Wet Climate Phases during the Pleistocene Probably Supported Higher Megafauna Populations in Southeastern North America

I love the fungus that grows on manure. I know that sounds weird, but the dung fungus spore concentration in sediment samples is the best evidence paleo-ecologists have of determining past megafauna populations. It is the perfect proxy because if dung fungus spores are high in a sample, megafauna populations must have been high during that time period. There is no hiding all the defecation that was occurring then. Low dung fungus spore concentrations are evidence of low megafauna populations. The latest dung fungus spore study was from a core of sediment taken beneath Lake Peten-Itza in Guatemala. The core was over 120 feet long and included radio-carbon dated time periods from 42,000 years BP-4,000 years BP. The dung fungus concentrations were compared with the pollen composition within each time period to determine what types of environments existed when megafauna populations were high or low. The types of environments fluctuated with known climate phases, alternating between oak and myrtle-dominated woodlands, pine-dominated woodlands, dry acacia-grassland scrub, and seasonal rain forest (the predominating present day environment). Oak-dominated woodlands prevailed during wet interstadials; acacia scrub grasslands prevailed during dry stadials. Megafauna populations were highest in this region during phases of climate that favored oak-dominated woodlands. I also noticed on the chart below that grass pollen was higher during this phase as well, suggesting wildlife had abundant grass and acorns to eat. Nearby fossil sites show horse, llama, mammoth, gompothere, and glyptodont occurred in this region during these time periods. Megafauna populations were lowest during dry stadials.

Location of the study site. Image from the below referenced study. Scientists took the core from 1 of the deepest parts of the lake that never dried out during dry climate phases.

Chart showing abundance of dung fungus (sporomiella) with pollen composition from a >120 foot core taken from sediment beneath Lake Peten-Itza. Megafauna populations were most abundant during wetter climate phases. Chart from the below reference.

Lake Peten-Itza today. It is surrounded by a seasonal rain forest, but during different climatic phases of the Pleistocene the surrounding environment varied between oak-dominated woodlands, pine-dominated woodlands, poor quality scrub grasslands, and seasonal rain forests. This lake is old and over 500 feet deep in some places.

Lucky Oak Woodland in Indiana. Much of central Georgia probably looked like this during wet interstadials of the Pleistocene.

Oak woodland in Ellijay, Georgia. Over 10,000 years ago this was prime habitat for Jefferson’s ground sloths, long-nosed peccaries, and tapirs. At least deer still occur here.

Other regions of the world weren’t the same. The mammoth steppe, a grassland and forb-dominated environment, located from northern Europe across Asia to Beringia, supported higher megafauna populations during cold stadials than other climate phases that favored forests and woodlands. The arid acacia scrub grasslands that occurred in Central America during stadials may have been nutrient poor and just did not support high populations of megafauna. Much of the region may have been bare soil.

I hypothesize populations of megafauna in the piedmont region of southeastern North America were also higher during interstadials. Pollen evidence indicates oak trees increased in abundance during these climate phases. Wetlands expanded and more grass, herbaceous plant growth, and acorns were available with increased precipitation; thus providing more potential food for wildlife. I think megafauna were likely limited to oasis-like habitats in this region during cold dry stadials. These habitats probably occurred in river valleys where stream flow was much reduced, and instead of meandering continuous rivers like those of today, the waterway was more like a chain of pools clogged with sand bars.

Many folks imagine Pleistocene-environments to resemble the modern day Serengeti, but this was not always the case. During cold dry climatic phases large areas may have hosted scarce wildlife populations restricted to shrinking water holes. Wildlife populations rebounded whenever climate phases shifted to more moist conditions. I’m sure wildlife populations fluctuated in parts of North America just like they did in Guatemala.

Reference:

Rozas-Davila, A.; A. Correa-Metreo, N. McMichael, M. Bush

“When the Grass wasn’t Green: Megafaunal Ecology and Paleodroughts”

Quaternary Science Review 266 August 2021