Plant Diversity In Agroecosystems Research Articles

Composition and Food Web Structure of Aphid-Parasitoid Populations on Plum Orchards in Chile

By increasing plant diversity in agroecosystems, it has been proposed that one can enhance and stabilize ecosystem functioning by increasing natural enemies' diversity. Food web structure determines ecosystem functioning as species at different trophic levels are linked in interacting networks. We compared the food web structure and composition of the aphid- parasitoid and aphid-hyperparasitoid networks in two differentially managed plum orchards: plums with inter-rows of oats as a cover crop (OCC) and plums with inter-rows of spontaneous vegetation (SV). We hypothesized that food web composition and structure vary between OCC and SV, with network specialization being higher in OCC and a more complex food web composition in SV treatment. We found a more complex food web composition with a higher species richness in SV compared to OCC. Quantitative food web metrics differed significantly among treatments showing a higher generality, vulnerability, interaction evenness, and linkage density in SV, while OCC presented a higher degree of specialization. Our results suggest that plant diversification can greatly influence the food web structure and composition, with bottom-up effects induced by plant and aphid hosts that might benefit parasitoids and provide a better understanding of the activity, abundance, and interactions between aphids, parasitoids, and hyperparasitoids in plum orchards.

Soil Properties and Bacterial Communities Associated with the Rhizosphere of the Common Bean after Using Brachiaria brizantha as a Service Crop: A 10-Year Field Experiment

Intensive agricultural farming practices, such as monoculture, require long bare fallow periods and the overuse of agrochemicals, which compromise soil health over time. Increasing plant diversity in agroecosystems with service crops represents a promising alternative to achieving sustainability goals. However, how specific cover crop species influence the abundance and structure of soil bacterial communities remains to be solved. In this study, we assessed the effects of B. brizantha in two different agricultural cycles for 10 years in a common bean monoculture system in the northwestern region of Argentina (NWA) by measuring chemical, physical, and microbiological parameters in the rhizosphere, as well as by screening the rhizobiome using 16S rRNA sequencing. The ten-year inclusion of B. brizantha had a positive impact on properties in the rhizosphere compared to the common bean monoculture. The bacterial beta-diversity was different among treatments, but not the alpha-diversity. The most abundant phyla were Actinobacteria, Proteobacteria, Acidobacteria, Chloroflexi and Myxococcota. The predicted functions related to chemoheterotrophy and aerobic chemoheterotrophy were increased under B. brizantha treatments compared to the bean monoculture. The inclusion of the pasture B. brizantha contributed to restoring soil health and minimizing soil degradation.

Plant functional diversity is affected by weed management through processes of trait convergence and divergence.

Weed management involving tillage and/or herbicides has generally led to a decline of plant diversity in agroecosystems, with negative impacts on ecosystem services provision. The use of plant covers has become the predominant alternative in vineyard management, with numerous studies focusing on analyzing the advantages and disadvantages of plant covers compared to the aforementioned management. Although the impacts of weed management on taxonomic diversity have been widely studied, many gaps remain on their effects on plant functional diversity. As plant functional diversity is linked to the delivery of key ecosystem services in agroecosystems, understanding these effects could enable the development of more sustainable practices. From 2008 to 2018, a long-term trial was carried out in a Mediterranean vineyard to assess different agricultural practices. In this article, we examined how weed management, as well as irrigation use, could affect plant functional diversity. Based on 10 functional traits, such as plant height, specific leaf area or seed mass, we measured different indices of functional diversity and used null models to detect processes of trait convergence and divergence. Our results revealed that weed management and irrigation use had a significant effect on plant functional diversity. Mown plots showed the highest functional richness but were functionally convergent, since mowing was a strong functional filter on most of the traits. Tillage also behaved as a functional filter on some vegetative traits, but favored the divergence of certain reproductive traits. Herbicide-treated and irrigated plots showed the highest values of functional divergence by promoting more competitive species with more divergent trait values. The effect of weed management on these community assembly processes was shaped by the use of irrigation in vineyard rows, leading to functional divergence in those vegetative traits related to resource acquisition and seed mass. These results suggest that greater functional diversity may be associated with the bias caused by higher occurrence of competitive species (e.g. Convolvulus arvensis, Sonchus asper) with contrasting values for certain traits. Therefore, since these species are considered harmful to crops, higher plant functional diversity might not be a desirable indicator in agroecosystems.

Forbs differentially affect soil microbial community composition and functions in unfertilized ryegrass-red clover leys

Increasing plant diversity in agroecosystems is proposed to enhance multiple ecosystem services. Adding selected forbs such as caraway (Carum carvi L.) and plantain (Plantago lanceolata L.) to productive grass-clover mixtures can further enhance forage yields, root biomass, uptake of mineral nutrients and improve animal performance. Yet, it remains unclear whether and how adding these forbs to grass-clover mixtures can influence soil microbial communities and associated soil carbon (C) and nitrogen (N) cycling. Based on a three-year grassland experiment varying in species diversity and composition with and without fertilizer application, we determined soil microbial community composition and functions related to C and N cycling under laboratory incubations. Results showed that inclusion of caraway modified soil microbial community composition by enhancing fungal-to-bacterial phospholipid fatty acids of the ryegrass-red clover mixture. Adding plantain to the ryegrass-red clover mixture increased the relative decomposition rate of the labile C pool, but not of the recalcitrant C pool. Yet, β-glucosidase activity and net N mineralization were unchanged due to inclusion of either forb. Moreover, fertilization with cattle slurry generally weakened these forb-induced changes in soil microbial properties. These findings demonstrate that adding selected forbs to unfertilized grass-clover leys can modify soil microbial community composition and associated C and N cycling, implying a potential for promoting long-term soil C sequestration through enhanced fungi-to-bacteria ratio, but a limited role in improving soil N fertility.

Intercropping hampers the nocturnal biological control of aphids

AbstractIncreasing plant diversity in agroecosystems (i.e. intercropping) has been widely accepted as a means of promoting conservation biological control of mites and insect pests. Nevertheless, the contribution from underlying mechanisms such as the provision of non‐prey alternative food (i.e. pollen and nectar) and shelter have not been properly disentangled; and additionally, it remains unexplored whether the performance of nocturnal and diurnal natural enemies is improved when provided with diverse plant communities. Using open field experiments and a greenhouse microcosm, we investigated whether intercropping collards with parsley could create shelter for natural enemies in the lower stratum (parsley), and whether or not nocturnal and diurnal natural enemies would carry out aphid biological control equally well in this increased plant diversity scenario (intercropping). The results showed that the shelter alone provided by the lower stratum/companion plants (parsley) mediated an increase in the abundance of natural enemies without involving the provision of non‐prey alternative food. However, the biological control of aphids exerted by nocturnal predators was negatively affected by intercropping. The lower stratum (parsley) appeared to hamper the ability of nocturnal predators to reach aphids more quickly on the collard host plants (higher stratum). In total, our findings indicate that intercropping non‐flowering companion plants is likely enough to mediate an increase of natural enemies via shelter provision. In addition, the results suggest that nocturnal predators, or non‐flying predators for that matter, are hampered by complex lower stratum vegetation. Thus, considering natural enemy behaviour and plant characteristics when designing polyculture systems are vital for attaining conservation biological control success.

Positive effects of plant association on rhizosphere microbial communities depend on plant species involved and soil nitrogen level

How plant species associations affect the rhizosphere microbiota, in comparison to each plant species, has been overlooked. Here we investigated how bi-species plant associations affect total bacterial and N-cycling microbial communities (nitrate reducers and ammonia-oxidizers), as an example of functional communities, at low and high soil N levels. At low N level, the total bacterial abundance in the rhizosphere of the bi-species associations was significantly higher than that of either plant on its own for half of the bi-species associations. For the other associations, the bacterial abundance was not different from the corresponding mono-species cultures. The abundance of several N-cycling guilds was also enhanced by some of the bi-species associations with increases of up to 125% and no negative effects were recorded, which highlights the importance of plant diversity in agro-ecosystems.

Impact of herbicides on wild plant diversity in agro-ecosystems: a review

Plant diversity in agro-ecosystems is an important component of global biodiversity. However, wild plant diversity in agro-ecosystems is influenced by herbicide use. We reviewed recent studies on the impacts of herbicides on wild plant diversity in agro-ecosystems and the underlying mechanisms. It is found that herbicide application (i) decreases the number of plants that are sensitive to herbicides; (ii) increases the number of plants that are resistant to herbicides; (iii) changes species composition; and (iv) decreases genetic diversity and species diversity of wild plants in agro-ecosystems. Herbicide impacts simplify plant functional groups and hence add instability to communities. The underlying mechanism is that herbicides kill plants or change plant metabolism, resistance, reproduction and the habitat of plants. Furthermore, herbicides, human activities, and environmental factors have interactions. The impacts of herbicides on plants are plant-specific and the responses of various species or communities to the herbicides are also different. At present, the use of herbicides is increasing rapidly in agricultural production in China. We therefore suggest that more studies are needed on the impact of herbicides on wild plants and the underlying mechanisms, and the historical performances of herbicides use should be studied using long-term monitoring of wild plant changes carried out in China’s agro-ecosystems. Studies on policies and laws related to the use of herbicides also need to be strengthened, whicn can help to improve conservation of biodiversity in agro-ecosystem of China.

Biomass Production and Carbon/Nitrogen Ratio Influence Ecosystem Services from Cover Crop Mixtures

Increasing plant diversity in agroecosystems with cover crops has been a successful strategy to augment ecosystem services from agriculture, and increasing diversity of cover crops may provide even greater benefits. Productivity and ecosystem services from multi‐species cover crop mixtures were measured in a 2‐yr field study of 18 cover crop treatments preceding conventionally tilled corn in central Pennsylvania. Increasing the number of species in a stand increased cover crop biomass (R2 = 0.15). However, mixing cover crop species that were complementary in phenology or N acquisition strategy did not result in mixtures that produced more biomass than high yielding monocultures. Increasing cover crop biomass was positively correlated with several ecosystem services, namely weed suppression, prevention of nitrate leaching, and aboveground biomass N, but negatively impacted inorganic N availability and corn yield in the subsequent cropping season. The cover crop C/N ratio was another determinant of ecosystem services positively related to nitrate leaching prevention, but negatively related to inorganic N availability and corn yield. This study supports the long‐held assumption that increasing biomass can enhance certain ecosystem services from cover crops; however, because the mixtures tested did not produce more biomass than high yielding monocultures, opportunities to increase biomass‐driven services with mixtures may be limited. The correlation between biomass C/N ratio and ecosystem services in this study also indicates that functional traits, as opposed to biomass alone, will be important for predicting ecosystem services from cover crop mixtures.

Ant abundance and Cosmopolites sordidus damage in plantain fields as affected by intercropping

Intercropping is a practical way to increase plant diversity in agroecosystems and provide alternative food and habitat to arthropods, including generalist predators. In tropical agriculture, ants are important predators and have complex and often strong effects on pests. With the goal of optimizing control of the banana weevil, Cosmopolites sordidus, we studied maize (Zea mays), cocoyam (Xanthosoma sagittifolium), and bottle gourd (Lagenaria siceraria) as intercrops in a plantain field in Cameroon. We analyzed the effects of intercropping on ant abundance (by taxon) and on the damage caused by C. sordidus larvae to plantain corms. We also analyzed the relationship between ant abundance and C. sordidus damage. When added singly, the three intercropped plants significantly affected the abundance of each of the seven ant taxa but the sign of the effect depended on the intercropped plant species and on the ant taxon. Intercropping had clear effects on ant abundance, which was negatively correlated with C. sordidus damage for the Myrmicinae and positively correlated for the Formicinae and Ponerinae. Intercropping in plantain agroecosystems has the potential to alter ant community structure which contributes to C. sordidus control, but the effect of intercropped plant species remain unclear and further investigations are needed.

Epigeal arthropod communities in intensively farmed landscapes: Effects of land use mosaics, neighbourhood heterogeneity, and field position

Trophic guilds of epigeal arthropods regulating insect pest populations may be affected by factors acting across spatial scales. Although the influence of landscape factors associated with farming effects on arthropod assemblages has received increasing attention in the last decades, most research was carried out in fine-grain landscapes by comparing conventional and organic cropping systems. Here, we aimed at identifying the role of fencerows and crop heterogeneity in defining arthropod diversity in intensively farmed, coarse-grain landscapes. Hence, we developed a multi-scale heterogeneity approach based on fitting linear-mixed models to elucidate the effects of three spatial scales (local, neighbouring crop types, and landscape) on arthropod diversity. Mixed models were fitted to arthropod data obtained by pitfall trap samplings in 22 field pairs. Field position was a major determinant of arthropod species diversity at local scale, due to the contrasting disturbance regimes of fencerows and adjoining field edges. Fence density at landscape scale contributed to retain diverse arthropod assemblages in farmland mosaics by supplying habitats and refuges. In addition, extended fence network may function as corridors for dispersal, increasing connectivity between dissimilar habitats. Fence habitats, as well as their density in the landscapes, enhance both richness and abundance of beneficial arthropods. Our findings indicate that the overall arthropod diversity was benefited by landscape complexity, being the presence and density of fencerows key landscape attributes. Contrasting disturbance regimes at different field positions emerged as a major driver modulating arthropod species diversity in intensively managed farmland mosaics. Non-cropped habitats associated with fencerows and field margins play key ecological functions that are of vital importance for providing ecosystem services in agro-ecosystems. Complex landscapes may help to conserve overall plant diversity in agro-ecosystems, as well as the spillover of arthropod species from fencerows towards crop fields. Even in heavily intensified landscapes, where crop diversity is the main source of heterogeneity, promoting measures intended for both restoring and managing fencerow and non-cropped habitats through landscape planning will contribute to maintain arthropod richness (almost 70% of total richness) across the entire landscape.

Plant diversity affects behavior of generalist root herbivores, reduces crop damage, and enhances crop yield

Soil-dwelling pests inflict considerable economic damage in agriculture but are hard to control. A promising strategy to reduce pest pressure on crops is to increase the plant diversity in agroecosystems. This approach, however, demands a sound understanding of species' interactions, which is widely lacking for subterranean herbivore-plant systems. Here, we examine the effects of plant diversification on wireworms, the soil-dwelling larvae of click beetles that threaten crops worldwide. We conducted a field experiment employing plant diversification by adding either wheat or a mix of six associated plants (grasses, legumes, and forbs) between rows of maize to protect it from Agriotes wireworms. Wireworm feeding behavior, dispersal between crop and associated plants, as well as maize damage and yield were examined. The former was assessed combining molecular gut content and stable isotope analysis. The pests were strongly attracted by the associated plants in August, when the crop was most vulnerable, whereas in September, shortly before harvest, this effect occurred only in the plant mix. In maize monoculture, the larvae stayed in the principal crop throughout the season. Larval delta13C signatures revealed that maize feeding was reduced up to sevenfold in wireworms of the vegetationally diversified treatments compared to those of the maize monoculture. These findings were confirmed by molecular analysis, which additionally showed a dietary preference of wireworms for specific plants in the associated plant mix. Compared to the monoculture, maize damage was reduced by 38% and 55% in the wheat and plant mix treatment, which translated into a yield increase of 30% and 38%, respectively. The present findings demonstrate that increasing the plant diversity in agroecosystems provides an effective insurance against soil pests. The underlying mechanisms are the diversion of the pest from the principle crop and a changed feeding behavior. The deployment of diverse mixes of associated plants, tailored to the specific preferences of the soil herbivores, provides a promising strategy for managing subterranean pests while maintaining crop yield.

Weeds mediate the level of intraguild predation in arthropod food webs

AbstractIntraguild predation, which is common for generalist predators, is a specific form of omnivory that may suppress the biological control of a pest. The dietary flexibility of a given organism depends on the choice of the C3 (banana crop) and the C4 (weeds) pathways they use and on the trophic level on which they feed. Understanding the conditions in which intraguild predation decreases biological control is a major issue in agroecosystems. We tested whether the contribution of different primary producer pathways in diets of generalist predators mediates the level of intraguild predation. We studied 10 agroecosystems in which banana plants (C3 metabolism) were diversely associated with weeds (C4 metabolism). Diversity in litter macrofauna was relatively low, with a mean between three and eight species per trap. Measurement of stable isotopes showed a significant decrease in the δ15N values of generalist predators when the C4 pathway contributed more than the C3 pathway to their diet. We rejected hypotheses that an increase in the abundance of prey and that a decrease in prey's δ15N values occur when the C4 pathway contributes more than the C3 pathway to their diet. The results are consistent with the diet modification hypothesis, that is, intraguild predation is lower when the C4 (weeds) pathway is preferred to the C3 pathway. Our results suggest that when the C4 pathway of weeds is more exploited by herbivores (or detritivores), generalist predators tend to consume these herbivores and thus neglect the intraguild prey. The diverse C4 plant community probably supports a diverse herbivore community that provides alternative prey. Our results provide evidence that increasing plant diversity in agroecosystems should decrease intraguild predation of generalist predators and should therefore improve pest regulation. In an applied perspective, plant diversity could be increased by establishing a more diverse cover‐crop community.

A comparison of land‐sharing and land‐sparing strategies for plant richness conservation in agricultural landscapes

Strategies for conserving plant diversity in agroecosystems generally focus on either expanding land area in non-crop habitat or enhancing diversity within crop fields through changes in within-field management practices. In this study, we compare effects on landscape-scale species richness from such land-sharing or land-sparing strategies. We collected data in arable field, grassland, pasture, and forest habitat types (1.6 ha sampled per habitat type) across a 100-km2 region of farmland in Lancaster County, Pennsylvania, USA. We fitted species-area relationships (SARs) for each habitat type and then combined extrapolations from the curves with estimates of community overlap to estimate richness in a 314.5-ha landscape. We then modified these baseline estimates by adjusting parameters in the SAR models to compare potential effects of land-sharing and land-sparing conservation practices on landscape richness. We found that species richness of the habitat types showed a strong inverse relationship to the relative land area of each type in the region, with 89 species in arable fields (66.5% of total land area), 153 in pastures (6.7%), 196 in forests (5.2%), and 213 in grasslands (2.9%). Relative to the baseline scenario, major changes in the richness of arable fields produced gains in landscape-scale richness comparable to a conversion of 3.1% of arable field area into grassland habitat. Sensitivity analysis of our model indicated that relative gains from land sparing would be greatest in landscapes with a low amount of non-crop habitat in the baseline scenario, but that in more complex landscapes land sharing would provide greater gains. These results indicate that the majority of plant species in agroecosystems are found in small fragments of non-crop habitat and suggest that, especially in landscapes with little non-crop habitat, richness can be more readily conserved through land-sparing approaches.

Plant species coexistence alleviates the impacts of lead on Zea mays L.

Whether plant coexistence can reduce the impacts of lead (Pb) on crops in agroecosystems has not been well understood. We conducted a factorial experiment to investigate the effects of weeds coexisting with maize (Zea mays L.) on Pb accumulation in maize and soil microbes at two Pb levels (ambient and 300 mg/kg). Elevated Pb tended to increase the Pb concentration in maize and decreased soil microbial activity (indicated by the average well color development, AWCD), functional group diversity, as well as arbuscular mycorrhizal (AM) colonization and vesicle number of maize. Compared to the monoculture, weeds coexisting with maize reduced the Pb concentrations in the root, leaf, sheath and stem of maize at both seedling and mature stages. In maize-weed mixtures, soil microbial activity and functional group diversity tended to increase for both Pb treatments relative to the monoculture. Furthermore, principal component analysis revealed that the soil microbial community structure changed with the introduction of weeds. The highest Pb accumulation in weeds occurred for the elevated Pb treatment in a three species mixture. The results suggest that multiple plant species coexistence could reduce lead accumulation in crop plants and alleviate the negative impacts on soil microbes in polluted land, thereby highlighting the significance of plant diversity in agroecosystems.

Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: a review

Farmers are facing serious plant protection issues and phytosanitary risks, in particular in the tropics. Such issues are food insecurity, lower income in traditional low- input agroecosystems, adverse effects of pesticide use on human health and on the environment in intensive systems and export restrictions due to strict regulations on quarantine pests and limits on pesticide residues. To provide more and better food to populations in both the southern and northern hemispheres in a sustainable manner, there is a need for a drastic reduction in pesticide use while keeping crop pest and disease damage under control. This can be achieved by breaking with industrial agriculture and using an agro- ecological approach, whose main pillar is the conserva- tion or introduction of plant diversity in agroecosystems. Earlier literature suggest that increasing vegetational biodiversity in agroecosystems can reduce the impact of pests and diseases by the following mechanisms: (1) resource dilution and stimulo-deterrent diversion, (2)

Landscape complexity differentially affects alpha, beta, and gamma diversities of plants occurring in fencerows and crop fields

Agro-ecosystems still retain part of the original biodiversity, although agricultural intensification threatens to eliminate refuge patches from farmland mosaics. Landscape complexity resulting from networks of uncultivated corridors associated with fencerows may play a key role in sustaining biodiversity across scales, and may further influence diversity in adjacent, cultivated fields. We evaluated the relationship between farmland complexity and plant diversity of fencerows and crop fields at local and landscape scales in the Rolling Pampas of Argentina. We surveyed 222 fencerows and fields cultivated with winter or summer crops, and characterised farmland complexity by the perimeter/area ratio of cropland in 2-km diameter circles surrounding each field. Plant diversity was additively partitioned into alpha, beta, and gamma components. Fencerows had noticeably higher richness than cropped fields at local and landscape scales. Gamma and beta diversities of fencerows and fields were positively related to farmland complexity, supporting the role of spatial heterogeneity in maintaining plant diversity in agro-ecosystems. Landscape complexity did not influence alpha diversity of fencerows but significantly increased diversity within fields, a likely result of enhanced mass effects from uncultivated habitats in more varied farmland. More complex landscapes contained greater gamma diversity of exotic perennials in fencerows, and of exotic and native annuals within fields. Importantly, alpha and gamma diversities of native perennials from the pristine Pampa grassland increased with landscape complexity within cropped fields. In the face of ongoing landscape homogenisation under agricultural intensification, maintaining fencerow networks may become critical for conserving habitat heterogeneity and farmland biodiversity.