Multiple invasive species affect germination, growth, and photosynthesis of native weeds and crops in experiments

Alien plant species regularly and simultaneously invade agricultural landscapes and ecosystems; however, the effects of co-invasion on crop production and native biodiversity have rarely been studied. Secondary metabolites produced by alien plants may be allelopathic; if they enter the soil, they may be transported by agricultural activities, negatively affecting crop yield and biodiversity. It is unknown whether substances from different alien species in combination have a greater impact on crops and wild plants than if they are from only one of the alien species. In this study, we used a set of common garden experiments to test the hypothesis that mixed extracts from two common invasive species have synergistic effects on crops and weeds (defined as all non-crop plants) in European agricultural fields compared to single-species extracts. We found that both the combined and individual extracts had detrimental effects on the seed germination, seedling growth, biomass, and photosynthetic performance of both crops and weeds. We found that the negative effect of mixed extracts was not additive and that crop plants were more strongly affected by invasive species extracts than the weeds. Our results are important for managing invasive species in unique ecosystems on agricultural land and preventing economic losses in yield production

Patterns of ash (Fraxinus excelsior L.) colonization in mountain grasslands: the importance of management practices

International audienceWoody colonization of grasslands is often associated with changes in abiotic or biotic conditions or a combination of both. Widely used as fodder and litter in the past traditional agro-pastoral system, ash (Fraxinus excelsior L.) has now become a colonizing species of mountain grasslands in the French Pyrenees. Its present distribution is dependent on past human activities and it is locally controlled by propagule pressure and abiotic conditions. However, even when all favourable conditions are met, all the potentially colonizable grasslands are not invaded. We hypothesize that management practices should play a crucial role in the control of ash colonization. From empirical field surveys we have compared the botanical composition of a set of grasslands (present and former) differing in management practices and level of ash colonization. We have displayed a kind of successional gradient positively linked to both ash cover and height but not to the age of trees. We have tested the relationships between ash presence in grassland and management types i.e. cutting and/or grazing, management intensity and some grassland communities' features i.e. total and local specific richness and species heterogeneity. Mixed use (cutting and grazing) is negatively linked to ash presence in grassland whereas grazing alone positively. Mixed use and high grazing intensity are directly preventing ash seedlings establishment, when low grazing intensity is allowing ash seedlings establishment indirectly through herbaceous vegetation neglected by livestock. Our results show the existence of a limit between grasslands with and without established ashes corresponding to a threshold in the intensity of use. Under this threshold, when ash is established, the colonization process seems to become irreversible. Ash possesses the ability of compensatory growth and therefore under a high grazing intensity develops a subterranean vegetative reproduction. However the question remains at which stage of seedling development and grazing intensity these strategies could occur

TRY plant trait database – enhanced coverage and open access

Plant traits—the morphological, anatomical, physiological, biochemical and phenological characteristics of plants—determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait‐based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits—almost complete coverage for ‘plant growth form’. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait–environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives

Herbivore impact on grassland plant diversity depends on habitat productivity and herbivore size

Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies. [KEYWORDS: Cross-site ; fertility ; grazing ; plant–animal ;species richness]

Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration

Herbivores may influence the nitrogen (N) recycling rates and consequently increase or decrease the productivity of grasslands. Plant N concentration emerged as a critical parameter to explain herbivore effects from several conceptual models, which predict that herbivores decrease soil N availability when plant N concentration is low whereas they increase it when plant N concentration is high (Hobbs 1996, Ritchie et al. 1998, Pastor et al. 2006). However, a broader cross-site comparison among published studies to test these predictions is hampered by the different methodologies used to measure soil N availability or a proxy thereof, and a lack of measurements of plant N concentration. Therefore it remains unclear whether these model predictions are generally valid across a range of grasslands. We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6–8 year old exclosures (with an unfenced control) of vertebrate herbivores (>1 kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confir

Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi-dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle-bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores

Opposing community assembly patterns for dominant and jonnondominant plant species in herbaceous ecosystems globally

Biotic and abiotic factors interact with dominant plants—the locally most frequent or with the largest coverage—and nondominant plants differently, partially because dominant plants modify the environment where nondominant plants grow. For instance, if dominant plants compete strongly, they will deplete most resources, forcing nondominant plants into a narrower niche space. Conversely, if dominant plants are constrained by the environment, they might not exhaust available resources but instead may ameliorate environmental stressors that usually limit nondominants. Hence, the nature of interactions among nondominant species could be modified by dominant species. Furthermore, these differences could translate into a disparity in the phylogenetic relatedness among dominants compared to the relatedness among nondominants. By estimating phylogenetic dispersion in 78 grasslands across five continents, we found that dominant species were clustered (e.g., co-dominant grasses), suggesting dominant species are likely organized by environmental filtering, and that nondominant species were either randomly assembled or overdispersed. Traits showed similar trends for those sites (<50%) with sufficient trait data. Furthermore, several lineages scattered in the phylogeny had more nondominant species than expected at random, suggesting that traits common in nondominants are phylogenetically conserved and have evolved multiple times. We also explored environmental drivers of the dominant/nondominant disparity. We found different assembly patterns for dominants and nondominants, consistent with asymmetries in assembly mechanisms. Among the different postulated mechanisms, our results suggest two complementary hypotheses seldom explored: (1) Nondominant species include lineages adapted to thrive in the environment generated by dominant species. (2) Even when dominant species reduce resources to nondominant ones, dominant species could have a stronger positive effect on some nondominants by ameliorating environmental stressors affecting them, than by depleting resources and increasing the environmental stress to those nondominants. These results show that the dominant/nondominant asymmetry has ecological and evolutionary consequences fundamental to understand plant communities

Intraspecific and interspecific pre-adult competition on the neotropical region colonizer Zaprionus indianus (Diptera: Drosophilidae) under laboratory conditions Competição pré-adulta intra e interespecífica, em Zaprionus indianus (Diptera: Drosophilidae), espécie colonizadora da região neotropical, sob condições laboratoriais

This study analyzes the pre-adult interactions of Zaprionus indianus, a recently-introduced species in Brazil, with two others Drosophilidae under laboratory conditions. The effects of larval residues on the viability and on the developmental time of Z. indianus, Drosophila simulans and D. sturtevanti were used to evaluate pre-adult competitive interactions, conditioning the culture medium with larval residues. Pre-adult interactions between Z. indianus, D. sturtevanti and D. simulans may affect their relative abundance over time, since the viability of Z. indianus was negatively affected by residues of D. sturtevanti, and its residues reduced the viability of D. simulans and the developmental time of both D. simulans and D. sturtevanti.<br>Este estudo é uma análise das interações pré-adultas, sob condições laboratoriais, da mosca-do-figo Zaprionus indianus, espécie recentemente introduzida no Brasil, com dois outros drosofilídeos. A interferência de meio de cultura, acrescido de resíduos larvais, sobre a viabilidade e o tempo de desenvolvimento de Z. indianus, Drosophila simulans e D. sturtevanti foi utilizada para avaliar as interações competitivas pré-adultas. As interações pré-adultas entre Z. indianus, D. sturtevanti e D. simulans podem afetar sua abundância relativa ao longo do tempo, pois a viabilidade de Z. indianus foi negativamente afetada por resíduos de D. sturtevanti; os resíduos da mosca-do-figo reduziram a viabilidade de D. simulans e o tempo de desenvolvimento tanto de D. simulans como de D. sturtevanti