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Biological invasions are a defining feature of the Anthropocene, but the factors that determine the spatially uneven distribution of alien plant species are still poorly understood. Here, we present the first global analysis of the effects of biogeographic factors, the physical environment and socio-economy on the richness of naturalized and invasive alien plants. We used generalized linear mixed-effects models and variation partitioning to disentangle the relative importance of individual factors, and, more broadly, of biogeography, physical environment and socio-economy. As measures of the magnitude of permanent anthropogenic additions to the regional species pool and of species with negative environmental impacts, we calculated the relative richness of naturalized (= RRN) and invasive (= RRI) alien plant species numbers adjusted for the number of native species in 838 terrestrial regions. Socio-economic factors (per-capita gross domestic product (GDP), population density, proportion of agricultural land) were more important in explaining RRI (similar to 50 % of the explained variation) than RRN (similar to 40 %). Warm-temperate and (sub)tropical regions have higher RRN than tropical or cooler regions. We found that socio-economic pressures are more relevant for invasive than for naturalized species richness. The expectation that the southern hemisphere is more invaded than the northern hemisphere was confirmed only for RRN on islands, but not for mainland regions nor for RRI. On average, islands have similar to 6-fold RRN, and >3-fold RRI compared to mainland regions. Eighty-two islands (=26 % of all islands) harbour more naturalized alien than native plants. Our findings challenge the widely held expectation that socio-economic pressures are more relevant for plant naturalization than for invasive plants. To meet international biodiversity targets and halt the detrimental consequences of plant invasions, it is essential to disrupt the connection between socio-economic development and plant invasions by improving pathway management, early detection and rapid response.
Methane is an important greenhouse gas contributing to global climate change. Natural environments and restored wetlands contribute a large proportion to the global methane budget. Methanogenic archaea (methanogens) and methane oxidizing bacteria (methanotrophs), the biogenic producers and consumers of methane, play key roles in the methane cycle in those environments. A large number of studies revealed the distribution, diversity and composition of these microorganisms in individual habitats. However, uncertainties exist in predicting the response and feedback of methane-cycling microorganisms to future climate changes and related environmental changes due to the limited spatial scales considered so far, and due to a poor recognition of the biogeography of these important microorganisms combining global and local scales.
With the aim of improving our understanding about whether and how methane-cycling microbial communities will be affected by a series of dynamic environmental factors in response to climate change, this PhD thesis investigates the biogeographic patterns of methane-cycling communities, and the driving factors which define these patterns at different spatial scales. At the global scale, a meta-analysis was performed by implementing 94 globally distributed public datasets together with environmental data from various natural environments including soils, lake sediments, estuaries, marine sediments, hydrothermal sediments and mud volcanos. In combination with a global biogeographic map of methanogenic archaea from multiple natural environments, this thesis revealed that biogeographic patterns of methanogens exist. The terrestrial habitats showed higher alpha diversities than marine environments. Methanoculleus and Methanosaeta (Methanothrix) are the most frequently detected taxa in marine habitats, while Methanoregula prevails in terrestrial habitats. Estuary ecosystems, the transition zones between marine and terrestrial/limnic ecosystems, have the highest methanogenic richness but comparably low methane emission rates. At the local scale, this study compared two rewetted fens with known high methane emissions in northeastern Germany, a coastal brackish fen (Hütelmoor) and a freshwater riparian fen (Polder Zarnekow). Consistent with different geochemical conditions and land-use history, the two rewetted fens exhibit dissimilar methanogenic and, especially, methanotrophic community compositions. The methanotrophic community was generally under-represented among the prokaryotic communities and both fens show similarly low ratios of methanotrophic to methanogenic abundances. Since few studies have characterized methane-cycling microorganisms in rewetted fens, this study provides first evidence that the rapid and well re-established methanogenic community in combination with the low and incomplete re-establishment of the methanotrophic community after rewetting contributes to elevated sustained methane fluxes following rewetting.
Finally, this thesis demonstrates that dispersal limitation only slightly regulates the biogeographic distribution patterns of methanogenic microorganisms in natural environments and restored wetlands. Instead, their existence, adaption and establishment are more associated with the selective pressures under different environmental conditions. Salinity, pH and temperature are identified as the most important factors in shaping microbial community structure at different spatial scales (global versus terrestrial environments). Predicted changes in climate, such as increasing temperature, changes in precipitation patterns and increasing frequency of flooding events, are likely to induce a series of environmental alterations, which will either directly or indirectly affect the driving environmental forces of methanogenic communities, leading to changes in their community composition and thus potentially also in methane emission patterns in the future.
Alexander von Humboldt
(2022)
This book aims to view and to understand Alexander von Humboldt from different perspectives and in varying disciplinary contexts. His contributions addressed numerous topics in the earth but also life sciences—spanning from geo-botany, climatology, paleontology, oceanography, mineralogy, resources, and hydrogeology to links between the environmental impact of humans, erosion, and climate change. From the very beginning, he paved the way for a modern, integrated earth system science approach to decipher, characterize, and model the different forcing factors and their feedback mechanisms. It becomes obvious that Humboldt’s holistic approach is far beyond simple description and empiric data collection. As documented and analyzed in the different texts of this volume, he combines observation and analysis with emotions and subjective perceptions in a very affectionate way. However, this publication does not intend to add another encyclopedic text compilation but to observe and critically analyze this unique personality´s relevance in a modern context, particularly in discussing environmental and social key issues in the twenty-first century.