Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain.

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Effects of biodiversity on ecosystem functioning: a consensus of current knowledge

Microbes are the unseen majority in soil and comprise a large portion of lifes genetic diversity. Despite their abundance, the impact of soil microbes on ecosystem processes is still poorly understood. Here we explore the various roles that soil microbes play in terrestrial ecosystems with special emphasis on their contribution to plant productivity and diversity. Soil microbes are important regulators of plant productivity, especially in nutrient poor ecosystems where plant symbionts are responsible for the acquisition of limiting nutrients. Mycorrhizal fungi and nitrogenfixing bacteria are responsible for c. 5‐20% (grassland and savannah) to 80% (temperate and boreal forests) of all nitrogen, and up to 75% of phosphorus, that is acquired by plants annually. Free-living microbes also strongly regulate plant productivity, through the mineralization of, and competition for, nutrients that sustain plant productivity. Soil microbes, including microbial pathogens, are also important regulators of plant community dynamics and plant diversity, determining plant abundance and, in some cases, facilitating invasion by exotic plants. Conservative estimates suggest that c. 20 000 plant species are completely dependent on microbial symbionts for growth and survival pointing to the importance of soil microbes as regulators of plant species richness on Earth. Overall, this review shows that soil microbes must be considered as important drivers of plant diversity and productivity in terrestrial ecosystems.

/pdf/the-unseen-majority-soil-microbes-as-drivers-of-plant-1yjo7xbywu.pdf

The unseen majority: Soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems

Evidence is mounting that the immense diversity of microorganisms and animals that live belowground contributes significantly to shaping aboveground biodiversity and the functioning of terrestrial ecosystems. Our understanding of how this belowground biodiversity is distributed, and how it regulates the structure and functioning of terrestrial ecosystems, is rapidly growing. Evidence also points to soil biodiversity as having a key role in determining the ecological and evolutionary responses of terrestrial ecosystems to current and future environmental change. Here we review recent progress and propose avenues for further research in this field.

Belowground biodiversity and ecosystem functioning

Understanding the relative abundance of species in plant communities is an unsolved problem
1,2,3,4,5,6,7,8,9,10. Mechanisms such as competition, resource partitioning5, dispersal ability10 and predation tolerance6,7,8,9 do not adequately explain relative abundance under field conditions11,12. Recent work suggests that interactions between plants and soil microbes is important13,14,15,16,17,18,19,20,21. Here I show that such interaction explains a significant proportion of the variance in the relative abundance of species in plant communities. Rare plants exhibited a relative decrease in growth on ‘home’ soil in which pathogens had had a chance to accumulate, whereas invasive plants benefited from interactions with mycorrhizal fungi. Some plant species accumulate pathogens quickly and maintain low densities as a result of the accumulation of species-specific pathogens, whereas others accumulate species-specific pathogens more slowly and do not experience negative feedback until plant densities reach high levels13,15,21. These results indicate that plants have different abilities to influence their abundance by changing the structure of their soil communities, and that this is an important regulator of plant community structure.

Feedback with soil biota contributes to plant rarity and invasiveness in communities.

The colonization of land by plants appears to have coincided with the appearance of mycorrhiza-like fungi. Over evolutionary time, fungi have maintained their prominent role in the formation of mycorrhizal associations. In addition, however, they have been able to occupy other terrestrial niches of which the decomposition of recalcitrant organic matter is perhaps the most remarkable. This implies that, in contrast to that of aquatic organic matter decomposition, bacteria have not been able to monopolize decomposition processes in terrestrial ecosystems. The emergence of fungi in terrestrial ecosystems must have had a strong impact on the evolution of terrestrial bacteria. On the one hand, potential decomposition niches, e.g. lignin degradation, have been lost for bacteria, whereas on the other hand the presence of fungi has itself created new bacterial niches. Confrontation between bacteria and fungi is ongoing, and from studying contemporary interactions, we can learn about the impact that fungi presently have, and have had in the past, on the ecology and evolution of terrestrial bacteria. In the first part of this review, the focus is on niche differentiation between soil bacteria and fungi involved in the decomposition of plant-derived organic matter. Bacteria and fungi are seen to compete for simple plant-derived substrates and have developed antagonistic strategies. For more recalcitrant organic substrates, e.g. cellulose and lignin, both competitive and mutualistic strategies appear to have evolved. In the second part of the review, bacterial niches with respect to the utilization of fungal-derived substrates are considered. Here, several lines of development can be recognized, ranging from mutualistic exudate-consuming bacteria that are associated with fungal surfaces to endosymbiotic and mycophagous bacteria. In some cases, there are indications of fungal specific selection in fungus-associated bacteria, and possible mechanisms for such selection are discussed.

Living in a fungal world: impact of fungi on soil bacterial niche development⋆

ECOLOGICAL study of the role of soil microorganisms in vegetation succession has focused mainly on organisms affecting plant nutrition, such as mycorrhiza and nitrogen-fixing bacteria. But, soil-borne diseases are involved in the degeneration of Ammophila arenaria (Marram grass) and Hippophae rhamnoides (Sea buckthorn), two plant species that dominate the coastal foredunes of Europe and are widely planted for sand stabilization. We have used reciprocal transplantation and report here that soil-borne diseases may contribute to the succession of foredune plant species. In pot experiments, plant species that succeed A. arenaria were tolerant of the soil-borne diseases of this species. Plant species that were grown in soils from both previous and later succession stages were reduced most in soils from the later stages. During foredune succession, therefore, plants disappear from sites where the soil has become colonized with specific growth-depressing microorganisms. The soil-borne diseases must have considerable importance for the outcome of interspecific competition and may be involved in patterns of clonal growth. The different sensitivities of plant species for the soil-borne pathogens could be an evolutionary response to selection pressures of the succession stage to which a species is confined by the combined effect of local abiotic and biotic environmental factors.

Plant-specific soil-borne diseases contribute to succession in foredune vegetation

Dopamine (DA) in the dorsal and ventral striatum is associated with different aspects of locomotor activity control. The ventral striatum may form an interface between the limbic system and the extrapyramidal motor system. The distribution of dopaminergic fibers in this interface position was studied in detail with a method applying antibodies against DA. Furthermore, the ultrastructural morphology of the DA fibers was examined by means of immuno-electron microscopy. The results show that DA immunoreactivity is distributed over the ventral striatum in a highly compartmentalized fashion. In the dorsal striatum few compartments were found. The DA fibers in the ventral striatum establish mainly symmetric synaptic contacts, preferably with dendritic shafts and spines. The results are discussed in relation to previous data concerning the light and electron-microscopic identification of catecholaminergic fibers in the ventral and dorsal striatum.

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The dopaminergic innervation of the ventral striatum in the rat: a light- and electron-microscopical study with antibodies against dopamine.

Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens.

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach.

https://pure.rug.nl/ws/files/3620166/2000BiochemPharmacolvDijk.pdf

The uncoupling efficiency and affinity of flavonoids for vesicles

To study the origin of replant disease of Ammophila arenaria (L.) Link the growth and development in sand originating from the rhizosphere of a natural Ammophila vegetation was compared with the growth in sand from the sea-floor. In a greenhouse experiment, the growth of Ammophila seedlings in rhizosphere sand, when compared with that in sea sand, was significantly reduced. As sterilization by means of gamma-irradiation increased the biomass production of Ammophila seedlings significantly, it was concluded that the rhizosphere sand contained biotic factors that were harmful to Ammophila. In rhizosphere sand the roots of Ammophila were brown and poorly developed, and the specific uptake rates of N, P and K were reduced. The shoot weight proportion of the total plant dry matter was hardly influenced. In an outdoor experiment with Ammophila seedlings and cuttings, using both sands, the mortality was high and the plants were feeble in rhizosphere sand whereas plants in sea sand grew vigorously. It seems plausible that the plants in rhizophere sand were dessicated because the root system was shallow and badly developed. In the greenhouse experiments, Ammophila cuttings were less sensitive to the inhibiting factors in the rhizosphere than seedlings. This was confirmed in the outdoor experiment. Calammophila baltica (Fluegge ex Schrader) Brand, however, was hardly affected by the harmful biotic factors in the greenhouse. These results are discussed with reference to the ecology of Ammophila. It is assumed that the catching of fresh windblown sand provides Ammophila with a way to escape from harmful biotic soil factors, and it was concluded that degeneration of Ammophila is caused mainly by self-intolerance due to these biotic soil factors.

C. van Dijk

Papers

Plant-specific soil-borne diseases contribute to succession in foredune vegetation

The dopaminergic innervation of the ventral striatum in the rat: a light- and electron-microscopical study with antibodies against dopamine.

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach.

The uncoupling efficiency and affinity of flavonoids for vesicles

Biotic soil factors affecting the growth and development of Ammophila arenaria.