Bertarelli Summer2024 FINAL - Flipbook - Page 39
E NV IRO NM E NT
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RE W I L D I NG
Seabird guano delivers nitrogen
and phosphorus in the proper
ratios to enhance coral health.
times higher on rat-free islands than on “ratty” islands. Graham wondered
if the presence of seabirds affected other elements of the ecosystem, such
as fish and corals. “And the penny sort of dropped that, well, those seabirds
could be having a big influence on the reefs,” he says.
Graham had a hypothesis as to why. Because of the way global ocean
currents flow, tropics are nutritional deserts with very little of two critical
nutrients, nitrogen and phosphorus, which fertilize phytoplankton, the
base of the food chain. Seabirds can fly long distances and forage in places
that are richer in fish because they are richer in nutrients. When they come
back to islands to roost, they transport and then deposit those nutrients in
their guano. “However,” Graham notes, at the time, “very little was understood about the flow of nutrients from seabird islands to reefs.”
Graham chose six islands full of seabirds and six islands full of rats, and
extensively surveyed their soils, plants, and reefs for nutrients and the stable
isotopes of nitrogen. In nature, nitrogen atoms come in two forms, or isotopes: one with 15 neutrons and one with 14. The ratios of those two isotopes
act as a nutrient tracer, showing its flow through the ecosystem. For example,
seabird guano is enriched in the nitrogen isotope with 15 neutrons, called ¹⁵N.
If a seaweed is similarly enriched in ¹⁵N, it shows guano fertilized it.
“And as the data then started rolling in over the coming months,” Graham says, “It told quite a clean story about the role of the seabird nutrients
in fertilizing and boosting the productivity of the reef.”
Graham found that the nitrogen near seabird islands was a whopping 251
times greater than near ratty islands. On seabird islands, ¹⁵N was enriched
in soils, shrubs, in seaweeds, and in fish, showing that the guano was flowing from the island, onto the nearshore reef, and even farther offshore.
Near the seabird islands the total abundance of fish was nearly 50 percent
higher than near ratty islands.
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