Walking around Redonda

One of our first goals on Redonda was to get a good sense for the density of lizards on the island. This is one of the key metrics that has changed following rat eradications elsewhere in the Lesser Antilles. To do that we used a few different methods, we’re hoping that each will give us a slightly different sense for densities that we can compare one year to the next.

The first was a mark-recapture study in two different locations. We’d survey a fixed area (one of the big fig trees at the north of the island and a boulder field at the south) and catch all the lizards we could find. Every time we caught a lizard we’d write a number on its belly and put a little white dot on its backside (see below). The paint spot will help us see the lizard from a distance without spending lots of time trying to catch a lizard that’d already been caught that day.

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Lizard number 22 getting a number on its belly. Photo credit: Geoffrey Giller

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An anole hanging out on a branch after getting a white spot. Photo credit: Geoffrey Giller

We spent two long days marking all of the lizards we could find in the tree and the rocks. After each site had had a day to rest, we returned. By figuring out which lizards we’d previously caught and how many new lizards had shown up we can get an estimate of just how many lizards are using that area. The lizards in the tree were particularly plentiful – we’re estimating a population of around 50! with unlimited time we’d continue marking, revisiting, and recapturing a few more days but our island time was short so we compromised with a single recapture day at each site. We’ll repeat the survey next year in the same places to see if there are changes in density.

Our next survey method was the tried-and-true walk around, look on both sides of your path, and count ’em up. For this Anthony did a big hike around the perimeter of the island counting all of the lizards he saw. He did this twice and found a lot of lizards! Here’s a page from my notebook showing the track. Unfortunately the map only really makes sense to us, but as you can see, he saw 61 anoles and 65 ameiva that day. The next day he ended up seeing 73 anoles and 118 ameiva! Those are pretty wide margins, but they’ll provide a baseline for next year’s repeat transects.

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Now, I decided to strap a GoPro to Anthony’s head for the second survey. It’s two hours long though so I had to speed the video up. Grab your dramamine, you’re going to need it!

This video is just a short segment of the hike in the vicinity of the “Hanging Gardens.” It’s one of the prettiest parts of the island I think. You can tell too how tricky some of the traversing is. On this section Anthony saw 10 ameiva and 24 anoles. Can you find them? I slowed down the first ameiva so you can get a good look. You’ll also notice a few other surprises along the way.

 

Describing A. nubilus

 

img_4456-e1487604077456.jpgDr. James “Skip” Lazell Jr. was the first to formally describe Anolis nubilus in a 1972 publication from the Bulletin of the Museum of Comparative Zoology at Harvard. Above are two of his drawings of the species from the publication. He’s a colorful and evocative writer so I’m going to let his description of his 1964 trek to find the species and his subsequent notes on their ecology stand on their own.

From “The Anoles (Sauria, Iguanidae) of the Lesser Antilles” Volume 143, No.1. 1972.

Distribution. Anolis nubilus occurs only on Redonda. This tiny islet is exceedingly steep-to, and rises nearly 1000 feet out of the sea. There is virtually no surrounding bank, and the full swell of the western North Atlantic pounds Redonda’s cliffs. A tiny, nearly vertical gut on the leeward side provides the only access to the top of the islet up the cliffs; great blocks of basalt lie at the foot of this gut, and one’s original entrance to Redonda is made by jumping onto these blocks as the boat goes past them. It is about like jumping from a moving elevator onto a card table, except that elevators give more notice of directional reversals…but getting on is just the beginning.

The islet is a great block of igneous extrusive: strata of basalt and the peculiarly conglomeratelike, porphyritic material so often the result of Antillean vulcanism. The top of Redonda is a rolling wold, and a favorite place of innumerable nesting sea birds; the gut provides a route for their guano to descend the cliffs, and it dries to a thick powder there. Because of its leeward location, a chimney effect is produced in the gut, and the guano dust, mixed with the volcanic sand weathered from the parent rock, tends to rise when disturbed. As one toils up the gut under the tropical sun, one is accompanied by a cloud of this dust, which soon mingles with ones’ own sweat to produce a wondrously aromatic and abrasive, though rather gluey, bath. At the top, jumbles of rocks and clumps of prickly pear rise gently to the old ruins, complete with a hedge of Bougainvillea and the single tree. This is the home of Anolis nubilus.

Population structure and ecology. Anolis nubilus is not abundant, but occurs all over Redonda. Owing to the lack of trees, it seems to dwell mostly in the shade of large rocks close to ground. In the ruins of the old building and on the one tree (a Casuarina, apparently inedible even to goats), A. nubilus climb as high as they can get: about fifteen feet. This species must compete with the large, glossy black ground lizard, Ameiva atrata, for at least some of its food. Surely Redonda once supported more vegetation and presumably Anolis nubilus then had an easier life. The feral goats should be extirpated on this remarkable island, whose only known nonflying vertebrates are species found nowhere else on earth.