Splendid Toadfish (Sanopus splendidus) are endemic to the coastal reefs of Isla Cozumel
Photo: Randall McNeely
Rough and wild was the 30 minute ferry crossing from the Mexican mainland to Isla Cozumel. The boat, big enough to hold a couple hundred passengers, lurched its way across the 19 km wide channel, tossing about on the waves as though it was a mere canoe. Suspecting, based on the crew's cavalier attitude, that this was par for the course, I sat back and enjoyed the dramatic heave-to of the waves and veils of spray that blasted from the bow. The ride reminded me more of the North Atlantic than the western Caribbean. Despite the waves, our capable Mexican captain brought us dockside without incident. Half-domesticated Brown Pelicans (Pelecanus occidentalis) and skeins of undomesticated tourists lined the wharf. The pelicans were a welcome sight, and so were the Ruddy Turnstones (Arenaria interpres) that foraged among the feet of passersby, like some kind of maritime pigeons. I had stilled myself for tourists, but the sight of half a dozen massive cruise ships anchored nearby caused me to recalibrate my expectations. Cozumel, like Playa del Carmen, the mainland port from which I sailed, is a tourist trap, attracting sun-seekers from all over the north to hotels and resorts, including some that (disappointingly, but not surprisingly) have their own private pods of captive dolphins! But, I wasn't on the island for a luxury vacation, and I certainly wasn't heading to play with the caged cetaceans. Instead, I was in search of something far more interesting and far more worthwhile: Cozumel's unique endemic species.
Isla Cozumel amounts to only about 10% of Quintana Roo's land area, but it holds an estimated 40% of the state's animal diversity; and a great deal of that diversity is found only on Cozumel.
The Cozumel Harvest Mouse (Reithrodontomys spectabilis), Pygmy Raccoon (Procyon pygmaeus), and Dwarf Coati (Nasua nelsoni) are all endemic. So is the enigmatic Cozumel Fox (Urocyon sp.), a very rare species, presumably similar to its mainland counterpart the Grey Fox (Urocyon cinereoargenteus), but it has never actually been scientifically described. Both the Cozumel Emerald (Chlorostilbon forficatus) and the Cozumel Vireo (Vireo bairdi) are endemic. A third endemic bird, the Cozumel Thrasher (Toxostoma guttatum) is exceedingly rare, indeed almost extinct. The Cozumel Whiptail (Aspidoscelis cozumela) is the only endemic reptile. The coral reefs which fringe the island's shores are home to the endemic, Splendid Toadfish (Sanopus splendidus). There is also the very unusual cave-dwelling sea star Copidaster cavernicola, and at least three endemic species of crustaceans: Agostocaris bozanici, Yagerocaris cozumel, and Bahadzia setodactylus.
The fun doesn't stop there. Isla Cozumel is also home to endemic subspecies of Common Opossum (Didelphis marsupialis cozumelae), Coues' Rice Rat (Oryzomys couesi cozumelae), White-footed Mouse (Peromyscus leucopus cozumelae), Collared Peccary (Pecari tajacu nanus), Great Curassow (Crax rubra griscomi), House Wren (Troglodytes aedon beani), Blue-grey Gnatcatcher (Polioptila caerulea cozumelae), Black Catbird (Dumetella glabrirostris cozumelana), Yucatan Woodpecker (Melanerpes pygmaeus pygmaeus), Golden-fronted Woodpecker (Melanerpes aurifrons leei), Yucatan Flycatcher (Myiarchus yucatanensis lanyoni), Brown-crested Flycatcher (Myiarchus tyrannulus cozumelae), Bright-rumped Attila (Attila spadiceus cozumelae), Rufous-browed Peppershrike (Cyclarhis gujanensis insularis), Yellow Warbler (Setophaga petechia rufivertex), Rose-throated Tanager (Piranga roseogularis cozumelae), Western Spindalis (Spindalis zena benedicti) and Northern Cardinal (Cardinalis cardinalis saturata). In addition, three subspecies are near-endemic: Both the Roadside Hawk (Buteo magnirostris gracilis) and the Yellow-faced Grassquit (Tiaris olivacea intermedius) are found on Cozumel as well as Holbox Island, off the Yucatan's north coast. The Bananaquit (Coereba flaveola caboti) is found on Cozumel and some other islands off the Yucatan Peninsula.
Why does Cozumel have so many endemic species and subspecies? What makes this relatively dry, rocky, hurricane swept, Caribbean Thatch Palm (Thrinax radiate) clothed, 486 sq km island such a hotspot of biodiversity? I've always wondered...
It's not unusual for islands to possess unique fauna. It's by virtue of their isolation that islands tend be relatively depauperate in total species, but of the ones that do occur there, a good many may be endemic. Madagascar and only Madagascar boasts lemurs. Tasmania has its eponymous Devil (Sarcophilus harrisii). Eil Malk has a lake teeming with Stingless Golden Jellyfish (Mastigias papua etpisoni). Isla Socorro has the Socorro Mockingbird (Mimus graysoni). Santa Cruz Island has the Island Scrub Jay (Aphelocoma insularis). The main islands of New Zealand have the Lesser Short-tailed Bat (Mystacina tuberculata). Indeed, most of these islands have pantheons of endemic species. From Galapagos to Borneo, from Sri Lanka to South Georgia, islands are hotbeds of endemicity. But many (not all, but many) islands that have particularly rich endemic diversity are rather isolated. Oceanic or microcontinental islands, those disconnected from the nearest continental shelf, are so isolated that when a wayward bird or reptile comes ashore, they are not likely to be joined by others of their kind. When there's no gene flow between an island and the mainland, a host of evolutionary processes like founder effect, genetic drift, and good old fashion natural selection, cause island colonists to diverge in form and behaviour from their continental ancestors.
Cozumel, as far as islands go, is not very isolated from the mainland. Though only 19 km wide, the channel that separates Cozumel from continental Quintana Roo, is also some 900 m deep. That's deep enough to ensure that ever since the island first rose out of the sea some 200,000 years ago, it has never had a physical connection (a land bridge, if you like) to the mainland. Cozumel has been completely submerged by the ocean during times of high water (it's highest point is only about 10 m above sea level), but it's never been connected to the mainland, not even during periods of low sea levels, such as during the last ice age. It's not nearly as isolated as a typical oceanic island, but then again, some of the most diverse islands on Earth are not particularly isolated either: Borneo, Sumatra and New Guinea, for example. Evidentially, it doesn't take extreme distance, just a certain degree of isolation, to promote island endemicity.
Take the carnivores of Cozumel, for instance. There's the endemic Pygmy Raccoon and the Dwarf Coati. As their names suggest, they're small compared to their mainland relatives. Dwarf Coatis, for example, are only about 75% the size of mainland White-nosed Coatis (Nasua narica). The very rare (apparently not a single museum specimen exists) Cozumel Fox is also a dwarf, being essentially a reduced version of the mainland's Grey Fox. Why does the Cozumel carnivore fauna have a decidedly dwarfish aspect? In fact, dwarfism extends beyond the carnivores; the island's Collared Peccaries, Great Curassows, and Cozumel Thrashers are all miniaturized when compared to their mainland counterparts. So what's the deal, why evolve towards smallness?
A general pattern among island fauna the world over is that big creatures get smaller on islands, while little creatures get bigger. Biogeographers call this Foster's Rule. Think of the Komodo Dragon (Varanus komodoensis), a supersized monitor lizard. Or consider the diminutive White-tailed Deer (Odocoileus virginianus clavium), the so-called Key Deer, of the Florida Keys. Big things get small, small things get big. I'd love to call it a law of nature, but it's not: as far as rules go, Foster's is one that's fraught with exceptions. As David Quammen tells us in his wonderful exploration of island biogeography, Song of the Dodo, "Many kinds of animal are likely to grow larger on islands, yes, except under exceptional circumstances, which instead make them grow smaller. But to the exceptional circumstances there are other exceptions, which might again make them grow larger or, on the other hand, smaller". But even setting aside those exceptions, and their exceptions too, it can be difficult to say exactly what leads to dwarfism (or gigantism for that matter).
In general terms, it seems that on islands animals shrink when resources are scare (exceptions abound). Cozumel is relatively dry, having only localized permanent fresh surface water. It's relatively rocky. And it's prone to catastrophic disturbance in the form of hurricanes. Potential prey for predatory raccoons, coatis and foxes would also be rather small in size - birds, whiptails, insects, and seashore creatures like crabs. No need to be large to subdue small prey. Perhaps these factors could lead to dwarfism, if being small made coping with island life more efficient. By way of interest, there is a fourth carnivore (in this case a carnivore that eats mostly fruit, go figure) on Cozumel, but it's not endemic: it's the Kinkajou (Potos flavus), a species that is widespread in the neotropics, though reportedly becoming rather rare on Cozumel. The providence of the Kinkajou is questionable, with some suggesting that it was only recently introduced to the island by humans. Kinkajous on Cozumel are not dwarfs.
Western Spindalis (Spindalis zena)
Photo: Laura Gooch
Most of Cozumel's endemic species and subspecies seem to have an ancestral affinity with the Yucatan Peninsula. One particular exception is the very striking Western Spindalis, a tanager-like songbird that, along with a suite of similar congeners, occurs across the Greater Antilles. I found a few Western Spindalises along the overgrown roads of an abandoned subdivision project on a pleasantly overcast morning. Well, it was pleasant right up until two highly aggressive, but thankfully also very stupid, feral dogs put the run on me for the better part of a kilometre. Nonetheless, I saw the birds, and was rather happy to do so, partly because of their unique distribution (it's the only place in Mexico where they regularly occur) and because of the interesting taxonomic quandary they present. All spindalis species, there are four of them, were once classified as conspecific. Now they've been split, with separate species on Puerto Rico, Hispaniola and Jamaica, in addition to the widespread Western Spindalis. As for their general placement among the other passerines, there's still some debate. I'm intrigued by incertae sedis, species whose place in the taxonomic order is confused at best, or just simply unknown. For many years, the spindalises were considered to be tanagers, indeed the whole complex of species and subspecies was called the Stripe-headed Tanager. But genetic and traditional comparative taxonomic approaches tell us that spindalises are not tanagers. We don't yet know where to place them instead though. If they're not tanagers, what are they? Time will tell, I'm sure, but for the present I was quite content to stare at a mystery, until I heard those damned dogs coming for me!
Feral dogs aren't just a problem for birders, but they're also a problem for the endemic island wildlife. Introduced species threaten Cozumel's biodiversity. There are the usual culprits, that afflict island ecosystems all over the world: Domestic Dogs (Canis familiaris) and Domestic Cats (Felis catus), as well as House Mice (Mus musculus) and rats. The newest threat on Cozumel though, seems to be the Boa Constrictor (Boa constrictor). Boas are found on the mainland, in fact not far from the port of Playa del Carmen I saw a pair of Northern Caracaras (Caracara cheriway) ripping apart a massive road killed Boa Constrictor. But boas never made it Cozumel on their own. They were apparently released from the set of some B-rated movie about 40 years ago. The boas reproduced quickly, feeding on the island's birds, laying waste to Cozumel's once abundant, and not uncommon, Yellow-lored Parrots (Amazona xantholora).
Another island bird that has declined precipitously is the Cozumel Thrasher, but the degree to which boas are to blame is uncertain in this case. The thrasher, once an iconic Cozumel bird, was locally common until Hurricane Gilbert came ashore in 1988. After that, the thrashers virtually disappeared. Researchers searched throughout the 90's, seeing only a handful of thrashers and even capturing some of the last survivors. Subsequent storms seemed to push the already small population even closer to the brink. The last definitive sight record was of a single bird in 2006, but since then there have been no confirmed observations. If not totally extinct, the thrasher is most certainly functionally extinct. That is to say, even if there are a few remaining survivors, they are unlikely to ever re-establish a viable breeding population.
Many biologists and naturalists have asked, why did Hurricane Gilbert and subsequent storms, such as 1995's Hurricane Roxanne knock back the thrasher population so severely? After all, didn't this endemic species evolve to deal with the catastrophic habitat alterations that result from the hurricanes and tropical storms which sweep the island periodically? Perhaps, on pristine Cozumel, before the introduction of cats, mice, rats and boas, the thrasher population would have been able to recover from a devastating hurricane. Just maybe, the toll taken by so many non-native predators in addition to the effects of hurricanes (not to mention other possible adverse factors such as anthropogenic habitat changes, or even an unidentified invasive disease), was too much for the thrasher to endure.
Those feral dogs that ruined my spindalis watching, were something of a blessing in disguise. They forced me to relocate, and it just so happened that I came upon a cenote, and one that was guarded by a rather large and statuesque American Crocodile (Crocodylus acutus) to boot. Cenotes are water-filled sinkholes, and they're often part of complex subterranean karst (cave) networks. The cenotes on Cozumel are sort of like islands within the island, because they are really the only permanent sources of surface freshwater. Cozumel certainly isn't a desert island, it's covered in vegetation, and rainfall is frequent, if not sometimes torrential. But the limestone bedrock and thin soils drain rainwater very rapidly, making cenotes the only reliable surface waters. Most of Cozumel's cenotes, including Aerolito, the one I'd stumbled upon, connect to one another through a series of erosion-carved tunnels. The cave system also connects to the ocean, meaning that most of Cozumel's cenotes are anchialine in nature: they contain both fresh- and saltwater. Because freshwater is less dense than saltwater, the lower reaches of Cozumel's cenotes are salty, while the surface waters are fresh.
I wandered around the Red Mangroves (Rhizophora mangle) which fringed the cenote, watching for more crocodiles and hoping to find the Pygmy Raccoons that left their tracks in the mud. Sure enough, after a little stealthy tracking and mud up to my knees, I spotted one endemic raccoon among the tangle of strut-like mangrove roots. It recalled a slightly smaller, slightly greyer Northern Raccoon (Procyon lotor), the species with which I am familiar back home. Happy with that, I turned my attention to the Great-tailed Grackles (Quiscalus mexicanus), those large and gregarious blackbirds, as they worked their way through the trees, and within centimetres of the basking croc. Brazen or calculating, I wondered? Small fish swam in the clear cenote waters, colourful and plentiful. When a Mexican couple appeared, I left them to enjoy the cenote and its guardian crocodile. It wasn't until after I returned to town and began reading, that I began to understand just how impressive this cenote actually was.
American Crocodile (Crocodylus acutus)
Photo: Mark Conboy
Without some pretty serious dive training there aren't too many options for exploring cenotes. Aerolito, as one of the largest cenotes on Cozumel attracts the attention of cave divers, who can travel more than a kilometre underground through chambers and tunnels decorated with stalagmites and stalactites. Luckily YouTube provides a glimpse of what the cenote looks like deep underground. The décor is nice, but the video shows only a single example of the supposed abundance of organisms that apparently inhabit Aerolito. Including the endemic sea star, Copidaster cavernicola (unfortunately not the species featured ever so briefly at 2:03 in the video). Endemic crustaceans live here too. Both freshwater and brackish water fishes swim here. Aerolito is sort of like an underground estuary, with its mixing of freshwater and saltwater ecosystems.
Don't let the tourist trap reputation of Cozumel turn you off from the island's wondrous natural history. The island's diving is noteworthy (unfortunately I didn't have time to get offshore on this trip), but the lesser known facets of Cozumel, the dwarfed carnivores, the endemic species, and the deep cenotes, are all worth putting up with the crush of Hawaiian shirts and Bermuda shorts. Cozumel is a surprise, waiting to be discovered. Just watch out for the dogs.
