Tuesday, November 22, 2011

Spelunking for the Farallon Cave Cricket

In the early 1900’s, Milton Ray, a poet and scientist who visited the Farallones several times, described the Lost World Cave in the following poem:

                More strange the Lost World Cave. Ah me!
                How few have trod its rough dark floor,
                Where chambers weird in endless maze
                Far downward lead, through darksome ways.

Ray photographed the cave, but after his last visit, he stated that its entrance had been collapsed by islanders to prevent children from entering and getting lost.  Although, the reputed photograph of the Lost World Cave shows limestone features, which contradict the granitic composition of the island, contemporary islanders have always wondered about its existence and where it might be located.

Reproduced from "The Farallones, The Painted World, and Other Poems of California, vol. 2" by Milton Ray
On 11 and 12 November 2011, we explored all of the known, accessible Farallon caves with the crew of Island Conservation to estimate the population of the endemic Farallon Cave Cricket. The Farallon Islands are pockmarked with wave-carved sea caves.  Most of these caves are still at sea level, with waves rolling in and out, excavating minute quantities of material with each thundering slap.  Most of these caves, though, are difficult to access and unlikely to harbor crickets anyhow, so we did not enter them.  However, at some point in the island’s geologic past, the island was either uplifted 50 feet, or the sea level was 50 feet higher.  This left four caves that are deep enough for a cricket to evolve into a species found nowhere else on the planet.

The Farallon Cricket (Farallonophilus cavernicolus) was first described by David C. Rentz in 1972.  It is a member of the camel or cave cricket family (Rhaphidophoridae), which is quite diverse in California.  Species in this group are wingless and have a brownish, humpbacked appearance with large hindlegs and long antennae.  The Farallon Cave Cricket is no exception.  However, several anatomical features are sufficiently distinct from other members of the camel cricket family to warrant it a unique genus.  Behaviorally, the Farallon Cave Cricket frequently gathers together in small to large groups that can number up to 100 individuals.  When they feel threatened, as when a bright light is held up to them, they may drop off the cave wall or ceiling to the floor below.  They require moist areas and darkness. Little is known about their natural diet, but they will eat oats in captivity; it is thought that they may forage on organic material brought into the caves by nesting seabirds.

Farallon Cave Cricket.  November 12, 2011.
The first cave we explored was the Rabbit Cave, which is the largest known cave on the island and is located on the southeast facing slope of Lighthouse Hill.  This cave received its name from the rabbits that were introduced to the island in the 1800’s and used to plague islanders and wildlife alike.  After many failed attempts by previous islanders, PRBO biologists succeeded in eradicating the rabbits in 1975.  The entrance to this cave starts out as a 50 foot crawl before opening up into a spacious cavern that is nearly 20 feet high.  This open cavern extends back another 50 feet or so before petering out.  Although this is the largest cave on the island, the crickets were small (~ half an inch) and not especially numerous, with 700-800 present.  Perhaps this is due to the southeast orientation of the cave entrance that produces a relatively dry interior.

Spelunking crew exploring Rabbit Cave.
Dan Grout recording cricket data.
We then visited a small cave between E-seal Blind Hill and Pointy Cliff that has a 5 x 5 foot opening and extends back about 15 feet.  One islander recently got the creeps here, so this cave now goes by the name Spooky Cave.  This cave faces northwest so that the prevailing winds keep it moist and mossy.  Despite the cave's shallowness, there were approximately 300-500 crickets, a quarter of which were relatively large (nearly an inch long).

The approach to Spooky Cave.  Aren't you scared?
Spooky Cave's interior isn't too spooky
Corm Blind Cave was next on our spelunking tour.  This cave is only about 4 feet high and extends back about 12 feet.  It also faces northwest and is quite moist, but differs from the others in that it lacks a large, protective chamber behind a small opening that can shelter the crickets from the full force of the winds.  In this cave, we found about 100 crickets and a Burrowing Owl!

Corm Blind Hill Cave
On the northwest side of Shubrick Point is a large cave that we have recently named Cricket Cave.  The entrance is 6 x 9 feet, and the initial chamber is ~60 feet deep.  This chamber tapers down in the back and appears to end.  A narrow passage remains hidden until you walk right up to the end.  When we first discovered this passage last year, we wondered if we might have stumbled upon Lost World Cave.  The second chamber is impressive, and goes another 70 feet before tapering down again to a crawl space that apparently dead ends.  This is a much wetter cave than the others, with stalactites and many active formations.  There were 300-600 crickets within 25m of entrance, and even one in a spider web at the entrance itself.  There were ~500-700 crickets in the back chamber, at least 150 feet from entrance, and probably more than 1,000 crickets in total in this cave.

Dan Grout preparing to enter Cricket Cave
Cricket Cave's initial chamber narrowing down.
Interesting cave wall patterns
Cricket congregation showing different sizes
Nice stalactite formation 
Mushroom  stalactites growing off the walls
Although it seems the majority of Farallon Cave Crickets live in these large caves, they can also be found in smaller crevices.  During the night of November 18, while watching Cassin's Auklets and Arboreal Salamanders wander about, we saw ~150 crickets in and around the small crevices near "The Gap," an area on the northwest side of the island.  Perhaps these small crevices are used as refuges during dispersal events.

A small crevice that held several crickets
Although we didn't find the Lost World Cave, this spelunking expedition taught us a bit more than we knew before about the Farallon Cricket's behavior and demography.

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