Sporormiella and the late Holocene extinctions in Madagascar - PubMed (original) (raw)

Sporormiella and the late Holocene extinctions in Madagascar

David A Burney et al. Proc Natl Acad Sci U S A. 2003.

Abstract

Fossil spores of the dung fungus Sporormiella spp. in sediment cores from throughout Madagascar provide new information concerning megafaunal extinction and the introduction of livestock. Sporormiella percentages are very high in prehuman southwest Madagascar, but at the site with best stratigraphic resolution the spore declines sharply by approximately 1,720 yr B.P. (radiocarbon years ago). Within a few centuries there is a concomitant rise in microscopic charcoal that probably represents human transformation of the local environment. Reduced megaherbivore biomass in wooded savannas may have resulted in increased plant biomass and more severe fires. Some now-extinct taxa persisted locally for a millennium or more after the inferred megafaunal decline. Sites in closed humid forests of northwest Madagascar and a montane ericoid formation of the central highlands show only low to moderate Sporormiella percentages before humans. A subsequent rise in spore concentrations, thought to be evidence for livestock proliferation, occurs earliest at Amparihibe in the northwest at approximately 1,130 yr B.P.

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Figures

Fig. 1.

Fig. 1.

Sporormiella photomicrographs. (a) Tetrad. These are not often intact as fossils in sediments. (b) Terminal spore segment showing characteristic sigmoid aperture. (c) Note characteristic aperture in medial segment. In sediments, generic-level Sporormiella spp. identifications are readily made from pollen slides. (Scale bars, 10 μm.)

Fig. 2.

Fig. 2.

Location of sites in Madagascar. The southwestern sites are in the arid coastal strip. Ambolisatra receives ≈400 mm of annual precipitation, with high interannual variability. Belo-sur-Mer is likewise variable and receives ≈500 mm. Ambolisatra was excavated and cored at an interdunal pond site known today as Andolonomby (S23°04′/E43°45′). Excavations and coring were carried out at the Ankilibehandry site (S20°44′/E44°01′) near Belo-sur-Mer. Benavony (S13°43′/E48°29′) is a riparian marsh on the Sambirano River in the humid (2,500 mm/yr) northwest interior. A deep crater lake was cored at Lac Amparihibe (S13°18′/E48°13′) on the humid shelf island of Nosy Be (2,000 mm/yr). In the central highlands, cores were analyzed from the shallow crater lake Tritrivakely (S19°47′/E46°55′; 1,800-m elevation) and the volcanic barrier lake at Kavitaha (S19°02′/E49°34′; 1,210-m elevation).

Fig. 3.

Fig. 3.

Sporormiella spore and charcoal particle analyses for six sites throughout Madagascar. Spores are expressed as percent of pollen sum + Sporormiella, and charcoal is expressed as projected area in slides per volume of sediment. (a) Coastal sites in the arid southwest show very high percentages of Sporormiella before human arrival approximately two millennia ago. Late Holocene sediments are devoid of charcoal before human arrival and for approximately two centuries after Sporormiella decline. From ≈1,720 yr B.P. to a few centuries ago, Sporormiella is absent, and charcoal reaches an initial peak and then declines. Both Sporormiella and charcoal increase in recent centuries. (b) Humid sites of the northwest, Benavony marsh on the Sambirano River and Amparihibe crater lake on the shelf island of Nosy Be, provide information on low-elevation rain-forest sites. Benavony shows low to moderate values for Sporormiella in the late Holocene before an inferred human presence. The spore is absent there and also at Amparihibe, beginning approximately two millennia ago, until 1,130 yr B.P. when a moderate Sporormiella signal may correspond to the introduction of cattle husbandry. Charcoal increases simultaneously with this spore increase at Amparihibe and several centuries later in the dense rainforest of the interior at Benavony. (c) Sporormiella results from two central highland sites show that low to moderate spore values were recorded at the Tritrivakely crater lake throughout the prehuman Holocene. At the lower and less remote site of a volcanic barrier lake at Kavitaha in the central highlands, a core spanning the last 1,500 yr shows Sporormiella increasing to moderate percentages beginning ≈960 yr B.P. Charcoal particles (data not shown; see ref. 20) increase above moderate background levels approximately four centuries earlier. See Table 1 for key 14C details and calibrations.

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