Tag Archives: grazing

Looking for Ecological Impacts? Urine Luck!

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I came across a copy of one of my all-time favorite research articles the other day.  The paper tells a great story about the kinds of complex interactions that occur between the biotic and abiotic components of an ecosystem, including feedback loops and ecological hierarchies.  But more importantly, it’s a story about a guy who dumps bison urine on the prairie to see what happens.

The research paper, written by Ernie Steinauer and Scott Collins was actually a report from the second of two projects they conducted on bison urine impacts at the Konza Prairie Biological Station in Kansas.  Both projects were interesting, but I like the second one best because it focused on how bison urine can affect the prairie community at multiple scales.

Bison and cattle urine creates patches of high nitrogen concentration in the soil.  You can identify those patches in a pasture because they look like dinner plate-sized areas of extra dark green grass that, if left ungrazed, are taller than the surrounding vegetation.  Previous studies had shown that livestock urine increased the aboveground production of plants and altered plant species composition.  Other researchers had also found that herbivores preferentially graze urine-treated areas because of increased plant production and higher N concentration in leaves.  Steinauer and Collins fully expected to see the same patterns, but they also wanted to see whether or not those urine patches acted as initiation points for grazing lawns.

If you were to walk around a pasture being grazed at a moderate stocking rate, you’d see patches of uniformly short vegetation (often bathtub to bathroom-sized or so) surrounded by a matrix of somewhat taller vegetation.  Those short-cropped areas are called “grazing lawns”, and often consist of a single species of grass such as smooth brome (Bromus inermis) or big bluestem (Andropogon gerardii) that is particularly favored by grazers.  Grazing lawns are a result of livestock repeatedly feeding from the same plants all season long while ignoring others.  They do this because the regrowth from plants that have already been grazed tends to be more nutritious than more mature leaves of plants that haven’t. 

In their study, Steinauer and Collins wanted to see whether small deposits of bison urine would stimulate much larger grazing lawns around them.  If so, a small fertilization event by a bison would actually lead to a much larger disturbance to the prairie community.  In order to test their idea, they first needed some bison urine.  (How would you like to be handed that task?)  Fortunately, the great wealth of scientific literature includes a publication from 1978 that has a recipe for simulated bison urine.  Don’t you love science?  Steinauer and Collins helpfully reprinted the recipe in their research paper so the rest of us could join in the fun.

 (“…Excuse me, I’d like to buy a big batch of urea so I can mix me up some artificial bison urine.  Yes, I can wait a moment.  Oh, hello officer, where’d you come from?  You’d like me to step that way?  Sure.  Hey! Where are we going??”)

I’ve included links to both bison urine research papers by Steinauer and Collins at the bottom of this post, but in summary, they confirmed that bison preferentially grazed patches treated with urine (65% of urine-treated plots were grazed, compared to only 18% of untreated plots).  However, the bigger story is that those urine patches also acted as initiation points for larger grazing lawns.  In fact, many fewer untreated plots would have been grazed if they hadn’t been enveloped by grazing lawns that started in nearby treated plots.

What do we learn from this?  We learn that when a bison (or cow) decides to relieve itself in a particular place, it’s very likely starting a cascade of impacts…

  1. Urine fertilization increases the growth rate and nitrogen content of the plants in a small area.
  2. Those particularly nutritious plants attract a hungry bison, which after devouring the vegetation in that little spot, keeps its head down and eats a lot more of the same plants in the vicinity.
  3. The competitive balance between the plant species within the grazing lawn temporarily changes as grazed grasses lose root mass, opening up space for other plants that couldn’t otherwise compete with them.
  4. Grazing lawns improves habitat for a number of wildlife species, including insects and reptiles, that depend upon the availability of open sunny areas adjacent to taller shady vegetation.

A cow selflessly initiates a cascade of ecological processes in a prairie. This one happens to be within the burned patch of one of our patch-burn grazed prairies - thus, the uniformly short vegetation.

While Steinauer and Collins studied changes in plant composition within their urine patches, they only followed those plants for a single season, and found some conflicting results between their two studies.  Some of those may have come from the fact that their two studies took place on somewhat different soil types.  However, my own grazing research has shown that many of the effects of grazing are most evident in the year following the grazing event, after colonizing plants have had time to move into areas left open by weakened grasses.  It would have been very interesting to have followed the urine patches through several seasons to see how plant composition changed over that time period.

I’ve never met Ernie Steinauer but I bet I’d like him.  I say this because in addition to being crazy enough to mix up and dump bison urine on the prairie just to see what happened, Ernie also kept track of other interesting things that happened within his urine-treated patches.  One thing he noticed was that insect herbivory – especially on switchgrass (Panicum virgatum) – seemed to be influenced by urine treatments as well.  Now there’s the beginning to a great follow up project for someone!  After you try it, let me know what you find.

Here are the links to the two articles.

1995 article

2001 article

Sweet Clover: Ugly but Harmless? Or Dangerous Invasive Species?

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Why is sweet clover the target of aggressive control by some prairie managers and largely ignored by others?  After talking to a number of people across the Midwest and Great Plains, I think there are a couple of things happening.  First, the usually biennial sweet clover can be very abundant and showy in the years it blooms, but is harder to find in other years.  I think some prairie managers see those big flushes and mistake abundance for aggressiveness.  However, I also think that some soil/precipitation/latitude(?) conditions may lead to real negative impacts from sweet clover on plant diversity.

One of the lessons that’s been strongly reinforced for me this summer is that it can be difficult to extrapolate successful prairie management/restoration strategies from one region to another.  Just during the last several months, I’ve visited prairie managers in Nebraska, Indiana, Missouri, and South Dakota and I’ve seen tremendous variation between (and even within) those states in terms of which species are invasive and which are not.  It’s dangerous to assume that just because a species like sweet clover isn’t causing problems in one prairie, it won’t cause problems in another.  I hope we’ll eventually learn enough to accurately predict when to worry and when not to, but in the meantime, it behooves prairie managers to carefully evaluate species at their own sites.

Yellow sweet clover. This exotic species is still planted in some wildlife and ground cover grassland plantings because of its purported wildlife value and cheap seed. However, it appears to be invasive in some places and/or situations.

I’ve been working with prairies along Nebraska’s Platte River for nearly 20 years now, and my observations have led me to conclude that sweet clover is more of a big ugly plant than a true invasive species in those prairies.  Years of data collection on my plant communities support those observations.  That annual monitoring work entails listing the plant species I find in each of about 100 1m2 plots across a prairie.  Those plots are stratified across the prairie so the site is evenly sampled.  Once I have those plotwise species lists, I calculate the floristic quality  (FQI) inside each plot, a calculation that takes into account both the number of species present and the average “conservatism” value of those species.  I can then look at changes in mean floristic quality over time to help me see how the plant community changes over time.  I monitor a few prairies annually, and others on a periodic basis.

Those data show the same thing I’ve seen observationally – sweet clover changes in abundance from year to year (though not as much as it appears visually), but the species doesn’t increase in abundance over the long term and doesn’t appear to negatively impact floristic quality.  Below are graphs from three sites that show both sweet clover frequency (% of plots occupied by sweet clover) and mean floristic quality.  Two of those sites were annually grazed during the data collection period, and the other was only grazed once – toward the end of the sampling period.  Cattle grazing almost certainly helps control sweet clover because it is one of their favorite plants to eat, but I don’t think sweet clover is causing me problems where I don’t graze either.

What my data don’t show is the flush of tall blooming plants that happens every other year or so.  I’m just counting whether at least once sweet clover plant is present in each of my small plots – not how big it is, or whether or not it’s blooming.  Nevertheless, sweet clover frequency changes from year to year but doesn’t appear to correlate at all with changes in mean floristic quality.

Nine of years of annual data from a 1995 prairie restoration seeding. The site has been under patch-burn grazing during each of the nine years of data collection. Sweet clover is never abundant at this site, but has also not increased over time, even through years of drought and heavy grazing. Error bars for floristic quality indicate 95% confidence intervals.

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Sweet clover was present in between 22% and 42% of 1m plots in this restored crop field, planted in 2002. Though the sweet clover frequency varied from year to year, the mean floristic quality of the plant community increased between 2004 and 2008 before leveling off after that - apparently independent of sweet clover. Sweet clover data from 2008 was eliminated from this graph because I later questioned whether I'd confused black medick and sweet clover in some plots. This site was not grazed except in 2009.

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These data are from a remnant mesic prairie under patch-burn grazing. Instead of being stratified across the entire prairie, these data are from a single patch that was burned in 2008, and that same patch was sampled again in 2009 and 2010. It was grazed very hard in 2008. In 2009, no new patch was burned, so it was grazed hard again, but then fenced out in early June. In 2010, the site was not fenced out, but received only light grazing while cattle focused on another portion of the site that was burned. These data are from only about 30 plots per year.

I feel pretty good about ignoring sweet clover and focusing on more invasive species on our prairies.  Both my observations and data support that strategy.  However, as I said earlier, just because the species doesn’t appear to be problematic for me doesn’t mean it isn’t an invasive species in other prairies.  It’d be great if we could compare data similar to what I’m presenting here from a number of sites to see if sweet clover is acting differently in different places.  Without data, it’s hard to know whether or not people are just interpreting the “invasiveness” of sweet clover in different ways.  For now, my answer to the question,  “Is sweet clover really invasive?” is still the same…

Maybe.