The Myth of Self-Sustaining Prairies

Here’s a question I get asked occasionally:  “At what point will my prairie become self-sustaining?”

There are lots of ways “self-sustaining” can be defined, of course, but usually the person is hoping that at some point they can just step back and let the prairie do its thing with very little or no human input.  In other words, they hope the prairie will function like a machine.  Once you have it tuned up correctly, it’ll hum along just fine with only occasional inputs of fuel or maintenance.

Ah, that it would be so easy.  Unfortunately, there is a short answer to the question, and it’s a disappointing one.  The answer is, “It just doesn’t work that way.”

Here’s the short explanation of that short answer:

A prairie with no management at all accumulates thatch from each successive year of plant growth, and if not removed, that thatch eventually builds up to the point at which only a small number of plant species can survive.  Unfortunately, the most dominant of those surviving species tend to be either trees/shrubs or invasive plants.  In the eastern half of Nebraska, smooth brome tends to be a primary winner, along with tree species such as Siberian elm and eastern red cedar.

Besides the issue of thatch build-up, there are just too many threats, particularly from invasive species and trees, for prairies to maintain their species compositions and ecological functions without human management.  This is particularly true with tallgrass prairies in an agricultural matrix.  The degree of vulnerability to invasion depends upon soil type and the surrounding landscape.  Some soil types seem favor invasives more than others – oftentimes, high soil nitrogen levels can favor exotic grasses, for example.  The degree of invasive species pressure on a prairie is also influenced by the abundance and proximity of those invaders in the neighborhood around the prairie . However, all prairies (that I’m aware of) have some degree of vulnerability to invasive species.

Active management, such as the application of prescribed fire, is needed to prevent excessive thatch buildup and to help suppress invasive species.

That’s the short answer.  A longer and better answer is that tallgrass and mixed-grass prairies are not “climax communities” in the classic sense.  In my early ecology classes, I learned that terrestrial plant communities move through a process called succession from bare ground to some final stable state – usually a forest.   Bare ground is colonized by opportunistic species, which are eventually pushed out by longer-lived grasses and wildflowers.  Those grassland species are then replaced by various generations of tree species, each topping out the other until a final set of tall long-lived trees becomes dominant and creates a stable community.  Disturbances such as fire or severe weather events might set back succession temporarily, but the process keeps moving toward that climax community.

Prairies don’t fit that successional model very well.  Prairies are maintained (and defined?) by disturbances such as fire, grazing, and drought.  Without some combination of those ecological processes, prairies turn into woodlands.  Because of that, some might argue that prairies are simply an ephemeral stage of the longer successional process, and not really a stable ecosystem.  Others might argue that the whole idea of ecological succession is overly simplistic and not representative of the way all ecosystems function.

Without getting into that larger argument, the real point is that if we agree prairies are important, and we want to maintain them, active management is necessary.  Some people point to expansive prairies in Great Plains landscapes and wonder if those prairies could maintain themselves without humans if given the chance.  After all, lightning-caused fires and roaming herds of bison should be able to take care of things without interference from people, right?  In reality, we don’t have any historical precedent to back that up.  Today’s prairies have only been around since the last ice age –  about 10,000 years (less in the east, more in the west.)  During that entire time, people have been active managers of those prairies.  Fires set by Native Americans were much more abundant and extensive than lightning-caused fires.  Bison herds, and many other herbivores, responded to those fires by focusing grazing in those recently burned areas.  That intensive fire/grazing disturbance interacted with and compounded the impacts of long droughts, floods, and other weather-related events.  Cumulatively, those major disturbances maintained the integrity of prairies.

Along with climate and fire, bison (and other grazers/herbivores) were a major force that shaped prairies. However, people and their activities were also an important component of the process.

There’s really no way (and no reason) to separate people from prairie.  Regardless of the intent or motivation of the people who manage prairies – historically or now – their actions have tremendous impacts.  Similarly, inaction by people who control prairies has tremendous impacts as well.  In the natural resource management world, the phrase “No management is still management” is well-worn but nevertheless true.

Of course, defining the need for continual human management – even in the absence of today’s new challenges such as invasive species – doesn’t solve the problem.  What kind of management is needed?  How do we know when to do what?  The answers to those questions are complex, still being debated, and the primary subject of this blog, my book on prairie management, and myriad discussions among prairie managers around the world.

Some people who agree that prairies require some level of active management still search for a relatively simple management recipe to follow.  Annual haying or burning or two to three-year rotations of fire or grazing are examples of management regimes that are commonly used and advocated for.  This is really just a small step up from the idea that prairies should maintain themselves.  In this case, the argument is that prairies should be able to maintain themselves if we just provide them the right basic disturbance framework.

I’ve given my opinion on simple, repetitive management regimes often within this blog (see my Calendar Prairies post as an example).  I think repetitive management threatens plant diversity because there are always some plant species that are favored in a particular management regime and others who are not.  Over time, those species not favored will inevitably fade out of the community if the same regime is applied over and over.  Perhaps more importantly, animal species – including insects – with fairly specific habitat structure requirements are similarly affected.  Some species thrive under repetitive management if that management consistently favors them.  However, those animals that don’t find what they need in that management system can’t normally survive for many years in suboptimal habitat like many perennial plants can.   Animals without appropriate habitat either move or die – and in fragmented landscapes, or in landscapes where the same management is in place across the entire landscape, moving may not be a viable option.

Annual haying provides good growing conditions for many plants - especially those the bloom and produce seed prior to the haying date. On the other hand, some of the plant species favored by annual haying (including smooth brome) can become invasive. In addition, some desireable native plant species do poorly under annual hay management and eventually disappear from those prairies.

All of this adds up to one conclusion.  Diverse, functioning prairies require active, constant, and thoughtful management by humans.  There’s no getting out of that responsibility.  If we choose not to be active thoughtful managers, we are choosing to let prairies degrade, and we’ll have to live with the consequences (“No management is still management”).  Hopefully, though, most people with influence over the management of prairies will embrace their role, and be active managers – as well as active participants in ongoing discussions about the impacts of various management techniques and systems.

Though active prairie management is time-consuming, and often expensive, it’s also extremely rewarding.  Whether it’s a small backyard prairie garden, a 20,000 acre grassland, or something in-between, every year is a chance to try new things, see what happens, and learn from the experience.  More importantly, the diversity of plant, insect, and invertebrate species in well-managed prairies – large and small – is its own reward.  Who could ask for more than that?

Photo of the Week – April 21, 2011

Spring is the time for early wildflowers, cool rain showers, and northward-migrating birds.  It’s also the time during which smooth brome and other invasive cool-season grasses get their annual jump on the competition.

Suppressing invasive grasses and allowing native grasses and forbs to remain competitive in the plant community is one of the most important jobs for prairie managers in our area.  In some cases, well-timed herbicide treatments can be successful at knocking back those invasives, but it is usually very difficult to find a window of time when the temperature is warm enough for the herbicide to be effective but when most native plants are safely in dormancy.  More commonly, the best option is to suppress the growth of invasive grasses and decrease their competitive ability for that season.


Cow grazing smooth brome in a recently-burned restored prairie. Repeated defoliation throughout the spring season will greatly weaken this brome, allowing other plants to encroach upon its "territory".

We’ve found that grazing is particularly effective at suppressing cool-season grasses, because the grasses are repeatedly defoliated during their growth period, greatly reducing their vigor.  The strategy works particularly well for smooth brome because, at this time of the year, brome is by far the favorite food of cattle in our prairies.  They’ll eat other invasive grasses too (I watched them working on Kentucky bluegrass this morning, in fact) but if smooth brome is available, they’ll hit that first.

Sometimes we use short-term intensive grazing so that the cattle graze the brome hard, but then are removed from the pasture at about the time warm-season grasses are starting to emerge.  That works well to suppress cool-season grasses, but often we end up simply trading dominant cool-season grasses for dominant warm-season grasses – without helping forbs much.  This is because the warm-season grasses tend to take advantage of the weakened invasive grasses better than forbs do.

In the case of the above photo, we’re using a variation on patch-burn grazing, in which only a portion of the prairie is burned so that cattle will concentrate their grazing is focused within that patch.  In this version of patch-burn grazing, cattle will be present all season, but we will reduce the stocking rate in late May.  The idea is to get high grazing pressure on smooth brome during the spring, but also to leave enough cattle in the pasture during the summer so they will selectively graze the warm-season native grasses.  With a light summer stocking rate, the cattle eat almost exclusively grasses, leaving wildflowers to grow with less competition from both invasive cool-season grasses and native warm-season grasses.  Outside the burned patch, very little grazing takes place at all, especially after the stocking rate is reduced.  Next year, a different patch will be burned so we don’t repeatedly stress the same plants each year.

This photo is a good demonstration of the attraction of both recently-burned patches and smooth brome to cattle – even when the brome is only about 2 inches tall!  This cow is working over a patch of brome in the restored prairie, even though it also has access to the unburned parts of the same pasture (seen in the far distant background of the photo), which includes some degraded native prairie with lots of smooth brome and other grasses in the 5-8″ height range.