University of Bremen

About: University of Bremen is a education organization based out in Bremen, Germany. It is known for research contribution in the topics: Population & Glacial period. The organization has 14563 authors who have published 37279 publications receiving 970381 citations. The organization is also known as: Universität Bremen.

Effects of life-history traits on responses of plant species to forest fragmentation

Abstract: : Knowing the general principles of plant-environment relationships is required to be able to predict changes in species occurrence and abundance in changing landscapes. Because habitat fragmentation may affect the dispersal, establishment, and persistence of species in various ways, we expected associations between species life-history traits related to these processes and their responses to fragmentation. We tested (1) whether groups of plant species with specific biological attributes are especially affected by forest fragmentation and (2) whether regionally rare species are more negatively affected than more common species. We surveyed 145 deciduous forest patches in northwestern Germany for the presence of a large set of forest plant species. For each of 82 species, we collected data on eight life-history traits and estimated species' responses to decreased patch size and increased distance to other occupied forest patches. We classified species into two emergent groups that differed strongly with respect to most considered life-history traits. The group of species that was more negatively affected by isolation mostly consisted of clonal forest specialist species characterized by few and heavy diaspores, lack of dispersal structures, small size, short-lived seeds, and insect pollination. There was no effect of patch area. Univariate analyses revealed (marginally) significant relationships between species' responses to isolation and diaspore number and mass, plant height, and habitat preference and between responses to patch area and seed-bank longevity, plant height, and habitat preference. Regional frequency of occurrence was not correlated to species' responses to fragmentation and did not differ between the two emergent groups. Rare species, however, were smaller and produced fewer and shorter-lived diaspores than common species. Forest fragmentation may thus threaten species differently, depending on their specific biological characteristics. Approaches based on life-history traits potentially allow prediction of species' responses to habitat fragmentation and may therefore aid in the assessment of the endangerment of plant species and ultimately in the conservation of biological diversity. Resumen: Se requiere conocimiento de los principios basicos de las relaciones planta-ambiente para predecir cambios en la ocurrencia de especies en paisajes cambiantes. Debido a que la fragmentacion del habitat puede afectar la dispersion, establecimiento y persistencia de las especies de varias maneras, esperabamos asociaciones entre caracteristicas de la historia de vida relacionadas con estos procesos y sus respuestas a la fragmentacion. Probamos (1) si grupos de especies de plantas con atributos biologicos especificos son especialmente afectados por la fragmentacion del bosque y (2) si especies regionalmente raras son afectadas mas negativamente que las especies mas comunes. Examinamos 145 parches de bosque deciduo en el noroeste de Alemania para buscar un conjunto grande de especies de plantas de bosque. Para cada una de 82 especies, recolectamos datos de ochos caracteristicas de la historia de vida y estimamos la respuesta de las especies al decremento del tamano del parche y al incremento de la distancia a otros parches de bosque. Clasificamos a las especies en dos grupos emergentes, con diferencias notables en la mayoria de las caracteristicas consideradas. El grupo de especies que fue afectado mas negativamente por el aislamiento consistio principalmente de especies clonales especialistas de bosque caracterizadas por diasporas escasas y pesadas, carencia de estructuras dispersoras, tamano pequeno, semillas de vida corta y polinizacion por insectos. No hubo efecto de la superficie del parche. Analisis univariados revelaron (marginalmente) relaciones significativas entre las respuestas de las especies al aislamiento y el numero y masa de las diasporas, la altura de plantas y la preferencia de habitat y entre respuestas a la superficie del parche y la longevidad del banco de semillas, la altura de plantas y la preferencia de habitat. La frecuencia de ocurrencia regional no se correlaciono con las respuestas de las especies a la fragmentacion y no hubo diferencia entre los dos grupos emergentes. Sin embargo, las especies raras fueron mas pequenas y produjeron diasporas mas pequenas y menos longevas que las especies comunes. Por lo tanto, la fragmentacion del bosque puede amenazar a las especies de manera diferente, dependiendo de sus caracteristicas biologicas especificas. Los metodos basados en caracteristicas de la historia de vida potencialmente permiten la prediccion de las respuestas de las especies a la fragmentacion del habitat y por lo tanto pueden ayudar a la evaluacion del peligro de extincion de una especie de planta y, a la postre, a la conservacion de la diversidad biologica.

Optimal Hubbard models for materials with nonlocal Coulomb interactions: graphene, silicene, and benzene.

Abstract: To understand how nonlocal Coulomb interactions affect the phase diagram of correlated electron materials, we report on a method to approximate a correlated lattice model with nonlocal interactions by an effective Hubbard model with on-site interactions ${U}^{*}$ only. The effective model is defined by the Peierls-Feynman-Bogoliubov variational principle. We find that the local part of the interaction $U$ is reduced according to ${U}^{*}=U\ensuremath{-}\overline{V}$, where $\overline{V}$ is a weighted average of nonlocal interactions. For graphene, silicene, and benzene we show that the nonlocal Coulomb interaction can decrease the effective local interaction by more than a factor of 2 in a wide doping range.

On the complexity of mod-2l sum PLA's

Abstract: Consideration is given to the realization of logic functions by using PLAs with an exclusive-OR (EXOR) array, where a function is represented by mod-2 (EXOR) sum-of-products (ESOPs) and both true and complemented variables are used. The authors propose a new PLA structure using an EXOR array. They derive upper bounds on the number of products of this type of PLA that are useful for estimating the size of a PLA as well as for assessing the minimality of the solutions obtained by heuristic ESOP minimization algorithms. Computer simulation using randomly generated functions shows that PLAs with the EXOR array require, on the average, fewer products than conventional PLAs. For symmetric functions, the authors conjecture that the PLAs with an EXOR array require, at most, as many products as the conventional PLAs. The proposed PLAs can be made easily testable by adding a small amount of hardware. >

The PREDICTS database: a global database of how local terrestrial biodiversity responds to human impacts

Abstract: Biodiversity continues to decline in the face of increasing anthropogenic pressures such as habitat destruction, exploitation, pollution and introduction of alien species Existing global databases of species’ threat status or population time series are dominated by charismatic species The collation of datasets with broad taxonomic and biogeographic extents, and that support computation of a range of biodiversity indicators, is necessary to enable better understanding of historical declines and to project – and avert – future declines We describe and assess a new database of more than 16 million samples from 78 countries representing over 28,000 species, collated from existing spatial comparisons of local-scale biodiversity exposed to different intensities and types of anthropogenic pressures, from terrestrial sites around the world The database contains measurements taken in 208 (of 814) ecoregions, 13 (of 14) biomes, 25 (of 35) biodiversity hotspots and 16 (of 17) megadiverse countries The database contains more than 1% of the total number of all species described, and more than 1% of the described species within many taxonomic groups – including flowering plants, gymnosperms, birds, mammals, reptiles, amphibians, beetles, lepidopterans and hymenopterans The dataset, which is still being added to, is therefore already considerably larger and more representative than those used by previous quantitative models of biodiversity trends and responses The database is being assembled as part of the PREDICTS project (Projecting Responses of Ecological Diversity In Changing Terrestrial Systems – wwwpredictsorguk) We make site-level summary data available alongside this article The full database will be publicly available in 2015