Rapidly declining biodiversity may be a contributing factor to another global megatrend—the rapidly increasing prevalence of allergies and other chronic inflammatory diseases among urban populations worldwide. According to the “biodiversity hypothesis,” reduced contact of people with natural environmental features and biodiversity may adversely affect the human commensal microbiota and its immunomodulatory capacity. Analyzing atopic sensitization (i.e., allergic disposition) in a random sample of adolescents living in a heterogeneous region of 100 × 150 km, we show that environmental biodiversity in the surroundings of the study subjects’ homes influenced the composition of the bacterial classes on their skin. Compared with healthy individuals, atopic individuals had lower environmental biodiversity in the surroundings of their homes and significantly lower generic diversity of gammaproteobacteria on their skin. The functional role of the Gram-negative gammaproteobacteria is supported by in vitro measurements of expression of IL-10, a key anti-inflammatory cytokine in immunologic tolerance, in peripheral blood mononuclear cells. In healthy, but not in atopic, individuals, IL-10 expression was positively correlated with the abundance of the gammaproteobacterial genus Acinetobacter on the skin. These results raise fundamental questions about the consequences of biodiversity loss for both allergic conditions and public health in general.

Tropical forests, which harbor high levels of biodiversity, are being lost at an alarming speed. Madagascar, a biodiversity hotspot, has lost more than half of its original forest cover. Most of the remaining forests are small fragments of primary and secondary forest with differing degrees of human impact. These forests, as well as coffee and fruit plantations, may be important in supporting the forest‐dependent biodiversity in Madagascar but this has been little studied. In Madagascar, dung beetles, which offer important ecosystem services, are largely restricted to forests. We examined the ability of fragmented and degraded forests to support dung beetle diversity, compared to the large areas of primary forest in eastern Madagascar. We found a general trend of a reduction of species with a loss of forest connectivity. In contrast, a higher level of forest disturbance was associated with higher species diversity. In several sites of low‐quality forest as many or more species were found as in less disturbed and primary forests. The average size of dung beetles was smaller in the lower quality localities than in the primary forests. These findings suggest that many forest dung beetles in Madagascar are better adapted to forest disturbance than earlier expected, although they require some level of connectivity to surrounding forest. Abstract in Malagasy is available with online material.

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Intensive agriculture is detrimental to soil biodiversity and functioning. Promoting communities of key soil organisms, such as earthworms and arbuscular mycorrhizal (AM) fungi, may help improve agricultural sustainability by replacing inputs with ecosystem services. In this thesis, I explore ways to manage earthworm communities for improved soil functioning via adjusted agricultural practices, promotion of source habitats in the landscape, and inoculation. I also explore the effects of soil compaction on AM fungal symbiosis in wheat varieties, and test application of grassland soil as a method to increase AM fungal diversity in agricultural soils. I show that diverse earthworm communities and bioturbation can be promoted by reducing tillage intensity and total earthworm densities can be increased via diversifying crop rotations. Moist and fertile semi-natural grasslands with high small-scale habitat heterogeneity may serve to sustain earthworm diversity in agricultural landscapes. Inoculation with commercially obtained Lumbricus terrestris earthworms may help restore populations of this tillage sensitive species and improve wheat growth but the long-term establishment of these worms is uncertain. Application of grassland soil may increase AM fungal diversity in agricultural soils but this may not be reflected in AM fungal taxa colonizing wheat roots. I also show that soil compaction differently affects AM colonization in wheat varieties. Further research needs to identify which earthworm and AM fungal community properties best result in functional benefits under different conditions. Nevertheless, I show that earthworm and AM fungal communities can be promoted in various ways, individually or in combination, suggesting potential to enhance functional effects of these key organisms in agricultural soils.