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.