The effects of geographic range size and abundance on extinction during a time of “sluggish”’ evolution

Geographic range size and abundance are important determinants of extinction risk in fossil and extant taxa. However, the relationship between these variables and extinction risk has not been tested extensively during evolutionarily “quiescent” times of low extinction and speciation in the fossil record. Here we examine the influence of geographic range size and abundance on extinction risk during the late Paleozoic (Mississippian–Permian), a time of “sluggish” evolution when global rates of origination and extinction were roughly half those of other Paleozoic intervals. Analyses used spatiotemporal occurrences for 164 brachiopod species from the North American midcontinent. We found abundance to be a better predictor of extinction risk than measures of geographic range size. Moreover, species exhibited reductions in abundance before their extinction but did not display contractions in geographic range size. The weak relationship between geographic range size and extinction in this time and place may reflect the relative preponderance of larger-ranged taxa combined with the physiographic conditions of the region that allowed for easy habitat tracking that dampened both extinction and speciation. These conditions led to a prolonged period (19–25 Myr) during which standard macroevolutionary rules did not apply.

4.2 ka BP Megadrought and the Akkadian Collapse

The Akkadians, of southern Mesopotamia, created the first empire ca. 2300 BC with the conquest and imperialization of southern irrigation agriculture and northern Mesopotamian dry-farming landscapes. The Akkadian Empire conquered and controlled a territory of roughly 30,000 square kilometers and, importantly, its wealth in labor and cereal crop-yields. The Empire maintained a standing army, weaponry, and a hierarchy of administrators, scribes, surveyors, craft specialists, and transport personnel, sustainable and profitable for about one hundred years. Archaeological excavations indicate the empire was still in the process of expansion when the 2200 BC–1900 BC/4.2–3.9 ka BP global abrupt climate change deflected or weakened the Mediterranean westerlies and the Indian Monsoon and generated synchronous megadrought across the Mediterranean, west Asia, the Indus, and northeast Africa. Dry-farming agriculture domains and their productivity across west Asia were reduced severely, forcing adaptive societal collapses, regional abandonments, habitat-tracking, nomadization, and the collapse of the Akkadian Empire. These adaptive processes extended across the hydrographically varied landscapes of west Asia and thereby provided demographic and societal resilience in the face of the megadrought’s abruptness, magnitude, and duration.

Megadrought and Collapse

This is the first book to treat the major examples of megadrought and societal collapse, from the late Pleistocene end of hunter–gatherer culture and origins of cultivation to the 15th century AD fall of the Khmer Empire capital at Angkor, and ranging from the Near East to South America. Previous enquiries have stressed the possible multiple and internal causes of collapse, such overpopulation, overexploitation of resources, warfare, and poor leadership and decision-making. In contrast, Megadrought and Collapse presents case studies of nine major episodes of societal collapse in which megadrought was the major and independent cause of societal collapse. In each case the most recent paleoclimatic evidence for megadroughts, multiple decades to multiple centuries in duration, is presented alongside the archaeological records for synchronous societal collapse. The megadrought data are derived from paleoclimate proxy sources (lake, marine, and glacial cores; speleothems, or cave stalagmites; and tree-rings) and are explained by researchers directly engaged in their analysis. Researchers directly responsible for them discuss the relevant current archaeological records. Two arguments are developed through these case studies. The first is that societal collapse in different time periods and regions and at levels of social complexity ranging from simple foragers to complex empires would not have occurred without megadrought. The second is that similar responses to megadrought extend across these historical episodes: societal collapse in the face of insurmountable climate change, abandonment of settlements and regions, and habitat tracking to sustainable agricultural landscapes. As we confront megadrought today, and in the likely future, Megadrought and Collapse brings together the latest contributions to our understanding of past societal responses to the crisis on an equally global and diverse scale.

Megadrought, Collapse, and Causality

Recent discoveries of megadroughts, severe periods of drought lasting decades or centuries, during the course of the Holocene have revolutionized our understanding of modern climate history. Through advances in paleoclimatology, researchers have identified these periods of climate change by analyzing high-resolution proxy data derived from lake sediment cores, marine cores, glacial cores, speleothem cores, and tree rings. Evidence that megadroughts occurred with frequency and abruptly over the last 12,000 years, a timespan long assumed to be stable compared to earlier glacial periods, has also altered our understanding of societies’ trajectories. The fact that severe, multi-decadal or century-scale droughts coincided with societal collapses well known to archaeologists has challenged established multi-causal analyses of these events. Megadroughts, impossible to predict and impossible to withstand, may have caused political collapse, regional abandonment, and habitat tracking to still-productive regions. The nine megadrought and societal collapse events presented in this volume extend from the foraging-to-agriculture transition at the dawn of the Holocene in West Asia to the fifteenth-century AD collapse of the Khmer Empire in Angkor (Cambodia). Inevitably, this collection of essays also raises challenges to causal analyses of societal collapse and for future paleoclimatic and archaeological research.

Quantifying the effect of seasonal and vertical habitat tracking on planktonic foraminifera proxies

The composition of planktonic foraminiferal (PF) calcite is routinely used to reconstruct climate variability. However, PF ecology leaves a large imprint on the proxy signal: seasonal and vertical habitats of PF species vary spatially, causing variable offsets from annual mean surface conditions recorded by sedimentary assemblages. PF seasonality changes with temperature in a way that minimises the environmental change that individual species experience and it is not unlikely that changes in depth habitat also result from such habitat tracking. While this behaviour could lead to an underestimation of spatial or temporal trends as well as of variability in proxy records, most palaeoceanographic studies are (implicitly) based on the assumption of a constant habitat. Up to now, the effect of habitat tracking on foraminifera proxy records has not yet been formally quantified on a global scale. Here we attempt to characterise this effect on the amplitude of environmental change recorded in sedimentary PF using core top δ18O data from six species. We find that the offset from mean annual near-surface δ18O values varies with temperature, with PF δ18O indicating warmer than mean conditions in colder waters (on average by −0.1 ‰ (equivalent to 0.4 °C) per °C), thus providing a first-order quantification of the degree of underestimation due to habitat tracking. We use an empirical model to estimate the contribution of seasonality to the observed difference between PF and annual mean δ18O and use the residual Δδ18O to assess trends in calcification depth. Our analysis indicates that given an observation-based model parametrisation calcification depth increases with temperature in all species and sensitivity analysis suggests that a temperature-related seasonal habitat adjustment is essential to explain the observed isotope signal. Habitat tracking can thus lead to a significant reduction in the amplitude of recorded environmental change. However, we show that this behaviour is predictable. This allows accounting for habitat tracking, enabling more meaningful reconstructions and improved data–model comparison.

Quantifying the effect of seasonal and vertical habitat tracking on planktonic foraminifera proxies

The composition of planktonic foraminiferal (PF) calcite is routinely used to reconstruct climate change and variability. However, PF ecology leaves a large imprint on the proxy signal. The seasonal and vertical habitat of planktonic foraminifera (PF) species varies spatially, causing variable offsets from annual mean surface conditions recorded by sedimentary assemblages. PF seasonality changes with temperature in a way that minimises the environmental change that individual species experience. While such habitat tracking could lead to an underestimation of spatial or temporal trends and variability in proxy records, most paleoceanographic studies are based on the assumption of a constant habitat. Although the controls on depth habitat variability are less well constrained, it is not unlikely that habitat tracking also affects PF depth habitat. Despite the implications, the effect of this behaviour on foraminifera proxy records has not yet been formally quantified on a global scale. Here we attempt to characterise the effect of habitat tracking on the amplitude of environmental change recorded in sedimentary PF using core top δ18O data from six species, which we compare to predicted δ18O. We find that the offset from mean annual near-surface δ18O values varies with temperature, with PF δ18O indicating warmer than mean conditions in colder waters (on average by −0.1 ‰ (or 0.4°C) per °C), thus providing a first-order quantification of the degree of underestimation due to habitat tracking. We then use an empirical model to estimate the contribution of seasonality to the observed difference between PF and annual mean δ18O and use the residual Δδ18O to assess trends in calcification depth. Our analysis indicates that in all species calcification depth increases with temperature. Consistent with hydrographic conditions, vertical habitat adjustment is dominant in tropical species, whereas cold-water species mainly changes their seasonality when tracking their "optimum" habitat. Assumptions of constant PF depth or seasonal habitat made when interpreting proxy records are thus invalid. The approach outlined here can be used to account for these effects, enabling more accurate reconstructions and improved data-model comparison.