Isla Rasa, while being very flat and very small, is used as a nesting grounds every year for as many as 500,000 seabirds at a time. About 95 percent of the world’s Heermann’s Gulls and Elegant Terns go there to breed.
Isla Rasa ls not easy to find. It lies between Isla Angel de la Guarda and Isla San Lorenzo, west of Isla Tiburon, in the Gulf of California about 37 miles southeast of Bahia de Los Angeles, Baja California.
The island’s highest elevation is only 115 feet (35 m), compared to elevations of 500 feet with the other midriff islands in the group. Isla Rasa’s landscape is made up of rolling hills of volcanic rock along with valleys filled with guano deposits. The island’s flat sedimentary valleys provide ideal nesting sites for nesting Heermann’s Gulls and Elegant Terns, as well as a smaller population of royal terns.
Centuries of nesting lead to human predation
This yearly nesting routine has gone on for centuries with the guano-filled valleys growing and eventually changing the topography of the island. Since 1953, when the nesting phenomenon was first described by Lew W. Walker, the little island has drawn tourists, naturalists, documentary makers and seabird researchers.
For years, marauding humans would come to the island during the nesting season, robbing the nests of freshly laid eggs and taking them away to be sold in the markets in Santa Rosalia and other ports. The problem came to a head when in the spring of 1964, a survey under the auspices of the Belvedere Scientific Fund, discovered not a single chick was to be found.
This predatory human act prompted then President of Mexico, Lopez Mateos, to sign a decree establishing Isla Rasa as a Migratory Waterfowl Sanctuary. This move led to all the midriff islands in the Sea of Cortez becoming a protected refuge.
A warming ocean, along with El nino impact nesting
In the past two decades, researchers and scientists have documented the seabirds arriving in April every year, but some years, the birds would arrive and then soon after, leave without nesting. The first time this happened was in 1998 when El nino caused ocean productivity to collapse along the Eastern Pacific coast from Chile to California.
Then in 2003, it happened again when the colony deserted the island. Since then, colony desertion has been occurring with increasing frequency, in 2009, 2010, 2014, and 2015. Researchers and conservationists were left with two questions, where were the birds going to nest, and what was causing the abandonment of their traditional nesting site?
Finding answers with parallel studies
A group of researchers from the U.S. and Mexico began a study to determine what was happening to the nesting Elegant Terns (Thalasseus elegans), a model species used to monitor ocean dynamics. Their research showed that ocean warming and overfishing are producing an ecological collapse of the Gulf of California’s midriff islands region.
The scientists used actual nest counts in seabird colonies from Mexico and California. They discovered the nesting grounds of the Elegant Tern had expanded from the Gulf of California, in Mexico, into Southern California over the past two decades. But they also noted the expansion fluctuated from year to year.
“Whenever the terns perceive the conditions in the Gulf as inadequate to ensure successful reproduction,” says Enriqueta Velarde, project leader, “they move to alternative nesting grounds in Southern California including the San Diego Saltworks, Bolsa Chica Ecological Reserve, and Los Angeles Harbor.”
“When the Gulf waters get unusually warm,” explains Exequiel Ezcurra, a longtime collaborator of Velarde and a professor of ecology at the University of California, Riverside, “the sea becomes capped by a layer of warm surface water and the upwelling of cold, nutrient-rich waters fails to reach the surface. Productivity declines and, with it, the availability of small pelagic fish, on which the seabirds feed, also falls.”
The collapse in food availability for seabirds, caused by warmer waters, is compounded by a reduction in sardine populations due to overfishing in Mexico. With no food available, the seabirds seek better nesting grounds. The researchers concluded that “this behavior is new; before the year 2000, the terns stayed in the Midriff region, even when oceanographic conditions were adverse. Our results show that terns are responding dynamically to rapidly changing oceanographic conditions and fish availability by migrating 600 km northwest in search of more productive waters.”
The results of the study were published in AAAS journal Science Advances June 26, 2015, under the title: Warm oceanographic anomalies and fishing pressure drive seabird nesting north.
