Implications of this study

ability to calculate very reliable proxies for plant traits and plant trait diversity in the field based on information derived from trait databases and forces to make less precise assumptions. As a result no definitive conclusions can be drawn on the effects of trait diversity in the diversity-stability paradigm, as described in chapter 2. In order to It should be stressed that functional trait databases need to be expanded in the near future incorporating more plant species, more populations within species, and more habitat types.

Ecological indicators - Measurements of site-specific environmental factors are often not included in the vegetation databases. However, phytosociological logic states that the plant species composition can be used to assess abiotic factors at play. Ellenberg indicator values are based on that principal and are commonly used in environmental assessments where the vegetation composition is known. Whereas Ellenberg indicator values are originally developed for central-European plant species (Ellenberg et al., 1991), Wamelink developed and validated indicator values specifically for the Netherlands. In this thesis, we therefore applied Wamelink indicator values to asses ecological variables linked to the historical vegetation data (Wamelink et al., 2002; Wamelink et al., 2012). However, independently determined, site- and time-associated measurements of the vegetation plots are preferred (Zelený & Schaffers 2012; Wildi 2016). The need for including and measuring on-site abiotic environmental conditions should be stressed more in vegetation research as well as including this data along with species observations in vegetation databases.

6.5 Implications of this study

This thesis includes some of the first efforts in up-scaling diversity-stability relationships in natural (unmanipulated) ecosystems on larger temporal and spatial scales. The most notable result is the consistency of experimentally determined positive diversity-stability relationships in such ecosystems. The diversity-stability paradigm has been the topic of ecological debate for decades (Elton 1958; McNaughton 1977; Hector et al., 2007; Tilman et al., 2014).

Although many experimental studies support the theoretical framework, the extrapolation to other functions and scales remains an enormous challenge.

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In this thesis, we tried to up-scale experimental work to a real-world system and explored the possibilities of combining newly available data and information techniques to validate the diversity-stability paradigm in nature.

This study, confined to grasslands, may be a first step to provide tools and guidelines to further study diversity-stability relations and the mechanisms behind the insurance hypothesis on larger spatial and temporal scales in real-world ecosystems. Some of our findings suggest an important role for all individual species within their community, as plant species richness was the most important explanatory variable. For this reason, future research on diversity-stability mechanisms should also focus on the performance and role of individual species in the ecosystem’s response to a disturbance.

Physiological research on plant species and field experiments on the species and the community levels may provide new data which can be incorporated in the available trait information which can be used in large-scale observational studies such as this thesis. By doing this, we want to stress the complementarity of experimental work and observations in natural ecosystems and express the need for a closer collaboration between and combination of these scientific approaches in order to further increase our understanding of the natural world.

This study aimed to upscale diversity-stability research to scales relevant for nature management and nature policy. Data-based knowledge on such a scale may have implications for the way in which we manage and use our ecosystems in a changing world. Chapter 3 provides a good example, showing differences in resistance between semi-natural managed grassland and grassland managed using modern agricultural techniques. As our studies imply, more biodiverse ecosystems, with a higher plant species-richness, have a higher resistance to climate extremes compared to species-poor ecosystems. Although productivity remains higher in agricultural grasslands due to anthropogenic activity, our study suggests that adjusting agricultural management in order to preserve species or utilize a higher biodiversity in agricultural meadows, may result in a more stable productivity during climate extremes. Moreover, fewer measures may have to be taken to ensure a stable productivity when the ecosystem has a high resistance. This does not only apply for farmland alone but also other ecosystems where the function can

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be interpreted as a service. Forest and woodland ecosystems, for example, are exploited for timber and as a consequence biomass production is an ecosystem service that is strictly monitored. Where woodland ecosystems were some of the first systems where the diversity-productivity relationship was examined in large-scale ecosystems (often plantations), recent insights show the advantages of a high plant species diversity in forests in a changing environment, similar to our findings in grasslands (Paquette & Messier 2011;

Aspinwall et al., 2015). As this shows that the diversity-stability paradigm can be applied to both natural as well as agricultural ecosystems, more research has to be done into these mechanism in order to translate the diversity-stability paradigm for management and policy making. Overall, our results suggest an important role of plant species diversity in maintaining ecosystem functions under environmental stress. Loss of biodiversity may result in ecosystems that are more vulnerable to climate extremes with subsequent loss of ecosystem functions and services (Cardinale et al., 2012). While the results presented in this thesis suggest an important role of species diversity

Figure 6.1 Relationship between the additional relative loss of productivity as a result of the loss of resistance during a drought and the loss of biodiversity compared to the maximum number of species according to several linear regression models presented in this thesis. The lines represent the two most extreme models in this thesis and blue shaded area depicts the variation between the models.

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in the stability of ecosystem functioning, it remains a challenge to predict the consequences of species loss for ecosystem stability. All models presented in this thesis showed a significant diversity-stability relationship, resulting in a decreased resistance in relatively species-poor communities. In the studies presented here the diversity did not significantly change over time, however many ecosystems suffer species loss due to (anthropogenic) environmental disturbances like climate change, pollution, fragmentation, intensification and management. The loss of species in these systems decreases the chance of compensatory dynamics within an ecosystem during a potential drought.

Figure 6.1 visualizes the extrapolation of the outcomes of our models to the additional loss of productivity during a drought period, related to a decreased plant species-richness within a similar plant community.

The model depicted in Figure 6.1 can be illustrated as followed, assuming all species present have an equal role in ecosystem functioning. Under optimal circumstances a grassland has a certain resistance to drought. This means that it will lose some of its productivity, however complementary dynamics between the species present stabilizes the loss of productivity. Now, due to a new management regime, 50% of the species have disappeared from this grasslands. Under the same drought conditions, the grassland loses up to 27 percent of its resistance resulting in an additional 40% decrease of production (thus a 140% decrease of production compared to the initial situation under similar drought conditions).

At extensive species loss, productivity loss due to drought may increase over 200%. Note that species do not have equal roles within ecosystem functioning.

With the absence of key species, the loss of stability during drought perturbations will significantly increase.

Among other studies, Tilman et al. (2001) presented a positive relationship between plant species richness and long-term productivity, suggesting a decrease in productivity as species number decrease. Using the models presented in this thesis it can be theorized that the occurrence of a drought further impedes production. Moreover, a decreased productivity due to a lower plant species richness in combination with a decreased resistance related to low plant species richness within a community, causes an even bigger drop

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of productivity (Figure 6.2). Given the expected increase in frequency and magnitude of climate extremes and the current biodiversity crisis, we stress the need to conserve and protect global biodiversity on the species level.

Figure 6.2 Theoretical diversity-productivity relationships when plant species richness in a grassland community decreases relative to a theoretical maximum number of species. In green the diversity-productivity relationship based on Tilman et al. (2001) shows a decrease in productivity when species numbers decline (green). During a drought period productivity drops with a certain amount relative to the productivity maximum (yellow). However, including resistance models, the average of the correlations found in this thesis, to the diversity-productivity relationships adds to the hypothetical loss of productivity.

The additional loss increases as species numbers of a certain community further decline (red).

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