Janne Bengtsson

Reserves, Resilience and Dynamic Landscapes

Abstract In a world increasingly modified by human activities, the conservation of biodiversity is essential as insurance to maintain resilient ecosystems and ensure a sustainable flow of ecosystem goods and services to society. However, existing reserves and national parks are unlikely to incorporate the long-term and large-scale dynamics of ecosystems. Hence, conservation strategies have to actively incorporate the large areas of land that are managed for human use. For ecosystems to reorganize after large-scale natural and human-induced disturbances, spatial resilience in the form of ecological memory is a prerequisite. The ecological memory is composed of the species, interactions and structures that make ecosystem reorganization possible, and its components may be found within disturbed patches as well in the surrounding land-scape. Present static reserves should be complemented with dynamic reserves, such as ecological fallows and dynamic successional reserves, that are part of ecosystem management mimicking natural disturbance regimes at the landscape level.

Land‐use intensity and the effects of organic farming on biodiversity: a hierarchical meta‐analysis

Summary The benefits of organic farming to biodiversity in agricultural landscapes continue to be hotly debated, emphasizing the importance of precisely quantifying the effect of organic vs. conventional farming. We conducted an updated hierarchical meta‐analysis of studies that compared biodiversity under organic and conventional farming methods, measured as species richness. We calculated effect sizes for 184 observations garnered from 94 studies, and for each study, we obtained three standardized measures reflecting land‐use intensity. We investigated the stability of effect sizes through time, publication bias due to the ‘file drawer’ problem, and consider whether the current literature is representative of global organic farming patterns. On average, organic farming increased species richness by about 30%. This result has been robust over the last 30 years of published studies and shows no sign of diminishing. Organic farming had a greater effect on biodiversity as the percentage of the landscape consisting of arable fields increased, that is, it is higher in intensively farmed regions. The average effect size and the response to agricultural intensification depend on taxonomic group, functional group and crop type. There is some evidence for publication bias in the literature; however, our results are robust to its impact. Current studies are heavily biased towards northern and western Europe and North America, while other regions with large areas of organic farming remain poorly investigated. Synthesis and applications. Our analysis affirms that organic farming has large positive effects on biodiversity compared with conventional farming, but that the effect size varies with the organism group and crop studied, and is greater in landscapes with higher land‐use intensity. Decisions about where to site organic farms to maximize biodiversity will, however, depend on the costs as well as the potential benefits. Current studies have been heavily biased towards agricultural systems in the developed world. We recommend that future studies pay greater attention to other regions, in particular, areas with tropical, subtropical and Mediterranean climates, in which very few studies have been conducted.

Landscape heterogeneity and farming practice influence biological control

Summary We measured the impact of natural enemies on the population development on the bird cherry-oat aphid ( Rhopalosiphum padi ) on conventional and organic farms. By pairing farms with similar landscape features but different farming systems, we were able to separate the effects of farming practice and landscape features on the influence of natural enemies on R. padi abundance. Natural enemies had a greater impact on R. padi establishment on organic farms than on conventional farms. Irrespective of farming system, landscapes with abundant field margins and perennial crops were associated with low R. padi establishment. After establishment, there was no difference in ground-living enemy impact on R. padi population growth rate between farming systems, but impact was greater in landscapes where arable land was contiguous. Wir untersuchten die Wirkung naturlicher Feinde auf die Populationsentwicklung der Haferblattlaus (Rhopalosiphum padi) in konventionell und organisch bewirtschafteten landwirtschaftlichen Betrieben. Indem wir Betriebe paarten, die ahnliche Landschaftseigenschaften aber unterschiedliche Betriebsformen aufwiesen, konnten wir die Wirkung der Bewirtschaftung und der Landschaftseigenschaften auf den Einflus der naturlichen Feinde auf die Abundanz von R. padi trennen. Die naturlichen Feinde hatten in organisch bewirtschafteten Betrieben eine grosere Wirkung auf die Ansiedlung von R. padi als in konventionell bewirtschafteten Betrieben. Unabhangig von der Bewirtschaftungsform waren Landschaften mit zahlreichen Feldrainen und perennierenden Feldkulturen mit einer geringen Ansiedlung von R. padi verbunden. Die Wirkung bodenlebender Feinde auf die Wachstumsrate der Populationen von R. padi nach der Ansiedlung unterschied sich nicht zwischen den Bewirtschaftungsformen. Die Wirkung war jedoch in Landschaften groser, die zusammenhangende Ackerflachen aufwiesen.

RELATIONSHIP AMONG THE SPECIES RICHNESS OF DIFFERENT TAXA

Spatially explicit forecasting of changes in species richness is key to designing informative scenarios on the development of diversity on our planet. It might be possible to predict changes in the richness of inadequately investigated groups from that of groups for which enough information is available. Here we evaluate the reliability of this approach by reviewing 237 richness correlations extracted from the recent literature. Of the 43 taxa covered, beetles, vascular plants, butterflies, birds, ants, and mammals (in that order) were the most common ones examined. Forests and grasslands strongly dominated the ecosystem types studied. The variance explanation (R2) could be calculated for 152 cases, but only 53 of these were significant. An average correlation effect size of 0.374 (95% CI = +/- 0.0678) indicates positive but weak correlations between taxa within the very heterogeneous data set; None of the examined explanatory variables (spatial scale, taxonomic distance, trophic position, biome) could account for this heterogeneity. However, studies focusing on 10-km2 grid cells had the highest variance explanation. Moreover, within-phylum between-class comparisons had marginally significantly lower correlations than between-phylum comparisons. And finally, the explanatory power of studies conducted in the tropics was significantly higher than that of studies conducted in temperate regions. It is concluded that the potential of a correlative approach to species richness is strongly diminished by the overall low level of variance explanation. So far, no taxon has proved to be a universal or even particularly good predictor for the richness of other taxa. Some suggestions for future research are inclusion of several taxa in models aiming at regional richness predictions, improvement of knowledge on species correlations in human dominated systems, and a better understanding of mechanisms underlying richness correlations.

Yield increase attributable to aphid predation by ground-living polyphagous natural enemies in spring barley in Sweden

It has been claimed that natural enemies of pests perform important ecosystem services in agricultural landscapes. These services have rarely been evaluated in yield or monetary terms at farm level. We have examined the degree to which ground-living natural enemies of the bird cherry-oat aphid (Rhopalosiphum padi (L.)) reduced aphid abundance and influenced barley yields on ten commercial farms in central Sweden. On average, ground-living natural enemies of pests increased barley yields by 303 kg/ha. This corresponded to a potential 52% reduction in yield loss from R. padi compared with a scenario where no natural enemies were present. Measured as the percentage increase of actual yield, ground-living natural enemies of aphids increased the yield by 23%. The increase in yields in absolute terms was larger on conventional farms than organic farms, but the percentage increase of yields was larger on organic farms. The study only considered 1 year, with high aphid abundance. However, we also discuss the effects of predators as well as insecticides in the long-term. Our study shows that yield increases attributable to predators can be compared with yield increases from insecticide use for the evaluation of different management strategies.

The relationship between agricultural intensification and biological control: experimental tests across Europe

Agricultural intensification can affect biodiversity and related ecosystem services such as biological control, but large-scale experimental evidence is missing. We examined aphid pest populations in cereal fields under experimentally reduced densities of (1) ground-dwelling predators (-G), (2) vegetation-dwelling predators and parasitoids (-V), (3) a combination of (1) and (2) (-G-V), compared with open-fields (control), in contrasting landscapes with low vs. high levels of agricultural intensification (AI), and in five European regions. Aphid populations were 28%, 97%, and 199% higher in -G, -V, and -G-V treatments, respectively, compared to the open fields, indicating synergistic effects of both natural-enemy groups. Enhanced parasitoid: host and predator: prey ratios were related to reduced aphid population density and population growth. The relative importance of parasitoids and vegetation-dwelling predators greatly differed among European regions, and agricultural intensification affected biological control and aphid density only in some regions. This shows a changing role of species group identity in diverse enemy communities and a need to consider region-specific landscape management.

LANDSCAPE COMPLEXITY AND FARMING PRACTICE INFLUENCE THE CONDITION OF POLYPHAGOUS CARABID BEETLES

Previous work has suggested that the condition of polyphagous predatory carabid beetles can be influenced by landscape structure and farming practice. However, earlier studies aimed at testing the effect of farming practice on insect condition and reproduction have not been designed to test this accurately because farms representing different cultivation systems have not had similar landscape complexity. In this study we measured the condition of the polyphagous predatory carabid beetles Harpalus rufipes, Pterostichus cupreus, P. melanarius, and P. niger from five pairs of conventional and organic farms in central east Sweden. The farms within each pair were near to each other and had similar size, land use, and landscape structure. The condition of the beetles was measured as the residuals from the regression between ln(elytra length) and ln(body mass). We show that the residuals for P. melanarius were positively correlated with their fat reserves. For P. melanarius, residuals were higher on farms where the fields had high perimeter-to-area ratio. The pooled residuals of H. rufipes, P. cupreus, and P. niger were positively correlated with the perimeter-to-area ratio and crop diversity. Pterostichus me- lanarius from organic farms had better condition than conspecifics at conventional farms, and for H. rufipes, P. cupreus, and P. niger combined there was a tendency toward better condition on organic farms than at conventional farms. Harpalus rufipes, P. cupreus, P. melanarius, and P. niger, which are all polyphagous predatory carabid beetles, are natural enemies of pest aphids in the agroecosystem. We conclude that, by manipulating landscape structure and composition, improvement in the condition of polyphagous predatory carabid beetles could be achieved, thereby potentially increasing populations of these beneficial insects.

Long‐term effects of plant diversity and composition on soil nematode communities in model grasslands

An important component of plant-soil feedbacks is how plant species identity anddiversity influence soil organism communities. We examine the effects of grassland plant species growing alone and together up to a richness of 12 species on nematode diversity and feeding group composition, eight years after the establishment of experimental grassland plots at the BIODEPTH site in northern Sweden. This is a substantially longer time than most other experimental studies of plant effects on soil fauna. We address the hypotheses that (la) higher species or functional diversity of plants increases nematode diversity, as well as influences nematode community composition. Alternatively, (1b) individual plant species traits are most important for nematode diversity and community composition. (2) Plant effects on soil organisms will decrease with increasing number of trophic links between plants and soil fauna. Plant species identity was often more important than plant diversity for nematode community composition, supporting hypothesis 1b. There was a weak positive relation between plant and nematode richness;which could be attributed to the presence of the legume Trifolium pratense, but also to some other plant species, suggesting a selection or sampling effect. Several plant species in different functional groups affected nematode community composition. For example, we found that legumes increased bacterial-feeding nematodes, most notably r-selected Rhabditida, while fungal-feeding nematodes were enhanced by forbs. Other bacterial feeders and obligate root feeders were positively related to grasses. Plant effects were usually stronger on plant-, bacterial- and fungal-feeding nematodes than on omnivores/predators, which supports hypothesis 2. Our study suggests that plant identity has stronger effects than plant diversity on nematode community composition, but when comparing our results with similar previous studies the effects of particular plant species appear to vary. We also found that more productive plant species affected bacterial-feeding nematodes more than fungal feeders. Moreover, we observed stronger effects the fewer the number of trophic links there were between a nematode feeding group and plants. Although we found clear effects of plants on soil nematodes, these were probably not large enough to result in strong and persistent plant-soil-organism-plant feedback loops.

The Dynamics of Social-Ecological Systems in Urban Landscapes: Stockholm and the National Urban Park, Sweden

Abstract: This study addresses social‐ecological dynamics in the greater metropolitan area of Stockholm County, Sweden, with special focus on the National Urban Park (NUP). It is part of the Millennium Ecosystem Assessment (MA) and has the following specific objectives: (1) to provide scientific information on biodiversity patterns, ecosystem dynamics, and ecosystem services generated; (2) to map interplay between actors and institutions involved in management of ecosystem services; and (3) to identify strategies for strengthening social‐ecological resilience. The green areas in Stockholm County deliver numerous ecosystem services, for example, air filtration, regulation of microclimate, noise reduction, surface water drainage, recreational and cultural values, nutrient retention, and pollination and seed dispersal. Recreation is among the most important services and NUP, for example, has more than 15 million visitors per year. More than 65 organizations representing 175,000 members are involved in management of ecosystem services. However, because of population increase and urban growth during the last three decades, the region displays a quite dramatic loss of green areas and biodiversity. An important future focus is how management may reduce increasing isolation of urban green areas and enhance connectivity. Comanagement should be considered where locally managed green space may function as buffer zones and for management of weak links that connect larger green areas; for example, there are three such areas around NUP identified. Preliminary results indicate that areas of informal management represent centers on which to base adaptive comanagement, with the potential to strengthen biodiversity management and resilience in the landscape.

The Effects of Spatial Scale on Trophic Interactions

Food chain models have dominated empirical studies of trophic interactions in the past decades, and have lead to important insights into the factors that control ecological communities. Despite the importance of food chain models in instigating ecological investigations, many empirical studies still show a strong deviation from the dynamics that food chain models predict. We present a theoretical framework that explains some of the discrepancies by showing that trophic interactions are likely to be strongly influenced by the spatial configuration of consumers and their resources. Differences in the spatial scale at which consumers and their resources function lead to uncoupling of the population dynamics of the interacting species, and may explain overexploitation and depletion of resource populations. We discuss how changed land use, likely the most prominent future stress on natural systems, may affect food web dynamics by interfering with the scale of interaction between consumers and their resource.