Predator Dominated Reefs

By Brian Zgliczynski

Typical reef scene at Jarvis Island with large-bodied predatory species
patrolling the reef.

Upon entering the water, the fish team is typically greeted by numerous predatory fishes such as grey reef  sharks (Carcharhinus amblyrhynchos), twinspot snapper (Lutjanus bohar), black trevally (C. lugubris) , and coral grouper (Cephalopholis miniata). Large-bodied predatory species, which are common at Jarvis, are becoming increasingly rare throughout the tropical Pacific with fisheries exploitation exerting direct impact on reef-fish communities. Predatory species play an integral role in structuring coral reefs and the systematic removal of these important species can have detrimental impacts to the ecosystem.

Predatory species like jacks and sharks
are abundant at Jarvis

The fish team conducts surveys recording species composition as well as the number and size of all fishes observed in a predefined area. These data are converted into measures of abundance and biomass and used to estimate fish populations around an island or reef. At Jarvis, predatory species are highly abundant and account for over half of total fish biomass. Reef scenes like the one pictured above are commonplace. To put this into perspective, Jarvis has about 300 times more predatory fish biomass than the entire island of Oahu. The research conducted here has altered our perspective of the typical trophic pyramid in which predators (tertiary consumers) comprise a small fraction of total fish biomass in a reef ecosystem. At Jarvis Island, the trophic pyramid is inverted, with top predators accounting for a majority of fish biomass.

Trophic pyramids with species divided into their respective trophic categories.
Tertiary consumers = top-level predatory species, planktivores = species that
feed on microscopic organisms, Secondary consumers = lower-level carnivorous
species, and Primary consumers = herbivores. The Pyramid to the left represents
a degraded system with few predators (tertiary consumers) while the pyramid to
the right represents what researchers have observed at Jarvis Island,
where predators are highly abundant.

Grey reef shark, Jarvis.

As predator dominated coral reef ecosystems become increasingly rare in most parts of the world, contemporary ecological studies tend to concentrate efforts on systems that have already been degraded.  However, Jarvis Island represents one of the remaining examples of ecosystems in their natural state. Such systems provide an ecological baseline and an unprecedented opportunity for marine scientists to understand what ‘pristine’ coral reef ecosystems are like. A recently published article in the journal of Conservation Biology, Re-Creating Missing Population Baselines for Pacific Reef Sharks, is a great example of how the Coral Reef Ecosystem Division's monitoring program allows researchers to examine and compare the variations between these very different reef ecosystems across the Pacific. The article has gained national attention and was written up in Science Codex, by CNN, as well as the Washington Post.  It is studies such as this that support coral reef management and conservation efforts by providing the necessary science to aid in the development of effective ecosystem-based management and recovery plans towards the future.