Virginia Tech scientists report that the diversity of sea creatures has risen and fallen over hundreds of millions of years, side by side with encounters between predators and prey, according to fossil evidence
For decades there has been a debate between paleontologists, biologists and ecologists about how ecological interactions, such as hunting, affect the way animals evolve over time.
John Warren Huntley, a postdoctoral fellow in the Department of Geosciences at Virginia Tech, and professor of geosciences Micha Kowalowski, decided to examine the importance of ecology by reviewing the literature for hunting cases in the marine invertebrate population, such as oysters and their relatives.
"Today, certain predators leave easy-to-identify marks, such as round, smooth holes, on the shells of their prey," Huntley said. "You can also find similar holes drilled by predators on the fossilized shells."
The researchers also looked for scars on the shells, which indicate that the prey survived the attack, and regenerated the shell.
The research was conducted by examining studies that reported the prevalence of drill holes and restoration scars in fossil species from the last 550 million years.
The first thing that Natalie and Kowalewski found was that the prevalence of hunting increased sharply 480 million years ago - about 50 million years earlier than reported in other studies. "The earlier studies were based on changes in morphology - predators with stronger jaws and claws and prey with more ornate shells. We looked at the frequency of attacks, which increased about 50 million years before the changes in armor," Huntley said.
But the most impressive discovery is the observation that the incidence of drill holes and restoration scars remarkably parallels the Spalkoski diversity curve for marine invertebrates. This diversity curve was created by the late Jack Spekoski of the University of Chicago, and describes the formation and extinction of marine animals in the last 540 million years (the Phanerozoic period). "There is a strong correlation between the prevalence of hunting and the diversity of the marine population during the Phanerozoic period," he said Huntley.
In the two researchers' paper, "Strong coupling between hunting frequency and diversity in the Phanerozoic fossil record," the researchers propose three opposing hypotheses that might explain the correlation. "This is the classic problem with interpreting correlations between two types of data," Huntley said, "you have to be careful when you choose a cause. Say, for example, factors X and Y are correlated. A change in X could also cause a change in Y, and a change in - Y could also cause a change in X. But it could also be that X and Y are both controlled by another factor."
The first hypothesis is that the prevalence of hunting can lead to diversity. "In this case, the ecological interactions will be important in evolution," Huntley said. "Organisms evolve over a long period of time, in response to their enemies. At a time when there is more hunting, more species will evolve."
The second hypothesis is that when more biodiversity was created, predators with more complicated hunting strategies also evolved. "Hunting methods such as drilling into shells and culling are more evolutionarily driven than more primitive methods of hunting such as swallowing the prey whole. In this case, we would expect sophisticated forms of hunting to evolve only when diversity is high," Huntley said.
The third hypothesis is that something else drives both hunting and biodiversity. "Certain periods have more sedimentary rocks, and therefore more fossils, that are better preserved than others," Huntley said. "You can see less diversity when there are fewer fossils to examine. It's possible that this bias in examining evidence affects our ability to find specimens from other periods with high hunting prevalence."
"Now we will try to resolve the complication," Huntley said. "We can test these hypotheses by examining relevant links between the frequency of hunting and diversity in today's marine populations. Additionally, we need to understand that the Spekosky curve is actually a product of examining unbalanced samples, and this will help us interpret our findings."
