The antiquity of Madagascar’s grasslands and the rise of C4 grassy biomes

Aim Grasslands and savannas, which make up > 75% of Madagascar’s land area, have long been viewed as anthropogenically derived after people settled on the island c. 2 ka. We investigated this hypothesis and an alternative – that the grasslands are an insular example of the post‐Miocene spread of C₄ grassy biomes world‐wide. Location Madagascar, southern Africa, East Africa. Methods We compared the number of C₄ grass genera in Madagascar with that in southern and south‐central African floras. If the grasslands are recent we would expect to find fewer species and genera in Madagascar relative to Africa and for these species and genera to have very wide distribution ranges in Madagascar. Secondly, we searched Madagascan floras for the presence of endemic plant species or genera restricted to grasslands. We also searched for evidence of a grassland specialist fauna with species endemic to Madagascar. Plant and animal species endemic to C₄ grassy biomes would not be expected if these are of recent origin. Results Madagascar has c. 88 C₄ grass genera, including six endemic genera. Excluding African genera with only one or two species, Madagascar has 86.6% of southern Africa’s and 89.4% of south‐central Africa’s grass genera. C₄ grass species make up c. 4% of the flora of both Madagascar and southern Africa and species : genus ratios are similar (4.3 and 5.1, respectively). Turnover of grasses along geographical gradients follows similar patterns to those in South Africa, with Andropogoneae dominating in mesic biomes and Chlorideae in semi‐arid grassy biomes. At least 16 monocot genera have grassland members, many of which are endemic to Madagascar. Woody species in frequently burnt savannas include both Madagascan endemics and African species. A different woody flora, mostly endemic, occurs in less frequently burnt grasslands in the central highlands, filling a similar successional niche to montane C₄ grasslands in Africa. Diverse vertebrate and invertebrate lineages have grassland specialists, including many endemic to Madagascar (e.g. termites, ants, lizards, snakes, birds and mammals). Grassland use of the extinct fauna is poorly known but carbon isotope analysis indicates that a hippo, two giant tortoises and one extinct lemur ate C₄ or CAM (crassulacean acid metabolism) plants. Main conclusions The diversity of C₄ grass lineages in Madagascar relative to that in Africa, and the presence of plant and animal species endemic to Madagascan grassy biomes, does not fit the view that these grasslands are anthropogenically derived. We suggest that grasslands invaded Madagascar after the late Miocene, part of the world‐wide expansion of C₄ grassy biomes. Madagascar provides an interesting test case for biogeographical analysis of how these novel biomes assembled, and the sources of the flora and fauna that now occupy them. A necessary part of such an analysis would be to establish the pre‐settlement extent of the C₄ grassy biomes. Carbon isotope analysis of soil organic matter would be a feasible method for doing this.     

Climate‐driven rampant speciation of the Cape flora

Aim To test whether the radiation of the extremely rich Cape flora is correlated with marine‐driven climate change. Location Middle to Late Miocene in the south‐east Atlantic and the Benguela Upwelling System (BUS) off the west coast of South Africa. Methods We studied the palynology of the thoroughly dated Middle to Late Miocene sediments of Ocean Drilling Program (ODP) Site 1085 retrieved from the Atlantic off the mouth of the Orange River. Both marine upwelling and terrestrial input are recorded at this site, which allows a direct correlation between changes in the terrestrial flora and the marine BUS in the south‐east Atlantic. Results Pollen types from plants of tropical affinity disappeared, and those from the Cape flora gradually increased, between 10 and 6 Ma. Our data corroborate the inferred dating of the diversification in Aizoaceae c. 8 Ma. Main conclusions Inferred vegetation changes for the Late Miocene south‐western African coast are the disappearance of Podocarpus‐dominated Afromontane forests, and a change in the vegetation of the coastal plain from tropical grassland and thicket to semi‐arid succulent vegetation. These changes are indicative of an increased summer drought, and are in step with the development of the southern BUS. They pre‐date the Pliocene uplift of the East African escarpment, suggesting that this did not play a role in stimulating vegetation change. Some Fynbos elements were present throughout the recorded period (from 11 Ma), suggesting that at least some elements of this vegetation were already in place during the onset of the BUS. This is consistent with a marine‐driven climate change in south‐western Africa triggering substantial radiation in the terrestrial flora, especially in the Aizoaceae.     

Evidence of a tipping point in a southern African savanna?

Many ecosystems exhibit threshold behaviour, where periods of relative stability are punctuated by rapid transitions between alternate stable states when an ecological threshold, or tipping point, is reached. This is of concern in grass-dominated habitats, many of which appear to be on the point of conversion to more wooded vegetation assemblages. However, changes in grass-dominated ecosystems are often difficult to interpret, because it is not always clear whether grasslands are ancient or are anthropogenically derived from past deforestation. As a result, the conservation, maintenance and restoration of ancient grasslands are sometimes neglected.In this study, the history of vegetation change in the savannas of the Hluhluwe-iMfolozi Park, KwaZulu-Natal, South Africa, are investigated by analysing stable carbon isotopes (δ¹³C) from soil profiles. Without exception, the data show that C₃ dominated thicket, forest, and densely wooded savanna now occur on sites that were previously C₄ grassland or open savanna. Although the drivers of this change are not clear, there is potential for management intervention because tree density can be manipulated through fire, a natural part of this dynamic landscape. The study identified two sites which are at a threshold between C₄ and C₃ dominance, and highlighted them as priorities for conservation management intervention.     

When is a ‘forest’ a savanna,and why does it matter?

Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high‐rainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree–C₄ grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C₄ grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life‐history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire‐resistant tree species in a C₄ grass‐dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.     

Paleogeographic variations of pedogenic carbonate delta13C values from Koobi Fora, Kenya: implications for floral compositions of Plio-Pleistocene hominin environments

Plio-Pleistocene East African grassland expansion and faunal macroevolution, including that of our own lineage, are attributed to global climate change. To further understand environmental factors of early hominin evolution, we reconstruct the paleogeographic distribution of vegetation (C(3)-C(4) pathways) by stable carbon isotope (delta(13)C) analysis of pedogenic carbonates from the Plio-Pleistocene Koobi Fora region, northeast Lake Turkana Basin, Kenya. We analyzed 202 nodules (530 measurements) from ten paleontological/archaeological collecting areas spanning environments over a 50-km(2) area. We compared results across subregions in evolving fluviolacustrine depositional environments in the Koobi Fora Formation from 2.0-1.5 Ma, a stratigraphic interval that temporally brackets grassland ascendancy in East Africa. Significant differences in delta(13)C values between subregions are explained by paleogeographic controls on floral composition and distribution. Our results indicate grassland expansion between 2.0 and 1.75 Ma, coincident with major shifts in basin-wide sedimentation and hydrology. Hypotheses may be correct in linking Plio-Pleistocene hominin evolution to environmental changes from global climate; however, based on our results, we interpret complexity from proximate forces that mitigated basin evolution. An approximately 2.5 Ma tectonic event in southern Ethiopia and northern Kenya exerted strong effects on paleography in the Turkana Basin from 2.0-1.5 Ma, contributing to the shift from a closed, lacustrine basin to one dominated by open, fluvial conditions. We propose basin transformation decreased residence time for Omo River water and expanded subaerial floodplain landscapes, ultimately leading to reduced proportions of wooded floras and the establishment of habitats suitable for grassland communities.     

Pedogenic carbonate stable isotopic evidence for wooded habitat preference of early Pleistocene tool makers in the Turkana Basin

The origin and evolution of early Pleistocene hominin lithic technologies in Africa occurred within the context of savanna grassland ecosystems. The Nachukui Formation of the Turkana Basin in northern Kenya, containing Oldowan and Acheulean tool assemblages and fossil evidence for early members of Homo and Paranthropus, provides an extensive spatial and temporal paleosol record of early Pleistocene savanna flora. Here we present new carbon isotopic (δ¹³C_VPDB) values of pedogenic carbonates (68 nodules, 193 analyses) from the Nachukui Formation in order to characterize past vegetation structure and change through time. We compared three members (Kalochoro, Kaitio, and Natoo) at five locations spanning 2.4–1.4 Ma and sampled in proximity to hominin archaeological and paleontological sites. Our results indicate diverse habitats showing a mosaic pattern of vegetation cover at each location yet demonstrate grassland expansion through time influenced by paleogeography. Kalochoro floodplains occurred adjacent to large river systems, and paleosols show evidence of C₃ woodlands averaging 46–50% woody cover. Kaitio habitats were located along smaller rivers and lake margins. Paleosols yielded evidence for reduced portions of woody vegetation averaging 34–37% woody cover. Natoo environments had the highest percentage of grasslands averaging 21% woody cover near a diminishing Lake Turkana precursor. We also compared paleosol δ¹³C_VPDB values of lithic archaeological sites with paleosol δ¹³C_VPDB values of all environments available to hominins at 2.4–1.4 Ma in the Nachukui and Koobi Fora Formations. Grassy environments became more widespread during this interval; woody canopy cover mean percentages steadily decreased by 12%. However, significantly more wooded savanna habitats were present in the vicinity of lithic archaeological sites and did not mirror the basin-wide trend of grassland spread. Hominin lithic archaeological sites consistently demonstrated woody cover circa 40% throughout our study interval and were 4–12% more woody than coeval basin environs. We propose that Turkana Basin early tool makers may have preferred a more wooded portion of the savanna ecosystem to reduce heat stress and to gain differential access to potable water, raw materials, animal carcasses, and edible plants.     

Forest people: The role of African rainforests in human evolution and dispersal

Conventionally, the African continent has been partitioned in two evolutionary domains. One of them, the rainforest, is home to apes and covers central and West Africa. The other one extends through the woodlands and savannas of East and Southern Africa and has been traditionally perceived as home to humanity. The morphology of early humans is well‐adapted to open environments.¹ In addition, food procurement in savannas is known to be easier and more reliable than is provisioning in the rainforest, with its dispersed and cryptic faunal resources and fickle carbohydrates and fat. In the late 1980s, human ecologists and socio‐cultural anthropologists demonstrated that full‐fledged foraging without some agricultural support has been virtually undocumented in tropical forests today or in the recent past.² This research portrayed the present‐day rainforest ecosystem as an unfriendly environment that is unable to support purely foraging groups, and questioned whether hominids ever lived in it.²     

Paleosols and paleoenvironments of the middle Miocene,Maboko Formation,Kenya

The middle Miocene (15 Ma) Maboko Formation of Maboko Island and Majiwa Bluffs, southwestern Kenya, has yielded abundant fossils of the earliest known cercopithecoid monkey (Victoriapithecus macinnesi), and of a kenyapithecine hominoid (Kenyapithecus africanus), as well as rare proconsuline (Simiolus leakeyorum, cf. Limnopithecus evansi) and oreopithecine apes (Mabokopithecus clarki, M. pickfordi), and galagids (Komba winamensis). Specific habitat preferences can be interpreted from large collections of primate fossils in different kinds of paleosols (pedotypes). Fossiliferous drab-colored paleosols with iron-manganese nodules (Yom pedotype) are like modern soils of seasonally waterlogged depressions (dambo). Their crumb structure and abundant fine root-traces, as well as scattered large calcareous rhizoconcretions indicate former vegetation of seasonally wet, wooded grassland. Other fossiliferous paleosols are evidence of nyika bushland (Ratong), and early-successional riparian woodland (Dhero). No fossils were found in Mogo paleosols interpreted as saline scrub soils. Very shallow calcic horizons (in Yom, Ratong, and Mogo paleosols) and Na-montmorillonite (in Mogo) are evidence of dry paleoclimate (300-500 mm MAP=mean annual precipitation). This is the driest paleoclimate and most open vegetation yet inferred as a habitat for any Kenyan Miocene apes or monkeys. Victoriapithecus was abundant in dambo wooded grassland (Yom) and riparian woodland (Dhero), a distribution like that of modern vervet monkeys. Kenyapithecus ranged through all these paleosols, but was the most common primate in nyika bushland paleosols (Ratong), comparable to baboons and macaques today. Mabokopithecus was virtually restricted to riparian woodland paleosols (Dhero), and Simiolus had a similar, but marginally wider, distribution. Habitat preferences of Mabokopithecus and Simiolus were like those of modern colobus monkeys and mangabeys. A single specimen of Komba was found in dambo wooded grassland paleosol (Yom), a habitat more like that of the living Senegal bushbaby than of rainforest galagids. A shift to non-forest habitats may explain the terrestrial adaptations of Victoriapithecus, basal to the cercopithecid radiation, and of Kenyapithecus, basal to the hominoid radiation. Both taxa are distinct from earlier Miocene arboreal proconsulines, oreopithecines and galagids.     

Pliocene forest dynamics as a primary driver of African bird speciation

Aim Montane tropics are areas of high endemism, and mechanisms driving this endemism have been receiving increasing attention at a global scale. A general trend is that climatic factors do not explain the species richness of species with small to medium‐sized geographic ranges, suggesting that geological and evolutionary processes must be considered. On the African continent, several hypotheses including both refugial and geographic uplift models have been advanced to explain avian speciation and diversity in the lowland forest and montane regions of central and eastern Africa; montane regions in particular are recognized as hotspots of vertebrate endemism. Here, we examine the possible role of these models in driving speciation in a clade of African forest robins. Location Africa. Methods We constructed the first robustly supported molecular phylogenetic hypothesis of forest robins. On this phylogeny, we reconstructed habitat‐based distributions and geographic distributions relative to the Albertine Rift. We also estimated the timing of lineage divergences via a molecular clock. Results Robust estimates of phylogenetic relationships and clock‐based divergences reject Miocene tectonic uplift and Pleistocene forest refugia as primary drivers of speciation in forest robins. Instead, our data suggest that most forest robin speciation took place in the Late Pliocene, from 3.2 to 2.2 Ma. Distributional patterns are complex, with the Albertine Rift region serving as a general east–west break across the group. Montane distributions are inferred to have evolved four times. Main conclusions Phylogenetic divergence dates coincide with a single period of lowland forest retraction in the late Pliocene, suggesting that most montane speciation resulted from the rapid isolation of populations in montane areas, rather than montane areas themselves being drivers of speciation. This conclusion provides additional evidence that Pliocene climate change was a major driver of speciation in broadly distributed African animal lineages. We further show that lowland forest robins are no older than their montane relatives, suggesting that lowland areas are not museums which house ‘ancient’ taxa; rather, for forest robins, montane areas should be viewed as living museums of a late Pliocene diversification event. A forest refugial pattern is operating in Africa, but it is not constrained to the Pleistocene.     

Fire and fire-adapted vegetation promoted C4 expansion in the late Miocene

Large proportions of the Earth's land surface are covered by biomes dominated by C(4) grasses. These C(4)-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C(4) grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO(2), seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C(4) grassland expansion. We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO(2), temperature, precipitation, fire and the tolerance of vegetation to fire influence C(4) grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model. We show that physiological differences between the C(3) and C(4) photosynthetic pathways cannot explain C(4) grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C(4)-dominated biomes can persist. We propose that three mechanisms were involved in C(4) expansion: the physiological advantage of C(4) grasses under low atmospheric CO(2) allowed them to invade C(3) grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.     

Contrasting long‐term records of biomass burning in wet and dry savannas of equatorial East Africa

Rainfall controls fire in tropical savanna ecosystems through impacting both the amount and flammability of plant biomass, and consequently, predicted changes in tropical precipitation over the next century are likely to have contrasting effects on the fire regimes of wet and dry savannas. We reconstructed the long‐term dynamics of biomass burning in equatorial East Africa, using fossil charcoal particles from two well‐dated lake‐sediment records in western Uganda and central Kenya. We compared these high‐resolution (5 years/sample) time series of biomass burning, spanning the last 3800 and 1200 years, with independent data on past hydroclimatic variability and vegetation dynamics. In western Uganda, a rapid (<100 years) and permanent increase in burning occurred around 2170 years ago, when climatic drying replaced semideciduous forest by wooded grassland. At the century time scale, biomass burning was inversely related to moisture balance for much of the next two millennia until ca. 1750 ad , when burning increased strongly despite regional climate becoming wetter. A sustained decrease in burning since the mid20th century reflects the intensified modern‐day landscape conversion into cropland and plantations. In contrast, in semiarid central Kenya, biomass burning peaked at intermediate moisture‐balance levels, whereas it was lower both during the wettest and driest multidecadal periods of the last 1200 years. Here, burning steadily increased since the mid20th century, presumably due to more frequent deliberate ignitions for bush clearing and cattle ranching. Both the observed historical trends and regional contrasts in biomass burning are consistent with spatial variability in fire regimes across the African savanna biome today. They demonstrate the strong dependence of East African fire regimes on both climatic moisture balance and vegetation, and the extent to which this dependence is now being overridden by anthropogenic activity.     

Cascading biodiversity and functional consequences of a global change–induced biome switch

Aim At a regional scale, across southern Africa, woody thickening of savannas is becoming increasingly widespread. Using coupled vegetation and faunal responses (ants), we explore whether major changes in woody cover in savannas represent an increase in the density of savanna trees (C₄ grass layer remains intact) or a ‘regime shift’ in system state from savanna to thicket (=dry forest) where broad‐leaved, forest‐associated trees shade out C₄ grasses. Location Hluhluwe Game Reserve, South Africa. Methods We sampled paired open (low woody cover) and closed (high cover that have undergone an increase in tree density) sites. Vegetation was sampled using belt transects, and a combination of pitfall trapping and Winkler sampling was used for ants. Results Closed habitats did not simply contain a higher density of woody savanna species, but differed significantly in structure, functional composition (high prevalence of broad‐leaved trees, discontinuous C₄ grasses) and system properties (e.g. low flammability). Ant assemblage composition reflected this difference in habitat. The trophic structure of ant assemblages in the two habitats revealed a functional shift with much higher abundances of predatory species in the closed habitat. Main conclusions The predominance of species with forest‐associated traits and concomitant reduction of C₄ grasses in closed sites indicate that vegetation has undergone a shift in fundamental system state (to thicket), rather than simply savanna thickening. This biome shift has cascading functional consequences and implications for biodiversity conservation. The potential loss of many specialist savanna plant species is especially concerning, given the spatial extent and speed of this vegetation switch. Although it is not clear how easily the habitat switch can be reversed and how stable the thicket habitats are, it is likely in the not‐too‐distant future that conservation managers will be forced to make decisions on whether to actively maintain savannas.     

Impacts of climate change on the vegetation of Africa: an adaptive dynamic vegetation modelling approach

Recent IPCC projections suggest that Africa will be subject to particularly severe changes in atmospheric conditions. How the vegetation of Africa and particularly the grassland–savanna–forest complex will respond to these changes has rarely been investigated. Most studies on global carbon cycles use vegetation models that do not adequately account for the complexity of the interactions that shape the distribution of tropical grasslands, savannas and forests. This casts doubt on their ability to reliably simulate the future vegetation of Africa. We present a new vegetation model, the adaptive dynamic global vegetation model (aDGVM) that was specifically developed for tropical vegetation. The aDGVM combines established components from existing DGVMs with novel process‐based and adaptive modules for phenology, carbon allocation and fire within an individual‐based framework. Thus, the model allows vegetation to adapt phenology, allocation and physiology to changing environmental conditions and disturbances in a way not possible in models based on fixed functional types. We used the model to simulate the current vegetation patterns of Africa and found good agreement between model projections and vegetation maps. We simulated vegetation in absence of fire and found that fire suppression strongly influences tree dominance at the regional scale while at a continental scale fire suppression increases biomass in vegetation by a more modest 13%. Simulations under elevated temperature and atmospheric CO₂ concentrations predicted longer growing periods, higher allocation to roots, higher fecundity, more biomass and a dramatic shift toward tree dominated biomes. Our analyses suggest that the CO₂ fertilization effect is not saturated at ambient CO₂ levels and will strongly increase in response to further increases in CO₂ levels. The model provides a general and flexible framework for describing vegetation response to the interactive effects of climate and disturbances.     

Long‐term variability and rainfall control of savanna fire regimes in equatorial East Africa

Fires burning the vast grasslands and savannas of Africa significantly influence the global carbon cycle. Projecting the impacts of future climate change on fire‐mediated biogeochemical processes in these dry tropical ecosystems requires understanding of how various climate factors influence regional fire regimes. To examine climate–vegetation–fire linkages in dry savanna, we conducted macroscopic and microscopic charcoal analysis on the sediments of the past 25 000 years from Lake Challa, a deep crater lake in equatorial East Africa. The charcoal‐inferred shifts in local and regional fire regimes were compared with previously published reconstructions of temperature, rainfall, seasonal drought severity, and vegetation dynamics to evaluate millennial‐scale drivers of fire occurrence. Our charcoal data indicate that fire in the dry lowland savanna of southeastern Kenya was not fuel‐limited during the Last Glacial Maximum (LGM) and Late Glacial, in contrast to many other regions throughout the world. Fire activity remained high at Lake Challa probably because the relatively high mean‐annual temperature (~22 °C) allowed productive C₄ grasses with high water‐use efficiency to dominate the landscape. From the LGM through the middle Holocene, the relative importance of savanna burning in the region varied primarily in response to changes in rainfall and dry‐season length, which were controlled by orbital insolation forcing of tropical monsoon dynamics. The fuel limitation that characterizes the region's fire regime today appears to have begun around 5000–6000 years ago, when warmer interglacial conditions coincided with prolonged seasonal drought. Thus, insolation‐driven variation in the amount and seasonality of rainfall during the past 25 000 years altered the immediate controls on fire occurrence in the grass‐dominated savannas of eastern equatorial Africa. These results show that climatic impacts on dry‐savanna burning are heterogeneous through time, with important implications for efforts to anticipate future shifts in fire‐mediated ecosystem processes.     

Comparative phylogeography of three endemic rodents from the Albertine Rift, east central Africa

The major aim of this study was to compare the phylogeographic patterns of codistributed rodents from the fragmented montane rainforests of the Albertine Rift region of east central Africa. We sampled individuals of three endemic rodent species, Hylomyscus denniae, Hybomys lunaris and Lophuromys woosnami from four localities in the Albertine Rift. We analysed mitochondrial DNA sequence variation from fragments of the cytochrome b and control region genes and found significant phylogeographic structuring for the three taxa examined. The recovered phylogenies suggest that climatic fluctuations and volcanic activity of the Virunga Volcanoes chain have caused the fragmentation of rainforest habitat during the past 2 million years. This fragmentation has played a major role in the diversification of the montane endemic rodents of the region. Estimation of the divergence times within each species suggests a separation of the major clades occurring during the mid to late Pleistocene.     

Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: Implications for early hominin paleoecology

Major morphological and behavioral innovations in early human evolution have traditionally been viewed as responses to conditions associated with increasing aridity and the development of extensive grassland-savanna biomes in Africa during the Plio-Pleistocene. Interpretations of paleoenvironments at the Pliocene locality of Laetoli in northern Tanzania have figured prominently in these discussions, primarily because early hominins recovered from Laetoli are generally inferred to be associated with grassland, savanna or open woodland habitats. As these reconstructions effectively extend the range of habitat preferences inferred for Pliocene hominins, and contrast with interpretations of predominantly woodland and forested ecosystems at other early hominin sites, it is worth reevaluating the paleoecology at Laetoli utilizing a new approach. Isotopic analyses were conducted on the teeth of twenty-one extinct mammalian herbivore species from the Laetolil Beds (∼ 4.3–3.5 Ma) and Upper Ndolanya Beds (∼ 2.7–2.6 Ma) to determine their diet, as well as to investigate aspects of plant physiognomy and climate. Enamel samples were obtained from multiple localities at different stratigraphic levels in order to develop a high-resolution spatio-temporal framework for identifying and characterizing dietary and ecological change and variability within the succession. In general, dietary signals at Laetoli suggest heterogeneous ecosystems with both C₃ and C₄ dietary plants available that could support grassland, woodland, and forested communities. All large-bodied herbivores analyzed yielded dietary signatures indicating mixed grazing/browsing strategies or exclusive reliance on C₃ browse, more consistent with wooded than grassland-savanna biomes. There are no clear isotopic patterns documenting shifting ecology within the Laetolil Beds or between the Laetolil and overlying Upper Ndolanya Beds, although limited data from the U. Ndolanya Beds constrains interpretations. Comparison of the results from Laetoli with isotopic enamel profiles of other African fossil and modern communities reveals significant differences in dietary patterns. Relative to extant taxa in related lineages, carbon isotopic ranges of a number of Laetoli fossil herbivores are anomalous, indicating significantly more generalized intermediate C₃/C₄ feeding behaviors, perhaps indicative of dietary niches and habitat types with no close modern analogs. Enamel oxygen isotope ranges of fossil taxa from Laetoli are consistently more ¹⁸O depleted than modern E. African herbivores, possibly indicating more humid conditions during that interval in the past. These data have important implications for reconstructing dietary trajectories of mammalian herbivore lineages, as well as the evolution of ecosystems in East Africa. Isotopic analyses of similar or related taxa at other hominin fossil sites yield signatures generally consistent with Laetoli, suggesting that mammalian communities in East Africa were sampling ecosystems with similar proportions of browse and grass. Collectively, the isotopic dietary signatures indicate heterogeneous habitats with significant wooded or forested components in the Laetoli area during deposition of the Laetolil and Upper Ndolanya Beds. Early hominin foraging activity in this interval may have included access to forest or woodland biomes within this ecosystem, complicating traditional interpretations linking early human evolutionary innovations with a shift to savanna habitats.     

Simulated distribution of vegetation types in response to climate change on the Tibetan Plateau

Abstract. Questions: What is the relationship between alpine vegetation patterns and climate? And how do alpine vegetation patterns respond to climate changes? Location: Tibetan Plateau, southwestern China. The total area is 2500000 km² with an average altitude over 4000 m. Methods: The geographic distribution of vegetation types on the Tibetan Plateau was simulated based on climatology using a small set of plant functional types (PFTs) embedded in the biogeochemistry‐biography model BIOME4. The paleoclimate for the early Holocene was used to explore the possibility of simulating past vegetation patterns. Changes in vegetation patterns were simulated assuming continuous exponential increase in atmospheric CO concentration, based on a transient ocean‐atmosphere simulation including sulfate aerosol effects during the 21st century. Results: Forest, shrub steppe, alpine steppe and alpine meadow extended while no desert vegetation developed under the warmer and humid climate of the early Holocene. In the future climate scenario, the simulated tree line is farther north in most sectors than at present. There are also major northward shifts of alpine meadows and a reduction in shrub‐dominated montane steppe. The boundary between montane desert and alpine desert will be farther to the south than today. The area of alpine desert would decrease, that of montane desert would increase. Conclusions: The outline of changes in vegetation distribution was captured with the simulation. Increased CO2 concentration would potentially lead to big changes in alpine ecosystems.     

A palaeoecological investigation into the role of fire and human activity in the development of montane grasslands in East Africa

Human activity has been widely implicated in the origin and expansion of montane grasslands in East Africa, yet little palaeoecological evidence exists to test whether these grasslands are natural or secondary. Pollen and charcoal data derived from two Holocene records in the Eastern Arc mountains of Tanzania are used as a case study to investigate the supposed secondary nature of montane grasslands in Africa. Fossil pollen data are used to detect vegetation change, and charcoal analysis is used to reconstruct fire history. The pollen data are characterised by stable proportions of local taxa suggesting permanence of grasslands throughout the past ~13,000 years. Recent increases in fire adapted taxa such as Morella point towards the development of a grassland/forest patch mosaic possibly associated with burning. However, robust evidence of human activity is absent from the records, which may be attributed to the late human occupation of the mountains. The records indicate long-term persistence of grasslands which, coupled with a lack of evidence of human activity, suggests that these grasslands are not secondary. These data support the hypothesis that grasslands are an ancient and primary component of montane vegetation in Africa, but that they experienced some expansion during the late Holocene as a result of changing fire regime.     

Oldest Evidence of Toolmaking Hominins in a Grassland-Dominated Ecosystem

Background

Major biological and cultural innovations in late Pliocene hominin evolution are frequently linked to the spread or fluctuating presence of C₄ grass in African ecosystems. Whereas the deep sea record of global climatic change provides indirect evidence for an increase in C₄ vegetation with a shift towards a cooler, drier and more variable global climatic regime beginning approximately 3 million years ago (Ma), evidence for grassland-dominated ecosystems in continental Africa and hominin activities within such ecosystems have been lacking.

Methodology/Principal Findings

We report stable isotopic analyses of pedogenic carbonates and ungulate enamel, as well as faunal data from ∼2.0 Ma archeological occurrences at Kanjera South, Kenya. These document repeated hominin activities within a grassland-dominated ecosystem.

Conclusions/Significance

These data demonstrate what hitherto had been speculated based on indirect evidence: that grassland-dominated ecosystems did in fact exist during the Plio-Pleistocene, and that early Homo was active in open settings. Comparison with other Oldowan occurrences indicates that by 2.0 Ma hominins, almost certainly of the genus Homo, used a broad spectrum of habitats in East Africa, from open grassland to riparian forest. This strongly contrasts with the habitat usage of Australopithecus, and may signal an important shift in hominin landscape usage.     

Ancient forest fragmentation or recent radiation? Testing refugial speciation models in chameleons within an African biodiversity hotspot

Aim East Africa is one of the most biologically diverse regions, especially in terms of endemism and species richness. Hypotheses put forward to explain this high diversity invoke a role for forest refugia through: (1) accumulation of new species due to radiation within refugial habitats, or (2) retention of older palaeoendemic species in stable refugia. We tested these alternative hypotheses using data for a diverse genus of East African forest chameleons, Kinyongia. Location East Africa. Methods We constructed a dated phylogeny for Kinyongia using one nuclear and two mitochondrial markers. We identified areas of high phylogenetic diversity (PD) and evolutionary diversity (ED), and mapped ancestral areas to ascertain whether lineage diversification could best be explained by vicariance or dispersal. Results Vicariance best explains the present biogeographic patterns, with divergence between three major Kinyongia clades (Albertine Rift, southern Eastern Arc, northern Eastern Arc) in the early Miocene/Oligocene (> 20 Ma). Lineage diversification within these clades pre‐dates the Pliocene (> 6 Ma). These dates are much older than the Plio‐Pleistocene climatic shifts associated with cladogenesis in other East African taxa (e.g. birds), and instead point to a scenario whereby palaeoendemics are retained in refugia, rather than more recent radiations within refugia. Estimates of PD show that diversity was highest in the Uluguru, Nguru and East Usambara Mountains and several lineages (from Mount Kenya, South Pare and the Uluguru Mountains) stand out as being evolutionarily distinct as a result of isolation in forest refugia. PD was lower than expected by chance, suggesting that the phylogenetic signal is influenced by an unusually low number of extant lineages with long branch lengths, which is probably due to the retention of palaeoendemic lineages. Main conclusions The biogeographic patterns associated with Kinyongia are the result of long evolutionary histories in isolation. The phylogeny is dominated by ancient lineages whose origins date back to the early Miocene/Oligocene as a result of continental wide forest fragmentation and contraction due to long term climatic changes in Africa. The maintenance of palaeoendemic lineages in refugia has contributed substantially to the remarkably high biodiversity of East Africa.