Thursday, 25 October 2018

The owl, the mouse and the murrelet. How manmade climate change could be pushing species to the brink in ways rarely imagined.

by Larry Powell
A Scripp's murrelet chick (Synthliboramphus scrippsi). Humans hunted its cousin, 
the Great Auk, to extinction in the 18 hundreds. A U.S. Nat'l. Park Service photo.
A new study finds, complex changes in climate are threatening yet another species - this time a little diving seabird known as the Scripp's murralet (above). But this time, it isn't because of direct impacts  from severe weather events, as is often the case. Rather, it is how those events are interfering with traditional interactions between a predator, the barn owl (below) and its two main prey, the deer mouse and the murralet. The three species breed on the channel islands, off the coast of California. The study focused on Santa Barbara, the smallest.

Barn owl (Tyto alba). Photo by Peter K. Burian.
In-depth research by two American and two Canadian scientists, documents a fascinating but insidious train of events that could  be leading to the little seabird's demise. During prolonged, torrential rainfalls, vegetation on the island flourishes, providing plenty of food for island populations such as the deer mouse (below). This is quickly followed by a spike in barn owl numbers, the mouse's main predator. But when droughts set in, mice populations plummet. Apparently as a survival instinct, the owl then turns to the murralet. Estimates are that, at such times, the owls consume 15 times more murralet's than before. So, in this seemingly unjust scenario, when the mice diminish, it's the murralet, not the owl, which seems to pay the higher price.

Deer mouse. Peromyscus maniculatus.
Photo by Gregory "Siobirdr" Smith.

The Scripp's murralet is listed as "vulnerable" with, at most, 650 breeding pairs left on the island. Their numbers are believed to be declining and the scientists fear, because of this so-called  "hyperpredation," they may be headed for extinction.

But this disturbing prognosis may not be confined to Santa Barbara. Many ecosystems share the same set of conditions, where more than one species share a common predator. So it's feared the Santa Barbara experience may be set to repeat itself elsewhere, as well.

A condition known as "El Niño /Southern Oscillation, (ENSO)" is believed to be playing a role here. El Niños  have been naturally-occurring phenomena for hundreds of years (warming the Pacific ocean off South America and affecting weather events, worldwide). But some recent studies conclude El Niños, too, may be becoming more intense and numerous because of manmade climate change. So, rather than occurring roughly once every 20 years, we might expect El Niños closer to one every decade. This does not bode well for species like the murralet.

The study was published this week by The Royal Society. The lead author of the study was Prof. S. Thomsen of Simon Fraser University, B.C.   

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