Researchers from the University of Washington and NOAA's Northwest Fisheries Science Center found the opposite of what they expected when they used a new scientific method to sample the waters of Puget Sound.
Like humans, sea creatures are constantly leaving their DNA behind -- shedding, oozing from wounds or pooping into the environment. University of Washington professor Ryan Kelly said DNA testing has advanced so far that just one scoop of water can reveal what animals were nearby.
This relatively new capability is called environmental DNA, or eDNA for short. It can be used in two basic ways. One is to confirm the presence or absence of a specific critter, typically an invasive or endangered species. The other is to identify the suite of creatures around a place.
The paper published this week is one of the first to present results from the latter approach in saltwater. For this study, Kelly's team found four urban shoreline environments to compare to four that were less developed, all in the central Puget Sound region.
"We thought for sure we would see urbanization lead to a decrease in the number of different species we saw,” Kelly said. “Instead we found the opposite."
Kelly called this "great news" given how many stories there are about humans ruining the environment. He said these results should not give license to pave over paradise, but do raise questions for further investigation about how worthwhile spending millions of dollars on Puget Sound restoration would be.
The eDNA screening of seawater samples identified hundreds of species simultaneously. The study methodology excluded plants to focus solely on animals ranging from starfish, barnacles, limpets, snails, clams and crabs to kelp flies, marine mammals and salmon. Even eagles and bats that flew overhead left their identities behind.
"Much to our surprise, it seemed like diversity overall tended to increase in urban settings," Kelly said in an interview Thursday.
The researchers added the caveat that they found more local variation in the sea life composition in the eelgrass habitats next to less-urbanized shorelines.
"While... diversity strongly increased with upland urbanization, more-urban sites were significantly more homogeneous (within sites) than less-urban sites," the paper reported.
"We see these results as a counterexample to the idea that humans uniformly decrease biodiversity. Rather, the observation that more urbanized areas support larger, but more homogeneous, suites of species indicates a more nuanced effect of human alteration on nearshore communities," the paper concluded.
Kelly said the genetic traces left behind by marine life degrades beyond detection in a matter of hours to days and surprisingly doesn't drift very far from its source in that time.
The study was published this week in the peer-reviewed online science journal PeerJ. A grant from the David and Lucile Packard Foundation supported the research.
The researchers offered several possible explanations for what land-sea processes would cause more urbanized waters to have a greater richness of sea life. One possibility is that urban runoff brings more nutrients, which acts as fertilizer for ecosystem productivity. Another mechanism could be that upland development leads to a muddier seafloor, which supports more mollusk species. Or a reverse correlation is possible where the richness of the nearshore influences where humans settle most densely.
The broad scale marine species survey at the heart of this study could not disentangle which mechanisms were responsible for the trend.
"I thought the results were interesting, the analysis well executed and interpreted," wrote Rupert Collins, a University of Bristol researcher who provided an independent peer review.
The relatively new journal that published this study follows an "open access" philosophy, which means the scientific paper, the supporting data and peer-review comments are publicly accessible for free.