Biology in the Anthropocene: Challenges and insights from young fossil records

Abstract

With overwhelming evidence of change in habitats, biologists today must assume that few, if any, study areas are natural and that biological variability is superimposed on trends rather than stationary means. Paleobiological data from the youngest sedimentary record, including death assemblages actively accumulating on modern land surfaces and seabeds, provide unique information on the status of present-day species, communities, and biomes over the last few decades to millennia and on their responses to natural and anthropogenic environmental change. Key advances have established the accuracy and resolving power of paleobiological information derived from naturally preserved remains and of proxy evidence for environmental conditions and sample age so that fossil data can both implicate and exonerate human stressors as the drivers of biotic change and permit the effects of multiple stressors to be disentangled. Legacy effects from Industrial and even pre-Industrial anthropogenic extirpations, introductions, (de)nutrification, and habitat conversion commonly emerge as the primary factors underlying the present-day status of populations and communities; within the last 2 million years, climate change has rarely been sufficient to drive major extinction pulses absent other human pressures, which are now manifold. Young fossil records also provide rigorous access to the baseline composition and dynamics of modern-day biota under pre-Industrial conditions, where insights include the millennial-scale persistence of community structures, the dominant role of physical environmental conditions rather than biotic interactions in determining community composition and disassembly, and the existence of naturally alternating states.

Keywords: biodiversity; conservation; ecology; paleobiology; paleoecology.

Fig. 1.

Historical trends in environmental conditions and technology over the last 5 million years, culminating in the Anthropocene, when human activities achieved a global signature. Most biological data (Top) are from studies conducted after World War II, and thus short-term variability has occurred in the context of trends rather than stationary means in climate, nutrients, and other human pressures. The deeper historical reach of paleobiological data challenge assumptions of stability and equilibrium and show that many populations, species, communities, and biomes had undergone significant changes in abundance, structure, and function at regional scales well before the mid-20th century. Note changes in scale along the x axis, where the present day is operationally set at 1950 CE (radiocarbon definition). Based on signals in sedimentary records and notwithstanding earlier effects on biota, geologists will likely formalize the start of the Anthropocene Epoch either at 1950 CE (i.e., when bomb-generated radionuclides appear and isotopically depleted industrial nitrates increase strongly) or at 1850 CE, with the culmination of the Industrial Revolution in Europe and North America, when atmospheric pCO₂ first reached the upper limit of Holocene variability (the previous ∼12,000 y) (2, 9). Sources are as follows: ocean pH (adapted from ref. 8); temperature anomalies from a 1961–1990 baseline for the Plio-Pleistocene (reprinted from ref. with permission from AAAS), Holocene to 1850 CE (10), and post-Industrial Revolution (HarCRUT3 data from ref. 11), all lowered by 0.3 °C to fit the 1986–2005 baseline used to project future changes (reprinted with permission from ref. 12); human cultural evolution (2); land-animal extinction phases, regional fisheries (13), and catch-based global data (reprinted with permission from ref. 14); land conversion (reprinted from ref. , by permission of the Royal Society); and anthropogenic reactive nitrogen input to biosphere (reprinted with permission from ref. 16), which now equals total natural N₂ fixation (dashed line).