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How diversity of life is generated, maintained, and distributed across space and time is the central question of community ecology. Communities are shaped by three assembly processes: (I) dispersal, (II) environ-mental, and (III) interaction filtering. Heterogeneity in environmental conditions can alter these filtering processes, as it increases the available niche space, spatially partitions the resources, but also reduces the effective area available for individual species. Ultimately, heterogeneity thus shapes diversity. However, it is still unclear under which conditions heterogeneity has positive effects on diversity and under which condi-tions it has negative or no effects at all. In my thesis, I investigate how environmental heterogeneity affects the assembly and diversity of diverse species groups and whether these effects are mediated by species traits.
In Chapter II, I first examine how much functional traits might inform about environmental filtering pro-cesses. Specifically, I examine to which extent body size and colour lightness, both of which are thought to reflect the species thermal preference, shape the distribution and abundance of two moth families along elevation. The results show, that assemblages of noctuid moths are more strongly driven by abiotic filters (elevation) and thus form distinct patterns in colour lightness and body size, while geometrid moths are driven by biotic filters (habitat availability), and show no decline in body size nor colour lightness along elevation. Thus, one and the same functional trait can have quite different effects on community assembly even between closely related taxonomic groups.
In Chapter III, I elucidate how traits shift the relative importance of dispersal and environmental filtering in determining beta diversity between forests. Environmental filtering via forest heterogeneity had on aver-age higher independent effects than dispersal filtering within and among regions, suggesting that forest heterogeneity determines species turnover even at country-wide extents. However, the relative importance of dispersal filtering increased with decreasing dispersal ability of the species group. From the aspects of forest heterogeneity covered, variations in herb or tree species composition had overall stronger influence on the turnover of species than forest physiognomy. Again, this ratio was influenced by species traits, namely trophic position, and body size, which highlights the importance of ecological properties of a taxo-nomic group in community assembly.
In Chapter IV, I assess whether such ecological properties ultimately determine the level of heterogeneity which maximizes species richness. Here, I considered several facets of heterogeneity in forests. Though the single facets of heterogeneity affected diverse species groups both in positive and negative ways, we could not identify any generalizable mechanism based on dispersal nor the trophic position of the species group which would dissolve these complex relationships.
In Chapter V, I examine the effect of environmental heterogeneity of the diversity of traits itself to evalu-ate, whether the effects of environmental heterogeneity on species richness are truly based on increases in the number of niches. The results revealed that positive effects of heterogeneity on species richness are not necessarily based on an increased number of niches alone, but proposedly also on a spatially partition of resources or sheltering effects. While ecological diversity increased overall, there were also negative trends which indicate filtering effects via heterogeneity.
In Chapter VI, I present novel methods in measuring plot-wise heterogeneity of forests across continental scales via Satellites. The study compares the performance of Sentinel-1 and LiDar-derived measurements in depicting forest structures and heterogeneity and to their predictive power in modelling diversity. Senti-nel-1 could match the performance of Lidar and shows high potential to assess free yet detailed infor-mation about forest structures in temporal resolutions for modelling the diversity of species.
Overall, my thesis supports the notion that heterogeneity in environmental conditions is an important driv-er of beta-diversity, species richness, and ecological diversity. However, I could not identify any general-izable mechanism which direction and form this effect will have.
Modern agriculture is the basis of human existence, a blessing, but also a curse. It provides nourishment and well-being to the ever-growing human population, yet destroys biodiversity-mediated processes that underpin productivity: ecosystem services such as water filtration, pollination and biological pest control. Ecological intensification is a promising alternative to conventional farming, and aims to sustain yield and ecosystem health by actively managing biodiversity and essential ecosystem services. Here, I investigate opportunities and obstacles for ecological intensification. My research focuses on 1) the relative importance of soil, management and landscape variables for biodiversity and wheat yield (Chapter II); 2) the influence of multi-scale landscape-level crop diversity on biological pest control in wheat (Chapter III) and 3) on overall and functional bird diversity (Chapter IV). I conclude 4) by introducing a guide that helps scientists to increase research impact by acknowledging the role of stakeholder engagement for the successful implementation of ecological intensification (Chapter V).
Ecological intensification relies on the identification of natural pathways that are able to sustain current yields. Here, we crossed an observational field study of arthropod pests and natural enemies in 28 real-life wheat systems with an orthogonal on-field insecticide-fertilizer experiment. Using path analysis, we quantified the effect of 34 factors (soil characteristics, recent and historic crop management, landscape heterogeneity) that directly or indirectly (via predator-prey interactions) contribute to winter wheat yield. Reduced soil preparation and high crop rotation diversity enhanced crop productivity independent of external agrochemical inputs. Concurrently, biological control by arthropod natural enemies could be restored by decreasing average field sizes on the landscape scale, extending crop rotations and reducing soil disturbance. Furthermore, reductions in agrochemical inputs decreased pest abundances, thereby facilitating yield quality.
Landscape-level crop diversity is a promising tool for ecological intensification. However, biodiversity enhancement via diversification measures does not always translate into agricultural benefits due to antagonistic species interactions (intraguild predation). Additionally, positive effects of crop diversity on biological control may be masked by inappropriate study scales or correlations with other landscape variables (e.g. seminatural habitat). Therefore, the multiscale and context-dependent impact of crop diversity on biodiversity and ecosystem services is ambiguous. In 18 winter wheat fields along a crop diversity gradient, insect- and bird-mediated pest control was assessed using a natural enemy exclusion experiment with cereal grain aphids. Although birds did not influence the strength of insect-mediated pest control, crop diversity (rather than seminatural habitat cover) enhanced aphid regulation by up to 33%, particularly on small spatial scales. Crop diversification, an important Greening measure in the European Common Agricultural Policy, can improve biological control, and could lower dependence on insecticides, if the functional identity of crops is taken into account. Simple measures such as ‘effective number of crop types’ help in science communication.
Although avian pest control did not respond to landscape-level crop diversity, birds may still benefit from increased crop resources in the landscape, depending on their functional grouping (feeding guild, conservation status, habitat preference, nesting behaviour). Observational studies of bird functional diversity on 14 wheat study fields showed that non-crop landscape heterogeneity rather than crop diversity played a key role in determining the richness of all birds. Insect-feeding, non-farmland and non-threatened birds increased across multiple spatial scales (up to 3000 m). Only crop-nesting farmland birds declined in heterogeneous landscapes. Thus, crop diversification may be less suitable for conserving avian diversity, but abundant species benefit from overall habitat heterogeneity. Specialist farmland birds may require more targeted management approaches.
Identifying ecological pathways that favour biodiversity and ecosystem services provides opportunities for ecological intensification that increase the likelihood of balancing conservation and productivity goals. However, change towards a more sustainable agriculture will be slow to come if research findings are not implemented on a global scale. During dissemination activities within the EU project Liberation, I gathered information on the advantages and shortcomings of ecological intensification and its implementation. Here, I introduce a guide (‘TREE’) aimed at scientists that want to increase the impact of their research. TREE emphasizes the need to engage with stakeholders throughout the planning and research process, and actively seek and promote science dissemination and knowledge implementation. This idea requires scientists to leave their comfort zone and consider socioeconomic, practical and legal aspects often ignored in classical research.
Ecological intensification is a valuable instrument for sustainable agriculture. Here, I identified new pathways that facilitate ecological intensification. Soil quality, disturbance levels and spatial or temporal crop diversification showed strong positive correlations with natural enemies, biological pest control and yield, thereby lowering the dependence on agrochemical inputs. Differences between functional groups caused opposing, scale-specific responses to landscape variables. Opposed to our predictions, birds did not disturb insect-mediated pest control in our study system, nor did avian richness relate to landscape-level crop diversity. However, dominant functional bird groups increased with non-crop landscape heterogeneity. These findings highlight the value of combining different on-field and landscape approaches to ecological intensification. Concurrently, the success of ecological intensification can be increased by involving stakeholders throughout the research process. This increases the quality of science and reduces the chance of experiencing unscalable obstacles to implementation.
Endophytes live in partial symbiosis inside a plant and have been detected in all tested plants. They belong to the group of fungi or bacteria and their ecological function is mostly unknown. The fungal endophytes of the genus Epichloë belong to a special group of endophytes. Epichloë endophytes live symbiotically inside cool season grass species and some of them are able to produce alkaloids toxic to vertebrates and insects. Their symbiosis is seen as mutualistic for the following reasons: the fungus provides the plant herbivore resistance by producing alkaloids, and it increases the plant’s drought tolerance as well as its biomass production. In return, the grass provides the fungus shelter, nutrients and dispersal. Epichloë endophytes are host specific and the ability to produce alkaloids differs between species. In order to estimate intoxication risks in grasslands, it is necessary to detect infection rates of different grass species with Epichloë endophytes, and to determine the genotypes and chemotypes of the Epichloë species as well as the produced alkaloid concentrations. Factors like land-use intensity or season may have an influence on infection rates and alkaloid concentrations. Also, different methodological approaches may lead to different results. In this doctoral thesis my general aim was to evaluate intoxication risks in German grasslands caused by Epichloë endophytes. For that I investigated infection rates of different grass species and the genotypes and chemotypes of their Epichloë endophytes in German grasslands (Chapter II). Furthermore, I compared alkaloid concentrations detected with dry and fresh plant weight and different analytical methods. I also detected possible changes on the influence of season or land-use intensity (Chapter III). Additionally, I examined infections with Epichloë endophytes and alkaloid concentrations in commercially available grass seed mixtures and determined how that influences the intoxication risk of grazing animals in Europe (Chapter IV).
It is of agricultural interest to estimate intoxication risks for grazing livestock on German grasslands due to Epichloë infected grass species. Therefore, it is important to investigate which grasses are infected with the Epichloë endophyte, if the endophytes have the ability to produce vertebrate and invertebrate toxic alkaloids and if the alkaloids are indeed produced. I showed that Epichloë festucae var. lolii infecting agriculturally important Lolium perenne lacked the starting gene for ergovaline biosynthesis. Hence, vertebrate toxic ergovaline was not detected in the majority of the collected L. perenne plants. The detection of alkaloid concentrations is an important tool to estimate intoxication risk for vertebrates, but also invertebrates. My studies showed that the usage of dry plant material is crucial to quantify the correct alkaloid concentrations, and that alkaloid concentrations can vary depending on the detection method. Hence, the usage of validated, similar detection methods is important to be able to compare alkaloid concentrations from different studies. Nevertheless, the trends of seasonal changes and the influence of land-use intensity stayed the same, regardless if dry or fresh plant weight was used. Also, alkaloid concentrations were below toxicity thresholds on population level, regardless of the method used. Two commercially available forage grass and two commercially available turf grass seed mixtures were infected with Epichloë endopyhtes and alkaloids were detected. This might contribute to the spreading of Epichloë endopyhtes in Germany, therefore seed mixtures should be tested for Epichloë infections. My results indicate that the intoxication risk is generally low in Germany at the moment, although that might change due to climate change, an increase of monocultural land-use, or the seeding of Epichloë infected grass seeds.
My dissertation comprises three studies: (1) an assessment of honey bee colony losses in the USA between 2014 and 2015, (2) an exploration of the potential of reclaimed sand mines as bee habitat, and (3) an evaluation of native and non-native pollinator friendly plants in regard to their attraction to bees. While the first study focuses on honey bees, the latter two studies primarily take wild bees or entire bee communities in focus.
The study on honey bee colony losses was conducted within the framework of the Bee Informed Partnership (BIP, beeinformed.org) and aligns with the annual colony loss surveys which have been conducted in the USA since the winter of 2006/2007. It was the fourth year for which summer and annual losses were calculated in addition to winter losses. Among participants, backyard beekeepers were the largest group (n = 5690), although sideline (n = 169) and commercial (n = 78) beekeepers managed the majority (91.7 %) of the 414 267 surveyed colonies. Overall, 15.1 % of the estimated 2.74 million managed colonies in the USA were included in the study. Total honey bee colony losses (based on the entirety of included colonies) were higher in summer (25.3 %) than in winter (22.3 %) and amounted to 40.6 % for the entire 2014/2015 beekeeping year. Average colony losses per beekeeper or operation were higher in winter (43.7 %) than in summer (14.7 %) and amounted to 49 % for the entire 2014/2015 beekeeping year. Due to the dominance of backyard beekeepers among participants, average losses per operation (or unweighted loss) stronger reflected this smaller type of beekeeper. Backyard beekeepers mainly named colony management issues (e.g., starvation, weak colony in the fall) as causes for mortality, while sideline and commercial beekeepers stronger emphasized parasites or factors outside their control (e.g., varroa, nosema, queen failure).
The second study took place at reclaimed sand mines. Sand mines represent anthropogenically impacted habitats found worldwide, which bear potential for bee conservation. Although floral resources can be limited at these habitats, vegetation free patches of open sandy soils and embankments may offer good nesting possibilities for sand restricted and other bees. We compared bee communities as found in three reclaimed sand mines and at adjacent roadside meadows in Maryland, USA, over two years. Both sand mines and roadsides hosted diverse bee communities with 111 and 88 bee species, respectively. Bee abundances as well as richness and Shannon diversity of bee species were higher in sand mines than at roadsides and negatively correlated with the percentage of vegetational ground cover. Species composition also differed significantly between habitats. Sand mines hosted a higher proportion of ground nesters, more uncommon and more ‘sand loving’ bees similar to natural sandy areas of Maryland. Despite the destruction of the original pre-mining habitat, sand mines thus appear to represent a unique habitat for wild bees, particularly when natural vegetation and open sand spots are encouraged. Considering habitat loss, the lack of natural disturbance regimes, and ongoing declines of wild bees, sand mines could add promising opportunities for bee conservation which has hitherto mainly focused on agricultural and urban habitats.
The third study was an experimental field study on pollinator friendly plants. Bees rely on the pollen and nectar of plants as their food source. Therefore, pollinator friendly plantings are often used for habitat enhancements in bee conservation. Non-native pollinator friendly plants may aid in bee conservation efforts, but have not been tested and compared with native pollinator friendly plants in a common garden experiment. In this study, we seeded mixes of 20 native and 20 non-native pollinator friendly plants in two separate plots at three sites in Maryland, USA. For two years, we recorded flower visitors to the plants throughout the blooming period and additionally sampled bees with pan traps. A total of 3744 bees (120 species) were sampled in the study. Of these, 1708 bees (72 species) were hand netted directly from flowers for comparisons between native and non-native plants. Depending on the season, bee abundance and species richness was either similar or lower (early season and for richness also late season) at native plots compared to non-native plots. Additionally, the overall bee community composition differed significantly between native and non-native plots. Furthermore, native plants were associated with more specialized plant-bee visitation networks compared to non-native plants. In general, visitation networks were more specialized in the early season than the later seasons. Four species (Bombus impatiens, Halictus poeyi/ligatus, Lasioglossum pilosum, and Xylocopa virginica) out of the five most abundant bee species (also including Apis mellifera) foraged more specialized on native than non-native plants. Our study showed that non-native plants were well accepted by a diverse bee community and had a similar to higher attraction for bees compared to native plants. However, we also demonstrated alterations in foraging behavior, bee community assemblage, and visitation networks. As long as used with caution, non-native plants can be a useful addition to native pollinator friendly plantings. This study gives a first example of a direct comparison between native and non-native pollinator friendly plants.
Forest biodiversity conservation requires precise, area-wide information on the abundance and distribution of key habitat structures at multiple spatial scales. We combined airborne laser scanning (ALS) data with color-infrared (CIR) aerial imagery for identifying individual tree characteristics and quantifying multi-scale habitat requirements using the example of the three-toed woodpecker (Picoides tridactylus) (TTW) in the Bavarian Forest National Park (Germany). This bird, a keystone species of boreal and mountainous forests, is highly reliant on bark beetles dwelling in dead or dying trees. While previous studies showed a positive relationship between the TTW presence and the amount of deadwood as a limiting resource, we hypothesized a unimodal response with a negative effect of very high deadwood amounts and tested for effects of substrate quality. Based on 104 woodpecker presence or absence locations, habitat selection was modelled at four spatial scales reflecting different woodpecker home range sizes. The abundance of standing dead trees was the most important predictor, with an increase in the probability of TTW occurrence up to a threshold of 44–50 dead trees per hectare, followed by a decrease in the probability of occurrence. A positive relationship with the deadwood crown size indicated the importance of fresh deadwood. Remote sensing data allowed both an area-wide prediction of species occurrence and the derivation of ecological threshold values for deadwood quality and quantity for more informed conservation management.
I. Timing is a crucial feature in organisms that live within a variable and changing environment. Complex mechanisms to measure time are wide-spread and were shown to exist in many taxa. These mechanisms are expected to provide fitness benefits by enabling organisms to anticipate environmental changes and adapt accordingly. However, very few studies have addressed the adaptive value of proper timing. The objective of this PhD-project was to investigate mechanisms and fitness consequences of timing decisions concerning colony phenology and foraging activity in the honey bee (Apis mellifera), a social insect species with a high degree of social organization and one of the most important pollinators of wild plants and crops. In chapter II, a study is presented that aimed to identify the consequences of disrupted synchrony between colony phenology and the local environment by manipulating the timing of brood onset after hibernation. In a follow-up experiment, the importance of environmental factors for the timing of brood onset was investigated to assess the potential of climate change to disrupt synchronization of colony phenology (Chapter III). Chapter IV aimed to prove for the first time that honey bees can use interval time-place learning to improve foraging activity in a variable environment. Chapter V investigates the fitness benefits of information exchange between nest mates via waggle dance communication about a resource environment that is heterogeneous in space and time.
II. In the study presented in chapter II, the importance of the timing of brood onset after hibernation as critical point in honey bee colony phenology in temperate zones was investigated. Honey bee colonies were overwintered at two climatically different sites. By translocating colonies from each site to the other in late winter, timing of brood onset was manipulated and consequently colony phenology was desynchronized with the local environment. Delaying colony phenology in respect to the local environment decreased the capability of colonies to exploit the abundant spring bloom. Early brood onset, on the other hand, increased the loads of the brood parasite Varroa destructor later in the season with negative impact on colony worker population size. This indicates a timing related trade-off and illustrates the importance of investigating effects of climate change on complex multi-trophic systems. It can be concluded that timing of brood onset in honey bees is an important fitness relevant step for colony phenology that is highly sensitive to climatic conditions in late winter. Further, phenology shifts and mismatches driven by climate change can have severe fitness consequences.
III. In chapter III, I assess the importance of the environmental factors ambient temperature and photoperiod as well as elapsed time on the timing of brood onset. Twenty-four hibernating honey bee colonies were placed into environmental chambers and allocated to different combinations of two temperature regimes and three different light regimes. Brood onset was identified non-invasively by tracking comb temperature within the winter cluster. The experiment revealed that ambient temperature plays a major role in the timing of brood onset, but the response of honey bee colonies to temperature increases is modified by photoperiod. Further, the data indicate the involvement of an internal clock. I conclude that the timing of brood onset is complex but probably highly susceptible to climate change and especially spells of warm weather in winter.
IV. In chapter IV, it was examined if honey bees are capable of interval time-place learning and if this ability improves foraging efficiency in a dynamic resource environment. In a field experiment with artificial feeders, foragers were able to learn time intervals and use this ability to anticipate time periods during which feeders were active. Further, interval time-place learning enabled foragers to increase nectar uptake rates. It was concluded that interval time-place learning can help honey bee foragers to adapt to the complex and variable temporal patterns of floral resource environments.
V. The study presented in chapter V identified the importance of the honey bee waggle dance communication for the spatiotemporal coordination of honey bee foraging activity in resource environments that can vary from day to day. Consequences of disrupting the instructional component of honey bee dance communication were investigated in eight temperate zone landscapes with different levels of spatiotemporal complexity. While nectar uptake of colonies was not affected, waggle dance communication significantly benefitted pollen harvest irrespective of landscape complexity. I suggest that this is explained by the fact that honey bees prefer to forage pollen in semi-natural habitats, which provide diverse resource species but are sparse and presumably hard to find in intensively managed agricultural landscapes. I conclude that waggle dance communication helps to ensure a sufficient and diverse pollen diet which is crucial for honey bee colony health.
VI. In my PhD-project, I could show that honey bee colonies are able to adapt their activities to a seasonally and daily changing environment, which affects resource uptake, colony development, colony health and ultimately colony fitness. Ongoing global change, however, puts timing in honey bee colonies at risk. Climate change has the potential to cause mismatches with the local resource environment. Intensivation of agricultural management with decreased resource diversity and short resource peaks in spring followed by distinctive gaps increases the probability of mismatches. Even the highly efficient foraging system of honey bees might not ensure a sufficiently diverse and healthy diet in such an environment. The global introduction of the parasitic mite V. destructor and the increased exposure to pesticides in intensively managed landscapes further degrades honey bee colony health. This might lead to reduced cognitive capabilities in workers and impact the communication and social organization in colonies, thereby undermining the ability of honey bee colonies to adapt to their environment.
The present work investigates the influence of environmental stimuli on the building behavior of workers of the leaf-cutting ant Atta vollenweideri. It focuses on cues related to the airflow-driven ventilation of their giant underground nests, i.e., air movements and their direction, carbon dioxide concentrations and humidity levels of the nest air. First, it is shown that workers are able to use airflow and its direction as learned orientation cue by performing learning experiments with individual foragers using a classical conditioning paradigm. This ability is expected to allow workers to also navigate inside the nest tunnels using the prevailing airflow directions for orientation, for example during tasks related to nest construction and climate control.
Furthermore, the influence of carbon dioxide on the digging behavior of workers is investigated. While elevated CO2 levels hardly affect the digging rate of the ants, workers prefer to excavate at locations with lower concentrations and avoid higher CO2 levels when given a choice. Under natural conditions, shifting their digging activity to soil layers containing lower carbon dioxide levels might help colonies to excavate new or to broaden existing nest openings, if the CO2 concentration in the underground rises.
It is also shown that workers preferably transport excavated soil along tunnels containing high CO2 concentrations, when carbon dioxide levels in the underground are elevated as well. In addition, workers prefer to carry soil pellets along outflow tunnels instead of inflow tunnels, at least for high humidity levels of the air. The material transported along tunnels providing outflow of CO2-rich air might be used by workers for the construction of ventilation turrets on top of the nest mound, which is expected to promote the wind-induced ventilation and the removal of carbon dioxide from the underground.
The climatic conditions inside the nest tunnels also influence the structural features of the turrets constructed by workers on top the nest. While airflow and humidity have no effect on turret structure, outflow of CO2-rich air from the nest causes workers to construct turrets with additional openings and increased aperture, potentially enhancing the airflow-driven gas exchanges within the nest.
Finally, the effect of airflow and ventilation turrets on the gas exchanges in Atta vollenweideri nests is tested experimentally on a physical model of a small nest consisting of a single chamber and two nest tunnels. The carbon dioxide clearance rate from the underground was measured depending on both the presence of airflow in the nest and the structural features of the built turrets. Carbon dioxide is removed faster from the physical nest model when air moves through the nest, confirming the contribution of wind-induced flow inside the nest tunnels to the ventilation of Atta vollenweideri nests. In addition, turrets placed on top of one of the tunnel openings of the nest further enhance the CO2 clearance rate and the effect is positively correlated with turret aperture.
Taken together, climatic variables like airflow, carbon dioxide and humidity levels strongly affect the building responses of Atta vollenweideri leaf-cutting ants. Workers use these environmental stimuli as orientation cue in the nest during tasks related to excavation, soil transport and turret construction. Although the effects of these building responses on the microclimatic conditions inside the nest remain elusive so far, the described behaviors are expected to allow ant colonies to restore and maintain a proper nest climate in the underground.
Plant-associated fungi can affect the plants‘ interaction with herbivores and
other microorganisms. For example, many common forage grasses are infected
with Epichloë endophytes. The endophytes systemically colonize the aerial
parts of the plants. They produce bioprotective alkaloids that can negatively
affect insects and livestock feeding on the grasses, and interact with other
fungal species which living from the plants‘ nutrients. Environmental conditions
strongly influence Epichloë endophytes. Endophyte-mediated effects
on herbivores are more pronounced under increased temperatures and the
endophytes may benefit from land use in managed grasslands. Under the
framework of the large-scale German project “Biodiversity Exploratories”, I
investigated whether infection rates and alkaloid concentrations of Epichloë
festucae var. lolii in Lolium perenne (Chapter I) and Epichloë endophytes (E.
uncinata, E. siegelii) in Festuca pratensis (Chapter II) depend on land use and
season. Further I analysed, whether foliar fungal assemblages of L. perenne
are affected by the presence of Epichloë endophytes (Chapter IV).
Solitary bees in seasonal environments have to align their life-cycles with favorable environmental conditions and resources. Therefore, a proper timing of their seasonal activity is highly fitness relevant. Most species in temperate environments use temperature as a trigger for the timing of their seasonal activity. Hence, global warming can disrupt mutualistic interactions between solitary bees and plants if increasing temperatures differently change the timing of interaction partners. The objective of this dissertation was to investigate the mechanisms of timing in spring-emerging solitary bees as well as the resulting fitness consequences if temporal mismatches with their host plants should occur. In my experiments, I focused on spring-emerging solitary bees of the genus Osmia and thereby mainly on O. cornuta and O. bicornis (in one study which is presented in Chapter IV, I additionally investigated a third species: O. brevicornis).
Chapter II presents a study in which I investigated different triggers solitary bees are using to time their emergence in spring. In a climate chamber experiment I investigated the relationship between overwintering temperature, body size, body weight and emergence date. In addition, I developed a simple mechanistic model that allowed me to unite my different observations in a consistent framework. In combination with the empirical data, the model strongly suggests that solitary bees follow a strategic approach and emerge at a date that is most profitable for their individual fitness expectations. I have shown that this date is on the one hand temperature dependent as warmer overwintering temperatures increase the weight loss of bees during hibernation, which then advances their optimal emergence date to an earlier time point (due to an earlier benefit from the emergence event). On the other hand I have also shown that the optimal emergence date depends on the individual body size (or body weight) as bees adjust their emergence date accordingly. My data show that it is not enough to solely investigate temperature effects on the timing of bee emergence, but that we should also consider individual body conditions of solitary bees to understand the timing of bee emergence.
In Chapter III, I present a study in which I investigated how exactly temperature determines the emergence date of solitary bees. Therefore, I tested several variants degree-day models to relate temperature time series to emergence data. The basic functioning of such degree-day models is that bees are said to finally emerge when a critical amount of degree-days is accumulated. I showed that bees accumulate degree-days only above a critical temperature value (~4°C in O. cornuta and ~7°C in O. bicornis) and only after the exceedance of a critical calendar date (~10th of March in O. cornuta and ~28th of March in O. bicornis). Such a critical calendar date, before which degree-days are not accumulated irrespective of the actual temperature, is in general less commonly used and, so far, it has only been included twice in a phenology model predicting bee emergence. Furthermore, I used this model to retrospectively predict the emergence dates of bees by applying the model to long-term temperature data which have been recorded by the regional climate station in Würzburg. By doing so, the model estimated that over the last 63 years, bees emerged approximately 4 days earlier.
In Chapter IV, I present a study in which I investigated how temporal mismatches in bee-plant interactions affect the fitness of solitary bees. Therefore, I performed an experiment with large flight cages serving as mesocosms. Inside these mesocosms, I manipulated the supply of blossoms to synchronize or desynchronize bee-plant interactions. In sum, I showed that even short temporal mismatches of three and six days in bee-plant interactions (with solitary bee emergence before flower occurrence) can cause severe fitness losses in solitary bees. Nonetheless, I detected different strategies by solitary bees to counteract impacts on their fitness after temporal mismatches. However, since these strategies may result in secondary fitness costs by a changed sex ratio or increased parasitism, I concluded that compensation strategies do not fully mitigate fitness losses of bees after short temporal mismatches with their food plants. In the event of further climate warming, fitness losses after temporal mismatches may not only exacerbate bee declines but may also reduce pollination services for later-flowering species and affect populations of animal-pollinated plants.
In conclusion, I showed that spring-emerging solitary bees are susceptible to climate change as in response to warmer temperatures bees advance their phenology and show a decreased fitness state. As spring-emerging solitary bees not only consider overwintering temperature but also their individual body condition for adjusting emergence dates, this may explain differing responses to climate warming within and among bee populations which may also have consequences for bee-plant interactions and the persistence of bee populations under further climate warming. If in response to climate warming plants do not shift their phenologies according to the bees, bees may experience temporal mismatches with their host plants. As bees failed to show a single compensation strategy that was entirely successful in mitigating fitness consequences after temporal mismatches with their food plants, the resulting fitness consequences for spring-emerging solitary bees would be severe. Furthermore, I showed that spring-emerging solitary bees use a critical calendar date before which they generally do not commence the summation of degree-days irrespective of the actual temperature. I therefore suggest that further studies should also include the parameter of a critical calendar date into degree-day model predictions to increase the accuracy of model predictions for emergence dates in solitary bees. Although our retrospective prediction about the advance in bee emergence corresponds to the results of several studies on phenological trends of different plant species, we suggest that more research has to be done to assess the impacts of climate warming on the synchronization in bee-plant interactions more accurately.
The rotation of the earth leads to a cyclic change of night and day. Numerous strategies evolved to cope with diurnal change, as it is generally advantageous to be synchronous to the cyclic change in abiotic conditions. Diurnal rhythms are regulated by the circadian clock, a molecular feedback loop of RNA and protein levels with a period of circa 24 hours. Despite its importance for individuals as well as for species interactions, our knowledge of circadian clocks is mostly confined to few model organisms.
While the structuring of activity is generally adaptive, a rigid temporal organization also has its drawbacks. For example, the specialization to a diurnal pattern limits the breadth of the temporal niche. Organisms that are adapted to a diurnal life style are often poor predators or foragers during night time, constraining the time budget to only diurnal parts of the day/night cycle.
Climate change causes shifts in phenology (seasonal timing) and northward range expansions, and changes in season or in latitude are associated with novel day length – temperature correlations. Thus, seasonal organisms will have some life history stages exposed to novel day lengths, and I hypothesized that the diurnal niche determines whether the day length changes are beneficial or harmful for the organism. I thus studied the effects of day length on life-history traits in a multi-trophic system consisting of the pea aphid Acyrthosiphon pisum and predatory larvae of Chrysoperla carnea (common green lacewing) and Episyrphus balteatus (marmalade hoverfly). In order to identify the mechanisms for phenological constraints I then focused on diurnal rhythms and the circadian clock of the pea aphid.
Aphids reacted to shorter days with a reduced fecundity and shorter reproductive period. Short days did however not impact population growth, because the fitness constraints only became apparent late in the individual’s life. In contrast, E. balteatus grew 13% faster in the shorter day treatment and preyed on significantly more aphids, whereas C. carnea grew 13% faster under longer days and the elevation of predation rates was marginally significant. These results show that day length affects vital life-history traits, but that the direction and effect size depends on species.
I hypothesized that the constraints or fitness benefits are caused by a constricted or expanded time budget, and hence depend on the temporal niche. E. balteatus is indeed night-active and C. carnea appears to be crepuscular, but very little data exists for A. pisum. Hence, I reared the pea aphid on an artificial diet and recorded survival, moulting and honeydew excretion. The activity patterns were clearly rhythmic and molting and honeydew excretion were elevated during day-time. Thus, the diurnal niche could explain the observed, but weak, day length constraints of aphids.
The diurnal niche of some organisms is remarkably flexible, and a flexible diurnal niche may explain why the day length constrains were relatively low in A. pisum. I thus studied its circadian clock, the mechanism that regulates diurnal rhythms. First, I improved an artificial diet for A. pisum, and added the food colorant Brilliant Blue FCF. This food colorant stained gut and honeydew in low concentration without causing mortalities, and thus made honeydew excretion visible under dim red light. I then used the blue diet to raise individual aphids in 16:08 LD and constant darkness (DD), and recorded honeydew excretion and molting under red light every three hours. In addition, we used a novel monitoring setup to track locomotor activity continuously in LD and DD. Both the locomotor rhythm and honeydew excretion of A. pisum appeared to be bimodal, peaking in early morning and in the afternoon in LD. Both metabolic and locomotor rhythm persisted also for some time under constant darkness, indicating that the rhythms are driven by a functional circadian clock. However, the metabolic rhythm damped within three to four days, whereas locomotor rhythmicity persisted with a complex distribution of several free-running periods. These results fit to a damped circadian clock that is driven by multiple oscillator populations, a model that has been proposed to link circadian clocks and photoperiodism, but never empirically tested.
Overall, my studies integrate constraints in phenological adaptation with a mechanistic explanation. I showed that a shorter day length can constrain some species of a trophic network while being beneficial for others, and linked the differences to the diurnal niche of the species. I further demonstrated that a flexible circadian clock may alleviate the constraints, potentially by increasing the plasticity of the diurnal niche.