(CN) – Coral reefs account for about a third of the all the biodiversity in Earth’s oceans and they are a vital resource for humanity as well. But they face threats from intensifying human activities that are changing the composition and survival rates of corals. Long-standing human stressors like agricultural runoff, overfishing and warming ocean temperatures brought on by climate change are in part responsible for large-scale coral reef die-offs.
"Coral reef ecosystems now appear to be unraveling before our eyes, with intensifying outbreaks of coral disease and bleaching threatening the persistence of reef habitats and the immense biodiversity they support," said Katie Cramer, an assistant research professor at the Julie Ann Wrigley Global Institute of Sustainability at Arizona State University and an Ocean Science Fellow at the Center for Oceans at Conservation International.
Coral reefs are one of the most sensitive ecosystems, requiring steady oxygen levels, pH, and temperatures to remain healthy. They are also one of the oldest ecosystems and have only become sensitive because of the constant effects of human activity weakening their resilience.
“Corals have withstood some major disturbances in the past, but these long-term human activities like fishing and farming have actually made reefs much less resilient to climate change,” said Cramer. “They are some of the oldest ecosystems and have withstood dramatic changes in climate, they just aren’t able to withstand us.”
They exist in shallow waters and accordingly face many physical threats from coastal development and recreational misuse. But agriculture and destructive fishing practices are some of the biggest culprits as well.
Cramer has conducted several research projects over the course of her career concerning marine life health and conservation efforts with a focus on coral reefs. Her work revolves around reconstructing long-term change in coral reef ecosystems by combining paleoecological, historical, and modern survey data to uncover the causes of reef declines and inform conservation efforts.
“We can get a good idea of the times of these changes by using radiometric dating, to get a good precise time of when these fossils lived. We can pinpoint when the change happened and link that with human activity of that time,” said Cramer. “We are using the tools that paleontologists use, but instead of going back through geological time we are just looking over human timescales.”
She added, “Along with the fossil data, we can also look at historical data. We looked at records of early European explorers from the 1500s, where they write about what they saw, and we could piece together those records to see what the reefs looked like compared to modern data.”
In her AAAS talk titled “Coral Reefs: Centuries of Human Impact,” she outlines evidence of the long-existing human ecological footprint that set off the incline of coral reef die-offs occurring in our oceans today. Her study provides proof that humans have affected reefs long before anyone knew what to look for, and with this knowledge scientists can better understand the causes of coral alteration and develop protections against them.
"I am interested in going back to the scene of the crime when humans first began to impact coral reefs centuries to millennia ago, to understand when, why, and how much reefs have been altered by humans," said Cramer. “This study goes back 125,000 years to track reefs before and since humans. We have found in the Caribbean that corals have been dying for a couple decades before climate change and bleaching events, likely due to fishing and pollution.”
In another of her studies, Cramer goes back in time and looks for the origins of coral reef decline in the Caribbean by tracking changes across the past 3,000 years. She does this by examining the composition of a variety of fossils discovered in reef sediment cores she collected herself from Panama, including various coral skeletons, fish teeth, urchin spines, mollusk shells and more.
“My colleagues and I hammered metal tubes and pulled sediments from below the modern reefs to look at these fossils spanning thousands of years to the present,” explained Cramer. “We can track the abundance of reef fish from the number of fish teeth fossils in these cores, and we can track the growth rate of corals by measuring the accumulation rate of reef sediments from coral fossils. We can track trace changes in composition by looking at coral fossils.
"These measurements showed that the decline in algae-eating reef fish due to overfishing directly caused a decline in coral growth rates over the past 1,000 years," Cramer continued. "Algae-eating fish such as parrotfish are very important to coral health because they prevent algae from overgrowing corals."
Her efforts have revealed that long-standing local human impacts including fishing and agriculture have been significantly altering coral reefs for centuries earlier than previously thought, if not longer. These harmful human activities have been doing invisible damage long before the rise of disease and bleaching epidemics, which are both commonly understood to be among the main drivers of coral loss.
“It's hard to understand the mechanisms of coral die-offs because there are so many different factors impacting them at once,” said Cramer. “In addition to fishing, there's land-based pollution in the form of sediments, fertilizers and sewage runoff. All these things are happening at once.”
Moving forward, Cramer will present the first evidence of her study that effectively reconstructed changes in coral communities from reefs across the Caribbean, spanning the pre-human period to the present. These projects reveal that coral communities were being altered by human activities much earlier than previously believed, allowing new conservation efforts to be put into place.
“By looking back in time before major human impact we get an idea of how these impacts have weakened corals' ability to withstand climate change. We need to take action and manage what we can, like fishing practices, to give reefs a fighting chance against climate change,” said Cramer.
Cramer hopes that through this work, and by listening to the echoes of past environmental change on coral reefs, her findings can help to better inform conservation efforts when coral reefs need help most.
"We need to resolve why and how much coral reefs have changed over human history to inform our responses to the current reef crisis. We need to understand how reefs have responded to past changes to best ensure their persistence into the future," said Cramer.
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