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Human forest modification is among the largest global drivers of terrestrial degradation
of biodiversity, species interactions, and ecosystem functioning. One of the most
pertinent components, forest fragmentation, has a long history in ecological research
across the globe, particularly in lower latitudes. However, we still know little how
fragmentation shapes temperate ecosystems, irrespective of the ancient status quo of
European deforestation. Furthermore, its interaction with another pivotal component
of European forests, silvicultural management, are practically unexplored. Hence,
answering the question how anthropogenic modification of temperate forests affects
fundamental components of forest ecosystems is essential basic research that has
been neglected thus far. Most basal ecosystem elements are plants and their insect
herbivores, as they form the energetic basis of the tropic pyramid. Furthermore, their
respective biodiversity, functional traits, and the networks of interactions they
establish are key for a multitude of ecosystem functions, not least ecosystem stability.
Hence, the thesis at hand aimed to disentangle this complex system of
interdependencies of human impacts, biodiversity, species traits and inter-species
interactions.
The first step lay in understanding how woody plant assemblages are shaped by
human forest modification. For this purpose, field investigations in 57 plots in the
hyperfragmented cultural landscape of the Northern Palatinate highlands (SW
Germany) were conducted, censusing > 4,000 tree/shrub individuals from 34 species.
Use of novel, integrative indices for different types of land-use allowed an accurate
quantification of biotic responses. Intriguingly, woody tree/shrub communities reacted
strikingly positive to forest fragmentation, with increases in alpha and beta diversity,
as well as proliferation of heat/drought/light adapted pioneer species. Contrarily,
managed interior forests were homogenized/constrained in biodiversity, with
dominance of shade/cold adapted commercial tree species. Comparisons with recently
unmanaged stands (> 40 a) revealed first indications for nascent conversion to oldgrowth
conditions, with larger variability in light conditions and subsequent
community composition. Reactions to microclimatic conditions, the relationship
between associated species traits and the corresponding species pool, as well as
facilitative/constraining effects by foresters were discussed as underlying mechanisms.
Reactions of herbivore assemblages to forest fragmentation and the subsequent
changes in host plant communities were assessed by comprehensive sampling of >
1,000 live herbivores from 134 species in the forest understory. Diversity was –
similarly to plant communities - higher in fragmentation affected habitats, particularly
in edges of continuous control forests. Furthermore, average trophic specialization
showed an identical pattern. Mechanistically, benefits from microclimatic conditions,
host availability, as well as pronounced niche differentiation are deemed responsible.
While communities were heterogeneous, with no segregation across habitats, (smallforest fragments, edges, and interior of control forests), vegetation diversity, herbivore
diversity, as well as trophic specialization were identified to shape community
composition. This probably reflected a gradient from generalistic/species poor vs.
specialist/species rich herbivore assemblages.
Insect studies conducted in forest systems are doomed to incompleteness
without considering ‘the last biological frontier’, the tree canopies. To access their
biodiversity, relationship to edge effects, and their conservational value, the
arboricolous arthropod fauna of 24 beech (Fagus sylvatica) canopies was sampled via
insecticidal knockdown (‘fogging’). This resulted in an exhaustive collection of > 46,000
specimens from 24 major taxonomic/functional groups. Abundance distributions were
markedly negative exponential, indicating high abundance variability in tree crowns.
Individuals of six pertinent orders were identified to species level, returning > 3,100
individuals from 175 species and 52 families. This high diversity did marginally differ
across habitats, with slightly higher species richness in edge canopies. However,
communities in edge crowns were noticeably more heterogeneous than those in the
forest interior, possibly due to higher variability in environmental edge conditions. In
total, 49 species with protective value were identified, of which only one showed
habitat preferences (for near-natural interior forests). Among them, six species (all
beetles, Coleoptera) were classified as ‘priority species’ for conservation efforts. Hence,
beech canopies of the Northern Palatinate highlands can be considered strongholds of
insect biodiversity, incorporating many species of particular protective value.
The intricacy of plant-herbivore interaction networks and their relationship to
forest fragmentation is largely unexplored, particularly in Central Europe. Illumination
of this matter is all the more important, as ecological networks are highly relevant for
ecosystem stability, particularly in the face of additional anthropogenic disturbances,
such as climate change. Hence, plant-herbivore interaction networks (PHNs) were
constructed from woody plants and their associated herbivores, sampled alive in the
understory. Herbivory verification was achieved using no-choice-feeding assays, as well
as literature references. In total, networks across small forest fragments, edges, and
the forest interior consisted of 696 interactions. Network complexity and trophic niche
redundancy were compared across habitats using a rarefaction-like resampling
procedure. PHNs in fragmentation affected forest habitats were significantly more
complex, as well as more redundant in their realized niches, despite being composed of
relatively more specialist species. Furthermore, network robustness to climate change
was quantified utilizing four different scenarios for climate change susceptibility of
involved plants. In this procedure, remaining herbivores in the network were measured
upon successive loss of their host plant species. Consistently, PHNs in edges (and to a
smaller degree in small fragments) withstood primary extinction of plant species
longer, making them more robust. This was attributed to the high prevalence of
heat/drought-adapted species, as well as to beneficial effects of network topography
(complexity and redundancy). Consequently, strong correlative relationships were
found between realized niche redundancy and climate change robustness of PHNs.
This was both the first time that biologically realistic extinctions (instead of e.g.random extinctions) were used to measure network robustness, and that topographical
network parameters were identified as potential indicators for network robustness
against climate change.
In synthesis, in the light of global biotic degradation due to human forest
modification, the necessity to differentiate must be claimed. Ecosystems react
differently to anthropogenic disturbances, and it seems the particular features present
in Central European forests (ancient deforestation, extensive management, and, most
importantly, high richness in open-forest plant species) cause partly opposed patterns
to other biomes. Lenient microclimates and diverse plant communities facilitate
equally diverse herbivore assemblages, and hence complex and robust networks,
opposed to the forest interior. Therefore, in the reality of extensively used cultural
landscapes, fragmentation affected forest ecosystems, particularly forest edges, can be
perceived as reservoir for biodiversity, and ecosystem functionality. Nevertheless, as
practically all forest habitats considered in this thesis are under human cultivation,
recommendations for ecological enhancement of all forest habitats are discussed.
The massive use of chemicals by humans is increasing pollution of the world’s ecosystems. Yet, knowledge about exposure and effects of chemicals in real-world ecosystems remains limited. Prediction of chemical effects in the context of ecotoxicological research and chemical regulation continues to focus on organism- or population-level responses established under simplified conditions while aiming to protect the functioning of ecosystems. A unified, comprehensive framework for the prediction of chemical effects in real-world ecosystems is still lacking. A major limitation of ecotoxicological studies considered in predictive modelling is that they rarely consider spatial dynamics (e.g. gene flow or species dispersal) as relevant processes influencing the trajectory of populations or communities, respectively. For instance, the spatial propagation of pesticide effects from polluted to least impacted sites has been predicted in several modelling studies but has not yet been characterised in the field.
The thesis starts in Chapter 1 with a brief introduction to chemical pollution in ecosystems, chemical effect prediction in ecotoxicology, and pesticides in freshwater ecosystems, then outlines the main objectives of the thesis. Subsequently, Chapter 2 presents a conceptual study about the current prediction of chemical effects in ecotoxicology and potential future avenues to improve ecological relevance of effect predictions by addressing the integration of different levels of biological organisation (termed biological levels). The study shows that approaches and tools that currently contribute to the prediction of chemical effects can be attributed to three idealised perspectives: the suborganismal, organismal and ecological perspective. The perspectives focus on different biological levels and are associated with distinct scientific concepts and communities. They complement each other so theoretical and empirical links between them may enhance prediction by capturing the entire phenomenon of chemical effects, from chemical uptake to ecosystem effects. Complex experimental studies accounting for eco-evolutionary dynamics are needed to cross barriers between biological levels as well as spatiotemporal scales. Overall, the conclusions of Chapter 2 may help to develop overarching frameworks for predicting chemical effects in ecosystems, including for untested species. Chapters 3 and 4 present a field study combined with laboratory analyses on the potential propagation of pesticides and their effects from agricultural stream sections to the edge of least impacted upstream sections, that can serve as refuges for many species. The study examines exposure and effects for different biological levels at three site types, the pesticide-polluted agricultural sites (termed agriculture), least impacted upstream sites (termed refuge) and transitional sites (termed edge) in six small streams of south-west Germany. The results in Chapter 3 show that regional transport of pesticides can lead to ecologically relevant pesticide exposure in forested sections within a few kilometres upstream of agricultural areas (i.e. at both edge and refuge sites). As further demonstrated in Chapter 3, the tested indicators of community responses (Jaccard Index, taxonomic richness, total abundance, SPEARpesticides) together suggest a species turnover from upstream refuge to downstream agricultural sites and a potential influence of adjacent agriculture on the edge sites. In contrast, Chapter 4 does not identify any particular edge effect that distinguish edge organisms and populations in edge sites from those in more upstream refuge sites. Gammarus fossarum populations at edges show equal levels of imidacloprid tolerance, energy reserves (i.e. lipid content) and genetic diversity to populations further upstream. Gammarus spp. from agricultural sites exhibit a lower imidacloprid tolerance compared to edge and refuge, potentially due to energy trade-offs in a multiple stressor environment, but related effects do not propagate to the edges (Chapter 4). Notwithstanding, the results of Chapter 4 indicate bidirectional gene flow between site types, supporting the hypothesis that adapted genotypes – if present at locally polluted sites – could spread to populations at least impacted sites. Taken together, Chapters 3 and 4, illustrate that pesticides and their effects can potentially propagate to least impacted upstream sections, empirically novel findings to our knowledge. These results of this thesis can help in predicting or explaining population and community dynamics in least impacted habitats and can ultimately inform pesticide management as well as freshwater restoration and protection of biodiversity.