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1. What can hippopotamus isotopes tell us about past distributions of C 4 grassy biomes on Madagascar?

   https://doi.org/10.1002/ppp3.10402  

   Crowley, Brooke Erin ; Godfrey, Laurie Rohde ; Samonds, Karen Elizabeth ( July 2023 , PLANTS, PEOPLE, PLANET)

   Today, expansive C₄grassy biomes exist across central, western, and northern Madagascar. Some researchers have argued that the island's now‐extinct pygmy hippopotamuses belonged to a megaherbivore grazing guild that maintained these grasslands prior to human arrival. However, the chemistry of hippo bones indicates that C₄grasses were only a minor part of hippo diet. This, in turn, suggests that C₄grasses were present but not widespread when hippos were alive and that grasses expanded only after Malagasy people shifted from hunting and foraging to agropastoralism approximately 1000 years ago. These results have important implications for environmental reconstructions and biodiversity management.

   Extinct hippopotamuses (Hippopotamusspp.) were part of Madagascar's megaherbivore guild. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C₄grassy biomes, then they should have regularly consumed C₄grasses, which have high δ¹³C values. However, if expansive C₄grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C₃plants with low δ¹³C values. Nitrogen isotopes can clarify foraging habitat (moist or dry).

   We assessed δ¹³C and δ¹⁵N values for hippos from different ecoregions of Madagascar and compared these with data for extinct herbivorous lemurs from the same ecoregions. We further explored the effects of wet/dry transitions on isotopic trends for hippos from the central highlands and spiny thicket ecoregions.

   Carbon isotopes suggest (1) limited C₄consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C₄resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions.

   Malagasy hippos did not regularly graze C₄grasses in dry, open habitats, even in regions blanketed by C₄grassy biomes today. Malagasy grasses are adapted to grazing and fire, but these are likely ancient adaptations that accompanied grasses when they initially spread to Madagascar. C₄grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.

    

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2. The past, present, and future of herbivore impacts on savanna vegetation

   https://doi.org/10.1111/1365-2745.13685  

   Staver, A. Carla ; Abraham, Joel O. ; Hempson, Gareth P. ; Karp, Allison T. ; Faith, J. Tyler ( June 2021 , Journal of Ecology)

   Herbivory is a key process structuring vegetation in savannas, especially in Africa where large mammal herbivore communities remain intact. Exclusion experiments consistently show that herbivores impact savanna vegetation, but effect size variation has resisted explanation, limiting our understanding of the past, present and future roles of herbivory in savanna ecosystems.

   Synthesis of vegetation responses to herbivore exclusion shows that herbivory decreased grass abundance by 57.0% and tree abundance by 30.6% across African savannas.

   The magnitude of herbivore exclusion effects scaled with herbivore abundance: more grazing herbivores resulted in larger grass responses and more browsing herbivores in larger tree responses. However, existing experiments are concentrated in semi‐arid savannas (400–800‐mm rainfall) and soils data are mostly lacking, which makes disentangling environmental constraints a challenge and priority for future research.

   Observed herbivore impacts were ~2.1× larger than existing estimates modelled based on consumption. Wildlife metabolic rates may be higher than are usually used for estimating consumption, which offers one clear avenue for reconciling estimated herbivore consumption with observed herbivore impacts. Plant‐soil feedbacks, plant community composition, and the phenological or demographic timing of herbivory may also influence vegetation productivity, thereby magnifying herbivore impacts.

   Because herbivore abundance so closely predicts vegetation impact, changes in herbivore abundance through time are likely predictive of the past and future of their impacts. Grazer diversity in Africa has declined from its peak 1 million years ago and wild grazer abundance has declined historically, suggesting that grazing likely had larger impacts in the past than it does today.

   Current wildlife impacts are dominated by small‐bodied mixed feeders, which will likely continue into the future, but the magnitude of top‐down control may also depend on changing climate, fire and atmospheric CO₂.

   Synthesis. Herbivore biomass determines the magnitude of their impacts on savanna vegetation, with effect sizes based on direct observation that outstrip existing modelled estimates across African savannas. Findings suggest substantial ecosystem impacts of herbivory and allow us to generate evidence‐based hypotheses of the past and future impacts of herbivores on savanna vegetation.

    

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3. Afrotropics on the wing: phylogenomics and historical biogeography of awl and policeman skippers

   https://doi.org/10.1111/syen.12455  

   Toussaint, Emmanuel F. A. ; Chiba, Hideyuki ; Yago, Masaya ; Dexter, Kelly M. ; Warren, Andrew D. ; Storer, Caroline ; Lohman, David J. ; Kawahara, Akito Y. ( October 2020 , Systematic Entomology)

   The Old‐World Tropics encompass many unique biomes and associated biotas shaped by drastic climate and geological changes throughout the Cenozoic. Disjunct distributions of clades between the Afrotropics and the Oriental regions are testament to these changes. Awl and policeman skippers (Hesperiidae: Coeliadinae) are disjunctly distributed with some genera endemic to the Afrotropics and others restricted to the Oriental and Australian regions. We reconstruct the phylogeny of these butterflies using target exon capture phylogenomics. We also generate a dated framework for this clade that uses the putatively oldest known butterfly fossil to estimate the historical biogeography of Coeliadinae using a model‐based approach. We infer a stable and robust phylogeny for the subfamily, with all but one Afrotropical lineage forming a derived clade. The African genusPyrrhiadessyn. nov.is placed in synonymy withCoeliadesto accommodate the new phylogeny. Our comparative dating exercise casts doubt on the assignment of the fossilProtocoeliades kristensenias a derived Coeliadinae and suggests, along with our biogeographic estimation, a split of Coeliadinae from the rest of skippers in the Palaeoceneca. 70 million years ago. The origin of crown Coeliadinae skippers is estimated in Indomalaya during the late Eoceneca. 36 million years ago, with subsequent Oligocene colonisation events toward the Australian region and the Afrotropics. Colonisation of the Afrotropics from the Indian region occurred during climatic transition, associated biome shifts, and the closure of the Tethys Ocean, which likely allowed geodispersal through the Arabian Peninsula. The current disjunct distribution of Coeliadinae in the Old World Tropics may result from the emergence of savannahs in the Miocene that progressively replaced woodlands and forests in the Arabian Peninsula and western Asia. Coeliadinae skippers are almost exclusively dicot feeders and were likely extirpated as grasslands became dominant, resulting in the present‐day disjunct distribution of these butterflies.

    

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4. Facilitation by isolated trees triggers woody encroachment and a biome shift at the savanna–forest transition

   https://doi.org/10.1111/1365-2664.13994  

   Abreu, Rodolfo C. R. ; Durigan, Giselda ; Melo, Antônio C. G. ; Pilon, Natashi A. L. ; Hoffmann, William A. ( August 2021 , Journal of Applied Ecology)

   Woody encroachment into grassy biomes is a global phenomenon, often resulting in a nearly complete turnover of species, with savanna specialists being replaced by forest‐adapted species. Understanding the mechanisms involved in this change is important for devising strategies for managing savannas.

   We examined how isolated trees favour woody encroachment and species turnover by overcoming dispersal limitation and environmental filtering. In a savanna released from fire in south‐eastern Brazil (Cerrado), we sampled woody plants establishing under 40 tree canopies and in paired treeless plots. These trees comprised eight species selected for habitat preference (savanna or forest) and dispersal syndrome (bird dispersed or not). We recorded dimensions of each tree, dispersal syndrome and habitat preference of recruits, and quantified the physical environment within each plot, aiming at a mechanistic understanding of woody encroachment.

   We found clear evidence that isolated trees cause nucleation and drive changes in functional composition of savanna. Effectiveness as nucleator differed among species, but was unrelated to their functional guilds (habitat preference or dispersal syndrome). The density of saplings in nuclei was partially explained by soil moisture (+), daily temperature amplitude (−) and sum of bases (−).

   Our results indicate that isolated trees act first as perches, strongly favouring bird‐dispersed species. They then act as nurse trees, considerably changing the environment in favour of forest‐adapted recruits. In the long term, as the nuclei expand and merge, savanna specialists tend to disappear and the savanna turns into a low‐diversity forest.

   Synthesis and applications. Fire suppression has allowed the nucleation process and consequently the woody encroachment and fast replacement of savanna specialists by forest species in the Cerrado. By elucidating the mechanisms behind woody encroachment, we recommend using prescribed fires to burn forest seedlings and to reduce tree canopy size wherever the management goal is to maintain the typical savanna structure and composition.

    

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5. Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal

   https://doi.org/10.1002/eap.2830  

   Dodds, Walter K. ; Ratajczak, Zak ; Keen, Rachel M. ; Nippert, Jesse B. ; Grudzinski, Bartosz ; Veach, Allison ; Taylor, Jeffery H. ; Kuhl, Amanda ( March 2023 , Ecological Applications)

   Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure from local (land use/land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here we report on a decade‐long watershed‐scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before–after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, associated with a decline in streamflow, loss of grassy plant species, and other ecosystem‐scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for 3 years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when reseeded with grassland species. Rapid expansion of shrubs (Cornus drummondii,Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every 2 years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands, resulting in inexorable movement toward a new ecosystem state. Human pressures, such as climate change, atmospheric CO₂increases, and elevated atmospheric nitrogen deposition, could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well‐studied sites.

    

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