Predator Dominated Reefs

By Brian Zgliczynski

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

If you could ask any of the scientists aboard the Hi'ialakai to describe what it's like to dive at Jarvis Island, you would hear something like: “mind-blowing, intimidating, exhilarating, intense, eye-opening“.  If you heard these words alone you would think we were out here in the central Pacific filming an energy drink commercial, or certainly something other than conducting scientific research.  However, this is definitely not the case, and Jarvis Island is all of this, and more. The first thing we notice upon arriving at a dive site are ominous shadows circling below. As we perform pre-dive checks and review survey protocols, you can’t help but wonder what awaits. The few minutes just before a dive can be filled with anticipation, and quite an adrenaline rush.

Predatory species like jacks and sharks
are abundant at Jarvis

Upon entering the water the ecological monitoring team is 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.

CRED divers conduct 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.

As predator dominated coral reef ecosystems become increasingly rare in most parts of the world, contemporary ecological studies concentrate efforts on systems that have already been degraded.  However, Jarvis Island and other U.S. Pacific islands represent some 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, aiding in the formulation of appropriate metrics necessary for developing effective ecosystem-based management and recovery plans towards the future.