Variable phylogeographic histories of five forest birds with populations in Upper and Lower Guinea: implications for taxonomy and evolutionary conservation

This study evaluates phylogeographic structure in selected forest bird species distributed on either side of the Dahomey Gap in West Africa to extend analyses of avian diversification across the Guineo-Congolean forest. Mitochondrial DNA sequence data were collected for five species, including four with a break in distribution across the Dahomey Gap and one with a more continuous distribution. Data for the five focal species were included in a single analysis along with appropriate outgroup taxa, allowing estimates of divergence time on a common timescale. Both phylogeographic patterns and estimated divergence times were highly variable. In two cases (Green Hylia Hylia prasina and Yellow-whiskered Greenbul Eurillas latirostris), samples from east and west of the Dahomey Gap belong to divergent mitochondrial lineages, but with substantially older divergence in Eurillas (～4.1 MY). In Western Bluebill Spermophaga haematina, relatively divergent eastern and western lineages (～1.1 MY) were sampled in the same locality in eastern Ghana, suggesting east to west gene flow across the gap following earlier vicariance. In Forest Robin Stiphrornis erythrothorax, birds sampled within Ghana were from two highly divergent lineages (～1.9 MY), one of which is more closely related to birds from Cameroon and Equatorial Guinea, such that divergence across the Dahomey Gap was comparatively recent (～0.36 MY). For both Hylia and Stiphrornis, we corroborate recent studies documenting multiple divergent lineages within each genus, some of which may represent distinct species. Consistent with previous studies, our analysis shows that the Dahomey Gap can be a significant barrier to gene flow for species restricted to forest habitats. At the same time, individual species exhibit a broad range of divergence times and phylogeographic histories of variable complexity. We discuss the taxonomic implications of our results for each focal species.     

Pyrenean ptarmigans decline under climatic and human influences through the Holocene

In Europe, the Quaternary is characterized by climatic fluctuations known to have led tomany cycles of contraction and expansion of species geographical ranges. In addition,during the Holocene, historical changes in human occupation such as colonization orabandonment of traditional land uses can also affect habitats. These climatically oranthropically induced geographic range changes are expected to produce considerableeffective population size change, measurable in terms of genetic diversity andorganization. The rock ptarmigan (Lagopus muta) is a small-bodied grouseoccurring throughout Northern hemispheric arctic and alpine tundra. This species is notconsidered threatened at a continental scale, but the populations in the Pyrenees are ofconcern because of their small population size, geographical isolation and low geneticdiversity. Here, we used 11 microsatellites to investigate genetic variations anddifferentiations and infer the overall demographic history of Pyrenean rock ptarmiganpopulations. The low genetic variability found in these populations has been previouslythought to be the result of a bottleneck that occurred following the last glacial maximum(i.e., 10 000 years ago) or more recently (i.e., during the last 200 years). Ourresults clearly indicate a major bottleneck affecting the populations in the last tenth ofthe Holocene. We discuss how this decline can be explained by a combination of unfavorableand successive events that increased the degree of habitat fragmentation.     

Elaeis guineensis Jacq. (oil palm) fluctuations in central Africa during the late Holocene: climate or human driving forces for this pioneering species?

The oil palm (Elaeis guineensis) is naturally abundant in all the African rain forests. Because this tree is a heliophytic, pioneering species, it can rapidly colonize the periphery of the rain forest and gaps created inside, regardless of the cause. In the majority of traditional African societies, people exploit natural oil palm groves; no attempt is made to propagate the palm by sowing or transplanting seedlings. The late Holecene phase of dramatic forest decline, around 2500 uncal. B.P., was particularly favourable to oil palm expansion. The scale and timing of this palaeoenvironmental variation, and the subsequent response of this pioneer tree, cannot be interpreted as indicating human activities, let alone an indication of cultivation. Received December 4, 2000 / Accepted March 8, 2001     

First report on new evidence for the occurrence of Podocarpus and possible human presence at the mouth of the Amazon during the Late-glacial

Palynological studies on late Quaternary lake sediments from the region of the Amazon estuary, 100 km north-east of Belém, Pará State, Brazil, enable reconstruction of lowland Amazonian rain forest during the Late-glacial and Holocene periods. Late-glacial forests included populations of Podocarpus which suggests a distinct climatic cooling. Ilex was abundant in the early Holocene. Records of the mangrove taxon, Rhizophora, indicate rapid Atlantic sea-level rise in the beginning of the Holocene. High charcoal representation may reflect the first arrival of Amerindians in the Amazon coastal area, probably about 10 800 B.P.     

Impact of a drier Early-Mid-Holocene climate upon Amazonian forests

This paper uses a palaeoecological approach to examine the impact of drier climatic conditions of the Early-Mid-Holocene (ca 8000-4000 years ago) upon Amazonia's forests and their fire regimes. Palaeovegetation (pollen data) and palaeofire (charcoal) records are synthesized from 20 sites within the present tropical forest biome, and the underlying causes of any emergent patterns or changes are explored by reference to independent palaeoclimate data and present-day patterns of precipitation, forest cover and fire activity across Amazonia. During the Early-Mid-Holocene, Andean cloud forest taxa were replaced by lowland tree taxa as the cloud base rose while lowland ecotonal areas, which are presently covered by evergreen rainforest, were instead dominated by savannahs and/or semi-deciduous dry forests. Elsewhere in the Amazon Basin there is considerable spatial and temporal variation in patterns of vegetation disturbance and fire, which probably reflects the complex heterogeneous patterns in precipitation and seasonality across the basin, and the interactions between climate change, drought- and fire susceptibility of the forests, and Palaeo-Indian land use. Our analysis shows that the forest biome in most parts of Amazonia appears to have been remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning. Only in ecotonal areas is there evidence of biome replacement in the Holocene. From this palaeoecological perspective, we argue against the Amazon forest 'dieback' scenario simulated for the future.