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.

10 thoughts on “Living on the Edge

  1. Thanks for writing about ecotones, Heather. You are absolutely right about their vital importance to wildlife for habitat, food sources, and travel corridors.

    • Oh, and I forgot to mention that Virginia is working on a proposal to designate our watershed as the Buffalo Creek Wildlife Corridor with conservation easements offered as an incentive to participate. The project aims to conserve a wildlife travel corridor that runs under Interstate 81 and connects two mountain ridges in the central part of the state.

      • Thanks, Jo Ann. The Buffalo Creek project sounds terrific. Maintaining corridors such as that are absolutely critical for long-term species survival and diversity. I hope it goes well and would love an update once it gets going. Good luck.

    • Thanks, Teresa. I strive to be good at explaining things. It’s especially important for my students that I continue to hone the skill. As for your question, large-scale farming will always result in large swaths of natural habitat being removed from a system. Farmland does provide habitat for some animals, but the diversity is a lot lower than what would be there. To help mitigate those impacts, farmers can maintain corridors between their fields.

      What those corridors would look like depends on what was there originally. In aspen parkland, shelterbelts of aspen and willow are good habitat and can work as connectors between remaining woodlots. In grassland-dominated areas, maintaining right of ways with native prairie grasses between crop fields would go a long way to help reconnect the landscape from the point of view of wildlife. Yes, these corridors will facilitate predator movements and brood parasitism by brown-headed cowbirds, but in the face of intensive agriculture destroying all native habitat, it’s better than nothing.

      The challenge, however, is encouraging farmers to leave shelterbelts, ditches and pothole ponds for wildlife. With GPS-equipped farm equipment designed to go in straight lines to maximize efficiency and production, there is a push to drain wetlands and cut down the trees that are left. There needs to be an incentive program to give these farmers a reason to set aside some of their productive land for wildlife.

  2. A new word for me, Heather and one I’m not likely to forget since it is “subtle gradation of shades from soft, sunny aspen leaves to the dark mossy needles of the boreal forest” we experience when we climb this hill from any direction. Also, the unnatural “linear accessibility” created by clear cuts, seismic lines and industry roads are what is responsible for the local grizzly populations decline and the influx of other species like white-tailed deer. Thanks for this very educational piece and the pleasure of your art to grace it.

    • Thanks, Cindy. It’s not a word that gets used much outside of biology circles, but I really like it. I remember very clearly the subtle shift from aspen to conifers as I climbed the hill up to your place this spring. It was really very striking. Unfortunately, linear accessibility (love that term) is a problem all over the world as humans punch roads and other trails through once-intact habitats. Thankfully, were beginning to understand the importance of connectivity and are working to rebuild natural corridors. The Yellowstone to Yukon Initiative ( is a large-scale example of that. Other pushes to keep new roads from being built in fragile areas have been somewhat successful as well, but it will always be a tug-of-war between human interests and nature’s needs. Still the natural world is adaptable and though it may look different than it one did, habitats often find a way to adjust. Thanks for stopping by. I’m glad you enjoyed the piece.

  3. There is a trend here recently, to remove the natural verges, as you say millennial pathways, vernal streams; anything that would impede the passage of the behemoth that pass as farm machinery these days. Centuries-old hedgerows are out of fashion, a mere annoyance to be uprooted, wetland drained with miles of black plastic tubing. Can there really be that much of an advantage in destroying the homes of so many necessary insects, birds and animals? Does the water so callously dumped into runoff ditches have such little value? I suppose so, now that it’s been enhanced with “fertiliser” and RoundUpReady detritus – after all, we have an endless supply of the stuff, so who cares really?
    Natural pest control, genetic diversity, soil erosion, microbiota and water conservation are so obviously unimportant, mere impedimenta of the past…

      • Thanks, very much, Deb, for your kind words and thoughtful insight. The trend you speak of, unfortunately exists all over Canada, at least that I know and likely many other places. Technology has made it easier to farm in straight lines. However, I am encourages by other trends I’m seeing in development that is going in the opposite direction, maintaining natural corridors and habitat, so there is still hope. I think as we see a generational turnover in places where policy is made, we’ll see positive changes in regulations and management. There are still going to be people who flaunt the rules and do it their way (I see it all the time, unfortunately), but climate change is striking fear in the hearts of more and more people and fear is a great motivator to protect what we have left and rebuild what we’ve lost.

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