#ecosystem engineer

The waters I walk beside begin to swell, flooding out among the trees and flowers. Eventually, the rising waters necessitate a change in the trail and the dusty path becomes a wooden bridge winding its way through a sea of tall, green grasses. Slowly running water gurgles beneath the boards and in the distance the beaver lodge, a mound of interwoven limbs, rises above the waves of greenery. Every few feet, a channel cuts through the vegetation, criss-crossing back and forth under the footbridge. These are the beaver highways; canals they use to reach the saplings growing on the ponds periphery. Using little more than the wood at hand and hours of determined construction, the local beavers have transformed this stretch of high sierra forest into something altogether different. They have engineered an entirely novel ecosystem.

The term “ecosystem engineer” seems to have been coined by Clive G. Jones, John H. Lawton, and Moshe Shachak in a 1994 paper entitled Organisms as Ecosystem Engineers. In this paper they used beavers as the landmark example of a species that has impacts beyond their physical form. That is to say, while all species impact their ecosystems by eating some things and being eaten by others, some special species live and behave in ways that cause widespread changes. In some cases this results to major modifications to the original environment. In others, like my backyard beavers, their actions actually generate new ecosystems within the larger environment.

When they first move into an area and bring down trees, beavers are selective. They tend to choose certain species over others and this serves to change population dynamics in the surrounding woodland. The large rodents then turn their cut timber into water blocking dams which convert swift running streams into still and tranquil ponds. This change drowns some plants while watering others. My own beaver pond is filled with the still standing skeletons of drowned pines while those growing around the ponds edge reap the rewards of an elevated water table. Swift-current specialists who are of no concern to the beavers abandon the tributary blocked by the dam. These effects can be shockingly far reaching. Migratory salmon for instance, returning to their natal tributaries from a life at sea stop using rivers with upstream beaver dams and in the Arctic at least, beluga whales who specialize in hunting those salmon stop entering the dammed rivers as well. The ecosystem modifications don’t stop there. A host of subtle and varied changes begin to take place. Still-water specialists move into the niche abandoned by the swift-water specialists. Sediment brought downstream begins to collect in the pond, bringing with it a rich supply of nutrients. Grasses and brushy plants take advantage of the lack of competition from trees and the newly fertilized soil. It is a vibrant cascade of changes as life adapts and shifts to capitalize on the novel environment.

Beavers are the gold standard by which we understand ecosystem engineers, but they are far from the sole example. When the concept was originally proposed, it was suggested that ecosystem engineers fall into two distinct categories, allogenic and autogenic. The distinction is fairly simple. Allogenic engineers, like the beavers, shape the environment through their actions. The goals of their actions are the attainment of food and shelter but the results of these actions are far-reaching, dramatic, and exist on a scale far beyond what is considered normal for a species of their general size and population density. As for the autogenic engineers, they are the species that modify the environment simply by existing. The simplest autogenic examples are trees. The primary factor that makes a forest ecosystem a forest ecosystem is the presence and persistence of trees. The diverse and interconnected life of a forest ecosystem is intrinsically dependent upon the existence of the trees. A forest quite literally cannot exist without trees. As with the beavers, the impact of the pines in my backyard stretches far beyond the nutrients they consume and those they supply. Their massive structures provide shelter for animals, many of which the trees are completely oblivious too. The leaf litter of fallen pine needles and sloughed bark provides cover for innumerable insects and smaller animals while also providing a nutrient source and seed bed for germinating plants. The vast shaded areas they create prohibit colonization by some shade-intolerant plants and provide ideal living conditions for others. As with the beavers, the list of effects to an ecosystem generated by trees is both lengthy and subtle.

As it turns out, many of the species in the Safari West collection qualify as ecosystem engineers as well. In that initial paper published by Jones et al, they mentioned crested porcupines as allogenic engineers. The two species of crested porcupine are found throughout much of subsaharan Africa and into the Indian subcontinent. Like beavers, they are large rodents with industrious attitudes. Crested porcupines aren’t builders however, they’re excavators. They dig large holes in their constant quest for edible tubers and roots and over time, those holes accumulate runoff and organic matter, which turns them into ideal spots for germinating seeds. Studies have shown vastly increased plant diversity in porcupine pits as compared to control plots.

The many hoofed animals that make their home at Safari West have also been called ecosystem engineers although their engineering contributions are less friendly toward humans than others we’ve considered. In Africa, heavy-bodied hoofed animals congregate around water holes and rivers across the continent. As they do, their hooves create deep impressions in the mud that then fill with warm stagnant water; the ideal breeding ground for mosquitos. It has been argued that without the abundance of larval nurseries inadvertently constructed by thirsty ungulates, the mosquitos of Africa would be far less ubiquitous than they are. In a less itchy example, it has also been suggested that massive herds of ungulates, like our zebras and wildebeest have the additional engineering impact of widespread fertilizing and tilling action on the soil. Essentially, as these large migratory herds move, they transfer nutrients from where they cropped a mouthful of grass to where they dropped their dung. The action of literally millions of hooves on this fertilized soil tills and turns it, preparing the land for new growth; an effect similar to how we humans plow and fertilize our fields. These actions have little to no direct impact on the herds but indirectly impact available resources for other organisms within the ecosystem.

These few examples are interesting but they also hint at a potential problem with the idea of ecosystem engineers. If we look closely enough, won’t we find that all organisms shape their environment in some form or another? After all, hoofed animals aren’t the only things to leave footprints in the mud and shrubs also produce leaf litter and shade. In point of fact, this idea has long been a major point of contention in the academic community. The general consensus now is that in order to qualify as an ecosystem engineer, an organism must make a substantial impact on its environment, on par with or exceeding the impacts of purely physical forces (erosion, wind, fire, etc). This makes for a vague and readily contested definition but it doesn’t diminish the primary point which is this: we humans and the other forms of life crawling, swimming, and flying around this planet are not mere occupants on this world. We are equal parts inhabitants, destroyers, and ultimately, builders of the ecosystems in which we live our lives.

Now that we’ve established the role that these ecosystem engineers play, it’s appropriate to wonder: what does this have to do with conservation? I believe that part of what makes the fight to save the endangered species and ecosystems of this world so difficult and frustrating stems from a misconception in how the world works. Species are born, evolve, succeed, fail, and go extinct all the time. These ongoing processes are part of the natural pattern of life and came into play the moment the very first single-celled organisms bloomed into existence. Just as children are meant to grow into adults, so species are meant to develop and change as they interact with each other and their environment. Likewise, the ecosystems these interacting species make up will shift and transform, grow and recede, ebb and flow as the underlying interactions governing their existence vary.

For example, once not all that long ago, my backyard was an unbroken stretch of forest. The pines and the aspens struggled with one another for their place in the sun. Some thrived, others starved, and the forest remained healthy and strong. Then a pair of beavers moved in and started logging. Had I lived here then, I may have viewed those beavers as an invasive species, a destructive pest wreaking havoc on the stability of the forest. They cut down some trees and drowned others. They flooded the forest floor and in their wake other species of plant and animal, alien to that stretch of forest moved in. The system changed.

Now my backyard contains a luscious pond. Greenery abounds and with it I get to enjoy the chirping and trilling of many species of bird. I get to watch the deer wading in the shallows. I get to watch the corn lilies and ranger’s buttons and wolfsbane blooming in the bright springtime sun. It’s a healthy and vibrant ecosystem and one which never would’ve existed under a paradigm of conservation focused on the idea of preserving what is right now at the expense of what once was or potentially could be.

Eventually the beavers will leave. Maybe the primary pair will die and none of their offspring will take their place, or we’ll have an exceptionally wet spring and their dams will fail and wash away, or maybe one of the local coyotes will catch them unaware someday. One way or another, at some point, the time of the beavers in this location will come to a close. When it does, the dams will break down and the pond will drain. The stream will cut a new course across a swath of open land rich with accumulated sediment and sunlight. A meadow will form filled with tall grasses and the herbivores that graze on them. The entire habitat will shift from what it is now into something new. To be honest, I do not look forward to that day. In time, the aspens and pines that wait patiently on the edge of the pond will drop seeds in that someday meadow. Over time it will be recolonized by the forest. The ecosystem will change yet again. The key to a healthy ecosystem isn’t stasis but stability. The pond isn’t permanent and the meadow that replaces it won’t be either. The important thing is that when one of those habitats fade, something must arise in its place.

Too often, those of us who care about the natural world focus on how it is in the now. We want to preserve the world as it exists in the brief moments in which we are lucky enough to experience it. Alternatively, we want to turn back the clock to some idealized and Edenic past. This is a false paradigm. The idea of “ecosystem engineers” may be a bit vague and scientifically problematic, but it is real enough to demonstrate the flaw in that idealized way of viewing the world. We should not, and in fact, cannot preserve the world in amber. When we focus our efforts on an endangered plant or animal, the question should be asked; is this species failing because of a breakdown in the system or is it failing because of the system itself?