Coral reef study proves humans are major ecosystem disruptors

In August 2019, the picturesque Molokini island in Hawaii saw an influx of more than 40,000 tourists, eager to experience its vibrant underwater world through snorkeling and diving. However, the COVID-19 pandemic and subsequent worldwide lockdown in March 2020 brought this bustling tourist destination to a standstill. 

This unforeseen event presented scientists with a unique opportunity to study the impact of underwater tourism on marine fish populations. The findings of this study, recently published in PLOS One, will aid resource managers in better protecting Molokini and other vulnerable marine ecosystems.

Dr. Kevin Weng, the study’s lead author from William & Mary’s Virginia Institute of Marine Science, explained that “the COVID-related tourism freeze provided a unique natural experiment to measure the effects of decreased tourism on fish behavior in a high-use, no-take marine protected area.” 

Collaborating with Dr. Weng were Dr. Alan Friedlander and Whitney Goodell from the National Geographic Society, as well as Dr. Laura Gajdzik and Russell Sparks from the Hawaii Department of Land and Natural Resources. Friedlander and Goodell also have affiliations with the University of Hawaii at Mānoa.

Molokini island proves to be perfect location for this study

Located three miles off the coast of Maui, Molokini was designated a “no-take” marine protected area (MPA) in 1977 due to concerns raised by tour operators about the impact of fishing and other consumptive activities. 

Russell Sparks noted that “tour operators have always been interested in the conservation of Molokini, and have worked with the State on several measures.”

With the growth of non-consumptive activities such as snorkeling and SCUBA diving, tour operators collaborated with the State to implement a limited-entry permit system for tour boats, as well as replacing anchoring with permanent moorings to protect coral reefs. The study focused on examining the effects of these non-consumptive activities on marine life. 

“Our research demonstrates that human presence alone can alter the community structure and possibly the functioning of an ecosystem,” said Dr. Weng. This implies that improvements can be made to reduce the impact of tourism on marine ecosystems in Hawaii and globally.

Community structure refers to the variety and population of species within an ecosystem. During Hawaii’s COVID lockdown – starting in March 2020 and gradually easing until tourist numbers returned to pre-pandemic levels in May 2021 – the researchers conducted SCUBA surveys on five separate occasions. 

These surveys recorded the species, abundance, size, and location of predatory and herbivorous fish in Molokini’s submerged crater. Additionally, the team tracked the movement of predatory species using electronic tags.

By comparing their observations with data collected in the years before and after the lockdown, the scientists were able to identify differences in fish community structure caused by human presence. Data on human presence was obtained through logbooks maintained by the 40 charter boat companies permitted to bring tourists into Molokini’s waters. 

A natural experiment with disturbing results

The results of a unique natural experiment, brought about by the COVID-19 pandemic, have revealed the impact of human presence on marine ecosystems. 

“When tourism shut down due to COVID, species that had been displaced from shallow habitats by high human presence moved back in on a timescale of months, increasing fish biomass as well as the proportion of larger predators,” explained study co-author Dr. Alan Friedlander.

The primary species contributing to the observed increase in biomass during the lockdown were fast-swimming predatory fish known as jacks. These fish tend to fear humans, as they are often targeted by anglers. 

As tourism resumed, the predators retreated to deeper waters, causing fish biomass and habitat use to return to pre-pandemic levels. Biomass refers to a combined measure of fish abundance and size.

Changes in predator biomass were also reflected in the behavior of these fish. Prior to the COVID lockdown, jacks were known to vacate the inside of Molokini’s crater during peak morning tourist visits. However, during the lockdown, they remained in the shallow, sheltered interior of the crater. 

As tourism resumed, the predators were once again displaced from their shallow-water habitat. This displacement is particularly concerning as their summertime spawning season coincides with the annual peak in marine tourism.

The human-induced displacement of predatory fish from Molokini’s crater is likely to have cascading effects on the local food web. Previous studies have demonstrated that a decline in the abundance of predatory fish impacts not only the herbivorous fish that serve as their prey but also the algae and other primary producers that herbivores consume.  

“Predators have diverse ecosystem roles and their loss can reduce the resistance and resilience of ecosystems to other stressors,” said Friedlander.

“Molokini is being overused, and that management is needed to improve not only ecosystem health but the visitor experience,” said Sparks. 

“Our findings indicate that the business-as-usual conditions of high tourism alter community structure by displacing predatory fishes to deeper environments,” said Weng. 

In addition, a 2011 study found that more than two thirds of visitors to Molokini felt crowded during their trip. Most people surveyed supported actions that would reduce visitor numbers.

“As Hawaii formulates marine management plans and undertakes the Sustainable Hawai`i Initiative, lessons from Molokini can help inform managers and help facilitate an effective response. As part of this process, we need to think strategically about the scale and configuration of tourism in Hawaii to optimize earnings and employment without damaging the environment,” said Gajdzik.

“Our study indicates that the intensity of non-consumptive uses, especially in heavily visited MPAs, should be considered for the long-term health and resilience of these ecosystems,” said Weng. “Management of tourism should be guided by biological research, and include clear and well-enforced rules, adaptive management, and broad stakeholder involvement.”

This research emphasizes the importance of understanding and addressing the consequences of human presence on marine ecosystems, particularly in popular tourist destinations like Molokini. By identifying these effects, scientists and conservationists can work together to implement more sustainable tourism practices, ensuring the long-term health of these critical marine environments.

Impact of climate change and booming population on global reefs

Climate change and population growth are two major factors contributing to the degradation of coral reefs worldwide. These fragile ecosystems, often referred to as the “rainforests of the sea,” support a vast array of marine life and provide numerous benefits to human populations, such as coastal protection, food, and tourism revenue. However, the combined impacts of climate change and population growth are putting immense pressure on coral reefs and threatening their survival.

1. Climate change:

• Ocean warming: As global temperatures rise due to climate change, so do ocean temperatures. Warmer waters can cause coral bleaching, a stress response in which corals expel the symbiotic algae living within their tissues. This algae provides corals with essential nutrients and gives them their vibrant colors. Bleached corals are more susceptible to disease and have a higher mortality rate, which can lead to the collapse of entire reef ecosystems.
• Ocean acidification: The ocean absorbs about 30% of the carbon dioxide (CO2) released into the atmosphere, causing a chemical reaction that lowers the water’s pH, making it more acidic. This process, known as ocean acidification, reduces the availability of calcium carbonate, a critical building block for coral skeletons. As a result, corals struggle to grow and maintain their structures, making them more vulnerable to erosion and breakage.
• Sea level rise: Rising sea levels, driven by melting ice caps and thermal expansion, can impact coral reefs in several ways. They can alter the light environment, change water circulation patterns, and increase sedimentation, all of which can negatively affect coral growth and overall reef health.

2. Population growth:

• Coastal development: As human populations grow, the demand for coastal development increases, leading to habitat destruction and modification. This can result in the loss of vital reef ecosystems and the reduction of the natural barriers that protect shorelines from erosion and storm damage.
• Overfishing: Growing populations also lead to increased demand for seafood, often resulting in overfishing. The removal of key herbivorous and predatory fish species can disrupt the balance of coral reef ecosystems, leading to algal overgrowth and reduced resilience to other stressors.
• Pollution: Increased human populations generate more pollution, including agricultural runoff, sewage, and industrial waste, which can harm coral reefs. Excess nutrients from pollution can cause eutrophication and algal blooms, which deplete oxygen levels and suffocate coral reefs. Additionally, chemicals and toxins can directly damage corals and other marine life.
• Increased tourism: Population growth and increased global mobility have contributed to a boom in marine tourism. While tourism can provide economic benefits, it can also lead to physical damage to coral reefs from boat anchors, snorkelers, and divers, as well as pollution from increased human presence in coastal areas.

In conclusion, climate change and population growth pose significant threats to coral reefs, leading to habitat degradation, loss of biodiversity, and the disruption of vital ecosystem services. It is crucial to implement sustainable management practices, reduce greenhouse gas emissions, and raise awareness about the importance of coral reefs to mitigate these impacts and preserve these unique and valuable ecosystems for future generations.

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