Elisabeth Obermaier

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.

Selection of large host plants for oviposition by a monophagous leaf beetle: nutritional quality or enemy‐free space?

Abstract.  1. Oviposition site selection is crucial for the reproductive success of herbivorous insects. According to the preference–performance hypothesis, females should oviposit on host plants that enhance the performance of their offspring. More specifically, the plant vigour hypothesis predicts that females should prefer large and vigorously growing host plants for oviposition and that larvae should perform best on these plants.

Hide and seek on two spatial scales – vegetation structure effects herbivore oviposition and egg parasitism

Abstract Habitat parameters as well as host density and distribution play an important role in host-parasitoid interactions. Vegetation structure can affect both, herbivorous insects searching for places for egg deposition and foraging egg parasitoids. Herbivores might escape egg parasitism by depositing their eggs on sites with vegetation factors unfavourable for host searching parasitoids. The importance of a particular vegetation parameter for the host-parasitoid relationship, however, might depend on the spatial scale. In this study the influence of vegetation structure on oviposition of the polyphagous tansy leaf beetle Galeruca tanaceti L. (Coleoptera: Chrysomelidae) and on egg parasitism by the specialist egg parasitoid Oomyzus galerucivorus was investigated on two spatial scales in the field. Factors like vegetation height and coverage, presence of host plants and microclimate were measured and correlated with presence of herbivore eggs as well as incidence or rate of parasitism. On a larger scale 25 sites (patch size = 300–2000 m 2 ) of suitable semi-arid grassland in different successional stages were studied. The probability that egg clutches of G. tanaceti were present at a site increased with a decreasing area covered by shrubs. Parasitism rates on the macro-site scale showed the same trend and were also higher at sites with a lower percentage of shrubs covering the area. On a smaller scale vegetation structure was investigated using 40 patches (patch size = 1 m 2 ). While the best predictor for the presence of beetle egg masses on this scale was the complexity of vegetation, parasitism was influenced by the presence of a host plant, Achillea millefolium , and a sunny microclimate. In Wirt-Parasitoid Interaktionen spielen Wirtsdichte und -verteilung, aber auch Habitat-Parameter eine wichtige Rolle. Die Vegetationsstruktur kann sowohl herbivore Insekten auf der Suche nach Ei- ablageplatzen als auch ihre Eiparasitoide auf der Suche nach Wirten beeinflussen. Herbivore Insekten konnten einer Eiparasitierung dadurch entgehen, dass sie ihre Eier an Orte legen, die fur Parasitoide ungunstige Vegetations-Parameter aufweisen. Die Bedeutung eines Vegetations-Parameters auf die Wirt-Parasitoid Beziehung kann von der raumlichen Skala abhangen, die untersucht wird. Diese Arbeit untersucht den Einfluss der Vegetationsstruktur auf die Eiablage des polyphagen Rainfarnblattkafers Galeruca tanaceti L. (Coleoptera: Chrysomelidae) und auf die Parasitierung durch den spezialisierten Ei-Parasitoiden Oomyzus galerucivorus auf zwei raumlichen Skalen im Freiland. Verschiedene Vegetations-Parameter wie die Vegetationshohe, der Deckungsgrad, das Wirtspflanzenvorkommen und das Mikroklima wurden gemessen und mit dem Vorhandensein von Eigelegen und Parasitierung korreliert. Auf der grosflachigen Skala wurden 25 Flachen (Grose = 300–2000 m 2 ) semiariden Grasslands in verschiedenen Sukzessionsstadien untersucht. Die Wahrscheinlichkeit, dass Eigelege von G. tanaceti auf einer Flache gefunden wurden nahm mit sinkendem Verbuschungsgrad zu. Die Parasitierungsrate auf der grosflachigen Skala nahm ebenfalls mit sinkendem Verbuschungsgrad zu. Kleinflachig wurde die Vegetationsstruktur in 40 Quadraten (Grose = 1 m 2 ) untersucht. Wahrend die Eiablage des Kafers am starksten von der Komplexitat der Vegetation beeinflusst wurde, wurde die Parasitierungsinzidenz durch das Vorkommen der Wirtspflanze Achillea millefolium und durch ein sonniges Mikroklima bestimmt.

Mother's choice of the oviposition site: balancing risk of egg parasitism and need of food supply for the progeny with an infochemical shelter

Summary.Oviposition site selection of herbivorous insects depends primarily on host plant presence which is essential for offspring survival. However, parasitoids can exploit host plant cues for host location. In this study, we hypothesised that herbivores can solve this dilemma by ovipositing within high plant diversity. A diverse plant species composition might represent an ‘infochemical shelter’, as a potentially complex volatile blend can negatively affect the host location ability of parasitoids. We examined this exemplarily for the egg-laying response of the generalist leaf beetle, Galeruca tanaceti, in relation to (1) host plant availability and (2) plant species diversity in the field. Further, we investigated the effect of odours from mixed plant species compositions on (3) leaf beetle oviposition site selection and on (4) the orientation of its specialised egg parasitoid, Oomyzus galerucivorus. In the field, egg clutch occurrence was positively related to the presence and quantity of two major host plants, Achillea millefolium (yarrow) and Centaurea jacea, and to the number of herbaceous plant species. In two-choice bioassays, female beetles oviposited more frequently on sites surrounded by an odour blend from a diverse plant species composition (including yarrow) than on sites with a pure grass odour blend. In the presence of yarrow odour and an odour blend from a diverse plant mixture (including yarrow) no difference in the oviposition response was recorded. Experienced parasitoid females were attracted to yarrow odours, but showed no response when yarrow odours were offered simultaneously with odours of a non-host plant. In conclusion, it could be shown in laboratory bioassays that the parasitoid responds only to pure host plant odours but not to complex odour blends. In contrast, the herbivore prefers to oviposit within diverse vegetation in the field and in the laboratory. However, the laboratory results also point to a priority of host plant availability over the selection of a potential ‘infochemical shelter’ for oviposition due to high plant diversity.

Olfactory versus Contact Cues in Host Plant Recognition of a Monophagous Chrysomelid Beetle

The importance of olfactory versus contact cues for host plant recognition was investigated in the tortoise beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae), which is strictly monophagous on meadow sage. The reaction of adult beetles to olfactory and contact host cues was tested using three bioassays (locomotion compensator, six-chamber-olfactometer, ‘stem arena’) to account for different behavioral contexts. Bioassay-guided fractionation of plant extracts was elaborated to characterize the nature of contact stimuli. The beetles were only slightly attracted to odors from small amounts of leaf material. However, when contact cues were provided additionally, the beetles showed strong preferences for samples of their host plant over controls. Bioassay-guided fractionation led to isolation of at least two non-polar contact stimuli acting in concert that are sufficient for host plant identification in C. canaliculata.

Connectivity counts: disentangling effects of vegetation structure elements on the searching movement of a parasitoid

1. A heterogeneous habitat structure can have a profound impact on foraging carnivorous arthropods. In the present study, we examined which elements of complex vegetation structure influence the searching movement of a parasitoid model organism.

Habitats as Complex Odour Environments: How Does Plant Diversity Affect Herbivore and Parasitoid Orientation?

Plant diversity is known to affect success of host location by pest insects, but its effect on olfactory orientation of non-pest insect species has hardly been addressed. First, we tested in laboratory experiments the hypothesis that non-host plants, which increase odour complexity in habitats, affect the host location ability of herbivores and parasitoids. Furthermore, we recorded field data of plant diversity in addition to herbivore and parasitoid abundance at 77 grassland sites in three different regions in Germany in order to elucidate whether our laboratory results reflect the field situation. As a model system we used the herb Plantago lanceolata, the herbivorous weevil Mecinus pascuorum, and its larval parasitoid Mesopolobus incultus. The laboratory bioassays revealed that both the herbivorous weevil and its larval parasitoid can locate their host plant and host via olfactory cues even in the presence of non-host odour. In a newly established two-circle olfactometer, the weeviĺs capability to detect host plant odour was not affected by odours from non-host plants. However, addition of non-host plant odours to host plant odour enhanced the weeviĺs foraging activity. The parasitoid was attracted by a combination of host plant and host volatiles in both the absence and presence of non-host plant volatiles in a Y-tube olfactometer. In dual choice tests the parasitoid preferred the blend of host plant and host volatiles over its combination with non-host plant volatiles. In the field, no indication was found that high plant diversity disturbs host (plant) location by the weevil and its parasitoid. In contrast, plant diversity was positively correlated with weevil abundance, whereas parasitoid abundance was independent of plant diversity. Therefore, we conclude that weevils and parasitoids showed the sensory capacity to successfully cope with complex vegetation odours when searching for hosts.

Land use and host neighbor identity effects on arbuscular mycorrhizal fungal community composition in focal plant rhizosphere

Arbuscular mycorrhizal fungi (AMF) provide a number of ecosystem services as important members of the soil microbial community. Increasing evidence suggests AMF diversity is at least partially controlled by the identities of plants in the host plant neighborhood. However, much of this evidence comes from greenhouse studies or work in invaded systems dominated by single plant species, and has not been tested in species-rich grasslands. We worked in 67 grasslands spread across the three German Biodiversity Exploratories that are managed primarily as pastures and meadows, and collected data on AMF colonization, AMF richness, AMF community composition, plant diversity, and land use around focal Plantagolanceolata plants. We analyzed the data collected within each Exploratory (ALB Schwäbische Alb, HAI Hainich-Dün, SCH Schorfheide-Chorin) separately, and used variance partitioning to quantify the contribution of land use, host plant neighborhood, and spatial arrangement to the effect on AMF community composition. We performed canonical correspondence analysis to quantify the effect of each factor independently by removing the variation explained by the other factors. AMF colonization declined with increasing land use intensity (LUI) along with concurrent increases in non-AMF, suggesting that the ability of AMF to provide protection from pathogens declined under high LUI. In ALB and HAI mowing frequency and percent cover of additional P. lanceolata in the host plant neighborhood were important for AMF community composition. The similar proportional contribution of land use and host neighborhood to AMF community composition in a focal plant rhizosphere suggests that the diversity of this important group of soil microbes is similarly sensitive to changes at large and small scales.

Enemies in low places - insects avoid winter mortality and egg parasitism by modulating oviposition height

Oviposition site selection in insects is essential in terms of low egg mortality, high offspring survival and therefore a high reproductive output. Although oviposition height could be a crucial factor for the fitness of overwintering eggs, it has rarely been investigated. In this study the oviposition height of a polyphagous leaf beetle, Galeruca tanaceti Linnaeus in different habitats and at different times of the season was examined and its effect on egg clutch mortality was recorded. The leaf beetle occurs as an occasional pest on several agricultural plants. It deposits its eggs within herbaceous vegetation in autumn. Eggs are exposed to numerous biotic and abiotic mortality factors summarized as egg parasitism and winter mortality. Oviposition height of the leaf beetle was not uniform, but changed significantly with the structure of the habitat and during the season. Mean oviposition height per site (70.2+/-4.9 cm) was significantly higher than mean vegetation height (28.4+/-2.4 cm). Height of plants with egg clutches attached and oviposition height were significantly positively correlated. The results suggest that females try to oviposit as high as possible in the vegetation and on the plants selected. In accordance with this, the probability of egg parasitism and of winter egg clutch mortality significantly declined with increasing oviposition height. A preference of G. tanaceti for oviposition sites high up in the vegetation might therefore have evolved due to selection pressures by parasitoids and winter mortality.

Effect of vegetation density, height, and connectivity on the oviposition pattern of the leaf beetle Galeruca tanaceti

Vegetation structure can profoundly influence patterns of abundance, distribution, and reproduction of herbivorous insects and their susceptibility to natural enemies. The three main structural traits of herbaceous vegetation are density, height, and connectivity. This study determined the herbivore response to each of these three parameters by analysing oviposition patterns in the field and studying the underlying mechanisms in laboratory bioassays. The generalist leaf beetle, Galeruca tanaceti L. (Coleoptera: Chrysomelidae), preferentially deposits its egg clutches on non‐host plants such as grasses. Earlier studies revealed that oviposition within structurally complex vegetation reduces the risk of egg parasitism. Consequently, leaf beetle females should prefer patches with dense, tall, or connected vegetation for oviposition in order to increase their reproductive success. In the present study, we tested the following three hypotheses on the effect of stem density, height, and connectivity on oviposition: (1) Within habitats, the number of egg clutches in areas with high stem densities is disproportionately higher than in low‐density areas. The number of egg clutches on (2) tall stems or (3) in vegetation with high connectivity is higher than expected for a random distribution. In the field, stem density and height were positively correlated with egg clutch presence. Moreover, a disproportionately high presence of egg clutches was determined in patches with high stem densities. Stem height had a positive influence on oviposition, also in a laboratory two‐choice bioassay, whereas stem density and connectivity did not affect oviposition preferences in the laboratory. Therefore, stem height and, potentially, density, but not connectivity, seem to trigger oviposition site selection of the herbivore. This study made evident that certain, but not all traits of the vegetation structure can impose a strong influence on oviposition patterns of herbivorous insects. The results were finally compared with data on the movement patterns of the specialised egg parasitoid of the herbivore in comparable types of vegetation structure.