Browsing by Person "Tietjen, Britta"

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Geometric and demographic effects explain contrasting fragmentation‐biodiversity relationships across scales
(2025) Gelber, Stav; Blowes, Shane A.; Chase, Jonathan M.; Huth, Andreas; Schurr, Frank M.; Tietjen, Britta; Zeller, Julian W.; May, Felix
There is consensus that habitat loss is a major driver of biodiversity loss, while the effects of fragmentation, given a constant total habitat amount, are still debated. Here, we use a process‐based metacommunity model to show how scale‐ and context‐dependent fragmentation–biodiversity relationships can emerge from the interplay of two types of fragmentation effects – geometric and demographic. Geometric effects arise from the spatial distributions of species and landscape modification, whereas demographic effects reflect long‐term changes in species demographic rates following landscape modification. Our spatial model considers sessile individuals in a heterogeneous landscape and dynamically simulates the processes of species reproduction, dispersal, competition, mortality, and immigration. We introduce a novel approach to partition geometric and demographic fragmentation effects that is based on model outputs directly after landscape modification and after a phase of community dynamics in the modified landscape. In detailed simulation experiments, we assessed how key ecological processes and factors, such as dispersal, habitat heterogeneity, and edge effects, influence geometric, demographic and net fragmentation effects across spatial scales. We found that increasing intraspecific aggregation due to short dispersal and/or environmental autocorrelation increased positive geometric fragmentation effects at the landscape scale. In our model, negative demographic fragmentation effects emerged at the local and landscape scale due to high dispersal mortality in the matrix and due to negative edge effects. We showed that the model can simultaneously predict positive fragmentation–biodiversity relationships at the local scale and negative relationships at the landscape scale as well as context‐dependent variation of these relationships at the landscape scale. We conclude that the framework of geometric and demographic effects can reconcile previous apparently conflicting results and hopefully unlock and advance the debate on biodiversity changes in modified landscapes.
Trade‐offs among restored ecosystem functions are context‐dependent in Mediterranean‐type regions
(2025) Fiedler, Sebastian; Perring, Michael P.; Monteiro, José A.; Branquinho, Cristina; Buzhdygan, Oksana; Cavieres, Lohengrin A.; Cleland, Elsa E.; Cortina‐Segarra, Jordi; Grünzweig, José M.; Holm, Jennifer A.; Irob, Katja; Keenan, Trevor F.; Köbel, Melanie; Maestre, Fernando T.; Pagel, Jörn; Rodríguez‐Ramírez, Natalia; Ruiz‐Benito, Paloma; Schurr, Frank M.; Sheffer, Efrat; Valencia, Enrique; Tietjen, Britta
Global biodiversity hotspots, including Mediterranean‐type ecosystems worldwide, are highly threatened by global change that alters biodiversity, ecosystem functions, and services. Some restoration activities enhance ecosystem functions by reintroducing plant species based on known relationships between plant traits and ecosystem processes. Achieving multiple functions across different site conditions, however, requires understanding how abiotic factors like climate and soil, along with plant assemblages, influence ecosystem functions, including their trade‐offs and synergies. We used the ModEST ecosystem simulation model, which integrates carbon, water, and nutrient processes with plant traits, to assess the relationships between restored plant assemblages and ecosystem functions in Mediterranean‐type climates and soils. We investigated whether maximised carbon increment, water use efficiency, and nitrogen use efficiency, along with their trade‐offs and synergies, varied across different abiotic contexts. Further, we asked whether assemblages that maximised functions varied across environments and among these functions. We found that maximised ecosystem carbon increment and nitrogen use efficiency occurred under moist, warm conditions, while water use efficiency peaked under drier conditions. Generally, the assemblage that maximised one function differed from those for other maximised functions. Synergies were rare, except between water and nitrogen use efficiencies in loam soils across most climates. Trade‐offs among maximised functions were common, varying in strength with abiotic context and plant assemblages, and were more pronounced in sandy loam soils compared to clay‐rich soils. Our findings suggest that due to variation in abiotic conditions within and across Mediterranean‐type regions at the global scale, site‐specific plant assemblages are required to maximise ecosystem functions. Thus, lessons from a single site cannot be transferred to another site, even where the same plant functional types are available for restoration. Our simulation results offer valuable insights into potential ecosystem performance under specific abiotic conditions following restoration with particular plant functional types, thereby informing local restoration efforts.