Jens-Christian Svenning

The Influence of Late Quaternary Climate-Change Velocity on Species Endemism

Regions with low glacial-interglacial climate-change velocity were essential refuges for many small-ranged species. The effects of climate change on biodiversity should depend in part on climate displacement rate (climate-change velocity) and its interaction with species’ capacity to migrate. We estimated Late Quaternary glacial-interglacial climate-change velocity by integrating macroclimatic shifts since the Last Glacial Maximum with topoclimatic gradients. Globally, areas with high velocities were associated with marked absences of small-ranged amphibians, mammals, and birds. The association between endemism and velocity was weakest in the highly vagile birds and strongest in the weakly dispersing amphibians, linking dispersal ability to extinction risk due to climate change. High velocity was also associated with low endemism at regional scales, especially in wet and aseasonal regions. Overall, we show that low-velocity areas are essential refuges for Earth’s many small-ranged species.

A review of natural vegetation openness in north-western Europe.

Secondary succession is threatening many species of open habitats in north-western Europe. This problem has caused an intense debate over whether the present-natural vegetation in this region would be closed forest or more open vegetation. Native large herbivores have been proposed as the key agents creating such open vegetation. Here I address this question by reviewing the palaeoecological evidence regarding vegetation openness in past oceanic interglacials and the pre-agricultural Holocene, i.e. before the onset of strong human impact. I conclude that closed forest would predominate, but include localized longer-lasting openings. Further, open vegetation would be frequent on floodplains, infertile soils, chalklands, and in continental and submeditteranean areas. Large herbivores and fire emerge as likely potential key factors in creating open vegetation in north-western Europe. Fire would probably also be important in the maintenance of light-demanding or short-statured woody species within closed upland forests.

Upper thermal limits of Drosophila are linked to species distributions and strongly constrained phylogenetically

Upper thermal limits vary less than lower limits among related species of terrestrial ectotherms. This pattern may reflect weak or uniform selection on upper limits, or alternatively tight evolutionary constraints. We investigated this issue in 94 Drosophila species from diverse climates and reared in a common environment to control for plastic effects that may confound species comparisons. We found substantial variation in upper thermal limits among species, negatively correlated with annual precipitation at the central point of their distribution and also with the interaction between precipitation and maximum temperature, showing that heat resistance is an important determinant of Drosophila species distributions. Species from hot and relatively dry regions had higher resistance, whereas resistance was uncorrelated with temperature in wetter regions. Using a suite of analyses we showed that phylogenetic signal in heat resistance reflects phylogenetic inertia rather than common selection pressures. Current species distributions are therefore more likely to reflect environmental sorting of lineages rather than local adaptation. Similar to previous studies, thermal safety margins were small at low latitudes, with safety margins smallest for species occupying both humid and dry tropical environments. Thus, species from a range of environments are likely to be at risk owing to climate change. Together these findings suggest that this group of insects is unlikely to buffer global change effects through marked evolutionary changes, highlighting the importance of facilitating range shifts for maintaining biodiversity.

Disequilibrium vegetation dynamics under future climate change

PREMISE OF THE STUDY Near-future climate changes are likely to elicit major vegetation changes. Disequilibrium dynamics, which occur when vegetation comes out of equilibrium with climate, are potentially a key facet of these. Understanding these dynamics is crucial for making accurate predictions, informing conservation planning, and understanding likely changes in ecosystem function on time scales relevant to society. However, many predictive studies have instead focused on equilibrium end-points with little consideration of the transient trajectories. METHODS We review what we should expect in terms of disequilibrium vegetation dynamics over the next 50-200 yr, covering a broad range of research fields including paleoecology, macroecology, landscape ecology, vegetation science, plant ecology, invasion biology, global change biology, and ecosystem ecology. KEY RESULTS The expected climate changes are likely to induce marked vegetation disequilibrium with climate at both leading and trailing edges, with leading-edge disequilibrium dynamics due to lags in migration at continental to landscape scales, in local population build-up and succession, in local evolutionary responses, and in ecosystem development, and trailing-edge disequilibrium dynamics involving delayed local extinctions and slow losses of ecosystem structural components. Interactions with habitat loss and invasive pests and pathogens are likely to further contribute to disequilibrium dynamics. Predictive modeling and climate-change experiments are increasingly representing disequilibrium dynamics, but with scope for improvement. CONCLUSIONS The likely pervasiveness and complexity of vegetation disequilibrium is a major challenge for forecasting ecological dynamics and, combined with the high ecological importance of vegetation, also constitutes a major challenge for future nature conservation.

Specialization of mutualistic interaction networks decreases toward tropical latitudes.

Species-rich tropical communities are expected to be more specialized than their temperate counterparts. Several studies have reported increasing biotic specialization toward the tropics, whereas others have not found latitudinal trends once accounting for sampling bias or differences in plant diversity. Thus, the direction of the latitudinal specialization gradient remains contentious. With an unprecedented global data set, we investigated how biotic specialization between plants and animal pollinators or seed dispersers is associated with latitude, past and contemporary climate, and plant diversity. We show that in contrast to expectation, biotic specialization of mutualistic networks is significantly lower at tropical than at temperate latitudes. Specialization was more closely related to contemporary climate than to past climate stability, suggesting that current conditions have a stronger effect on biotic specialization than historical community stability. Biotic specialization decreased with increasing local and regional plant diversity. This suggests that high specialization of mutualistic interactions is a response of pollinators and seed dispersers to low plant diversity. This could explain why the latitudinal specialization gradient is reversed relative to the latitudinal diversity gradient. Low mutualistic network specialization in the tropics suggests higher tolerance against extinctions in tropical than in temperate communities.

PHYLOGENETIC CONSTRAINTS IN KEY FUNCTIONAL TRAITS BEHIND SPECIES’ CLIMATE NICHES: PATTERNS OF DESICCATION AND COLD RESISTANCE ACROSS 95 DROSOPHILA SPECIES

Species distributions are often constrained by climatic tolerances that are ultimately determined by evolutionary history and/or adaptive capacity, but these factors have rarely been partitioned. Here, we experimentally determined two key climatic niche traits (desiccation and cold resistance) for 92–95 Drosophila species and assessed their importance for geographic distributions, while controlling for acclimation, phylogeny, and spatial autocorrelation. Employing an array of phylogenetic analyses, we documented moderate‐to‐strong phylogenetic signal in both desiccation and cold resistance. Desiccation and cold resistance were clearly linked to species distributions because significant associations between traits and climatic variables persisted even after controlling for phylogeny. We used different methods to untangle whether phylogenetic signal reflected phylogenetically related species adapted to similar environments or alternatively phylogenetic inertia. For desiccation resistance, weak phylogenetic inertia was detected; ancestral trait reconstruction, however, revealed a deep divergence that could be traced back to the genus level. Despite drosophilids’ high evolutionary potential related to short generation times and high population sizes, cold resistance was found to have a moderate‐to‐high level of phylogenetic inertia, suggesting that evolutionary responses are likely to be slow. Together these findings suggest species distributions are governed by evolutionarily conservative climate responses, with limited scope for rapid adaptive responses to future climate change.

On the role of microenvironmental heterogeneity in the ecology and diversification of neotropical rain-forest palms (Arecaceae)

Microenvironmental heterogeneity is important in the ecology and diversification of the rich palm flora that inhabits neotropical rain forests. At small-0.1-102 m-scales, neotropical rain forests exhibit high heterogeneity in numerous environmental factors: canopy conditions, conspecifics, other plants, litter, soil factors, topography, and animal mutualists and pests. These aspects of microenvironmental heterogeneity affect the performance and the small-scale distribution of palms in numerous ways, often affecting different species differently. Notably, even subtle environmental variation can be of crucial ecological importance.Microenvironmental heterogeneity promotes the local coexistence of palm species by niche differences among the species and probably also by mass effects and negative density dependence. Sympatric species of the same growth form often differ in terms of light requirements, edaphic-topographic preferences, and possibly also in seed-dispersal patterns, whereas mass effects are likely to account for the local occurrence of a share of the rare species. Density dependence seems to be frequent among large-seeded palms, but its importance needs to be assessed.Microenvironmental heterogeneity is proposed to be an important diversity-generating factor in the neotropical palm flora through the process of parapatric speciation. This hypothesis is based on the observation that, in species-rich palm genera and species complexes, sympatric species or morphs often differ in edaphic-topographic preferences or in characteristics that confer differing light requirements and in traits that favor reproductive isolation.ResumenLa heterogeneidad microambiental es importante en la ecología y la diversificación de la rica flora de palmas que habita los bosques húmedos neotropicales. A pequeña escala (0.1-102 m), los bosques húmedos neotropicales exhiben una alta heterogeneidad en numerosos factores ambientales: las condiciones del dosel, los coespecíficos, las otras plantas, la broza, los factores del suelo, la topografia, y los mutualistas y parásites animales. Estos aspectos de la heterogeneidad microambiental afectan, de muchas maneras, el funcionamiento y la distribución a pequeña escala de las palmas, afectando a menudo distintas especies de modo diferente. Notablemente, aún una variación ambiental sutil puede ser de importancia ecológica crucial.La heterogeneidad microambiental promueve la coexistencia local de las especies de palmas por diferencias de nichos entre las especies y probablemente también por “mass effects” y dependencia negativa de la densidad. Las especies simpátricas con la misma forma de crecimiento se diferencian a menudo en términos de sus requerimientos de luz, preferencias edáfico-topográficas y posiblemente también en los patrones de dispersión de semillas, mientras que “mass effects” probablemente explican la ocurrencia local de una parte de las especies raras. La dependencia de la densidad parece ser frecuente entre las palmas con semillas grandes, pero su importancia necesita ser evaluada.Se propone que la heterogeneidad microambiental por el proceso de especiación parapátrica es un factor importante en la diversificación de la flora neotropical de palmas. Esta hipótesis se basa en la observatión de que en géneros de palmas con muchas especies y en complejos de especies, las especies o formas simpátricas se diferencian a menudo en preferencias edáfico-topográficas o en caracteristicas que les atribuyen diferentes requerimientos lumínicos y en los rasgos que favorecen el aislamiento reproductivo.

Global late Quaternary megafauna extinctions linked to humans, not climate change

The late Quaternary megafauna extinction was a severe global-scale event. Two factors, climate change and modern humans, have received broad support as the primary drivers, but their absolute and relative importance remains controversial. To date, focus has been on the extinction chronology of individual or small groups of species, specific geographical regions or macroscale studies at very coarse geographical and taxonomic resolution, limiting the possibility of adequately testing the proposed hypotheses. We present, to our knowledge, the first global analysis of this extinction based on comprehensive country-level data on the geographical distribution of all large mammal species (more than or equal to 10 kg) that have gone globally or continentally extinct between the beginning of the Last Interglacial at 132 000 years BP and the late Holocene 1000 years BP, testing the relative roles played by glacial–interglacial climate change and humans. We show that the severity of extinction is strongly tied to hominin palaeobiogeography, with at most a weak, Eurasia-specific link to climate change. This first species-level macroscale analysis at relatively high geographical resolution provides strong support for modern humans as the primary driver of the worldwide megafauna losses during the late Quaternary.

Geographical ecology of the palms (Arecaceae): determinants of diversity and distributions across spatial scales

BACKGROUND The palm family occurs in all tropical and sub-tropical regions of the world. Palms are of high ecological and economical importance, and display complex spatial patterns of species distributions and diversity. SCOPE This review summarizes empirical evidence for factors that determine palm species distributions, community composition and species richness such as the abiotic environment (climate, soil chemistry, hydrology and topography), the biotic environment (vegetation structure and species interactions) and dispersal. The importance of contemporary vs. historical impacts of these factors and the scale at which they function is discussed. Finally a hierarchical scale framework is developed to guide predictor selection for future studies. CONCLUSIONS Determinants of palm distributions, composition and richness vary with spatial scale. For species distributions, climate appears to be important at landscape and broader scales, soil, topography and vegetation at landscape and local scales, hydrology at local scales, and dispersal at all scales. For community composition, soil appears important at regional and finer scales, hydrology, topography and vegetation at landscape and local scales, and dispersal again at all scales. For species richness, climate and dispersal appear to be important at continental to global scales, soil at landscape and broader scales, and topography at landscape and finer scales. Some scale-predictor combinations have not been studied or deserve further attention, e.g. climate on regional to finer scales, and hydrology and topography on landscape and broader scales. The importance of biotic interactions - apart from general vegetation structure effects - for the geographic ecology of palms is generally underexplored. Future studies should target scale-predictor combinations and geographic domains not studied yet. To avoid biased inference, one should ideally include at least all predictors previously found important at the spatial scale of investigation.

Ecological Determinism in Plant Community Structure Across a Tropical Forest Landscape

The ecological mechanisms hypothesized to structure species-rich communities range from strict local determinism to neutral ecological drift. We assessed the degree of ecological determinism in tropical plant community structure by analyses of published demographic data; a broad range of spatial, historical, and environmental variables; and the distributions of 33 herbaceous species (plot size = 0.02 ha) and 61 woody species (plot size = 0.09 ha) among 350 plots in a 16-km2 forest landscape (Barro Colorado Island, Panama). We found a strong degree of cross-landscape dominance by a subset of species whose identities were predictable from sapling survivorship rates under shade. Using canonical ordination we found that spatial and environmental–historical factors were of comparable importance for controlling within-landscape variability in species composition. Past land use had a strong impact on species composition despite ceasing 100–200 years ago. Furthermore, edaphic–hydrological factors, treefall gaps, and an edge effect all had unique impacts on species composition. Hence, ecological determinism was evident in terms of both cross-landscape dominance and within-landscape variability in species composition. However, at the latter scale, the large portion of the explained variance in species composition among plots uniquely attributed to spatial location pointed to an equally important role for neutral processes.