Moving up the food chain can beat being on top

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Credit: V. Rudolf/Rice University

When it comes to predators, the biggest mouths may not take the biggest bite. According to a new study from bioscientists at Rice University, some predators have their greatest ecological impacts before they reach adulthood.

"We live in a world where humans are impacting species at different stages of their lives, and this work shows the importance of considering the entire life cycle of a species rather than just looking at a snapshot in time," said ecologist Volker Rudolf, associate professor in Rice's Department of BioSciences and co-author of a new paper in Nature Ecology and Environment that explored the how predatory salamanders impacted pond ecosystems at various stages of their growth.

Rudolf said he and study co-author Benjamin Van Allen, a former Rice graduate student who is now a postdoctoral researcher at the University of California at San Diego, were each surprised at how large and long-lasting an impact younger predators could have on their surroundings. In a three-month experiment that covered 54 test ponds, Rudolf's team was able to compare exactly how each pond fared when salamanders of a certain age were and were not present. They found that the longest lasting and most significant changes to the ponds came from the earliest stages of salamander development — the stage when the tiny animals have small mouths, no legs and a far more limited diet than they do at adulthood.

"For predators, ecologists typically assume that size matters and that larger predators, which eat more, have the biggest impact on their environment," Rudolf said. "But aside from social mammals and birds, which require a lot of parental care when they're young, most predators hunt and eat almost from birth. Sharks, barracudas, reptiles and insects all start out eating stuff, and they just move up the food chain as they grow bigger."

As a predator grows, its diet typically expands as it goes after larger and more varied prey. Ecologists refer to this as a change in "trophic position," and they use a species' trophic position to determine how it's impacting its local food chain. Because of trophic changes, a single species of predator can impact its ecosystem in very different ways at different times in its life cycle.

"All creatures can only influence their present environment, but for most adult predators, that environment was shaped, in some way, by their former selves," Van Allen said. "We knew there had to be some sort of signal of the early stages that would carry over, but we didn't know how important or how strong that effect would be, partly because no one had ever done this sort of really big, in-depth experiment to actually test the effect."

Rudolf, who joined Rice's faculty in 2007, said the experiment was the largest and most complex his group has undertaken and required thousands of person hours.

"We didn't just set these ponds up and let them run," he said. "We were mimicking the inputs and outputs that we observed from natural ponds around Houston, but because our ponds were enclosed, we had to do all of that by hand."

Each of the ponds contained more than 60 species of plants and animals. And while the experiment ran for three months, the sample analysis took two years.

"In the end, we observed 257,522 individual animals, meaning that we identified, photographed and documented them as being in one of our samples," Rudolf said.

While he's glad the project is over and wouldn't want to repeat it any time soon, Rudolf said the work paid off because it allowed him and Van Allen to address ecological questions that had not been explored previously.

"We expected to find an effect (from the younger stages), but no one anticipated how large that effect would be," he said. "That was surprising.

"And I think it shows that if you want to think about the role of species' ecosystems, you have to look at the whole system, the whole life cycle," Rudolf said.

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The research was supported by the National Science Foundation.

High-resolution IMAGES are available for download at:

http://news.rice.edu/files/2017/01/0117_PREDATOR-vr-lg-25moja5.jpg CAPTION: Volker Rudolf (Photo by Tommy LaVergne/Rice University)

http://news.rice.edu/files/2016/06/0609_VOLKER-ben-lg-ygv2nf.jpg CAPTION: Benjamin Van Allen (Image courtesy of B. Van Allen/UCSD)

https://commons.wikimedia.org/wiki/File:Ambystoma_talpoideum.jpg CAPTION: A young salamander of the species Ambystoma talpoideum (Photo courtesy of B. Van Allen/UCSD)

https://commons.wikimedia.org/wiki/File:Ambystoma_talpoideum_UMFS_1.jpg CAPTION: An adult salamander of the species Ambystoma talpoideum (Photo courtesy of Fredlyfish4/Wikimedia Commons)

http://news.rice.edu/files/2017/01/0117_PREDATOR-field-lg-12q80r0.jpg CAPTION: Rice University undergraduates collecting samples from a Houston-area pond. (Photo courtesy of V. Rudolf/Rice University)

http://news.rice.edu/files/2017/01/0117_PREDATOR-sample-lg-1f6nn3c.jpg CAPTION: Rice University undergraduate sampling zooplankton from an experimental pond. (Photo courtesy of V. Rudolf/Rice University)

http://news.rice.edu/files/2017/01/0117_PREDATOR-tanks-lg-1owt4ht.jpg CAPTION: Experimental ponds at Rice University. (Photo courtesy of V. Rudolf/Rice University)

The DOI of the Nature Ecology & Evolution paper is: 10.1038/s41559-016-0038

A copy of the paper is available at: http://dx.doi.org/10.1038/s41559-016-0038

Related research from Rice:

Where were you born? Origin matters for species interactions — June 13, 2016 http://news.rice.edu/2016/06/13/where-were-you-born-origin-matters-for-species-interactions/

Ecosystems change long before species are lost — Aug. 12, 2013 http://news.rice.edu/2013/08/12/ecosystems-change-long-before-species-are-lost/

Study: Ecological effects of biodiversity loss underestimated — Nov. 30, 2010 http://news.rice.edu/2010/11/30/study-ecological-effects-of-biodiversity-loss-underestimated/

This release can be found online at news.rice.edu.

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Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,910 undergraduates and 2,809 graduate students, Rice's undergraduate student-to-faculty ratio is 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is ranked No. 1 for happiest students and for lots of race/class interaction by the Princeton Review. Rice is also rated as a best value among private universities by Kiplinger's Personal Finance. To read "What they're saying about Rice," go to http://tinyurl.com/RiceUniversityoverview.

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