ASU study examines why some environments have more species than others

Stalagmites tell the climate story about species diversity

March 21, 2023

Why do some regions of the world have so many species of plants while others have so few?

This is one of the great enduring research questions in the biological sciences, and there have been many ideas put forward as an answer. Close-up photo of a protea plant. Protea plant, native to South Africa Cape Floristic Region. Photo courtesy Kerstin Braun Download Full Image

One idea is that regions that have had relatively stable climates through the millennia will have greater numbers of species. In contrast, dramatic climate change of the type we are experiencing today will drive species extinct, thus winnowing away diversity.

A study“Climatic stability recorded in speleothems may contribute to higher biodiversity in the Cape Floristic Region.” Kerstin Braun, Richard M. Cowling, Miryam Bar-Matthews, Avner Ayalong, Tami Zilberman, Mark Difford, R. Lawrence Edwards, Xianglei Li, Curtis W. Marean. Journal of Biogeography. published in the Journal of Biogeography by an international transdisciplinary team of scientists, led by Arizona State University researchers Curtis Marean and Kerstin Braun, provides compelling evidence for this idea from the study of climate change from stalagmites.

The Cape Floristic Region on Africa’s southern tip has one of the most diverse floras on Earth despite having infertile soils and hot, dry summers. Normally, more productive, wetter regions have more diversity.

“To understand the reasons for this diversity, we need to develop long climate records from a large sample of regions in the world and study their climate stability relative to their floral species diversity,” said Marean, who is an ASU Foundation Professor, a research scientist with the Institute of Human Origins and School of Human Evolution and Social Change, and a Nelson Mandela University (South Africa) Honorary Professor.

To do that, Marean developed a program to find and study stalagmites throughout the Cape of South Africa and collaborate with a transdisciplinary team of scientists including South African botanist Richard Cowling of Nelson Mandela University.

Map of South Africa

Topography of South Africa with the location of the cave and major cities. Gray shading indicates the Cape Floristic Region and a dark grey outline marks the area dominated by winter rainfall (>60% rainfall between April and September). Image by Kerstin Braun PhD.

Why stalagmites?

Stalagmites can preserve long records of climate change; they grow in caves, sometimes for over tens to hundreds of thousands of years, and they preserve changes in stable isotopes of oxygen and carbon that depend on the rain water and vegetation above the cave site. They can also be accurately dated with a technique called uranium-thorium dating.

Marean and colleagues rescued a set of stalagmites destined for destruction by mining at Cape Limeworks in Robertson in the Cape Floristic Region. The stalagmites, which grew between about 670,000 and 240,000 years ago, were then studied by Braun, an assistant research scholar in the Institute of Human Origins and lead author of the study.

Stalagmite in a cave

Cape Lime Robertston cave. One of the studied stalagmites that grew in place for nearly 400,000 years. Photo by Curtis Marean

“We compared the Cape Limeworks records to those from other sites in similar summer-dry, Mediterranean-type climates but with lower diversities than the Cape Floristic Region," Braun said. "The results clearly show that the southwestern Cape was climatically much more stable through glacial-interglacial changes than the other areas."

The results are significant because the Cape experiences a Mediterranean climate with dry, hot summers and mild, wet winters. This means that temperatures are generally low during the main growth season.

Common hypotheses for the evolution of diverse floras were developed to explain the high diversity of the wet tropics and often postulate that warm and wet conditions are needed for the evolution of high diversity — for example, through increasing the speed of metabolism or interactions between organisms.

These hypotheses don’t apply in arid Mediterranean regions, yet many of the floras in those regions exceed those of many tropical rainforests in the number of species per area.

Researchers from Mediterranean climate regions have long recognized their exceptional floras and proposed alternative hypotheses for their evolution. Cowling thinks that environmental stability and low extinction rates are a major factor in the accumulation of species.

The study is the first time that a paleoclimate reconstruction spanning several hundreds of thousands of years backs up these claims.

Cowling notes that “These findings show that relative climate stability over evolutionary time explains patterns of diversity in Mediterranean regions: The more stable, the richer the flora, with the Cape at the head of the pack. It’s possible that the climate stability theory applies also to tropical rainforest regions.”

“Our results," said Marean, “provide a dire warning of the downstream impacts of rapid climate change that we are now experiencing. Our study shows that rapid climate change annihilates plant lineages, so the human-induced rapid climate change we see today will do the same with horrific consequences for the animals and humans that rely on those plants.”

This research was funded by the USA National Science Foundation, the Hyde Family Foundations, the John Templeton Foundation, Arizona State University, the European Commission, the South African National Research Foundation and Nelson Mandela University.

Julie Russ

Assistant director, Institute of Human Origins


A lost world and extinct ecosystem

May 15, 2020

Archaeological sites on the far southern shores of South Africa hold the world’s richest records for the behavioral and cultural origins of our species. At this location, scientists have discovered the earliest evidence for symbolic behavior, complex pyrotechnology, projectile weapons and the first use of foods from the sea.

The Institute of Human Origins' field study site at Pinnacle Point sits at the center of this record, both geographically and scientifically, having contributed much of the evidence for these milestones on the evolutionary road to being a modern human. Paleo Agulhas Plain Looking out at the Palaeo-Agulhas Plain from the cave entrance at the Pinnacle Point research site, (left) 200,000 years ago during glacial phases and lower sea levels, and (right) today where the ocean is within yards of the cave entrances at high tides. Image by Erich Fisher.

The scientists working on these sites, led by institute Associate Director Curtis Marean, have always faced a dilemma in understanding the context of these evolutionary milestones — much of the landscape used by these ancient people is now submerged undersea and thus poorly known to us.

Marean is a Foundation Professor with the School of Human Evolution and Social Change and Honorary Professor with Nelson Mandela University in South Africa.

The archaeological records come from caves and rockshelters that now look out on to the sea, and in fact, walking to many of the sites today involves dodging high tides and waves. However, through most of the last 200,000 years, lowered sea levels during glacial phases, when the ice sucks up the water, exposed a vast plain. The coast was sometimes as much as 90 kilometers distant. Our archaeological data shows that this was the prime foraging habitat for these early modern humans that, until recently, we knew nothing about.

That has now changed with the publication of 22 articles in a special issue of Quaternary Science Reviews titled “The Palaeo-Agulhas Plain: A lost world and extinct ecosystem.” About 10 years ago, Marean began building a transdisciplinary international team to tackle the problem of building an ecology of this ancient landscape. ASU, Nelson Mandela University, the University of Cape Town and the University of California, Riverside anchored the research team. Funded primarily by a $1 million National Science Foundation grant to Marean, with significant funding and resources from the Hyde Family Foundations, the John Templeton Foundation, ASU, the Institute of Human Origins and XSEDE, they developed an entirely new way to reconstruct “paleoecologies” or ancient ecosystems.

This began with using the high-resolution South African regional climate model — running on U.S. and South African supercomputers — to simulate glacial climate conditions. The researchers used this climate output to drive a new vegetation model developed by project scientists to recreate the vegetation on this paleoscape. They then used a wide variety of studies such as marine geophysics, deep-water diving for sample collection, isotopic studies of stalagmites and many other transdisciplinary avenues of research to validate and adjust this model output.

They also created a human “agent-based model” through modern studies of human foraging of plants, animals and seafoods, simulating how ancient people lived on this now extinct paleoscape.

“Pulling the threads of all this research into one special issue illustrates all of this science,” Marean said. “It represents a unique example of a truly transdisciplinary paleoscience effort and a new model for going forward with our search to recreate the nature of past ecosystems. Importantly, our results help us understand why the archaeological records from these South African sites consistently reveal early and complex levels of human behavior and culture.

"The Palaeo-Agulhas Plain, when exposed, was a ‘Serengeti of the South’ positioned next to some of the richest coastlines in the world. This unique confluence of food from the land and sea cultivated the complex cultures revealed by the archaeology and provided safe harbor for humans during the glacial cycles that revealed that plain and made much of the rest of the world unwelcoming to human life.”

Julie Russ

Assistant director, Institute of Human Origins