It’s been too long since the initial installment of what was intended to be a regular feature of this blog – the Prairie Word of the Day. Since one blog post does not make a series, I figured I’d better at least write one more. I will try to add more to the series relatively soon.
If you read this blog or any other source of information about prairie ecology or management, you’re likely to have seen the term “disturbance” used in some context (e.g., “ecological disturbance,” “disturbance regime,” “disturbance in the force”). Ok, that last one is from a different context, but nevertheless.
Outside of prairie ecology or other ecological conversations, a disturbance is often something that requires contacting the police. However, in almost every case, prairie managers would appreciate you not calling the police in response to disturbances in a prairie. So what does the word “disturbance” mean in an ecological context?
It’s probably easiest to think about a disturbance in a prairie as something that disrupts the lives and processes of prairie organisms and creates a shift in the competitive balance among those species. Historically, the big three disturbances in grasslands were fire, grazing, and drought. Fire and grazing are still used as management strategies today, along with mowing/haying and, sometimes, herbicide treatments. Drought (and other significant weather events such as a flood) is not something managers prescribe or apply to a prairie, but it certainly has significant impacts.
Both “natural” disturbances and those prescribed by prairie managers are critically important to the health of prairies, despite the fact that they have significant impacts on species. In fact, many animals, plants, and other organisms often die as a result of disturbances. Prairies, however, more than most other ecosystems, are not only well suited to survive disturbances, they are defined by and rely on those disturbances. Without fire and drought, for example, prairies wouldn’t even exist – they would be woodlands instead (and who wants that??).
Every time a fire burns through a prairie, grazers chomp off much of the vegetation, or an extended drought turns a prairie brown in mid-summer, the competitive balance in a prairie is altered. Depending upon the timing and intensity of a fire, for example, it’s likely to kill some trees and “top-kill” others, forcing them to restart from buds at or below the surface of the ground. Fires can also kill or suppress the growth of other plant species, especially those most actively growing at the time of the fire. At the same time, fires create opportunities for other plants – especially those in direct competition with the ones suppressed or killed. Plants that thrive best in full sunlight and don’t do well under layers of thatch or beneath tall plants also respond very favorably after a fire.
Of course, fires can also kill or injure wildlife or insect species (not to mention microorganisms and other life forms) that are aboveground at the time of the fire and can’t evade the heat and/or smoke. The season in which a fire burns plays a big role in which organisms are affected. Dormant season fires can kill insects and other species that overwinter in vegetation or along the surface of the ground, but growing season fires cause even more fatalities because so many more species are active during that time of year. Even animals that survive the fire itself may have to travel to unburned areas in order to find suitable habitat, and those trips can be hazardous as well.
Despite the immediate negative impacts of fire on individual insects, wildlife, and other species, however, periodic fire usually has longer-term benefits – even for those species that suffer fatalities. Without fire – or some other disturbance that removes vegetation – prairies can become so overgrown that most species can’t survive there. Suffocating layers of thatch, encroachment by trees and shrubs, and the potential for diseases and predator populations to build up over time are all conditions that can be found in prairies that go undisturbed for long periods of time. In addition, if the same habitat and growing conditions prevail year after year in a particular patch of prairie, some species will thrive but others will not – leading to a loss of species diversity and overall prairie health.
Without going into details here, the impacts of drought and grazing on prairie species are similar to those of fire – each event will favor some species over others. Prairie plants are well adapted to periodic burning, grazing, or drought. Perennial plants have numerous buds at their base, from which they can grow new shoots after they’ve been burned or cropped off, and most can enter dormancy during a severe drought in order to conserve resources for better days. Shorter-lived plants often rely on disturbances to create opportunities for them to grow with less competition from nearby perennials. Those annual and biennial plants produce copious amounts of seed that fall to the ground and wait until the next time conditions favor their germination and growth.
Every vertebrate and invertebrate animal species relies on a particular set of habitat conditions. Some thrive when vegetation is short and sparse, and others prefer tall dense vegetation. The highest diversity of animals and insects is usually found where perennial plants have been recently weakened by fire, grazing, and/or drought, and short-lived plants are thriving while those perennials recover. In any case, habitat conditions are tied to disturbances – some conditions are created during or in response to a disturbance, and others (tall dense vegetation) are created when a disturbance hasn’t occurred for a while.
As prairie managers, our job is to provide habitat and growing conditions for as many prairie species as possible. We burn, graze, and mow prairies at prescribed intervals (and usually in patches, rather than across an entire prairie) in order to maintain a competitive environment in which all those prairie species can survive. Whenever we burn a prairie or introduce large grazers to it, we know that some species will be harmed by that action, but that others will flourish because of it. As long as we allow a patch of prairie to recover between disturbances and create a patchwork of habitats (some recently disturbed and others not), the prairie community will remain diverse and resilient.
Prairie management is complicated, and it can be difficult to make decisions and take actions that you know are going to negatively impact some species. On the other hand, the other choice is to do nothing.
And that would be disturbing.
Thanks for including the concept of “patchwork” in habitat restoration.
Is there a lower size limit below which patchwork treatment is no longer effective? I’m working with a 30+ acre California coastal terrace prairie grassland remnant that was commercially grazed for decades, then left on its own. Now the city is attempting to restore the grassland to its pre-grazing state.
Thanks, from a western Nebraska grasslands native.
Great question. Small prairies are tough. Rather than giving you a long thesis here, you might check out a blog post I wrote a while back that deals with your question. https://prairieecologist.com/2012/03/05/how-should-we-manage-small-prairies/
As a retired but still practicing prairie manager in Missouri, I feel ten acre treatment blocks are optimum. From a grassland wildlife standpoint, anything smaller than ten acres is easily searched by nest predators so I try to work in ten acre blocks. That’s my minimum for patch-burn grazing too, with a year or two without grazing before starting back into the pbg cycle. Some feel that is too small but it’s just different.
Nice post! I have wonder though if it makes sense to think of fire and grazing as disturbances….
Disturbance is usually considered a source of density independent mortality that resets successional processes. In the case of fire, I think it depends on your perspective (that of a tree or prairie plant). Fire kills most trees and prevents a successional trajectory towards forest, but maintains dominance by long-lived, late successional prairie plants. As a prairie enthusiast, I have always thought of fire as an abiotic condition necessary for the exclusion of weedy tree species and the maintenance of late successional prairie.
Grazing is also often thought of as a disturbance, but the primary conservation benefit is usually presented as preventing competitive exclusion by dominant competitors, C4 grasses. In this case, is grazing a source of negative density dependence rather than a disturbance?
I am interested to hear other’s idea on this, and I enjoyed your post on these processes that are so important to prairie ecology!
I would certainly consider both fire and grazing as disturbances. As you point out, the succession of prairie to forest is reset. This in particular is vital to the existence of prairie in the landscape. And while the long-lived (late successional?) prairie species are adapted to and dependent on fire, I expect that you would also find a decline in the populations of annual (early successional) prairie species if burns were discontinued. These annuals take advantage of the open spaces caused by the consumption of thatch and are further encouraged by the soil disturbances created by birds and other animals as they hunt for insects post-burns.
As for grazing, I’m not sure the disturbance is limited to the consumption of the vegetation. In addition to the consumption of some of the competing vegetation, herds are responsible for the trampling of upright stems and the mechanical disturbance of the soil as they tromp across the landscape. This opens holes in the ‘canopy’ in the same way that a falling tree does in a wooded landscape. The difference is the scale.
Hi Jonathan. Interesting points, and thanks for making them. I would argue that fire doesn’t have to maintain dominance by long-lived, late successional prairie plants. That might be true in some places when fire is only employed during the spring, but if you burn prairies in the summer, for example, it suppresses dominance by those same plants. I’m someone who wants, both ecologically and (admittedly) aesthetically, to see a fair amount of what you’d probably call “early successional” prairie in my sites. I think it’s good for diversity and resilience, and I think fire is one tool that helps create those conditions.
In terms of grazing, it can sure do what you say – help reduce competition from C4 grasses, but it can do much more. As I said in the post, it has pretty big impacts on habitat conditions for many animals. In fact, using your definition, I’d say that grazing can “reset” the successional process of habitat structure in a way – grazing creates short structure that then proceeds through a recovery phase and toward a dense grass-dominated condition.
Also, I didn’t get into this in the post, but many would argue that it’s difficult to separate fire and grazing and that they should be considered part of the same disturbance. I don’t know that I agree completely with that, but I do think they can be strongly coupled – both historically and as management tools today. Fire-driven grazing is a big part of our management here, and creates effects on plant and animal communities that are different than either fire or grazing alone.
I’m glad you enjoyed the post, and I appreciate your input. Keep the thoughts coming – on this and other topics.
May the Fagus be with you!
Actually, no… no woodies allow. May the Fimbristylis be with you!
Better… : )
I have never liked the term disturbance when used to describe prairie fires. The text “Biology”, Campbell, fourth edition, pp. 1136 describes disturbance as follows.
“Disturbances are events that can disrupt communities, changing resource availability and creating opportunities for new species to become established.”
In prairies it seems exclusion of fire causes more of the above criteria to occur than regular prescribed burning. I would even rebuttal that burning promotes stability in prairies.
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