Stable carbon isotope values confirm a recent increase in grasslands in northwestern Madagascar

Abstract

Madagascar is home to some of the world’s most unique plants and animals. Unfortunately, anthropogenic forest loss has had a dramatic impact on both floral and faunal communities. In many regions the scale and timing of this loss remains poorly constrained. Such is the case for northwestern Madagascar. Pollen records for this region suggest that fires and grass abundance increased around 1000 calendar years before present (cal. BP), as well as c. 500 cal. BP, presumably reflecting growing human settlements. However, temporal and spatial resolution for these transitions is limited by a hiatus in the sediment record. We present a suite of carbon isotope data from 14C-dated vertebrate bones from Anjohibe Cave, northwestern Madagascar. In agreement with palynological records, isotope data indicate that the transition to open savannah may be a recent phenomenon in northwestern Madagascar. We document dramatic changes in the vertebrate community over the past 2000 years. Remains from extinct taxa are all older than 1500 cal. BP, locally extirpated taxa were in the region until c. 300 years ago, and introduced species are essentially modern. Stable carbon data suggest minimal consumption of C4 resources by extinct, locally extirpated and locally extant endemic taxa. On the contrary, introduced rats and shrews exhibit exceptionally high carbon isotope values; this may reflect a dietary shift to newly introduced grasses or consumption of seeds from C4 plants.