress treatments within the same experiment (greenhouse or field), than among the same type of stress applied in different experiments. Moreover, the effects of the field stress treatments were more correlated with those of the greenhouse competition treatments than to those of the other greenhouse stress experiments, suggesting that competition rather than abiotic stress is a major factor in the field. In conclusion, the introgression risk of stress tolerance (trans-)genes under field conditions cannot easily be predicted based on genomic background selection patterns from controlled QTL experiments in greenhouses, especially field data will be needed to assess potential (negative) ecological effects of introgression of these transgenes into wild relatives.