Letting nature take its course

The phrase “let nature take its course” is so widely used and accepted, it has gained idiom status.  The idea that nature is self-perpetuating and self-correcting is an attractive one.  The supporting evidence is strong too – I mean, look at how long nature thrived before humans were even a species! 

Unfortunately, the romantic notion that we should just back off and allow natural areas to manage themselves just doesn’t work in today’s world.  Or, to be more accurate, most of us would be uncomfortable with the results of that strategy – especially at individual sites.  Whether you like it or not, the earth today is heavily shaped and manipulated by human activity.  Within that context, deciding to suddenly back away and allow nature handle things on its own comes with some serious repercussions.  It’s akin to taking a bunch of athletic kids, training them for years to be elite volleyball players, and then entering them in a soccer tournament.  They might be great athletes, but they aren’t likely to fare well at a tournament for which they don’t know the rules and don’t have the appropriate skills to succeed.

Prairies like this one depend upon human management for their survival.

This isn’t a post about what would happen to the earth if humans suddenly disappeared.  That story has been told by others, and you can go explore and argue about that story with them.  This is a post about what happens when we walk away from natural areas – prairies, in particular – within the context of the world we inhabit today.  I also want to be clear that this post is not a criticism of the way humans have altered the earth.  There’s plenty to talk about on that topic, but today’s post is about how we manage (or not) natural areas in the contemporary world.

Let’s start by considering some of the ways in which humans have altered the playing field for species and natural communities.  We’ll focus on the grassland landscapes of central North America because that’s the setting most familiar to me.  First and foremost, we’ve converted much of the landscape to intensive agriculture and other human developments.  As a result, once expansive swaths of prairie are now divided into small isolated fragments, limiting the ability of animals and plants to migrate or otherwise move across the landscape.  We’ve brought plants and animals from the opposite side of the globe and released them into this fragmented landscape.  Many of those have become dominant competitors, with the ability to eliminate other species from their territories and reduce biological diversity. 

The fragmented nature of today’s landscape facilitates invasions, which most often occur along boundaries between prairies and nearby land uses, such as roads, crop fields, suburban areas, or other areas where invaders are established.  Introduced species are not the only invaders in this context, however.  Native trees and shrubs, which have battled prairie plants for dominance since the last Ice Age, have been given a huge advantage.  Instead of trying to spread into prairies from a few stream valleys or other fire-resistant sites, they now invade from countless locations – like an army that has dropped millions of paratroopers behind enemy lines.

Invasive grasses have become dominant in this grassland, which is not being actively managed for plant diversity. In addition, the numerous tree lines in the background provide a constant seed source for potential woody plant invasion.

Within our fragmented landscapes, our agricultural and industrial activities have increased the levels of nutrients such as nitrogen and phosphorus – and those chemicals enter prairies through both the water and the air.  We are essentially fertilizing prairies, which might sound positive, but usually favors invasive plants (e.g., reed canarygrass) or makes a few native plants exhibit the same aggressive diversity-reducing traits as invasives.  The inadvertent fertilization of prairies is most intense in areas near crop fields or factories, but the impacts are measurable even at great distances from those sources.

Adding a rapidly-changing climate to all those other stressors just seems unfair, doesn’t it?  We’ve introduced new enemies, provided them (and old foes) with access points and increased competitive advantages, and carved up the landscape to block escape routes and re-supply lines.  Now, we’re turning up the heat and quickly changing the basic growing conditions and living environment within the prairies that have managed to survive to this point. 

But, hey, prairies should be able to handle all that without our help, right?  (Good luck in the soccer tournament, kids!)

Before we address that question, here’s one more consideration.  The last glaciers retreated from central North America thousands of years ago, and tundra and spruce forests gradually gave way to grasslands.  During and after that transition, people have been present and active managers of those grasslands.  Human hunters influenced the composition and behavior of animal communities, and arguably helped eliminate a number of important animal species.  Perhaps most importantly, humans were actively using fire as a management tool (to attract grazing animals, for example) as well as for warfare and other purposes.  Those fires were an essential factor that helped perpetuate grasslands and prevent them from being taken over by encroaching woody vegetation.  As a result, today’s prairies have never been separated from people and human management.    

People have been actively burning prairies in central North America since those prairies first constituted themselves after the last ice age.

So, what would happen to a prairie today if we decided to just leave it alone?  It’s not a hypothetical question – any experienced prairie manager can tell you stories based on their own prairies, or on prairies they’ve watched suffer from insufficient or no management.  The only fires that occur in today’s fragmented landscapes are those set by people.  In the absence of those prescribed fires (or a substitute such as haying or grazing), prairies begin accumulating thatch – the dead stems and leaves from successive years of annual growth of grasses and wildflowers.  Within a few years, that thatch begins to smother many of the plants trying to grow through it.  It also creates inhospitable habitat for most prairie animals.  Biological diversity, an essential component to the resilience and survival of prairie communities, decreases dramatically.

As thatch builds up, so does the competitive pressure from invasive plants and trees.  In my area, for example, smooth brome, reed canarygrass and tall fescue tend to flourish in the absence of management.  They do particularly well in high nitrogen environments, but they also get by just fine without inadvertent fertilization.  In addition, there are numerous tree and shrub species that can quickly take advantage of a lack of fire, including eastern red cedar, white mulberry, honey locust, Siberian elm, smooth sumac, and rough-leaved dogwood. Especially in highly-fragmented landscapes, those and other trees are often growing right on the edges of prairies, and if not, their seeds can easily reach prairies by bird, wind, or other conveyance.  Complete conversion of a diverse prairie to a woodland with a brome understory can happen within a couple of decades or less.

By the end of a growing season, prairies can produce a tremendous amount of vegetative growth, which can accumulate after multiple years (if not burned, grazed, or mowed off) to the point where it greatly inhibits the growth of plants beneath it and provides poor habitat for many animal species.

You might be thinking, “well, sure, I can see how that might happen in a tiny prairie surrounded by cornfields, woodlands, and suburban sprawl, but what about some of the big prairie landscapes of Kansas, Nebraska, or the Dakotas?   Surely those prairies can take care of themselves, right?“

We can argue about whether lightning fires alone would be sufficient to prevent tree encroachment in a huge expanse of prairie.  I feel confident they wouldn’t, but it’s an argument that can be had.  However, invasive species, spurred on by nutrient pollution and climate change, are still going to be a killer threat to biological diversity and the subsequent vitality of those prairies in the absence of human management.  Depending upon location, already-present invasive plant species such as leafy spurge, sericea lespedeza, spotted knapweed, cheatgrass, Kentucky bluegrass, and many others would expand their reach and power to the point where they would dominate large swaths of land, if not entire prairies.  That invasion and dominance would trigger a cascade of other impacts, leading to reduced plant diversity, decreased habitat quality for animals, eventual extirpation of many plant and animal species, and an ecosystem that would be unrecognizable – and undesirable to most of us.

Invasive species, such as crown vetch (Securigera varia), are already present throughout many prairies, but are held in check by constant and thoughtful management. In the absence of that management, prairies would be overrun by these species, losing plant diversity and ecological function as a result.

Anticipating protests, here are couple more quick points.  If you’re an advocate of the broad idea that we should get out of the way and let nature take its course, you might say prairies are an unfair example because they are a transitional ecosystem that relies heavily on disturbances such as fire to avoid becoming a woodland.  That’s fine, but if you look around the world, there are lots of other fire-dependent ecosystems, including many (most?) forests and woodlands.  Most of those also have very long histories of human fire management. 

In addition, woodlands and other ecological communities suffer from invasive species, habitat fragmentation, nutrient overload, and climate change, just as prairies do.  We’ve created a world that puts those natural systems at a disadvantage, and whether we like it or not, they now rely on us to help mitigate those threats.  Arguing about whether human management is natural or not is a moot argument that distracts from the great challenges we face in conservation.  Let’s focus on the important discussions about how best to manage ecosystems, not the settled issue of whether we should be managing in the first place. 

People are an intrinsic part of nature and the world we live in.  That shouldn’t make nature seem any less fascinating or inspiring – in fact, recognizing our interconnection with nature should inspire us even more. We are part of an incredibly complex and beautiful web of interacting species and communities across the glove. As such, it’s up to us to play our roles responsibly.  Just as we don’t exist outside of nature, we also can’t survive without it.

Prairie Word of the Day – Phenology

Hello, and welcome to the fifth edition of the popular series, “Prairie Word of the Day.” This is the series that has previously brought you such inspiring words/phrases as Tiller, Habitat Heterogeneity, Disturbance and Shifting Mosaic of Habitat. Thank you for the many cards and letters expressing your gratitude for the explanations of these words, and suggesting future topics.

Today’s featured word is Phenology. In short, phenology is the study of the timing of various events in the lives of plants and animals and the factors that influence that timing. Phenology should not be confused with Phrenology, which is the long discredited study of how the shape and size of the human skull supposedly correlates with character traits and mental capacity. Phrenology has been used to bilk people of their money, support racist and sexist stereotypes, and bolster Nazi eugenics. Let’s not talk about that today.

Phenology, without the “r”, is a complex and important topic in ecology. You might hear someone talk about the phenology of plants related to when they begin emerging from the ground, when they flower, and when they begin to wilt and senesce at the end of the growing season. Additionally, however, phenology includes the timing of the emergence of insects from dormancy or their final molt into adulthood. It also includes the timing of animal migrations and hibernation, as well as many other events in the lives of myriad organisms.

This bee (either Melissodes agilis or M. trinodis) is a specialist feeder on sunflower pollen and is only active during the period of summer when sunflowers are blooming. If the bee emerged before sunflowers started blooming, it might not find anything to eat.

The factors that influence a species’ phenology often include temperature, light, and moisture – in combination with genetic signals. We still have a lot to learn about the phenology of most prairie species, especially in terms of how they might adapt to changing climate. In fact, rapid climate change has brought much recent attention to phenology because changes in the flowering time of plants, for example, have already helped illustrate the occurrence and impacts of climate change. In addition, there is great concern that species may not be able to adapt the timing of their lives quickly enough to match the changing climate, and/or that timing of interdependent species might not remain synchronized. For example, flowers might start blooming before or after their particular pollinators are active, or birds or insects might migrate to breeding areas before food is available at those sites. A couple years ago, monarch butterflies arrived in Nebraska way ahead of schedule, but fortunately they were still able to something to eat and lay eggs on.

When monarchs arrived in Nebraska much earlier than normal, dandelions were one of the few abundant wildflowers for them to feed on and they laid eggs on whorled milkweed because common milkweed hadn’t emerged yet.

Here in Nebraska, we got some interesting insight into the phenology of plants during 2012. The year ended up giving us the most severe single year drought in recorded history and it started out as a year of extraordinarily warm temperatures. In fact, spring and summer temperatures arrived so early that we recorded many plant species blooming weeks or months ahead of their typical schedule. I wrote a short blog post about this back in May of 2012 and a number of people from around North America responded with their own sightings. The observation that stood out most to me was the blooming of asters in May. I had never seen heath aster (Aster ericoides) or New England aster (Aster novae-engliae) bloom before late August or September.

Phenology is also important to land managers trying to sustain biological diversity in prairies. For example, around here, we are constantly fighting cool-season invasive grasses. The growth period for those species starts earlier and ends later than that of most native prairie plants. That gives us some opportunities to use herbicides to kill or suppress smooth brome, Kentucky bluegrass, or other invasive grasses when the chance of harming other plants is very low. In addition, we can use prescribed fire, grazing or mowing to target those grasses when they are most vulnerable. For example, we might try to burn a prairie right as those species are starting to bloom because it wipes out those plants’ entire season of energy investment in growth and flowering. The fire doesn’t kill those grasses, but it can knock them back enough to allow other plants – especially those just starting their growth periods – to flourish while the vigor of the invasive grasses is low.

We timed this burn to suppress cool-season invasive grasses, which were just starting to bloom. After the fire, many warm-season grasses (and other plants) responded quickly because they were just beginning their period of most active growth.

Timing of burns can also be aimed at suppressing many other kinds of plants. For example, we sometimes try to burn prairies when encroaching trees are just leafing out and highly vulnerable. Alternatively, burns can be timed to limit impacts on animal or plant species. That might include strategically scheduling a fire based on the emergence of rare insect species or before sensitive reptiles become active in the spring. Prescribed grazing can be employed in much the same way – strategically moving livestock in and out of an area to suppress the growth of particular plants or to create desired habitat structure prior to the arrival or emergence of particular animal species. In all these cases, land managers are acutely aware of the phenology of the species they are trying to suppress or assist.

If you’re someone who enjoys keeping track of when things happen each year, you might enjoy joining a citizen science effort to document changes in the phenology of many different phenomenon. You could start at the National Phenology Network and peruse some of the options they provide. Or, if you already have years of field notes that document when you see your first bumblebee, prairie clover flower, or grasshopper sparrow each year, I’d encourage you to contact a local expert on that/those particular species and let them know about your data. You might have information of great value to conservation.