Coexistence of specialist and generalist species is shaped by dispersal and environmental factors

Disentangling the mechanisms mediating the coexistence of habitat specialists and generalists has been a long-standing subject of investigation. However, the roles of species traits and environmental and spatial factors have not been assessed in a unifying theoretical framework. Theory suggests that specialist species are more competitive in natural communities. However, empirical work has shown that specialist species are declining worldwide due to habitat loss and fragmentation. We addressed the question of the coexistence of specialist and generalist species with a spatially explicit metacommunity model in continuous and heterogeneous environments. We characterized how species' dispersal abilities, the number of interacting species, environmental spatial autocorrelation, and disturbance impact community composition. Our results demonstrated that species' dispersal ability and the number of interacting species had a drastic influence on the composition of metacommunities. More specialized species coexisted when species had large dispersal abilities and when the number of interacting species was high. Disturbance selected against highly specialized species, whereas environmental spatial autocorrelation had a marginal impact. Interestingly, species richness and niche breadth were mainly positively correlated at the community scale but were negatively correlated at the metacommunity scale. Numerous diversely specialized species can thus coexist, but both species' intrinsic traits and environmental factors interact to shape the specialization signatures of communities at both the local and global scales

Cover Crop Productivity in Contrasting Growing Conditions and Influence on the Subsequent Crop

In arable systems, cover crops provide multiple ecosystem services involved in soil protection and fertility. They can efficiently compete against weeds, control erosion or recycle nutrients. The supply of these services is however largely dependent on cover crop growth as most of the services are related to biomass production. The choice of a cover crop adapted to the growing conditions is thus essential. Growing mixtures instead of sole crops is expected to be a way to overcome variable growing conditions and to insure high biomass production.Different field trials have been conducted from 2013 to 2016 in Nyon, Switzerland to assess cover crop biomass production and stability, and their influence on the following crop. Indian mustard, field pea, black oat and phacelia were sown as sole crops and in mixtures with different diversity levels (2, 3 and 4 species). A mixture of 11 species (50% of legumes and 50% of other species) was also tested. Biomass production was assessed about three months after seeding, just before the seeding of the next main crop, in two 0.5 m × 0.5 m quadrats. For each cover crop, a risk of failure, e.g. the probability of producing less than 3 t/ha (minimal biomass allowing to provide the services expected from cover crops), was estimated. Weed pressure was appraised by weed biomass in quadrats. The yield of the following main crop, here winter wheat, was determined after harvesting with a combine harvester, at wheat maturity, in summer. Sole crops showed very contrasting performance according to the growing conditions. Pea was the most productive in low yielding conditions with 2 t/ha while other sole crops produced only 0.5 t/ha. Pea was barely more productive in high yielding conditions, reaching only 3 t/ha when cover crop average was 6 t/ha. By contrast, mustard and oat showed high production potential in these conditions, exceeding 7 t/ha. Pea should thus be favoured when the growing conditions are clearly identified as limiting (low N availability), while oat or mustard should be chosen in favourable conditions. However, in general, conditions are hardly predictable. Our results showed that mixtures should be preferred as they were adapted to a wider range of environments than sole crops, performing well regardless of the conditions and resulting in a lower risk of failure than sole crops. The 11-specie mixture revealed that generally 4 to 5 species are sufficient to insure a good performance regardless of the conditions. Regarding cover crop effect on the following crop, we evidenced the importance of high biomass production for efficient weed control and positive influence on crop yield in no till treatments.Our results showed that cover crop mixtures rather than sole crops should be chosen as they insure high biomass production and thus a good supply of ecosystem services. In addition, cover crop cultivation, even for a short period, proved to be paramount to the maintenance of grain yield and soil fertility on the long term, especially in reduced tillage systems

Cover Crops to Secure Low Herbicide Weed Control Strategies in Maize Grown with Reduced Tillage

As a key-element of conservation agriculture, the occasional or systematic suppression of full-inversion ploughing implies an adaptation of the cropping system. To assess the ability of cover crops to control weeds in a subsequent maize crop grown with reduced tillage, three annual experiments were implemented at the research station of Agroscope Changins, Nyon, Switzerland. Ten non-wintering cover crop (CC) species were sown in mid-summer and compared to a bare soil treatment in strip-plot experiments including different weeding strategies according to integrated weed management rules. In case of a predictable impasse for weed control, an alternative management option was chosen 1) at the end of winter: total herbicide application instead of no herbicide application, 2) at the beginning of May before maize sowing: minimum soil tillage instead of no tillage. The ability of cover crop species to control weed was evaluated at the stage 2-4 leaves of maize. The shoot dry matter yield of maize was measured at harvest at the end of August. At the beginning of November, mean CC dry shoot biomass varied between 1.2 and 11.1 t DM ha-1 depending on experimental year and CC species. On average over the three years, Asteraceae (Helianthusannuus and Guizotiaabyssinica) showed the highest shoot dry matter among the tested species (> 6.0 t DM ha-1). Legume species (Pisumsativum arvense, Trifolium alexandrinum and Vicia sativa) and Brassicaceae species (Brassicacampestrisoleifera and Raphanussativuslongipinnatus) presented the lowest 3-year mean shoot biomass (≤4.0 t DM ha-1) At the end of winter, the three legume species and Avenastrigosa showed the highest plant residue soil cover and Brassicaceae species the lowest one. CC residue soil cover at the end of winter was only slightly positively correlated with CC autumn shoot biomass. In three out of eight cases, the chosen weeding strategy was very efficient in terms of weed control at the stage 2-4 leaves of maize. In the remaining five cases, the weeding strategy did not succeed in preventing weed infestation at the beginning of maize development. A mean weed cover higher than 15% was observed when no total herbicide and/or no tillage was applied before maize sowing. In three out of these five cases, a significant CC effect on weed cover could be observed. CC species able to produce high amounts of biomass in autumn appeared to be useful in terms of weed control. The most efficient CC species varied from year to year: G. abyssinica in 2011, H. annuus in 2012 and A. strigosa in 2014. CC effect on maize yield was significant in a single case, but the effect of CC species tended to be positive compared to the control treatment without CC. Despite only partial efficacy, the use of cover crops is recommended for limiting weed incidence in cropping systems aimed at reducing soil tillage and herbicide use

Specific interactions leading to transgressive overyielding in cover crop mixtures

Growing mixtures of species instead of sole crops is expected to increase the ecosystem services provided by cover crops. This study aimed at understanding the interactions between species and investigating how they affect the performance of the mixture. Four species were combined in six bispecific mixtures in a field experiment. The performance of each species when grown in a mixture was compared to its performance as a sole crop at different sowing densities, to characterise the influence of intra- and interspecific competition for each species. Intra- and interspecific competition coefficients were quantified using a response surface design and the hyperbolic yield-density equation. Interactions between the four species ranged from facilitation to competition. Most of the mixtures exhibited transgressive overyielding. Without nitrogen (N) fertilisation, high complementarity between species allowed to achieve the highest biomass. With N fertilisation, high dominance of one mixture component should be avoided to achieve good performance. A revised approach in the use of the land equivalent ratio for the evaluation of cover crop mixtures is also proposed in this study. It allows to better identify transgressive overyielding in mixtures and to better characterise the effect of one species on the other within the mixture

Comparison of visual assessment and digital image analysis for canopy cover estimation

Nowadays, in the context of agriculture, cover crops are crops cultivated with the sole aim of providing important ecosystem services such as erosion prevention. Many services offered by these crops are directly linked to the development of their vegetation, and especially of canopy cover. A proper estimation of this cover is thus necessary to evaluate cover crop performance. Many methods to estimate canopy cover exist, but differ in terms of effort and time needed to implement them. In this study, we compared visual assessment of canopy cover in the field with two methods of digital image analysis (Assess and Canopeo), for different cover crop species and vegetation types. Visual estimation was positively correlated with both type of image analysis estimations. However, it showed systematically lower values of canopy cover, especially at intermediate canopy cover values. The type of vegetation influenced the visual and digital image estimations, narrow leaf species being the most difficult to evaluate visually. This study showed that depending on its utilisation, visual canopy cover assessment could be useful, especially when only relative estimation of canopy cover is needed. When absolute canopy cover estimation is needed, the use of digital image analysis should be preferred

Importance of cover crops in alleviating negative effects of reduced soil tillage and promoting soil fertility in a winter wheat cropping system

Reduction of soil tillage is of paramount importance for agricultural soil preservation. However, it is often accompanied by yield reduction and weed management problems. In this perspective, cover crops could play an important role to alleviate weed infestation and sustain yield. In this study, the results from a three-year experiment of cover crop cultivation in different soil tillage treatments is presented, together with results from DayCent simulations on the long term evolution of soil organic carbon and total nitrogen. Eight cover crop treatments were set up as subtreatments in a long term experiment in Switzerland. Cover crops were cultivated for a short two-month period between two winter wheats. Substantial differences in cover crop growth were observed depending on cover crop species. In all tillage treatments, high cover crop biomass production allowed to supress weed biomass compared to the no cover crop control. Wheat grain yield was higher in the minimum tillage than in the plough treatment. In the no till treatment, wheat yield was notably low, except in the field pea treatments, where wheat yield reached values similar to that observed in the plough and minimum tillage treatments. In addition, these differences in biomass production translated into important differences in nutrient inputs, and even in soil nutrient concentration in some cases. Long term simulations showed that cover crop cultivation could increase drastically soil organic carbon and total nitrogen, especially in reduced tillage treatments. Altogether, these results demonstrated that the presence of a well-developed cover crop, even for only two months, allows to sustain wheat yield in a no till treatment. It impacts also soil fertility and nutrient cycling. This study shows that an accurate use and management of cover crops, in interaction with tillage reduction, could maintain yield and improve soil fertility in the long term

Genome size rather than content might affect call properties in toads of three ploidy levels (Anura: Bufonidae: Bufo viridis subgroup)

In vertebrates, genome size has been shown to correlate with nuclear and cell sizes, and influences phenotypic features, such as brain complexity. In three different anuran families, advertisement calls of polyploids exhibit longer notes and intervals than diploids, and difference in cellular dimensions have been hypothesized to cause these modifications. We investigated this phenomenon in green toads (Bufo viridis subgroup) of three ploidy levels, in a different call type (release calls) that may evolve independently from advertisement calls, examining 1205 calls, from ten species, subspecies, and hybrid forms. Significant differences between pulse rates of six diploid and four polyploid (3n, 4n) green toad forms across a range of temperatures from 7 to 27 °C were found. Laboratory data supported differences in pulse rates of triploids vs. tetraploids, but failed to reach significance when including field recordings. This study supports the idea that genome size, irrespective of call type, phylogenetic context, and geographical background, might affect call properties in anurans and suggests a common principle governing this relationship. The nuclear-cell size ratio, affected by genome size, seems the most plausible explanation. However, we cannot rule out hypotheses under which call-influencing genes from an unexamined diploid ancestral species might also affect call properties in the hybrid-origin polyploids

Risk of herbicide mixtures as a key parameter to explain phytoplankton fluctuation in a great lake: the case of Lake Geneva, Switzerland

Mixture risk assessment predictions have rarely been confronted with biological changes observed in the environment. In this study, long-term monitoring of a European great lake, Lake Geneva, provides the opportunity to assess to what extent the predicted toxicity of herbicide mixtures explains the changes in the composition of the phytoplankton community next to other classical limnology parameters such as nutrients. To reach this goal, the gradient of the mixture toxicity of 14 herbicides regularly detected in the lake was calculated using concentration addition and response addition models. A temporal gradient of toxicity was observed which decreased from 2004 to 2009. Redundancy analysis and partial redundancy analysis showed that this gradient explains a significant portion of the variation in phytoplankton community composition with and without having removed the effect of all other co-variables. Moreover, species that are significantly influenced, positively or negatively, by the decrease of toxicity in the lake over time are highlighted. It can be concluded that the herbicide mixture toxicity is one of the key parameters to explain phytoplankton changes in Lake Genev

No Till and Organic Farming Improve Soil Properties but Reduce Crop Yield Compared to Conventional Farming in a Swiss Farm Network

Soils are of vital importance for sustainable food production. In order to maintain or improve soil quality, it is necessary to develop strategies for a sustainable use of soil. Alternative cropping practices such as reduced tillage and improved crop rotation are more and more adopted with the aim of decreasing the impact of agriculture on the environment. However, their on-the-ground impact in Swiss farming systems still has to be assessed. In this study, we quantified the impact of three farming systems (conventional farming, no-till, and organic farming) on plant and soil chemical, biological and physical properties. Our study included 20 fields for each farming system. All selected fields were cultivated with winter wheat the year of sampling. Soil was sampled at four layers, 0-5 cm, 5-20 cm, 20-25 cm, 25-50 cm. The main variables analysed were grain yield, soil nutrient availability, organic carbon stocks, bulk density, aggregation, porosity and soil biology. This was complemented with a comprehensive survey to collect information about cropping practices at field and farm scale, including organic matter inputs, fertilisation, tillage, phytosanitary treatments, and crop rotation.Our results show a significant influence of cropping practices on plant and soil properties. Wheat yield in no till and organic systems was reduced by 10% and 30% compared to conventional systems. Bulk density was higher in no-till than in ploughed fields in the 5-20 cm layer but similar in the subsoil. A strong stratification with depth of nutrients and soil organic carbon was observed in no-till fields. No-till and organic fields showed larger soil aggregates and higher microbial biomass in the surface layer (0-5 cm). Mycorrhizal colonisation of wheat roots was on average 50% higher in organic fields. However, no differences in carbon stock in the 0-20 cm layer was observed and the ratio organic matter / clay shows a high variability (from poor to good) and was not dependent on the farming system.Our results show that an improvement of soil properties can be achieved with alternative cropping practices such as no-till and organic farming, but also depends on the other practices adopted by the farmers, such as input of organic amendments, crop rotation diversification, residue management

Accumulation of biologically fixed nitrogen by legumes cultivated as cover crops in Switzerland

Aims Biological nitrogen fixation by legumes is expected to play a greater role in future cropping systems. Our study evaluated 19 legume species grown as cover crops in Swiss agroecosystems. Methods Two field experiments were set up to monitor the biomass production and nitrogen content of 19 legumes and two non-legumes. The proportion of nitrogen derived from atmospheric N2 (%Ndfa) was assessed using the 15N natural abundance method. In parallel, a pot experiment was set up to determine the species-specific B values necessary to apply this method. Results Some species produced an important amount of biomass in 3 months, up to 6.86 t/ha for Vicia faba. Five species, Lathyrus sativus, Pisum sativum, Vicia sativa, Vicia villosa, and V. faba, acquired more than 100 kg/ha of N through biological fixation. Important amounts of nitrogen were also derived from the soil. %Ndfa values showed high variability between and within species, ranging from 0 % to almost 100 %. Conclusions Some legumes showed high N accumulation even in a short growing period, and could play an important role in fixing renewable nitrogen in crop rotation