Living on the Edge

Ecotone - a zone of transition, of overlapOur world is in a constant state of transition, both in time and space. Most of us are more aware of the former, noting the passing of minutes, days and years. However, for many species, it’s changes in habitat across space that have a significant impact on their survival.

Life needs edges, places where the shadows of the forest recede in the face of the sun, where waves of grasses dip their roots in murky waters, where ripples lap incessantly at a rock face, etching away the sand of the future. Edges create variety and when it comes to ecology, variety is truly the spice of life, at least in terms of its diversity.

The technical term for a transition zone between two types of habitat is ecotone. It’s a place where two communities meet, knitting together elements of each other, often bringing the best of both worlds.

Some ecotones are abrupt, like the striking boundary between forest edge and farmer’s field, a change so sudden, it can easily be seen from the air. Others are more gradual, such as the subtle gradation of shades from soft, sunny aspen leaves to the dark mossy needles of the boreal forest as one moves pole-ward throughout much of the northern hemisphere.  Some edges we we can’t even see, like the lines between distinct communities layered on top of each other in the depths of a lake. It’s all a matter of perspective. What might seem like a continuum to us, may be a stark contract to another species.  It all depends on the resources you value.

Regardless of how they’re defined, edges are important places. They’re interfaces, areas where two distinct worlds can influence each other for better or worse. Edge-effects can be positive or negative, depending on the organism whose point of view you are looking from and what type of edge it is.

Naturally occurring ecotones, like a reed bed bordering a lake shore, are hugely important areas, a bridge between the land and watery worlds, creating an interface where a greater number of species can thrive than would otherwise exist without these marshes. Whether they’re lines of trees along a winding stream, offering a windbreak in an otherwise open field, or a wet meadow cutting its way through a thick forest, edges can also provide natural thoroughfares, ancient pathways followed by generations of animals.

However, that same linear accessibility can also become a problem when the edge is not natural. Clear-cuts slicing into an normally intact forest, seismic lines cross-crossing though arctic tundra or farmland pushing into what’s left of tall-grass prairie can create novel and unnatural ecotones, opening corridors for predators and invasive species, irrevocably changing the landscape. In contrast, what may be right-of-ways for some organisms may also be barriers for others, with human-caused edges limiting normally wider-ranging movements of many habitat-sensitive species, such as songbirds and woodland caribou.

Anyway you cut it, the world is full of edges, both dividing and uniting this remarkable patchwork of landscapes in all three dimensions. Understanding the depth of that complexity and our impacts on it has kept biologists busy for decades and will continue to do so for many more to come. I, for one, welcome the chance to continue the exploration.

Given to Fly

Alight - Herring Gull LandingI never get tired of watching birds fly. It’s something that’s always entranced me: a warbler flitting between sun-dappled leaves, a gull wheeling lazily against the clear blue of a Manitoba summer sky, or the subtle whisper of an owl’s feathers as it returns to roost.

My fascination with flight started at an early age, much to the consternation of my parents who had to cart me off to the hospital to get my foot x-rayed after an ill-fated attempt to get airborne from the top of a ladder with willow branches strapped to my arms.

I’m pleased to report that there was no permanent damage and I now have a much better grasp on the mechanics of avian flight.

Physicists and biologists alike are still trying to sort out all of the details; but we get the general gist of how it works and much of that knowledge has resulted in the air travel we enjoy today.

A bird in the air has two forces to contend with: gravity (the inexorable force the earth exerts on everything, drawing us back to its core) and drag (the force of the air that pushes back against us whenever we try to move through it). In order to keep itself aloft, the wings of a bird must produce enough lift to counter gravity and reduce drag.

 

Much of that is achieved through the shape the wing. It takes a lot of energy to flap all the time to produce enough thrust to keep you up and moving forward, so having wings that can generate lift and reduce drag as you glide are a beneficial adaptation. Wings aren’t flat, whether they are on a bird or a plane. Diagram explaining how cambered wings create liftFlat wings don’t create lift. Air moving around a symmetrical wing passes over and under its surface at the same speed on both sides. However, if you curve the wing and create a cambered airfoil, then you’re getting somewhere. With a cambered wing, the air passing over the top moves much faster than the air passing below the wing. This creates a pressure differential, with lower pressure above the wing, where air is being swept away and high pressure below where air is piling up, pushing the wing and the bird attached to it, up into the sky. There wasn’t much camber to my willow branches, hence the crash landing.

 

Diagram explaning how the angle of attack of a wing can affect liftAnother way increase that pressure differential is to tilt the leading edge of the wing up, dropping the flight feathers down and building up more air underneath. However, you can go too far with this. Tilt more than about 15o and the airstream separates from the upper surface of the wing, creating turbulence, stalling the bird out. They use this to their advantage when landing, like the gull in the image above. To control the stall, most birds can raise their equivalent of a thumb called the alula. This nub of bone with usually about three feathers on it (you can just see it sticking up behind the top of the gull’s wing in the picture) can split the airstream at the leading edge, forcing it back over the surface of the wing.

 

 

 

Once they’ve vanquished gravity, there’s still the matter of drag threatening to push them back to the ground. Flapping, of course, will keep you moving; but there are several design considerations that birds have made over millenia of evolution.  Birds that do a lot of gliding (e.g. gulls) have long, tapered wings that concentrate any vortices that might form at the wing tips (turbulence caused by the feathers slicing through the air) into two small areas that are as far apart as possible, reducing what is called ‘pressure drag’. Soaring birds, like hawks and Sandhill Cranes, take a different approach, spreading out their primary feathers like fingers, splitting up the wingtip vortices and reducing their impact.

If you found wrapping your head around all that was a bit of a challenge (like I did the first time I had to teach it), understanding what’s going on when a bird is flapping will give you a veritable headache. Things get complicated as the wing starts to move and lift and thrust start happening simultaneously. In a nutshell, however, the lift is generated by the curve in the part of the wing closest to the body, while the tips of the primaries produce the thrust, creating momentum that propels the bird through the air with a grace that always amazes me.

Sometimes taking a phenomenon apart and learning how each component works destroys the magic of the whole thing; but I haven’t found that to be the case with the flight of birds. Understanding the forces that make it possible for them to shed the earth’s shackles only makes it all the more remarkable.

The Edge of Darkness

Owl SilhouetteAs I’ve mentioned before, I have always had a love for obscure words, especially those that find everyday use in the lexicon of certain specialties.

Crespuscular is one of those words.

I use it all the time, but it’s definitely not common knowledge, something that’s become increasingly obvious over the many years that I’ve been a nature interpreter. I’ll throw it out there, along with other natural history terms, like ‘nocturnal’ or ‘carnivore’. While my charges usually nod sagely in understanding at these other adjectives, ‘crespuscular’ usually elicits furrowed brows and working tongues as they try to wrap their mouths around the syllables, eyes rolled up towards their brains, as though watching it try to divine the word’s meaning.

It’s too bad, because it’s a good word. It’s also a great way to be. A crepuscular animal is one that is most active at twilight, straddling the line between night and day in the muted light of either dawn or dusk. It certainly my favourite time to be out and about, probably because I’m in such good company.

Many animals are crepuscular in their habits; the most notable of which,  for me, are the owls. Species, like the Great Gray Owl, are at their best at this hazy time of day, making use of their enormous eyes and highly-tuned hearing to pick up the slightest rustle of prey along the forest floor. Owls, however, are not the only birds that enjoy this shoulder time. Common Nighthawks and Wilson Snipe also come alive in the dusk, the former swooping and diving through the gloom, scooping up millions of flying insects that have taken to the air after the heat of the day before the cool night temperatures slows their metabolisms and forces them back to earth. Most songbirds reserve their choruses for the crepuscular hours; Olive-sided Flycatchers announcing the dawn and Hermit Thrushes heralding the dusk, their refrains rounded out by the harmonies of breeding frogs.

Most boreal mammals are also crepuscular in their habits. The dull grey winter coat of the white-tailed deer is at its most invisible in the murky hours of twilight, especially to the mostly colour-blind vision of their carnivorous predators. Bats join the nighthawks in their aerial quest for a meal and rabbits emerge from the shadows, taking advantage of the low light to grab a quick nibble before complete darkness makes it difficult to spot approaching danger.

In reality, the busiest time of day, in whatever habitat you might live, is twilight. So, whether you are an early bird, who rises before the dawn, or a night owl, like me, who takes comfort in the release of the day as the sun slips below the horizon, get outside at these tenuous moments and discover the beauty and wonder of becoming crepuscular in your habits.

Sounds of Silence

White-tailed deerWalking through the winter woods I can’t help but feel an overwhelming sense of closeness with the world around me. Snow is nature’s greatest silencer, muting the world as it bathes it in white and it’s this silence that breeds a feeling of intimacy with my forest brethren. Shrouded by heavy bows and intermittent shadows, I feel my senses stretch through the quiet, reaching out for any sign that I’m not alone in my wanderings.

As I make my silent progress, I find myself wondering how the other inhabitants of the forest perceive this winter world. Whenever I get into one of these moods, my mind usually strays to the white-tailed deer, a species I’m fortunate to meet often on my woodland rambles.

We’re about the same size, a doe and I, and their soft, forward-facing eyes and expressive faces make them easy to relate to.

Though I know she could easily outrun me (especially since I’m a rather slow runner, even for a human), we have a bit more in common than we might first realize. White-tailed deer and humans perceive the world in much the same way. Deer, for the most part, are just a lot better at it.  They have to be. When you live you life under the constant threat of predation, it’s in your best interest to develop a sophisticated arsenal of early-warning systems and deer have plenty.

In deer, the nose knows everything that’s going on around them. With over 290 million olfactory receptors, deer can detect the faintest whiff of danger, even more accurately than their canine pursuers (who only have about 220 million). Both, however, seriously outstrip humans, with our rather paltry 5 million. Where do they put them all? The nasal region of both cervid and canine skulls is actually quite long and full of thin bones in a delicate scroll-work called nasal turbinates. In the living creature, these bones are covered with olfactory epithelium (skin with scent receptors) that picks up the tiniest of molecules. When actively sniffing, they fill their nasal cavities with as much air as possible, giving scent molecules a better chance of being picked up.

To further improve things, deer have a small, fluid-filled sack lying just on top of the palette called the vomeronasal organ (or Jacobson’s organ). This seems to function in a very specific type of scent detection – pheromones, something most mammals use in abundance and deer are no exception.  Whether we have such a functioning organ too is still being debated, but there is evidence that suggests it might play a subtle role in our lives.

Whenever I come face-to-face with a deer, I’m always drawn in by those liquid doe-eyes and this is one place where we have a bit of an edge over our four-legged friend, at least when it comes to how we see our world. Most people will tell you that mammals, especially ones that are active in the dark, don’t see colour. That’s not entirely true. The retina of deer eyes do have cones (colour receptors); they just can’t quite distinguish the same spectrum. A deer’s world is tinted in blues and greens, which makes sense, considering their main concern is picking out the right plants to eat. Still, don’t think you’re invisible to them as you walk through the woods in a blaze-orange vest. Recent work has found that they can pick out at least a hint of these longer wavelengths and with a visual range of 300 degrees while standing still and eyes that are highly sensitive to the slightest movement, a deer will notice you long before you even know you’re not alone.

Besides, if the eyes fail them, the ears wont. No matter how carefully I tread, I know that somewhere, the crunch of my footsteps is being collected by the large, rotating pinna of a deer’s ear. Their range of hearing is considerably better than ours, picking out much higher frequencies than we could ever hope to detect. The wide placement of the ears on the head and their ability to rotate them independently also make it possible for a deer to triangulate the source of a sound, much like an owl.

I know that I will never experience the world on the same level as any of my fellow forest inhabitants, but on a silent, snowy afternoon, I can’t help but want to try.

 

Suspended Animation

My world is getting quieter. As winter descends on the boreal forest, it’s like a veil of silence wraps around us, stilling all motion, save for the whisper of the wind across the newly-formed desert of snow.  One by one, the waterways stop, frozen in time, held captive by the solid grip of truly frigid temperatures.

Around here, the last body of water to succumb to this relentless creep of ice is the great Lake Winnipeg. Staring out over its endless expanse during the summer, it’s hard to believe that this inland ocean, the tenth largest freshwater lake in the world, finally gives in and stills beneath over a metre of ice.

It’s not a quick process and it actually starts a lot earlier than most of us realize. As the days get shorter and the air gets cooler, heat from the lake is slowly released into the atmosphere, often creating lake-effect precipitation, but that’s a story for another day.

As the water cools down, it becomes denser because the molecules’ natural vibrations slow and they cluster together. This denser water sinks deeper into the lake, letting warmer water rise to the surface and release its energy to the skies. This cycle continues until all the water in the lake reaches the same temperature. For freshwater, the magic number is 4 degrees Celsius, not zero, like you may have guessed. The water still freezes at zero degrees, but from 4 degrees on, the water molecules start to form the lattice work that ultimately becomes ice and at this point, this water is less dense than the rest of the layers below it, keeping it at the surface, where it starts to freeze.

At over 23,000 square kilometres, Lake Winnipeg doesn’t go quietly. November is a restless month, with flinty skies and often driving winds. Wild weather churns up this giant, but relatively shallow cauldron, breaking apart the fragile skin that forms on the surface, forcing it to start over.  Still, the cold eventually wins. The ice thickens and sheets knit together, sealing off the water below as it creaks and moans like a giant humpback whale trapped below the surface.

By the end of December, the ice will usually be thick enough to hold the weight of snowmobiles and tank-like Bombardiers used by the commercial fishermen to get to their harvesting grounds. By January, it’s strong enough to hold the weight of fully-loaded semi-trailers charging across the barren ice roads to deliver goods and supplies to towns that in summer can only be reached by air or by boat.

All the while, the lake is still very much alive beneath its frozen shell, reminding us of its presence with rattles and muted groans rising up from the depths. It’s a sound I never tire of hearing because it reminds me that the lake I love is still there, restless and waiting to be released with the warmth of spring.

Size Really does Matter

Autumn is a magical time, full of the fresh scent of fallen leaves, crisp, blue skies and the plaintive grunts of horny ungulates.

Yes, folks, it’s mating season in the boreal forest, known in the deer world as the rut. As the days get shorter, hormones start running rampant. Lean, muscular bodies have reached peak condition after a summer’s diet of green, leafy vegetation. Pheromones are being produced in vast quantities to be splashed onto every available surface. But, for deer and moose, it’s really all about the antlers, which have now been scrubbed clean of their protective velvet layer to gleam in the warm autumn light like a warriors sword.

Unlike what us human females like to tell our potential mates, size, in cervids, truly does matter. It’s all about who has the biggest rack.  Think about it, ladies, which would you be most attracted to: the young, scrawny male with little knobs that just barely make it past his ears, or the magnificent bull with antlers stretching over a meter from tip to tip? In female ungulates, the choice is ingrained: bigger is better because she’s not just seeing an impressive display, she’s seeing good genes.

Those antlers are what behaviourists call ‘honest signals’. Only males healthy enough to carry around all that weight can display them. Take moose (Alces alces), for example. Antlers of a large bull can span up to 5 ft (1.5 m) across and weigh 60 – 85 lbs (27 – 39 kg).  Male moose spend 25% of their energy in the summer just growing them, using more resources than females put into gestating young.  Antlers grow fast, starting to form in mid-summer and reaching full size by September. In fact, moose antler is the fastest growing bone tissue known. Growing it is one thing; then they have to carry them around for another few months.

It’s for that reason, that antlers are such a reliable signal of male health and why they are so attractive to females. If the male is strong and healthy enough to carry around 70 lbs on his head just to look good, he should father some healthy calves.

Another theory explaining why females choose males with larger antlers and other ornaments is known as the ‘sexy sons hypothesis’. The idea is that females choose males with the largest antlers, longest tail or brightest colours because they figure they will father sons who are equally attractive, ensuring they will pass on their genes to future generations. Either way you look at it, it has resulted in some pretty amazing looking animals.

These racks aren’t only for show. While in most cases, it can easy to determine a winner when two competing males cross paths, sometimes the match-up is too close to call on sight alone. In those situations, a fight usually breaks out. Like two hockey players, they launch at each other and lock heads, pushing and grunting until one eventually gives up. Although death is rare, fights can be dangerous. You can get cut, or gouged or, like the one pair of bull elk I watched, one can push the other into oncoming traffic.

For this reason, male ungulates spend a lot of time sizing each other up before engaging in any combat and they’ve evolved some more sophisticated ways to do that besides standing around comparing sizes. Those pheromones they splash about contain a lot of information on each individual’s  health and status. Many species, like white-tailed deer (Odocoileus virginianus), create pheromone markers all over their territory by scraping bark of small trees and rubbing scent glands at the base of their antlers onto the exposed wood, leaving their calling card. This allows other males entering the territory to decide if its worth taking this guy on without having to see him.

As the days grow shorter still, the furor eventually comes to an end. The females are with calf and the hormone levels in males peter out, leaving them exhausted, but hopefully satisfied. Having served their purpose, the antlers drop off, likely affording a great deal of relief, but it’s only a few short months before the next rack starts growing and the cycle starts again.

Feed Me, Seymour

I have to admit that plants are not the first things that come to mind when I think of the word carnivore. However, after spending a morning mucking about in a peat bog last week, I was reminded that ‘meat-eaters’ can be found in pretty much any kingdom and  like their animal counterparts, carnivorous plants can be as beautiful as they are deadly.

In the boreal forests of Manitoba, pitcher plants (Sarracenia purpurea) are probably the most commonly encountered carnivorous plant. If you happen to stumble into the right habitat, they can be downright plentiful. We found dozens of them springing up from the carpet of sphagnum, looking like the bloodied tubes of an expansive green pipe organ.

These plants are truly a wonder of evolutionary design. The pitchers are modified leaves, curled in on themselves and fused to form a vessel that holds rainwater. The fluted edges are boldly pattered to be attractive to insects. However, what an unsuspecting bug doesn’t realize is those leaves are also covered in stiff, slippery downward-pointing hairs. When an insect lands on the rim, they immediately head for the mouth of the pitcher, in search of the nectar promised by the bold colours of the plant. However, the deeper they go, the more difficult it becomes to retreat. The hairs only go one way, drawing their quarry down into their watery doom. The ill-fated arthropod eventually drops into the water and ultimately drowns, its decomposing body providing much-needed nutrients for the plant.

But what would drive the evolution of such a set-up? Most plants are more than capable of feeding themselves, transforming carbon dioxide into energetic sugars through photosynthesis and drawing nutrients from the substrate they’re growing on. For pitcher plants, the big problem is finding enough nitrogen to grow and reproduce. Bogs are cold, acidic places and nitrogen is hard to come by. However, bogs have a lot of insects, flies, mosquitoes and all sorts of critters flitting about, their little nitrogen-filled bodies just there for the taking.

So plants, like pitcher plants and sundews have evolved a way to take advantage of the situation and as a result, thrive in an environment where many organisms could never get a footing and those that do only manage to barely eke out a living.

Although they may be hardy and can go where few vascular plants have gone before, pitcher plants are still vulnerable. Bogs are fragile ecosystems, often taking from decades to millennia to form. Forestry, oil and gas exploration, wetland draining and peat harvesting destroy these habitats, often permanently. The good news, however, is that in part because of just how hard most boreal wetlands are to get to, there are still over 100 million hectares of peat bogs and fens in Canada.

Most of us don’t realize just how important these regions are. These often bleak-looking stretches of greens and browns that wrap around the boreal belt can store on average 3.5 times more carbon per hectare than the forests that surround them. They also hold vast volumes of water, slowing run-off and filtering out pollutants from watersheds. Although Canada’s peatlands are still relatively intact, the world has already lost over 25% of these wetlands to agriculture and harvesting in a number of countries, releasing tonnes of CO2 into the atmosphere and changing water dynamics. It’s a slippery slope. We’re on the lip of the pitcher plant. If we as a species don’t pull back hard on the reins of our need for carbon and other natural resources, more of these valuable sinks will be lost and we will find ourselves tumbling down into our dark pool.

So, put on your boots and venture out into these wet and wonderful places. Admire the pitcher plants and other unique organisms that call this seemingly desolate place home and remember that just because something’s beautiful doesn’t mean it’s not dangerous and sometimes that which seems dull and ordinary is often extraordinary.