Twenty-five years ago, paleontologists Carl Brett and Gordon Baird conducted a study on the fossil record from the Devonian sea that once covered New York State. He found that the Hamilton fauna—as this area’s fossil community came to be known—was a thriving ecosystem 380 million years ago. The species composition of the area remained stable for 5 million years. It became known as the poster child for ecosystem stability in the fossil record.
Paleontologists took careful note of the two paleontologists’ study; but some were hesitant to dub the Hamilton ecosystem as stable. How could an ecosystem remain so ecologically sound over such a long period of time? Was species composition stability—even to this degree—indicative of ecological stability?
More studies emerged that dug deeper into the Hamilton fauna, but these new ones came up with confounding results. Some researchers after Brett and Baird found that species abundances –the number of creatures belonging to a specific species—were fluctuating.
Paleontologists were further baffled. Some argued that an ecosystem couldn’t possibly be stable when the species abundances varied so greatly, whether or not the species composition remained constant. They assumed that a stable head count meant a stable ecosystem.
A new study published May 15th in the journal PLOS ONE, refutes that assumption.
Greg Dietl, Judith Nagel-Myers and John C. Handley, all from PRI; worked with Carl Brett at the University of Cincinnati; to study the Hamilton fauna—this time focusing on clams—once again.
Abundance clearly wasn’t telling the whole story, so the research team measured two other proxies to determine what exactly was going on in this ecosystem 380 million years ago. In addition to head counts of clam species in the region, the research team charted predation scars on clams, and body size of clam species. Repair scars on the shells of these clams indicate that they were attacked by a predator, but survived. Body size is correlated with a number of ecological parameters and offers insight into how an organism acted and interacted within a particular ecosystem.
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The research team looked closelsy at repair scars on clam shells from failed predation attacks.
They discovered that predation and body size remained stable over a long period of time—800,000 years—even as abundances of clam species varied.
“Dynamics of food webs may be key to explaining the persistence of the Hamilton fauna in the face of widely fluctuating species abundances,” Dietl commented. For instance, predators often adaptively switch to feeding on different prey species, allowing the ecosystem to persist even in the event of a specific species declining in numbers. Another mechanism may be the ability of different species to occupy the same ecological niche in a particular ecosystem. Details of the system are constantly fluctuating, but this very fact allows the system as a whole to continue being stable.
“Scientists have to look beyond abundance to test ecological stability in the fossil record,” Nagel-Meyers explained. This study tells a cautionary tale to ecologists: simple head counts aren’t enough for determining what is going on in ecosystems past, present, or even future. Abundance is regularly used in paleoecology to test for ecological stability, but it isn’t always telling of the stability of an ecosystem. This isn’t to say abundances are uninformative—on the contrary, abundance tells a lot about how a system functions and how organism interactions contribute to food web dynamics—but pointing to fluctuating abundances as solid evidence of an unstable ecosystem isn’t a fair conclusion.
The study, Abundance Is Not Enough: The Need for Multiple Lines of Evidence in Testing for Ecological Stability in the Fossil Record, was published May 15 in the journal PLoS ONE.
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Greg Dietl is the Director of Collections at the Paleontological Research Insitituion in Ithaca, NY. He is a paleoecologist and Adjunct Assistant Professor of Earth and Atmospheric Sciences at Cornell University.
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Judith Nagel-Myers is PRI's Collections Manager.
The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.
