Ilja Sonnemann

Choosing and using diversity indices: Insights for ecological applications from the German Biodiversity Exploratories

Biodiversity, a multidimensional property of natural systems, is difficult to quantify partly because of the multitude of indices proposed for this purpose. Indices aim to describe general properties of communities that allow us to compare different regions, taxa, and trophic levels. Therefore, they are of fundamental importance for environmental monitoring and conservation, although there is no consensus about which indices are more appropriate and informative. We tested several common diversity indices in a range of simple to complex statistical analyses in order to determine whether some were better suited for certain analyses than others. We used data collected around the focal plant Plantago lanceolata on 60 temperate grassland plots embedded in an agricultural landscape to explore relationships between the common diversity indices of species richness (S), Shannon’s diversity (H’), Simpson’s diversity (D1), Simpson’s dominance (D2), Simpson’s evenness (E), and Berger–Parker dominance (BP). We calculated each of these indices for herbaceous plants, arbuscular mycorrhizal fungi, aboveground arthropods, belowground insect larvae, and P. lanceolata molecular and chemical diversity. Including these trait-based measures of diversity allowed us to test whether or not they behaved similarly to the better studied species diversity. We used path analysis to determine whether compound indices detected more relationships between diversities of different organisms and traits than more basic indices. In the path models, more paths were significant when using H’, even though all models except that with E were equally reliable. This demonstrates that while common diversity indices may appear interchangeable in simple analyses, when considering complex interactions, the choice of index can profoundly alter the interpretation of results. Data mining in order to identify the index producing the most significant results should be avoided, but simultaneously considering analyses using multiple indices can provide greater insight into the interactions in a system.

Biodiversity at multiple trophic levels is needed for ecosystem multifunctionality

Many experiments have shown that loss of biodiversity reduces the capacity of ecosystems to provide the multiple services on which humans depend. However, experiments necessarily simplify the complexity of natural ecosystems and will normally control for other important drivers of ecosystem functioning, such as the environment or land use. In addition, existing studies typically focus on the diversity of single trophic groups, neglecting the fact that biodiversity loss occurs across many taxa and that the functional effects of any trophic group may depend on the abundance and diversity of others. Here we report analysis of the relationships between the species richness and abundance of nine trophic groups, including 4,600 above- and below-ground taxa, and 14 ecosystem services and functions and with their simultaneous provision (or multifunctionality) in 150 grasslands. We show that high species richness in multiple trophic groups (multitrophic richness) had stronger positive effects on ecosystem services than richness in any individual trophic group; this includes plant species richness, the most widely used measure of biodiversity. On average, three trophic groups influenced each ecosystem service, with each trophic group influencing at least one service. Multitrophic richness was particularly beneficial for ‘regulating’ and ‘cultural’ services, and for multifunctionality, whereas a change in the total abundance of species or biomass in multiple trophic groups (the multitrophic abundance) positively affected supporting services. Multitrophic richness and abundance drove ecosystem functioning as strongly as abiotic conditions and land-use intensity, extending previous experimental results to real-world ecosystems. Primary producers, herbivorous insects and microbial decomposers seem to be particularly important drivers of ecosystem functioning, as shown by the strong and frequent positive associations of their richness or abundance with multiple ecosystem services. Our results show that multitrophic richness and abundance support ecosystem functioning, and demonstrate that a focus on single groups has led to researchers to greatly underestimate the functional importance of biodiversity.

Land-use intensification causes multitrophic homogenization of grassland communities.

Land-use intensification is a major driver of biodiversity loss. Alongside reductions in local species diversity, biotic homogenization at larger spatial scales is of great concern for conservation. Biotic homogenization means a decrease in β-diversity (the compositional dissimilarity between sites). Most studies have investigated losses in local (α)-diversity and neglected biodiversity loss at larger spatial scales. Studies addressing β-diversity have focused on single or a few organism groups (for example, ref. 4), and it is thus unknown whether land-use intensification homogenizes communities at different trophic levels, above- and belowground. Here we show that even moderate increases in local land-use intensity (LUI) cause biotic homogenization across microbial, plant and animal groups, both above- and belowground, and that this is largely independent of changes in α-diversity. We analysed a unique grassland biodiversity dataset, with abundances of more than 4,000 species belonging to 12 trophic groups. LUI, and, in particular, high mowing intensity, had consistent effects on β-diversity across groups, causing a homogenization of soil microbial, fungal pathogen, plant and arthropod communities. These effects were nonlinear and the strongest declines in β-diversity occurred in the transition from extensively managed to intermediate intensity grassland. LUI tended to reduce local α-diversity in aboveground groups, whereas the α-diversity increased in belowground groups. Correlations between the β-diversity of different groups, particularly between plants and their consumers, became weaker at high LUI. This suggests a loss of specialist species and is further evidence for biotic homogenization. The consistently negative effects of LUI on landscape-scale biodiversity underscore the high value of extensively managed grasslands for conserving multitrophic biodiversity and ecosystem service provision. Indeed, biotic homogenization rather than local diversity loss could prove to be the most substantial consequence of land-use intensification.

Locally rare species influence grassland ecosystem multifunctionality

Species diversity promotes the delivery of multiple ecosystem functions (multifunctionality). However, the relative functional importance of rare and common species in driving the biodiversity–multifunctionality relationship remains unknown. We studied the relationship between the diversity of rare and common species (according to their local abundances and across nine different trophic groups), and multifunctionality indices derived from 14 ecosystem functions on 150 grasslands across a land-use intensity (LUI) gradient. The diversity of above- and below-ground rare species had opposite effects, with rare above-ground species being associated with high levels of multifunctionality, probably because their effects on different functions did not trade off against each other. Conversely, common species were only related to average, not high, levels of multifunctionality, and their functional effects declined with LUI. Apart from the community-level effects of diversity, we found significant positive associations between the abundance of individual species and multifunctionality in 6% of the species tested. Species-specific functional effects were best predicted by their response to LUI: species that declined in abundance with land use intensification were those associated with higher levels of multifunctionality. Our results highlight the importance of rare species for ecosystem multifunctionality and help guiding future conservation priorities.

Does induced resistance in plants affect the belowground community

Induced resistance (IR) is a new technology for crop protection that is assumed to be much more environmentally sound than traditional pesticides. The aim of the study presented here was to quantify potential unintended side effects of IR on ecosystem functioning by changes in the composition and performance of the soil community. Resistance was induced by applying the plant activator BION ® to barley and fallow plots. Soil biota were studied by measuring a broad range of microbiological and zoological parameters. The comparative approach used in our study shows that land-use type-specific differences in the effect of BION ® treatment are mediated by plant-specific responses to IR. BION ® treatment significantly reduced the growth of barley roots and increased root infection by the parasitic nematode Pratylenchus, but did not cause measurable changes in plant productivity, in the composition of the free-living soil biota or in root infection by mycorrhizal fungi. In the short term, therefore, we do not expect adverse effects of BION ® treatment on ecosystem functioning via changes in the belowground community. The response patterns revealed in our study might nevertheless help to explain the variability in the effectiveness of IR. Strong reduction of root biomass and selective effects on root-associated soil biota might have long-term effects on ecosystem functioning.

Response of soil microflora to changes in nematode abundance : evidence for large scale effects in grassland soil

The effect of increasing nematode abundance on microbial biomass and activity in a temperate grassland soil was investigated in a microcosm experiment. The experiment lasted for 33 days. The natural nematode diversity, as well as relevant aspects of the spatial heterogeneity of the soil microhabitat in a 80 m2 sampling area were maintained in the microcosms. No correlation was found between nematode abundance and microbial biomass (CFE) or ergosterol content (as a measure of active fungal biomass). However, a doubling of nematode abundance reduced CO2 production by 11 % and increased bacterial substrate utilization (BIOLOG) by 18 %. A possible explanation is that fungal activity was strongly reduced at higher nematode density, overcompensating the simultaneous increase in bacterial activity. The results show that the nematode community in a grassland soil is capable of causing a considerable shift in soil microbial activities towards an increased bacterial metabolism, overriding the spatial heterogeneity of the soil habitat and the taxonomic diversity of the community itself, and thereby producing functional effects relevant at spatial scales that far exceed the activity domains of the organisms involved. Beeinflussung der Bodenmikroflora durch unterschiedliche Nematodenabundanzen — Beurteilung grosraumiger Effekte in Grunlandboden In einem Mikrokosmos-Experiment wurde der Einflus steigender Nematodenabundanz auf die Mikroflora und ihre Abbauleistungen in einem Grunlandboden untersucht. Das Experiment dauerte 33 Tage. Die naturliche Diversitat der Nematoden, sowie wesentliche Parameter der naturlichen raumlichen Heterogenitat des Bodenmikrohabitats einer Flache von 80 m2 wurden in den Mikrokosmen beibehalten. Es lies sich kein Einflus der Nematoden-Abundanz auf die mikrobielle Biomasse (CFE) oder den Ergosterolgehalt (als Mas fur die aktive Pilzbiomasse) nachweisen. Bei einer Verdopplung der Nematodenabundanz wurde die CO2-Produktion um 11 % gesenkt, der bakterielle Substrat-Abbau (BIOLOG) dagegen um 18 % gesteigert. Eine mogliche Erklarung hierfur ist eine starke Reduktion der Pilzaktivitat durch hohere Nematodendichten, die die gleichzeitige Forderung der bakteriellen Aktivitat uberwiegt. Die Ergebnisse zeigen, das die Nematodengemeinschaft von Grunlandboden in der Lage ist, uber die raumliche Heterogenitat des Bodenhabitats und uber die eigene taxonomische Diversitat hinweg eine erhebliche Verschiebung der mikrobiellen Aktivitat zugunsten der Bakterienflora aufzubauen, und auf einer raumlichen Skala wirksam zu werden, die den Aktivitatsbereich der einzelnen Organismen um viele Grosenordnungen uberschreitet.

Horizontal migration of click beetle (Agriotes spp.) larvae depends on food availability

It is generally thought that soil animals face specific foraging conditions because movement through soil is highly energy consuming. The hypothesis tested here is that root‐feeding click beetle larvae (wireworms, Agriotes spp.; Coleoptera: Elateridae), to minimize energy loss, only migrate horizontally through soil when located in food‐depleted surroundings. Larvae were placed at either end of a root density gradient created by the grass Holcus lanatus L. (Poaceae), and their position in the gradient was recorded after 12 days of migration. Larval migration depended on the food situation at the starting point, with larvae moving from food‐depleted to food‐rich areas, but not leaving food‐rich areas. Larvae spread further around food‐rich areas when together with conspecifics than when being alone, presumably to avoid cannibalism. Food density‐dependent migration may have to be taken into account when using trap‐crops to control Agriotes larvae. Success may depend on the timing of trap‐crop establishment relative to the target crop to generate an effective food gradient.

The Root Herbivore History of the Soil Affects the Productivity of a Grassland Plant Community and Determines Plant Response to New Root Herbivore Attack

Insect root herbivores can alter plant community structure by affecting the competitive ability of single plants. However, their effects can be modified by the soil environment. Root herbivory itself may induce changes in the soil biota community, and it has recently been shown that these changes can affect plant growth in a subsequent season or plant generation. However, so far it is not known whether these root herbivore history effects (i) are detectable at the plant community level and/or (ii) also determine plant species and plant community responses to new root herbivore attack. The present greenhouse study determined root herbivore history effects of click beetle larvae (Elateridae, Coleoptera, genus Agriotes) in a model grassland plant community consisting of six common species (Achillea millefolium, Plantago lanceolata, Taraxacum officinale, Holcus lanatus, Poa pratensis, Trifolium repens). Root herbivore history effects were generated in a first phase of the experiment by growing the plant community in soil with or without Agriotes larvae, and investigated in a second phase by growing it again in the soils that were either Agriotes trained or not. The root herbivore history of the soil affected plant community productivity (but not composition), with communities growing in root herbivore trained soil producing more biomass than those growing in untrained soil. Additionally, it influenced the response of certain plant species to new root herbivore attack. Effects may partly be explained by herbivore-induced shifts in the community of arbuscular mycorrhizal fungi. The root herbivore history of the soil proved to be a stronger driver of plant growth on the community level than an actual root herbivore attack which did not affect plant community parameters. History effects have to be taken into account when predicting the impact of root herbivores on grasslands.

Community- Weighted Mean Plant Traits Predict Small Scale Distribution of Insect Root Herbivore Abundance

Small scale distribution of insect root herbivores may promote plant species diversity by creating patches of different herbivore pressure. However, determinants of small scale distribution of insect root herbivores, and impact of land use intensity on their small scale distribution are largely unknown. We sampled insect root herbivores and measured vegetation parameters and soil water content along transects in grasslands of different management intensity in three regions in Germany. We calculated community-weighted mean plant traits to test whether the functional plant community composition determines the small scale distribution of insect root herbivores. To analyze spatial patterns in plant species and trait composition and insect root herbivore abundance we computed Mantel correlograms. Insect root herbivores mainly comprised click beetle (Coleoptera, Elateridae) larvae (43%) in the investigated grasslands. Total insect root herbivore numbers were positively related to community-weighted mean traits indicating high plant growth rates and biomass (specific leaf area, reproductive- and vegetative plant height), and negatively related to plant traits indicating poor tissue quality (leaf C/N ratio). Generalist Elaterid larvae, when analyzed independently, were also positively related to high plant growth rates and furthermore to root dry mass, but were not related to tissue quality. Insect root herbivore numbers were not related to plant cover, plant species richness and soil water content. Plant species composition and to a lesser extent plant trait composition displayed spatial autocorrelation, which was not influenced by land use intensity. Insect root herbivore abundance was not spatially autocorrelated. We conclude that in semi-natural grasslands with a high share of generalist insect root herbivores, insect root herbivores affiliate with large, fast growing plants, presumably because of availability of high quantities of food. Affiliation of insect root herbivores with large, fast growing plants may counteract dominance of those species, thus promoting plant diversity.

Soil Macro-Invertebrates: Their Impact on Plants and Associated Aboveground Communities in Temperate Regions

Soil macro-invertebrates, the so-called soil macrofauna, belong to different functional groups such as ecosystem engineers, detritivores, root herbivores, and predators. They have often profound impacts on physical, chemical, and biological soil characteristics. Effects of soil macrofauna on plants are mediated by direct (trophic) and indirect interactions with roots. The objective of this chapter is to summarize the knowledge and identify knowledge gaps on the connection between soil macrofauna, plants, and aboveground arthropod communities. Above–belowground interaction studies involving soil macrofauna mainly focused on insect root herbivores or earthworms, while other taxa of the soil macrofauna have been widely neglected. Root feeding insect larvae can induce defense mechanisms in the whole plant, while earthworms affect plant performance mainly indirectly by changing resource availability, soil structure, and/or impacting other soil biota. The resulting systemic changes in plant traits can further affect aboveground plant interactions with herbivores and higher trophic levels. Other, little studied aspects are legacy effects of soil macrofauna that may alter future plant performance and interactions mediated by changes in soil characteristics and plant traits. Global change factors, such as climatic or land use changes, have also been shown to alter the strength of interactions between soil macrofauna and aboveground organisms. We conclude that more realistic insights in the role of soil macrofauna on plant performance and ecosystem functions could be achieved by more encompassing multispecies interactions under different climatic conditions and at different temporal and spatial scales.