New Data Found to Help Understand Evolution, Extinction Impact
Newswise — Gregory Herbert, a University of South Florida paleontologist, and colleagues at Yale University and the University of California-Davis, have generated new data clarifying how the process of species extinction affects biological communities.
"When species become extinct, their losses can reverberate throughout the community causing changes in the abundance of other species, such as predators dependent upon them for food," Herbert said.
But that's not the only consequence. When predators go into decline, nature loses one of the primary agents of natural selection, and evolution can slow to a halt.
"Extinction does more than just reduce the number of species in a community," explained Herbert. "It changes the way communities function and the potential for future evolution."
To study the effects of extinction on predator abundance, Herbert and colleagues Gregory Dietl (Yale University) and Geerat Vermeij (UC-Davis) analyzed an ancient extinction event at the end of the Pliocene era, roughly 1.8 million years ago. But, the major breakthrough in the study was derived from a clue from their behavioral experiments on a modern predator-prey system.
"Muricid snails, which are common today in the Gulf of Mexico, drill holes in the shells of clams to extract the meat," explained Herbert. "The nice circular hole left in the clam shell is well-preserved in the fossil record, which means we can study drilling behaviors today as well as millions of years ago.
According to Herbert, the snails decide where on the clam shell to start drilling based on their own perceived risk of being eaten or losing food to predators, such as crabs, fish, and even drilling snails, while feeding. The positioning of drillholes in fossil clams may hold important clues about changes in abundances of predators during critical times in the past, such as before and after mass extinctions.
"Our job was to figure out what these clues might be," he said.
In a series of controlled laboratory experiments, the research team studied snails' responses to other predators. When snails were isolated from other predators they drilled the thickest parts of the shell away from the clam's valve edges to avoid getting crushed between the valves. However, when the same drilling snails were exposed to cues from other predators, they drilled clam prey at the thin edges of the valves. This reduced the predation process from seven days to about two days and gave drillers extra time to hide between feedings.
"Snails chose the edge drilling strategy only when the perceived risks of being eaten themselves, or having their food stolen, were greater than the risks of injury due to edge drilling," observed Herbert.
To investigate the effects of extinction on predator abundance, Herbert and colleagues also analyzed patterns of drillhole placement in 11,354 sample Chione clamshells from ancient shell beds in Florida and 5,328 modern Chione shells from modern Florida sea grass habitats. Edge drilling, they predicted, should characterize times of abundant predators, whereas drilling away from the edge should characterize times of ecosystem stress, such as mass extinctions, when predators are likely to be reduced in abundance.
As expected, their analyses showed edge drilling was common for a period of two to three million years prior to a mass extinction event, but declined significantly afterwards.
The results of their work challenge the consensus that Florida marine communities recovered rapidly from the extinctions.
"Our study paints a much bleaker picture," Herbert said. "The fact that edge drilling predation by muricids remains absent even today, when the diversity of mollusks is as high as it was prior to the extinctions, tells us ecological recovery has yet to happen."
Their study also sheds light on the mysterious reduction in evolutionary rates among marine organisms in the region over the last two million years.
"Without abundant predators, the primary impetus for evolution has been removed," concluded Herbert. "This should give us concern for present day biodiversity losses, which are heavily biased towards top predators."
Their study was published in the Dec. 24 issue of Science.
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