A study of reef fish on a chain of volcanic seamounts and islands off the coast of Brazil has enabled scientists to extend to marine organisms the landmark theory of island biogeography, which explains the distribution and diversity of species on islands over time.
Led by Hudson Pinheiro, who began work on the project as a graduate student at UC Santa Cruz and is now a postdoctoral ichthyologist at the California Academy of Sciences, the researchers have proposed a new conceptual model of island biogeography for marine organisms. The theory explores how different processes, like sea level fluctuations and geographic isolation, influence marine species diversity around islands.
The team found that, despite some similarities, the forces that tend to shape diversification and community assemblage on land are different from those that impact marine life around islands and seamounts. The researchers from UC Santa Cruz, California Academy of Sciences, and Universidade Federal do Espírito Santo published their findings August 30 in Nature.
A key difference between the marine and terrestrial realms is that many marine species disperse better than land-based animals, arriving quickly around inhabitable islands to fill available ocean niches. This fast action by skilled dispersers leaves no chance for species to fill new niches through adaptation and evolution, as is typically seen over time on terrestrial island habitats.
"Organisms disperse so well in marine systems because their larvae are moved by ocean currents over great distances. In terrestrial systems, dispersal to islands is much more limited," said coauthor Giacomo Bernardi, professor of ecology and evolutionary biology at UC Santa Cruz.
Pinheiro has spent years conducting surveys and studying the genetics of reef fish in the Vitória-Trindade Chain of seamounts and islands in the south Atlantic Ocean. “In order to crack the forces that shape ocean life around islands, we decided to dig into the evolutionary history of Brazilian reef fishes,” he said. “When we focused on 10 species in a chain of volcanic seamounts and islands off the coast of Brazil, we saw that older endemic species were larger. These large fishes were better dispersers than their ‘younger’ species counterparts, which were all small, bad dispersers. It’s clear that these unique marine environments need their own model to explain what lives where over time.”
Using DNA analysis to trace the evolutionary history of the reef fishes restricted to this specific chain of seamounts and islands, the scientists were able to better understand exactly how island geography, geological history, and sea level fluctuations generally impact the diversity of nearby marine species. Fish were collected from both shallow and deep reef locations. Pinheiro and his colleagues used specialized equipment called rebreathers for dives to depths of 200 to 500 feet beneath the ocean’s surface.
Their findings confirm that sea level changes and the locations of exposed seamounts play a critical role in marine evolution over time, mainly by intermittently providing stepping-stones for weak disperser species to colonize island habitats.
“We’re finally understanding how the complex, ever-changing web of island life on land differs from the same web in the ocean,” said senior author Luiz Rocha, Academy curator of ichthyology. “Think of iconic Galápagos finches—when their ancestor arrived in the region, there were many empty land niches to fill and adaptations emerged to help them specialize over time. It’s simply not the same in island marine habitats. Fast-dispersing marine organisms quickly colonize available niches, and we don’t see the same pattern of specialized adaptation over time.”
Biologists Robert H. MacArthur and E.O. Wilson pioneered the field of island biogeography in the 1960s as a way to explain the distribution of species around the world—how they move, change, and occasionally go extinct. Their seminal book on the subject was published in 1967. Islands isolated by water could be viewed as splintered habitats with higher extinction rates than their larger, continuous counterparts like continents and their coastlines.
“Our research team had quite an exciting brainstorming session about testing a marine biogeography theory,” Bernardi said. “We knew that MacArthur and Wilson came up with parts of their theory by drawing on restaurant napkins. We were drawing our hypotheses on paper towels in a lab. This type of scientific collaboration is exciting, and the foundational work of innovative predecessors helps make it happen.”
Understanding the forces that help shape what allows species to persist in (and adapt to) different environments over time is a useful tool in tailoring conservation work and modeling for the future. This field of research has prompted new generations of scientists—including the authors of the Nature paper—to push for a more detailed understanding of evolution during our modern time of mass species extinction and rapidly changing climate.
Terrestrial and marine environments are both vulnerable to the never-before-seen wave of species die-off—called the “sixth mass extinction”—currently underway. The global community risks losing valuable marine ecosystems like coral reefs—systems that support the livelihoods and well-being of billions of people worldwide—to the combined impacts of overfishing, habitat destruction, water pollution, climate change, and ocean acidification. At least a quarter of the world’s reefs have already been lost, with another 30 percent predicted to die in the next thirty years.
“We have hope,” Pinheiro said. “The more we uncover about dynamic marine environments and how they change over time, the better we can plan to protect them from future threats. This study sheds light on the ebb and flow of marine life on islands over evolutionary time. My role as a scientist is to chase the light, and encourage others to do the same.”
