Naturescape: Prairie Rattlesnake

A Prairie Rattlesnake (Crotalus viridis) on the southeastern Alberta prairie.

Photo: Mark Conboy

Naturescape: Common Snapping Turtle

Late spring is nesting season for Common Snapping Turtles (Chelydra serpentina) at Long Point, Ontario. This female is covered in sand from her long walk inland to search for a suitable nesting site among the sand dunes.

Photo: Mark Conboy

Isla de Pequeños Carnívoros: Cozumel

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.

BioBrevia: X-rayted Natural History

X-ray and schematic illustrations of the critically endangered Montserrat Galliwasp (Diploglossus montisserrati) and shell fragments of its prey, a freshwater snail (Omalonyx matheroni).

Imagery: Bohaton et al 2015 (Creative Commons Attribution 4.0)

A new paper in Royal Society Open Science by Bochaton et al, X-ray Microtomography Provides First Data About the Feeding Behaviour of an Endangered Lizard, the Montserrat Galliwasp (Diploglossus montisserrati), is certainly a novel investigation of lizard diets. The Montserrat Galliwasp is one of the rarest reptiles on Earth, so rare that decades pass between observations of it in the wild, and it has only found its way into museum collections twice. This is an interesting article; here's the abstract:

Reporting the diet of recently extinct or very rare taxa, only known by a few museum specimens, is challenging. This study uses X-ray microtomography, a non-destructive investigation method, to obtain the first data about feeding behaviours in the Montserrat galliwasp (Diploglossus montisserrati) by scanning one of the two specimens known to date. The scans revealed the occurrence of shell fragments of a freshwater snail (Omalonyx matheroni) in the digestive tract of the specimen. This data combined with morphological evidence shows the occurrence of a durophagous feeding habit and a possible tendency of association with freshwater environments. This information could be crucial to save this critically endangered lizard endemic on Montserrat island. (Reproduced under Creative Commons Attribution 4.0).

Two Days with Bruce

Massasauga Rattlesnake (Sistrurus catenatus).

Photo: Mark Conboy

A wrong turn on an unfamiliar trail can lead to confusion, frustration and the unenviable job of backtracking across some slippery talus slope or through a knee-deep swamp. On the other hand, it can also lead to a wonderful discovery. By way of example, I took a wrong turn off a Bruce Peninsula trail earlier this month, and I was rewarded for my buffoonery by an encounter with one of Ontario's least often seen and most misunderstood reptiles: the Massasauga Rattlesnake (Sistrurus catenatus). I went to the Bruce Peninsula specifically in search of rattlesnakes and much to my delight what I found was a land full of additional surprises.

The focus of my explorations was Lion's Head Provincial Nature Reserve, the real treasure of the Bruce Peninsula, with one of most dramatic coastlines on the Great Lakes: shinning white cliffs that support ancient forests of stunted Eastern White Cedars (Thuja occidentalis). The cliffs plunge into a band of forest which in turn sweeps down to the crystalline waters Georgian Bay. Upon the cliff tops themselves, the forest, dense with Eastern White Cedar and Balsam Fir (Abies balsamea), hides a geological wonderland of glacial potholes, acid-worn caves, bottomless crags, overhanging rock faces, and erosion-sculpted boulders of ten thousand different forms.

I started on the trail at six o-clock one evening, into a forest alive with the ethereal whistling of Swainson's Thrushes (Catharus ustulatus), which lent the evening air a subdued and angelic texture. I worked my way east along the ridge top from one spectacular lookout to another. To the north I could see the peninsula stretching far off, a vast swath of seemingly undisturbed forest and coastline. Not a breath of sound, not a light, not a tower, not a road, not a cabin betrayed the illusion of vast and insurmountable wilderness. I knew what truly lay out there, hidden by distance, green forest, and heaving geology. Of course I knew of the towns and cottages, the lighthouses, the roads and their stinking cars. But I let my mind imagine a world unsullied by humans, a wild land stretching from Lion's Head to the tip of the peninsula and beyond to Manitoulan Island, to the North Shore, across the boreal hinterlands, to the freezing waters of Hudson Bay. A world teeming with game and fish and winding trails. The cries of a raven brought me back to reality. And so I lifted my pack, and hit the trail once again.

The cliffs upon which I walked dropped down for ninety or one hundred metres into the forest below. Far from smooth, the cliffs were undercut, overhanging ominously, inviting me to stand upon many a narrow slab, as though to dare fate. The cliff faces were pocked by small depressions and shelves, and it was upon those little natural balconies their existed a most remarkable community of ancient trees, a vertical forest of centuries-old Eastern White Cedars. Many of the cedars, though small and spindly, more like shrubs than proper trees, were two, three, four or even eight centuries old. The oldest yet discovered at Lion's Head is over 1,300 years old and for all its age is only about seven metres tall.

Dolostone cliffs at Lion's Head Provincial Nature Reserve with Eastern White Cedars (Thuja occidentalis).

Photo: Mark Conboy

The cliff-side cedars are natural bonsais, sculpted by centuries of extraordinarily slow growth on water- and nutrient-poor dolostone, twisted by driving winds, polished by blowing snow, and cracked by frost. Their trunks and limbs are gnarled, knotted and ropy. They do look truly ancient, truly sage-like. Many of the cedars appear to be half dead, and indeed they are. Eastern White Cedars, perhaps unique among Ontario trees, grow in sections; one portion of the root mass feeds one portion of the crown. In this way the left side of a cedar may die as the roots which feed it run low on nutrients while the right side of the tree continues to thrive, its roots having managed to find sufficient resources. This segmenting, may be one of the reasons why cedars have such staying power, where other less versatile species simply can't survive. Over the past 10,000 years, fire has periodically burned along the cliff tops, but the cliff faces appear to have been spared, and so the trees were allowed to grow old, excessively old. Even that zealous craze for wood and civilization, the rampant forest clearing, left the difficult-to-access vertical forests unmolested. The longevity of the ancient cedar forests it would seem, is a lucky coincidence of topography: safe there on the cliff sides from the ravages of fire and man. But not entirely safe.

Perhaps the single biggest threat that the ancient cedars face todays is rock climbers. The cliffs of Lion's Head are a popular sport climbing destination. It's easy to spot the most well-climbed routes: lichens have been rubbed away, duff has been swept from the tiny ledges, and in some rare cases, cedar branches have been cut. It seems that most of the pruning, and it hasn't been excessive, was done before the agelessness of the cedars was discovered. Though, if a saw-wielding climber had bothered to look at the annual growth rings of the limbs they were pruning it may have been obvious just how old those trees were. Today, the climbing community has more awareness of the ancient trees, and thankfully, disturbance is less of an issue as it was in the past. To share the cliff faces with age-old trees, that can only add to the exhilaration of the climb, can it not?

Cryptoendolithic organisms (not from the Bruce Peninsula in this example) growing inside a rock.

Photo: Guillaume Dargaud (Wikimedia Commons)

Eastern White Cedar belongs to the family Arborvitae, from the Latin, tree (arbor) of life (vitae). Indeed a fitting appellation for this long-lived species. The cedars though are not the only ancient cliff dwellers. The roughly textured dolostone supports an array of lichens, tiny symbiotic organisms that give colour to the cliff faces. Hidden within the pores of the rocks themselves is even more unlikely and more bizarre life, cryptoendolithic organisms. Nine species of cyanobacteria and 13 species of green algae living 1-5 mm deep within the porous rock have been recorded on the Bruce Peninsula. From the right vantage point these colonies of microorganisms can be seen as black stains on the white dolostone. The cyanobacteria and algae aren't just innocuous cliff dwellers, but instead play a role in the nutrient cycling of the cliff ecosystem, absorbing sunlight through the semi-translucent rock, and imputing nutrients into an otherwise depauperate ecosystem. The Bruce Peninsula is one of only a handful of places on Earth where cryptoendolithic organisms have been studied.
After several kilometres of fairly rugged trail I began a descent towards the coastline past thick slabs of dolostone that fell from the cliffs centuries ago, each now supporting their own garden of trees and shrubs. The Swainson's Thrushes sang all around as I dropped further and further down. Just before reaching the cobble beach, the trail passed below a massive overhanging slab. The slab protruded from the forest like a great bird's beak. Eight metres long and almost as wide, it sheltered a patch of bare soil, starved of rain water and sunshine, nothing grew there, a stark contrast to the thick forests that surrounded the outcrop. It's a fantastic natural sculpture, unexpected along this trail, while simultaneously not out of place in this land of geological wonder.

Earlier on the trail I came across several potholes, including one with the whimsical name, the Giant's Cauldron. These potholes or kettles were formed beneath glacial ice, where free flowing water formed a small whorlpool. Stones gathered up by the whirlpool were spun around and around wearing holes into the rock. The potholes stopped growing when the water flow subsided or the stones that chiselled them eroded to nothing. The Giant's Cauldron was about the size of, well, a very large cauldron, but just down the trail was an even more impressive one, the Lion's Head Pothole. This one was at least three metres deep and about a metre and half in diameter. There was a wonderful little portal in its side, that allowed one to squeeze right inside of the pothole. From within, the smooth stone wall directed your out of a sky light towards a canopy of Sugar Maple (Acer saccharum) and American Beech (Fagus grandifolia). Standing literally within the rock, an eerie silence prevailed.

The Lion's Head Potholes claims another curious naturalist.

Photo: Mark Conboy

The trail stepped over dozens of grykes, joints in the rock widened by weathering of the basic dolostone by slightly acidic rainwater. Small caves peaked out of the leaf litter; I wondered if they led to larger caves, caverns, perhaps as yet unexplored. The trail was rocky and never flat, each step either up or down over pock-marked rock. The pocks, or vugs in geological parlance, were formed as the dolostone itself was formed. Dolostone is essentially limestone that has been infused with magnesium. The original limestone, the precursor to the dolostone I navigated over all evening, was laid down in a massive coral reef sometime during the Silurian period (443 to 416 million years ago). This reef, which essentially forms the backbone of the Niagara Escarpment (of which the Bruce Peninsula is the northernmost extension), was formed by the growth and death of countless generations of corals and other sea creatures whose calcium carbonate shells became consolidated into solid limestone over eons. That limestone was eventually pushed underground where it was infused with highly saline magnesium-rich groundwater. The magnesium replaced the lime, changing the limestone to dolostone. That conversion resulted in some loss of rock volume, and that's how the vugs were formed. Had this dolomitization process not occurred we might not have the dramatic scenery of the Bruce Peninsula we see today. Dolostone, though it can be weathered, as evidenced by the potholes, grykes and caves along the trail, and large piles of talus below the cliffs, is far less resistant to erosion than limestone. Perhaps the limestone would have been eroded millennia ago, leaving a more or less flat shoreline, similar to the Peninsula's western shore. But the dolostone has persisted, giving us the dramatic views and ancient cedar forests of the Lion's Head.

The trail lead to a pleasant campsite under the shade of cedars and Red Maples (Acer rubrum). I pitched my tent near the cobble beach and settled in for a night apart from the rest of humankind.

In the morning, I found myself ascending to the cliff tops once again. Black-throated Green Warblers (Setophaga virens) were singing in profusion; it seemed that I was never out of earshot of one all morning. The woods were simply overrun with those pretty little songsters, but it wasn't long before my attention turned from the birds to the trailside plants. The Bruce Peninsula supports over forty species of orchids, most of which I didn't expect to find the Lion's Head; they are inhabitants of bogs, fens, swamps and alavars - habitats I simply didn't pass through on my hike. But these high and dry cedar woods do support a handful of species, including two biogeographical oddities: Menzies' Rattlesnake Plantain (Goodyera oblongifolia) and Alaska Orchid (Piperia unalascensis).

The tiny flowers of an Alaska Orchid (Piperia unalascensis), a western species with a disjunct Great Lake population.

Photo: Mark Conboy

Both the rattlesnake plantain and the Alaska Orchid are western disjunct species. Both are primarily distributed in western North America, but they can also be found in pockets isolated pockets elsewhere. The Alaska Orchid is the more extreme example of this pattern. Its main distribution extends from southern Alaska to Baja California. Disjunct populations occur on the Great Plains, in eastern Canada and of course, in the Great Lakes Basin. It's likely that Alaska Orchid enjoyed a much broader prehistoric distribution, that linked all of the disjunct populations to the species' main western range. For some reason, the Alaska Orchid's range contracted into the discrete populations we see today. Though less dramatically, the rattlesnake plantain also has a similar  disjunct pattern and probably has a similar biogeographic history. Perhaps it was the presence of such disjunct species that caused American botanist M.L. Fernald in the 1920's, to hypothesize that the Bruce Peninsula was an unglaciated relict, harbouring species that once enjoyed a wide preglacial distribution in an ice-free refugium. We now know that unequivically, the Peninsula was completely ice-covered for several thousand years and that the disjunct distribution of orchids and other species must be explained by other means.

Another plant caught my attention on several occasions throughout the hike. The Northern Holly Fern (Polystichum lonchitis) is a relatively rare species in Ontario, but evidently there is a healthy population on the Bruce Peninsula. Beyond ancient trees, disjunct orchids, and unusual ferns, the Bruce Peninsula harbours other botanical treasures, including significant concentrations of range-restricted Dwarf Lake Iris (Iris lacustris), Lakeside Daisy (Hymenoxys herbacea), and Tuberous Indian-plantain (Arnoglossum plantagineum).

Northern Holly Fern (Polystichum lonchitis) and Maidenhair Spleenwort (Asplenium trichomanes).

Photo: Mark Conboy

But as I said, the Massasauga Rattlesnake was the main reason I visited the Bruce Peninsula. Rattlesnakes were extirpated from my home county of Norfolk and most of the rest of Southwestern Ontario long ago, although there are remnant isolated populations at either end of Lake Erie. The species' last true haven is the Bruce Peninsula and the Georgian Bay coast. In the case of Ontario's Massasauga Rattlesnakes, there is a significant amount of genetic differentiation and geographical isolation between the Bruce Peninsula and Georgian Bay populations. Even though these two rattlesnake populations exist in close proximity, they are actually rather isolated from one another, and that can have some important biological consequences. 

Gene flow is important to maintaining genetic diversity, and good genetic diversity can help with disease resistance and adaptation to changing environmental conditions. Isolated populations of snakes and other organisms tend to have limited genetic diversity because of a lack of immigration from other populations (immigrants bring new genetic materials) and inbreeding.

Massasauga Rattlesnake (Sistrurus catenatus).

Photo: Mark Conboy

The snakes of the Bruce Peninsula are isolated from those of eastern Georgian Bay by water (and also intensive agriculture and road development on the southern part of the peninsula). Massasaugas, despite their fondness for wetlands, are unable or unwilling to undertake long distance swims, so the Bruce Peninsula snakes are effectively stuck on their own island, separate from their conspecifics to the east. Indeed open water is such an effective barrier to rattlesnake dispersal that Lyal Island, a mere 1.3 km off the western shore of the Bruce Peninsula, harbours its own genetically distinct population of Massasaugas. At some time in the past, Masassaugas colonized the island, but since that time, emigration from the main peninsula population has been all but nonexistent, allowing the Lyal Island snakes to develop their own genetic identities, apart from the Bruce Peninsula snakes.

 

Isolated populations are not necessarily a bad thing, especially if they are large, and genetic diversity can be maintained to some degree through mutations; so the Bruce Peninsula rattlesnakes are not likely to disappear because of inbreeding or something like that, at least not any time soon. What is most interesting about the isolation and genetic uniqueness of the peninsula's rattlesnakes (and it's orchids too, no doubt), is that it demonstrates a simple biological fact: not all members of one species are the same. I argue that each and every distinctive population should be conserved as though it were a distinctive species. In the case of Massasauga Rattlesnakes, we need to insure that the Bruce Peninsula population is receives proper conservation, and so too does the Lyal Island population and the many genetically discrete populations on Georgian Bay's east coast, each as though they were a different species. It would certainly be a tall order, but it would be the right conservation approach.