First Impressions From My Insect Week

Well, as promised, here are some early impressions from the time I spent looking around our Platte River Prairies with Mike Arduser (bees) and James Trager (ants) last week.  I’ll know a lot more after Mike and James have time to sort through the all the insects we collected.  Both have promised to send me annotated lists of the species we found, along with the locations each was found in.  Once I get that information, the three of us will be able to have some more discussions about what the data tell us about the state of our prairies – from the perspective of bees and ants.  It’s unlikely that our single snapshot of data collection will provide us with any major conclusions, but I have high hopes that our efforts will point out some paths I can follow during the next couple of years to flesh out the story.

The Platte River Prairies through the eyes of bees:

Obviously, plant diversity is important to bees, but the majority of bee activity last week seemed to be taking place on purple prairie clover, a species that is pretty abundant in both our remnant and restored prairies.  I don’t know yet how many of the 40 or so bee species we collected were found (at least once) on prairie clover, but I bet it was at least 30%.  In addition to purple prairie clover, hoary vervain was another fairly heavily used species (though nothing like the clover).  In some ways, then, a few very accessible and productive flowering species seem to play a huge role in supporting bees – especially generalist species – in our prairies during this time of year.  From talking with Mike, it sounds like that kind of heavy use of a few species is fairly common in other prairies he’s seen.

Mike Arduser peers intently at a patch of silky prairie clover, hoping to find bees that specialize on that plant species.

On the other side of the coin, roughly half of the bee species in our prairies (and most other prairies, I think) are specialist pollinators on a single species, or small group of species of flower.  For these bees, the abundance and distribution of their particular host plants is obviously critical.  We found bees that specialize on a number of different flowers, including those in the genera Callirhoe (poppy mallow), Physalis (Ground cherry), and Oenothera (evening primrose), among others.  There were also a fair number of bees with slightly broader, but still restricted, diets.  Many of the flowers used by specialist bees have other pollinators to help them produce seed, so the relationship is usually more critical to the bees than the flowers – though that’s not universally true.  Some, like the ground cherries, have flower types that can’t easily be pollinated by insects other than those built to specialize on them.

I took this photo a couple weeks before Mike and James arrived. Mike and I found it again – and some more like it – and he figured out that it’s the nest of a bee that specializes on poppy mallow flowers. The yellow pollen grains around the rim of the hole are commonly seen – but no one knows for sure what they’re for.

Mike and I spent some time talking about our need to know more about what thresholds of flower abundance and distribution are really important for these specialist bees.  For example, a prairie that has only a couple individual plants of ground cherry isn’t likely to be able to support a bee species that can only use that kind of plant.  But how many plants does an individual bee need to raise a brood?  And how close together do those plants need to be in order for bees to find and pollinate them efficiently? 

From the plant species’ standpoint, how many individual plants (and at what density?) does it take to attract enough pollinators to provide sufficient pollination for the plant population?  It’s a particularly important question when talking about rare plants, which often occur in small scattered populations.  It’s also important for our degraded remnant prairies that have very few individuals of even some more common plant species.  If we don’t have enough plants to garner effective pollination, the population may not reproduce – or sustain itself in the prairie.  It makes me think about the way we approach our overseeding efforts in those degraded prairies.  Maybe we need to be using lots of seed in small areas to establish patches of flowers with enough density to attract pollinators, rather than spreading that seed more lightly across large areas.  Very thought provoking.  I need do some investigation, and see whether we’re getting seed production from plants that are scattered at low densities across degraded remnant prairies.

Mike needs to spend some more time looking at the data we collected this week to see if there are differences in the bee communities between our restored and remnant prairies.  We spent quite a bit of effort collecting bees from purple prairie clover and hoary vervain across a range of restored and remnant sites in the hope that we could see whether or not the same bee species were visiting the flowers in both site types.  Granted, we only had about three good days of field data collection to draw conclusions from, but I hope it was enough to give us at least an initial glance at what’s going on.  To do it right, of course, we’d need to look at the abundance and density of plants at each site, and focus on more than just the number of bee species using them at each site – the number of total daily visits, for example, would also be important, as would seed production and other measures.  I’m hoping to collect some more bees throughout the rest of this season to give us a bigger slice of the total picture to look at.  It’d be great to see a pattern interesting enough that we can build a graduate student project out of this, and take a much closer look at what’s happening.  Anyone looking for a good project?

We saw a lot of bees on prairie clovers last week. This green sweat bee was not photographed during the week – and interestingly, we saw relatively few green sweat bees in our prairies last week.

Again, Mike still needs to finish sorting through and identifying the bees we collected this week.  However, he did notice a few interesting discrepancies between what he expected to see at our sites and what we actually found.  For example, we really didn’t see many (any?) twig-nesting bees, and only two species of parasitic bees.  At this point, it’s hard to know if that’s because the bee fauna here is just different than what he’s used to seeing further east, if our sample was too small to fairly represent what’s actually here, or if there’s something functionally missing from our prairies. 

On the other hand, there were a fair number of bee species that are rare in Missouri and other eastern prairie states, but common in our prairies here.  One of those is often called the “ghost bee” because of its fuzzy white appearance and the difficulty of finding it in eastern tallgrass prairie.  When Mike saw one in my net on our first day of sampling, I could tell from his expression that it wasn’t just another fuzzy bee…  Unfortunately, after I came home that night and bragged to my family (who didn’t seem that excited) that I’d caught the elusive ghost bee, we ended up seeing them pretty often during the rest of the week – somewhat dampening my sense of accomplishment.  And, just to rub it in further, during our public field day on Friday, two Pheasants Forever biologists showed Mike some very nice photos they’d taken of the same bee species from another prairie about an hour north of us.  Sure, it’s great to know that the species appears to be doing well in this part of the world, but it did knock the air out of my balloon a little…  On the other hand, how great is it that there are Pheasants Forever biologists out taking photos of bees in prairies?!  Awesome.

What about ants?

It turns out that ants provide an interesting contrast to bees in terms of how they perceive prairie “quality”.  While the diversity and abundance of bees in a prairie is heavily dependent on the diversity of flower plants, ants tend to look more at habitat structure than at plant species.  For example, James says that one very important attribute of prairies is the attractiveness of its habitat for recently-mated females looking for a place to start a new colony.  One significant factor for those females is the presence of enough bare ground to allow them to see whether or not an area is already occupied by colonies of potential competitors.  More broadly, prairies that are most attractive to ants tend to be those that have a lot of variation in plant density and structure.  Prairies that are dominated mainly by grasses are usually too homogenous to be very ant-friendly.  So, while ants might not respond directly to plant diversity, prairies with good plant diversity may be more likely to have the kind of structural diversity that ants are looking for.

After seeing the kind of fire and grazing management we’re using on our sites, James’ opinion was that we’re probably doing a pretty good job of facilitating ant habitat.  Our grazing helps create the kind of structural variation that ants tend to like, and our stocking rates are light enough that we aren’t likely to be compacting the soil to the extent that it might negatively impact them.  However, as with Mike, James only had a few days to look at our prairies and the ants living there, so it’s hard to draw too many conclusions – and he still needs to finish sorting and analyzing the data we collected this week. 

James Trager (left) sits under a tent discussin ants and other insects with visitors during our Platte River Prairies Field Day last Friday.

James, along with Laura Winkler, a graduate student from South Dakota State University who joined us for the week, caught about twenty ant species in our prairies.  They collected some of those ants by just walking around and looking for them.  Other ants were found by setting pitfall traps or laying out baits that ranged from chicken skins and canned meat to pecan cookies.  From his initial impressions, James said we had a surprisingly high abundance of a few ant species, but there were at least a couple of ant species he expected to be common here that we never saw.  It’s hard to know whether that’s because they’re really not here or because we just missed them during our limited sampling time.  In general, however, James felt that we might be missing some of the more common eastern prairie ants, and he wasn’t seeing western species that he would have expected to fill the same kinds of roles.  If that’s an accurate picture of the ant communities in our prairies, it makes me wonder whether that’s just the way prairies in our part of Nebraska are, or if there is something going on that we should be addressing.  I guess I’ll wait to hear more from James before I start worrying too much about it… 

Because of the limited time we had, and the great variation in management and soil types across our prairies (not to mention this summer’s drought) it wasn’t really possible to do a good direct comparison of the ant communities between our remnant and restored prairies.  However, I hope that James’ data might still give us some hints about whether or not there is something to follow up on in that regard.  Because I know our restored prairies have some soil characteristics that are pretty different from remnant prairies that haven’t been farmed, I expect there to be some differences.  Whether or not they’re important differences – or correctable – is something I don’t know…yet.

THANK YOU!

I’m extremely grateful to both Mike and James – as well as Laura – for giving up an entire week of their time to come out to explore our prairies and help me learn to think differently about them.  As always happens when I spend time with intelligent people who are experts in their particular subject matter, I was awed both by their level of knowledge and the complexity of the worlds they study.  Everywhere we looked, Mike and James shared fascinating stories about nearly every insect that flew or crawled by.  In addition, both were very knowledgeable about ecology in general, which led to fantastic discussions about much broader topics than just invertebrates.  It was a week that will keep my brain buzzing for a very long time, and I can’t thank them enough for coming.

I’ll keep you updated as I get more information and have some time to synthesize it…

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

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.

An Early Attempt to Evaluate Prairie Restoration Success by Looking at Insect Use.

Back in February, I wrote a post that laid out some ideas about how to measure success when using prairie restoration (reconstruction) to stitch fragmented remnant prairies back together.  One of the main needs is to see whether species from the remnant are also using the restored prairie.  If I’m trying to make a small remnant prairie function as a larger prairie by adding restored prairie around it, the species in the remnant must be able to expand into and travel through the restored habitat.

I just got some data from a couple of volunteer amateur entomologists that apply to that kind of evaluation, so I thought I’d share what they found.  First, let me be clear that these are amateurs (“enthusiasts” may be a better term than “entomologists” – though they are much better at insect identification than I am!) and that these are not research data.  However, what they found was very interesting, and will make a good launching point for future work.

Four species of tiger beetles were among the insects found during the 2009 inventory work.

INTRODUCTION

In early 2009, I was approached by Chris Aubushon and Connie McCartney, who live in Grand Island, Nebraska – near our Platte River prairies.  Chris and Connie were initially just looking for new sites to hunt for insects, but as we talked, we developed a bigger idea.  They volunteered to come out once a month for during the 2009 growing season and collect as many insects as they could from 6 sites and to identify what they could.  The six sites were really three sites where I had a restored prairie next to a remnant prairie, and they collected insects from both the remnant and restored portions of those sites.

Chris Aubushon (left) and Connie McCartney (right) setting up plots for their insect inventory project in early spring 2009.

Again, this was not a research project, but Chris and Connie – and some helpers – did come out once a month for 5 months and collect a lot of insects.   They restricted their sampling to one 100 foot by 100 foot plot at each of the 6 sites, and used a combination of sweep netting and pitfall traps to catch the insects.

RESULTS

Chris and Connie were able to identify 136 species from those five months of collecting.  They also found (but didn’t identify to species) approximately 33 species of spiders.  When I got the list of species from Chris, I sent it to several friends of mine who are knowledgeable about insects to get their impressions.  The consensus among those entomologists was that the insects on the list were almost exclusively common species that are habitat generalists.  In other words, the list doesn’t include many rare species, or species tied to particularly “good” quality prairies.  Instead, Chris and Connie mostly caught insect species that can be found just about anywhere.   That doesn’t diminish their efforts, but it’s important to remember as you interpret the breakdown of results.

Of the 136 species caught, 28 were seen at only one site.  Interestingly, 8 of those 28 species were found only in remnant prairie, while 20 were found only in restored prairie.  (I don’t think it is necessarily important, but of the 20 species found only in restored habitat, half were beetles.)  Two species – a firefly and a burying beetle were found at two different sites that were not adjacent to each other.  In each case, they were found in one restored prairie plot and one remnant prairie plot.

The reference insect collection from Chris and Connie's work in 2009.

All of the remaining species (106 of them) were found in both halves of at least one remnant/restored prairie pair.  In other words, except for the two species mentioned above, every other species of insect found in more than one place was seen in both the restored and remnant portion of at least one prairie.  Remembering that these are all generalist species, I’m still surprised – and encouraged – by those results.  I had expected to see at least a few insect speces that were found in remnant prairies but that weren’t yet making the jump into the adjacent restored prairies.  In fact, I was kind of counting on seeing that so that I could design some follow-up data collection to figure out what the obstacles might be that were preventing them from using restored prairie.

CONCLUSIONS

So what have I learned?  First, the prairie restoration work we’ve been doing appears to provide adequate habitat for the most common and widespread insect species in our remnant prairies.  At least for those species, the restoration work we’re doing is successfully increasing the size and connectivity of fragmented remnant prairies.

Second, I’ve learned that I need to recruit a lot more entomologists to come collect insects from our prairies because every time I do, I learn something.  A little more than a decade ago, I worked with Kristine Nemec on her graduate school project to compare insects between remnant and restored prairies along the Platte River in Nebraska.  For her project, she inventoried insects in 3 restored and 3 remnant (degraded) prairies, but the restorations and remnants weren’t adjacent to each other.  Nevertheless, among grasshoppers, katydids, leafhoppers, planthoppers, and treehoppers, she found very few species that were found in remnant prairies but not restored prairies.  She also found very similar numbers of species between remnant and restored prairie for each of the insect groups she looked at.  You can see a brief summary of some of her thesis findings here. Nemec Results

Third, I’ve learned that the next steps in our efforts to evaluate insect use of our restored prairies will be harder.  We probably need to start by identifying species of insects in our remnant prairies that may not utilize nearby restored prairie – based on what we (meaning people besides me) know about their life history.  Then we need to do some targeted sampling to see whether they are or are not using those restored prairies.  It’d be even better if we could actually track individual insect movement to see if they cross the boundary between remnant and restored prairie.   (That should be easy, right?)  Broad sampling like we’ve done so far is useful, and a good first step, but now we need to hone in on a few target species and see what we can learn about them.  Species that rely on a particular plant species or that are tied to soil organic matter levels may be examples of those we need to study.

To sum up, there’s plenty of work to do.  To this point, we haven’t found any glaring problems with our restored prairies that are so severe that common insect species are restricted from using them.  That’s pretty nice to know, but it’s just the tip of the research iceberg.  Now we need to dive in and start working on the rest.

I’m extremely grateful to Chris Aubushon and Connie McCartney for all of their hard work collecting, sorting, and identifying insects for this project.  It was an amazing effort – and done for the simple love of exploration of the natural world.  Thank you.

I’m also grateful to James Trager, M.J. Hatfield, and Ted MacRae for helping me to interpret the results of the project.  Their comments and insight helped me tremendously as I worked to understand what conclusions I could and couldn’t pull from these data.