Comparing the paleoclimates of northwestern and southwestern
Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction.
The relative importance of climate and humans in the
disappearance of the Malagasy megafauna remains
under debate. Data from southwestern Madagascar
imply aridifcation contributed substantially to the late
Holocene decline of the megafauna (the Aridifcation
Hypothesis). Evidence for aridifcation includes
carbon isotopes from tree rings, lacustrine charcoal
concentrations and pollen assemblages, and
changes in fossil vertebrate assemblages indicative
of a local loss of pluvial conditions. In contrast,
speleothem records from northwestern Madagascar
suggest that megafaunal decline and habitat change
resulted primarily from human activity including
agropastoralism (the Subsistence Shift Hypothesis).
Could there have been contrasting mechanisms of
decline in different parts of Madagascar? Or are we
lacking the precisely dated, high resolution records
needed to fully understand the complex processes
behind megafaunal decline?
Reconciling these contrasting hypotheses
requires additional climate records from southwestern
Madagascar. We recovered a stalagmite (AF2)
from Asafora Cave in the spiny thicket ecoregion,
~10 km from the southwest coast and just southeast
of the Velondriake Marine Reserve. U-series and
14C dating of samples taken from the core of this
stalagmite provide a highly precise chronology
of the changes in hydroclimate and vegetation in
this region over the past 3000 years. Speleothem
stable oxygen and carbon isotope analyses provide
insight into past rainfall variability and vegetation
changes respectively. We compare these records
with those for a stalagmite (AB2) from Anjohibe
Cave in northwestern Madagascar. Lastly, odds
ratio analyses of radiocarbon dates for extinct and
extant subfossils allow us to describe and compare
the temporal trajectories of megafaunal decline in
the southwest and the northwest. Combined, these
analyses allow us to test the Aridifcation Hypothesis
for megafaunal extinction.
The trajectories of megafaunal decline differed
in northwestern and southwestern Madagascar.
In the southwest, unlike the northwest, there is no
evidence of decoupling of speleothem stable carbon
and oxygen isotopes. Instead, habitat changes in
the southwest were largely related to variation in
hydroclimate (including a prolonged drought). The
megafaunal collapse here occurred in tandem with
the drought, and agropastoralism likely contributed
to that demise only after the megafauna had already
suffered drought-related population reduction.
Our results offer some support for the Aridifcation
Hypothesis, but with three caveats: frst, that there
was no island-wide aridifcation; second, that
aridifcation likely impacted megafaunal decline
only in the driest parts of Madagascar; and third,
that aridifcation was not the sole factor promoting
Comparing the paleoclimates of northwestern and southwestern
Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction Faina et al.: Comparing the paleoclimates of northwestern and southwestern Madagascar 109 megafaunal decline even in the dry southwest.
A number of megafaunal species survived the
prolonged drought of the first millennium, and then
likely succumbed to the activities of agropastoralists
