Rain Forest Fragmentation and Environmental Dynamics on Nosy Be Island (NW Madagascar) at 1300 cal BP Is Attributable to Intensified Human Impact

Abstract

Madagascar houses one of the Earth’s biologically richest, but also one of most endangered, terrestrial ecoregions. Although it is obvious that humans substantially altered the natural ecosystems during the past decades, the timing of arrival of early inhabitants on Madagascar as well as their environmental impact is still intensively debated. This research aims to study the beginning of early human impact on Malagasy natural ecosystems, specifically on Nosy Be island (NW Madagascar) by targeting the sedimentary archive of Lake Amparihibe, an ancient volcanic crater. Based on pollen, fungal spore, other non-pollen palynomorph, charcoal particle and diatom analyses combined with high-resolution sediment-physical and (in)organic geochemical data, paleoenvironmental dynamics during the past three millennia were reconstructed. Results indicate a major environmental change at ca. 1300 cal BP characterized by an abrupt development of grass (C4) dominated and fire disturbed landscape showing the alteration of natural rain forest. Further, increased soil erodibility is suggested by distinct increase in sediment accumulation rates, a strong pulse of nutrient input, higher water turbidity and contemporaneous increase in spores of mycorrhizal fungi. These parameters are interpreted to show a strong early anthropogenic transformation of the landscape from rain forest to open grassland. After ca. 1000 cal BP, fires remain frequent and vegetation is dominated by forest/grassland mosaic. While natural vegetation should be dominated by rain forest on Nosy Be, these last results indicate that human continuously impacted the landscapes surrounding the lake. At a local scale, our data support the “subsistence shift hypothesis” which proposed that population expansion with development of herding/farming altered the natural ecosystems. However, a precise regional synthesis is challenging, since high-resolution multi-proxy records from continuous sedimentary archives as well as records located further north and in the hinterland are still scarce in Madagascar. The lack of such regional synthesis also prevents precise comparison between different regions in Madagascar to detect potential (dis)similarities in climate dynamics, ecosystem responses and anthropogenic influences at the island’s scale during the (late) Holocene.

A speleothem record of hydroclimate variability in northwestern Madagascar during the mid-late Holocene

“…The Rodrigues Island climate record has become a record of choice, taken as likely indicative of Madagascar's climate history by other studies of Madagascar's megafaunal population dynamics (e.g. Broothaerts et al, 2023;Domic et al, 2021;Hansford et al, 2021;Hixon et al, 2021Hixon et al, , 2022Reinhardt et al, 2022).…”

The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ10Be at the catchment and regional level

“…There are clear indications that the vegetation cover of Madagascar has undergone drastic changes over the past millennium with a general decline in forest cover that has been linked to anthropogenic impacts (Gasse & Van Campo, 2001; Joseph & Seymour, 2020; Joseph et al., 2021; McConnell & Kull, 2014; Reinhardt et al., 2022). We therefore did not include the current forest cover as an environmental variable but work with generalised bioclimatic regions.…”

Modern pollen rain reveals differences across forests, open and mosaic landscapes in Madagascar

“…In Madagascar, an increasing number of palaeoecological records covering the Holocene have been generated covering different vegetation types across the island and which tackle questions related to environmental change and its drivers (e.g. Broothaerts et al, 2023; Razafimanantsoa, 2021; Razanatsoa et al, 2022; Reinhardt et al, 2022; Teixeira et al, 2021), land use change, as well as human history, settlement and practices (Burney, 2003; Razanatsoa et al, 2021). These have been conducted despite the few available calibrations that date back to the 80s and 90s (Burney, 1988; Straka, 1991).…”