Showing papers by "Thorsten Wiegand published in 2019"

The Latitudinal Diversity Gradient: Novel Understanding through Mechanistic Eco-evolutionary Models

Abstract: The latitudinal diversity gradient (LDG) is one of the most widely studied patterns in ecology, yet no consensus has been reached about its underlying causes. We argue that the reasons for this are the verbal nature of existing hypotheses, the failure to mechanistically link interacting ecological and evolutionary processes to the LDG, and the fact that empirical patterns are often consistent with multiple explanations. To address this issue, we synthesize current LDG hypotheses, uncovering their eco-evolutionary mechanisms, hidden assumptions, and commonalities. Furthermore, we propose mechanistic eco-evolutionary modeling and an inferential approach that makes use of geographic, phylogenetic, and trait-based patterns to assess the relative importance of different processes for generating the LDG.

Predicting range shifts of Asian elephants under global change

Abstract: MBA acknowledges funding from Spanish Ministry of Education CGL2015-68438-P project. PD and J-PP thank USFWS Asian elephant program (F12APO1186) and the International Elephant Foundation for financial support.

Projected impacts of climate change on functional diversity of frugivorous birds along a tropical elevational gradient

Abstract: Climate change forces many species to move their ranges to higher latitudes or elevations. Resulting immigration or emigration of species might lead to functional changes, e.g., in the trait distribution and composition of ecological assemblages. Here, we combined approaches from biogeography (species distribution models; SDMs) and community ecology (functional diversity) to investigate potential effects of climate-driven range changes on frugivorous bird assemblages along a 3000 m elevational gradient in the tropical Andes. We used SDMs to model current and projected future occurrence probabilities of frugivorous bird species from the lowlands to the tree line. SDM-derived probabilities of occurrence were combined with traits relevant for seed dispersal of fleshy-fruited plants to calculate functional dispersion (FDis; a measure of functional diversity) for current and future bird assemblages. Comparisons of FDis between current and projected future assemblages showed consistent results across four dispersal scenarios, five climate models and two representative concentration pathways. Projections indicated a decrease of FDis in the lowlands, an increase of FDis at lower mid-elevations and little changes at high elevations. This suggests that functional dispersion responds differently to global warming at different elevational levels, likely modifying avian seed dispersal functions and plant regeneration in forest ecosystems along tropical mountains.

Disentangling the functional trait correlates of spatial aggregation in tropical forest trees.

Abstract: Environmental filtering and dispersal limitation can both maintain diversity in plant communities by aggregating conspecifics, but parsing the contribution of each process has proven difficult empirically. Here, we assess the contribution of filtering and dispersal limitation to the spatial aggregation patterns of 456 tree species in a hyperdiverse Amazonian forest and find distinct functional trait correlates of interspecific variation in these processes. Spatial point process model analysis revealed that both mechanisms are important drivers of intraspecific aggregation for the majority of species. Leaf drought tolerance was correlated with species topographic distributions in this aseasonal rainforest, showing that future increases in drought severity could significantly impact community structure. In addition, seed mass was associated with the spatial scale and density of dispersal-related aggregation. Taken together, these results suggest environmental filtering and dispersal limitation act in concert to influence the spatial and functional structure of diverse forest communities.

Combined effects of grazing management and climate on semi‐arid steppes: hysteresis dynamics prevent recovery of degraded rangelands

Abstract: Livestock grazing has degraded many arid and semi‐arid rangelands around the world, and the drier climate predicted by climate change scenarios may amplify these effects and even lead to catastrophic vegetation shifts. We assess the long‐term effects (1900–2100) of grazing and rainfall on various aspects of vegetation structure including the grass‐shrub balance, the maintenance of spatial vegetation patterns, and the decline or recovery of palatable grasses (e.g. Poa ligularis) on a cover and/or density basis. We used the eco‐hydrological and individual‐based simulation model DINVEG for this purpose, which describes the spatiotemporal dynamics of Patagonian grass‐shrub steppes based on six decades of field research (1955–2018). Rainfall and grazing affected the simulated vegetation structure in different ways. Total plant cover was mostly influenced by rainfall, but the cover of palatable grasses was mostly influenced by stocking rate. Dry conditions and low stocking rates (122 mm/year and 0.2 sheep/ha). High stocking rates and/or drier conditions caused only gradual shifts in spatial vegetation patterns, but maintained the observed positive association for grasses around shrubs. In contrast, shrub encroachment was associated with repulsion between grasses and shrubs and the formation of shrub clusters into a matrix of scattered less palatable grasses. Plant compositional changes occurred through grass species replacement (e.g. P. ligularis is replaced by Pappostipa humilis) and the associated hysteresis effect of palatable grass species: model simulations suggest that 2–3 decades of heavy and year‐long continuous grazing can drive palatable grasses to close to extinction, whereas natural recovery of degraded steppes may take 100 years or longer. Synthesis and applications. Desertification and climate change challenge grazing management in semi‐arid rangelands, especially in already degraded ecosystems. Management that alternates between years of grazing and resting was effective to maintain the cover of palatable grasses, but this allowed for only very slow recovery of degraded steppes. While drier climate and grazing may not change the overall spatial patterns of vegetation, our results are rather pessimistic regarding the short‐term recovery of palatable grasses. This will require increasing complexity in ecosystem restoration efforts, combined with interventions such as sowing, watering, reseeding or major changes in land use.