May 03, 2017 11:01 AM EDT
Can we see predators in diverse tropical ecosystems, making an influential impact on species merely two links away? Ecologists such as Brown University Professor Jon Witman and his team find that there can be indeed such a significant "trophic cascade" when triggerfish are involved.
There are two species of triggerfish that are overcoming continuous impact by sharks, sea lions, and hogfish that are trying to swallow pencil urchins, in order to bring down their algal consumption. These interactions are important to understand how humans also disturb the smooth functioning of global tropical ecosystems. So far, it has not really been understood how it happens. How can it be understood when and how trophic cascades occur along with who are the people involved? Studying the triggerfish in the sea world is one way to avoid or fix the problems, according to Phys.org.
The mid-level predator, the triggerfish, which is important to the coastal ecosystems, has been traced in the Galápagos. This area has not been hitherto disturbed. The new study indicates that this mid-level predator is vital to the coastal ecosystems.
Triggerfish are called 'keystone' predators in the subtidal food web. That is because they control the pencil urchins as well as the population of the algae, according to Witman and co-authors Franz Smith of Brown and Mark Novak of Oregon State University, who reported their findings in PLOS ONE.
The team was not satisfied to just watch the ecosystem or merely count the different species. Instead, through a number of controlled experiments between 2007 and 2013 that showed the cause and effect, the scientists collected thousands of time-lapse photographs that showed the results.
Scientists tied various urchins to rocks at diverse sites. This made them accessible to the prey so that they could easily observe the predators' action. Hence, they managed to find out the predators that could eat urchins. A couple of species of triggerfish called blunt head and a fine scale was found to eat all the urchins, especially the bigger ones. However, the smaller, green ones were not disturbed at all.
Green urchins were not such a favorite as the pencil urchin. Fish were always found to eat the pencil urchin first. However, sometimes the hogfish could carry off the green urchin. Experts found that the triggerfish faced a number of challenges to get a meal. Most of the time, the hogfish would try to gatecrash and carry off the food that the triggerfish began to eat. The hogfish would always eat the leftovers.
In another experiment, scientists built rings next to enclosed cages on the rocks under the sea. After a year, they allowed algal growth inside and then filled the cages with urchins. In the pens, urchins could graze on the algae but they were vulnerable to predators. In the enclosed cages, they could dine in peace. Some of the pens were left empty of urchins as experimental controls. Scientists found a difference in urchin predation as well as a difference in algae grazing. This showed "experimental evidence of a three-level consumptive trophic cascade from triggerfish to pencil urchins to benthic algae."
The triggerfish accomplished a rare ecological feat - they executed a "trophic cascade" in a tropical environment. "With 16 species of sea urchin predators known in the Galápagos subtidal, only two species, blunt head and fine scale triggerfish, preyed on large pencil urchins to the extent that they precipitated a trophic cascade," the authors wrote. "Consequently, blunt head and fine scale triggerfish are key consumers in the Galápagos subtidal ecosystem, ecologically important for indirect, positive effects on the abundance of benthic algae."
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