Effects of landscape complexity and stand factors on arthropod communities in poplar forests

The arthropod communities are influenced by both local conditions and features of the surrounding landscape. Landscape complexity and stand factors may both influence arthropod communities in poplar forests, but the multiscale effects of these factors on poplar defoliators and natural enemies are still poorly understood. We collected poplar arthropods at 30 sampling sites within five forest landscapes in Xinjiang, China, and assessed whether landscape complexity and stand factors influence species abundance and diversity of poplar arthropods. Landscape complexity was quantified by several independent metrics of landscape composition, configuration, and connectivity at three spatial scales. We also determined the most powerful explanatory variables and the scale effect of each arthropod. Results found that landscape complexity and stand factors had different effects on different poplar arthropod communities. Landscape complexity promoted natural enemies at different spatial scales, but it inhibited the population of poplar defoliators at the scale of 200 m. Specifically, the abundance and diversity of all defoliators decreased with increasing proportion of nonhost plants. Landscape diversity only had a negative effect on defoliator abundance. The shape complexity of habitat patches increased the abundance of carabid beetles but reduced the abundance of green leafhoppers and migratory locusts. The abundance and diversity of predators increased with increasing structural connectivity of forest landscape. Additionally, both the abundance and diversity of all defoliators were positively correlated with the average height of herbaceous plants. Diversity of all defoliators increased with increasing size of host trees. The distance from sampling site to the nearest village positively influenced the abundance and diversity of all predators. Arthropod abundance and diversity in poplar forests were driven by stand factors and landscape complexity. Therefore, maintaining complex shape and structural connectivity of habitat patches and keeping poplar stands away from the village are crucial for management of forest landscape to enhance natural enemies. And in order to reduce the abundance of defoliators in poplar forest, the diversity of surrounding habitat types should be promoted within 200 m radii.     

Gut microbiomes of mobile predators vary with landscape context and species identity

Landscape context affects predator–prey interactions and predator diet composition, yet little is known about landscape effects on insect gut microbiomes, a determinant of physiology and condition. Here, we combine laboratory and field experiments to examine the effects of landscape context on the gut bacterial community and body condition of predatory insects. Under laboratory conditions, we found that prey diversity increased bacterial richness in insect guts. In the field, we studied the performance and gut microbiota of six predatory insect species along a landscape complexity gradient in two local habitat types (soybean fields vs. prairie). Insects from soy fields had richer gut bacteria and lower fat content than those from prairies, suggesting better feeding conditions in prairies. Species origin mediated landscape context effects, suggesting differences in foraging of exotic and native predators on a landscape scale. Overall, our study highlights complex interactions among gut microbiota, predator identity, and landscape context.     

Population structure of Columbia spotted frogs (Rana luteiventris) is strongly affected by the landscape

Landscape features such as mountains, rivers, and ecological gradients may strongly affect patterns of dispersal and gene flow among populations and thereby shape population dynamics and evolutionary trajectories. The landscape may have a particularly strong effect on patterns of dispersal and gene flow in amphibians because amphibians are thought to have poor dispersal abilities. We examined genetic variation at six microsatellite loci in Columbia spotted frogs (Rana luteiventris) from 28 breeding ponds in western Montana and Idaho, USA, in order to investigate the effects of landscape structure on patterns of gene flow. We were particularly interested in addressing three questions: (i) do ridges act as barriers to gene flow? (ii) is gene flow restricted between low and high elevation ponds? (iii) does a pond equal a ‘randomly mating population’ (a deme)? We found that mountain ridges and elevational differences were associated with increased genetic differentiation among sites, suggesting that gene flow is restricted by ridges and elevation in this species. We also found that populations of Columbia spotted frogs generally include more than a single pond except for very isolated ponds. There was also evidence for surprisingly high levels of gene flow among low elevation sites separated by large distances. Moreover, genetic variation within populations was strongly negatively correlated with elevation, suggesting effective population sizes are much smaller at high elevation than at low elevation. Our results show that landscape features have a profound effect on patterns of genetic variation in Columbia spotted frogs.     

Avian top predator and the landscape of fear: responses of mammalian mesopredators to risk imposed by the golden eagle

Top predators may induce extensive cascading effects on lower trophic levels, for example, through intraguild predation (IGP). The impacts of both mammalian and avian top predators on species of the same class have been extensively studied, but the effects of the latter upon mammalian mesopredators are not yet as well known. We examined the impact of the predation risk imposed by a large avian predator, the golden eagle (Aquila chrysaetos, L.), on its potential mammalian mesopredator prey, the red fox (Vulpes vulpes, L.), and the pine marten (Martes martes, L.). The study combined 23 years of countrywide data from nesting records of eagles and wildlife track counts of mesopredators in Finland, northern Europe. The predation risk of the golden eagle was modeled as a function of territory density, density of fledglings produced, and distance to nearest active eagle territory, with the expectation that a high predation risk would reduce the abundances of smaller sized pine martens in particular. Red foxes appeared not to suffer from eagle predation, being in fact most numerous close to eagle nests and in areas with more eagle territories. This is likely due to similar prey preferences of the two predators and the larger size of foxes enabling them to escape eagle predation risk. Somewhat contrary to our prediction, the abundance of pine martens increased from low to intermediate territory density and at close proximity to eagle nests, possibly because of similar habitat preferences of martens and eagles. We found a slightly decreasing trend of marten abundance at high territory density, which could indicate that the response in marten populations is dependent on eagle density. However, more research is needed to better establish whether mesopredators are intimidated or predated by golden eagles, and whether such effects could in turn cascade to lower trophic levels, benefitting herbivorous species.     

Neither the devil nor the deep blue sea: larval mortality factors fail to explain the abundance and distribution of Tischeria ekebladella

Abstract.  1. The distribution, abundance and population dynamics of herbivorous insects may be affected by trophic interactions, by abiotic influences, or by intra-specific processes. Relatively little is known about how trophic influences vary across space. Here, we investigate spatial variation in mortality in the oak-feeding leaf miner Tischeria ekebladella as attributable to individual causal agents.
2. Leaf miners were experimentally introduced on 67 trees on an island 5 km² in area in south-western Finland. On each tree, some larvae were protected by a muslin bag, others by a glue barrier around the branch and some left exposed.
3. In the bagged transplants, 78.4% of larvae survived, compared with only 9.6% in the other two treatments. Most of the mortality was because of airborne agents: mortality on branches sheltered by a glue barrier was as high as on fully exposed branch tips.
4. We consider mortality caused by parasitoid wasps to be the main source of larval death and the primary factor driving general patterns of survival. The effects of bird predation and premature leaf abscission were negligible.
5. We detected spatial aggregation in larval survival and parasitism rates at the level of individual trees, but not across the landscape.
6. Spatial variation in overall leaf miner survival, parasitism and leaf abscission does not suffice to explain patterns of incidence and abundance of wild T. ekebladella on experimental trees. Rather, we identify metapopulation dynamics as a likely determinant of the spatial distribution of T. ekebladella in the landscape.     

Simplifying understory complexity in oil palm plantations is associated with a reduction in the density of a cleptoparasitic spider,Argyrodes miniaceus (Araneae: Theridiidae), in host (Araneae: Nephilinae) webs

Expansion of oil palm agriculture is currently one of the main drivers of habitat modification in Southeast Asia. Habitat modification can have significant effects on biodiversity, ecosystem function, and interactions between species by altering species abundances or the available resources in an ecosystem. Increasing complexity within modified habitats has the potential to maintain biodiversity and preserve species interactions. We investigated trophic interactions between Argyrodes miniaceus, a cleptoparasitic spider, and its Nephila spp. spider hosts in mature oil palm plantations in Sumatra, Indonesia. A. miniaceus co‐occupy the webs of Nephila spp. females and survive by stealing prey items caught in the web. We examined the effects of experimentally manipulated understory vegetation complexity on the density and abundance of A. miniaceus in Nephila spp. webs. Experimental understory treatments included enhanced complexity, standard complexity, and reduced complexity understory vegetation, which had been established as part of the ongoing Biodiversity and Ecosystem Function in Tropical Agriculture (BEFTA) Project. A. miniaceus density ranged from 14.4 to 31.4 spiders per square meter of web, with significantly lower densities found in reduced vegetation complexity treatments compared with both enhanced and standard treatment plots. A. miniaceus abundance per plot was also significantly lower in reduced complexity than in standard and enhanced complexity plots. Synthesis and applications: Maintenance of understory vegetation complexity contributes to the preservation of spider host–cleptoparasite relationships in oil palm plantations. Understory structural complexity in these simplified agroecosystems therefore helps to support abundant spider populations, a functionally important taxon in agricultural landscapes. In addition, management for more structurally complex agricultural habitats can support more complex trophic interactions in tropical agroecosystems.     

Effects of agricultural diversification on the abundance, distribution, and pest control potential of spiders: a review

A review of the literature showed that spider abundance was increased by diversification in 63% of studies. A comparison of diversification modes showed that spider abundance in the crop was increased in 33% of studies by `aggregated diversification' (e.g. intercropping and non-crop strips) and in 80% of studies by `interspersed diversification' (e.g., undersowing, partial weediness, mulching and reduced tillage). It is suggested that spiders tend to remain in diversified patches and that extending the diversification throughout the whole crop (as in interspersed diversification) offers the best prospects for improving pest control. There is little evidence that spiders walk in significant numbers into fields from uncultivated field edges, but diversification at the landscape level serves to foster large multi-species regional populations of spiders which are valuable as a source of aerial immigrants into newly planted crops. There are very few manipulative field studies where the impact of spiders on pests has been measured in diversified crops compared with undiversified controls. It is encouraging, however, that in those few studies an increased spider density resulted in improved pest control. Future work needs are identified.     

Seed dispersal of a fleshy-fruited invasive shrub is affected by changes in the frugivorous bird assemblage along an elevational gradient

Seed dispersal by birds constitutes an essential mechanism for ornithochorous exotic plants to successfully invade a new system. New biotic associations with native birds might facilitate the upward spread of exotic plants from the foothills into the high mountains. However, environmental changes associated with elevation are known to drive changes in bird assemblages, and it is not clear how elevation changes impact the seed dispersal service of ornithochorous invaders. We evaluated changes in frugivorous bird assemblages of one of the exotic shrubs (Cotoneaster franchetii, Rosaceae) with the broadest elevation range among woody invaders in the Córdoba Mountains (Argentina). We quantified frugivory interactions (including absolute and proportional fruit consumption by seed dispersers, pulp consumers, and seed predators) using 4-h observations of focal C. franchetii shrubs distributed across low-elevation, mid-elevation, and high-elevation sites (700, 1100, and 1800 m a.s.l., respectively; 15 individuals per elevational band and one site per elevation). Seed disperser richness was highest at the low- and mid-elevation sites (three species vs. one at the high-elevation site), but proportional and absolute fruit consumption of C. franchetii was highest at the high-elevation site (39.1%, 88 seeds at high-elevation and 7.7%, 20 seeds at low-elevation). The Chiguanco Thrush (Turdus chiguanco, Turdidae) was the only seed disperser species found at the highest elevation site. Fruit consumption by seed dispersers was positively related to their abundance and elevation. In a high mountain system, a single abundant generalist seed disperser, rather than a high richness of seed disperser species, can uphold an effective dispersal service for an invasive ornithochorous shrub. This pattern may facilitate the spread of such plants across higher elevational ranges, thereby promoting the invasion of other exotic ornithochorous plants into upper elevations.     

Shoot development in grapevine (Vitis vinifera) is affected by the modular branching pattern of the stem and intra- and inter-shoot trophic competition

BACKGROUND AND AIMS: Shoot architecture variability in grapevine (Vitis vinifera) was analysed using a generic modelling approach based on thermal time developed for annual herbaceous species. The analysis of shoot architecture was based on various levels of shoot organization, including pre-existing and newly formed parts of the stem, and on the modular structure of the stem, which consists of a repeated succession of three phytomers (P0-P1-P2). METHODS: Four experiments were carried out using the cultivar 'Grenache N': two on potted vines (one of which was carried out in a glasshouse) and two on mature vines in a vineyard. These experiments resulted in a broad diversity of environmental conditions, but none of the plants experienced soil water deficit. KEY RESULTS: Development of the main axis was highly dependent on air temperature, being linearly related to thermal time for all stages of leaf development from budbreak to veraison. The stable progression of developmental stages along the main stem resulted in a thermal-time based programme of leaf development. Leaf expansion rate varied with trophic competition (shoot and cluster loads) and environmental conditions (solar radiation, VPD), accounting for differences in final leaf area. Branching pattern was highly variable. Classification of the branches according to ternary modular structure increased the accuracy of the quantitative analysis of branch development. The rate and duration of leaf production were higher for branches derived from P0 phytomers than for branches derived from P1 or P2 phytomers. Rates of leaf production, expressed as a -function of thermal time, were not stable and depended on trophic competition and environmental conditions such as solar radiation or VPD. CONCLUSIONS: The application to grapevine of a generic model developed in annual plants made it possible to identify constants in main stem development and to determine the hierarchical structure of branches with respect to the modular structure of the stem in response to intra- and inter-shoot trophic competition.     

When beggars are choosers—How nesting of a solitary bee is affected by temporal dynamics of pollen plants in the landscape

Wild bees are declining in intensively farmed regions worldwide, threatening pollination services to flowering crops and wild plants. To halt bee declines, it is essential that conservation actions are based on a mechanistic understanding of how bee species utilize landscapes. We aimed at teasing apart how foraging resources in the landscape through the nesting season affected nesting and reproduction of a solitary bee in a farmland region. We investigated how availability of floral resources and potentially resource‐rich habitats surrounding nests affected nest provisioning and reproduction in the solitary polylectic bee Osmia bicornis. The study was performed in 18 landscape sectors dominated by agriculture, but varying in agricultural intensity in terms of proportion of organic crop fields and seminatural permanent pastures. Pasture‐rich sectors contained more oak (Quercus robur), which pollen analysis showed to be favored forage in early season. More oaks ≤100 m from nests led to higher proportions of oak pollen in nest provisions and increased speed of nest construction in early season, but this effect tapered off as flowering decreased. Late‐season pollen foraging was dominated by buttercup (Ranunculus spp.), common in various noncrop habitats. Foraging trips were longer with more oaks and increased further through the season. The opposite was found for buttercup. Oak and buttercup interacted to explain the number of offspring; buttercup had a positive effect only when the number of oaks was above the mean for the studied sectors. The results show that quality of complex and pasture‐rich landscapes for O. bicornis depends on preserving existing and generating new oak trees. Lignose plants are key early‐season forage resources in agricultural landscapes. Increasing habitat heterogeneity with trees and shrubs and promoting suitable late‐flowering forbs can benefit O. bicornis and other wild bees active in spring and early summer, something which existing agri‐environment schemes seldom target.