Tag Archives: insects

The Importance of Weedy Flowers for Butterflies

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In our Platte River Prairies, regal fritillaries and other butterflies appear to depend heavily on a few weedy wildflower species as nectar plants.

I was in graduate school when I first started learning to identify butterflies.  I participated in several July 4th butterfly counts around Nebraska, and got to know some people who could help me with the tougher-to-identify species.  I was a good birder at the time, and quickly found many parallels between bird and butterfly identification.  One of those was that skippers were just like sparrows – lots of small drab-colored species that were really difficult to tell apart.  The difference, of course, was that I could catch skippers with my net to see them up close.

One species that wasn’t difficult to identify was the regal fritillary.  Once I figured out how to tell the difference between regals and monarchs I started seeing regals everywhere.  In fact, when I helped with July 4th counts, regal fritillaries were usually one of the most abundant species we saw.  I didn’t realize until later how rare regals are in eastern prairie states, and how fortunate I am to live in Nebraska where their populations are still strong.

Regal fritillary butterflies are still common in many western tallgrass prairies and mixed grass prairies, but have largely disappeared from most eastern prairies.

As I learned more about the species and its conservation status, I saw some inconsistencies between what regals were thought to need for survival in eastern prairies and what our Platte River Prairies were providing.  For example, many people I talked to from the east told me that regal fritillary larvae needed prairie violets for a food source.  However, our Platte River Prairies – which are full of regal fritillaries – only have the heart-shaped-leaf blue violet (Viola pratincola), and no prairie violets.  In addition, plant species like purple coneflowers (Echinacea spp) that are heavily used as nectar plants by regal fritillaries aren’t present in the floodplain of the Central Platte River.  In 2010, with the help of Dr. Ray Moranz of Iowa State University, we started a research project to learn more about how regal fritillaries survive and thrive in our prairies.

Through the project, we’re investigating multiple aspects of regal fritillary habitat use – including nectar plant use, the use of remnant vs. reconstructed prairies, and the way in which regals interact with a landscape managed with a combination of fire and grazing.  We’ve only completed one field season so far, and will need more time to answer those questions, but we have already seen some very striking results in terms of nectar plant use.

To collect our data, we regularly walked transects through our prairies and counted butterflies – noting nectaring behavior when we saw it, and recording the species of flowers the butterflies were nectaring on.  I’m only referring to the nectaring data in this post because we’ve got more work to do to analyze the other data we collected.  It’s also important to remember that these are just pilot-year data, although they match what I and others have observed for many years.

Though we saw many hundreds of regal fritillaries on our transects, relatively few were actually nectaring.  Early in the season when numbers were highest, we saw mainly males – patrolling for newly emerging females.  When those females finally arrived on the scene we started to see nectaring behavior from both males and females, but observations of nectaring were still relatively rare.  Multiple years of data collection will be important to confirm the patterns we saw this first year.

There were really only two kinds of wildflowers that attracted the vast majority of nectaring regal fritillaries through the 2010 season, and both are usually called weeds.  The first was hoary vervain (Verbena stricta) and the second was a small group of thistle species – primarily Flodman’s thistle (Cirsium flodmanii) and tall thistle (Cirsium altissimum).  Hoary vervain is a native wildflower that is largely considered to be a pasture weed by many people because it thrives in overgrazed situations.  Cattle don’t like to eat it and it can spread quickly when the surrounding grasses are weakened.  (However, it also decreases in abundance very quickly when the grasses regain their vigor.)  Flodman’s and tall thistle are two native thistles that are also considered to be weedy plants, and that do well in overgrazed pastures.  The third favorite plant was actually several species of milkweeds lumped together (primarily weedy species like Asclepias syriaca and A. speciosa).

During the early part of the summer, the majority of regal fritillaries nectaring on flowers chose hoary vervain.

In late summer and early fall, vervain largely stopped blooming and other species became more important for nectaring, particularly native thistle species.

Most of our data collection centered on regal fritillaries, but we also collected data on other butterfly species.  So far, there doesn’t appear to be much difference in the nectaring preferences of regals and the larger butterfly community.  During the early part of the season, hoary vervain was the primary choice for most butterflies and a variety of other species got only infrequent visits.  Later in the season when hoary vervain was largely done blooming, thistles and a few other species, including wild bergamot – aka monarda or Monarda fistulosa and rosinweed (Silphium integrifolium), shared the load.

Observations of early summer nectaring across all butterfly species showed that hoary vervain was the dominant selection for nectaring.

Later in the season, other flower species split the nectaring observations fairly evenly.

One of the most interesting aspects of doing this research on butterflies has been that I’ve been able to look at our prairies through the eyes of butterflies – and that’s given me some surprising insights.  Our remnant prairies are largely degraded by years of overgrazing and broadcast herbicide use, and we’ve been working to slowly bring them back to higher floristic quality.  I’ve assumed that increasing floristic quality would also be good for many other species, including butterflies.  In addition, we’ve worked very hard at getting the highest plant diversity we can into our restored (reconstructed) prairies.  While we’re not restoring those prairies for butterflies, specifically, I’ve always assumed that butterflies would benefit from high plant diversity.  From our very preliminary data on butterflies, it appears that while our restored prairies have many more flower species, including many showy species, they may not be providing any better nectaring opportunities than our remnants!  The flower species most used by regal fritillaries and other butterflies are hoary vervain and thistles, which are at least as common in our remnants as in our restorations.

Regal fritillaries, and other butterflies, seem to rely heavily on hoary vervain for nectar in our Platte River Prairies.

I’m confident that there many other benefits of the plant diversity in our restorations, of course, including the availability of a variety of larval host plants for butterflies.  But it’s a little humbling – and intriguing – to see butterflies relying on plant species that do well in beat-up pastures rather than flocking to our showy restored prairies!  It’s important, of course, not to extrapolate these results to other parts of the country.  I’m not saying that regal fritillaries can survive in beat-up pastures in eastern states.  If that was so, we’d have many places in the east with lots of regal fritillaries.  Nebraska and other western states with high numbers of regal fritillaries also have landscapes with relatively high amounts of native prairie – and that may be the most important factor for regal fritillary survival.

The other lesson from our early data seems to be that the value of “weedy” species shouldn’t be discounted just because they thrive in conditions many flower species can’t take.  Vervain and thistles could be the primary plant species supporting our butterfly populations along the Platte River right now.  That’s a pretty good argument for being valuable.  In addition, the violet species that the regals must be using as a larval food plant along the Platte is considered to be relatively weedy because it does well in degraded pastures.  I’m sure glad we have a lot of it – and so are regal fritillaries!  Other weedy plant species have value as well – apart from their obvious role in filling space when “good” plants are weakened by fire, grazing, or drought.  The seeds of native ragweed and annual sunflower species, for example, seem to be excellent food sources for many small mammals and birds (judging from the density of tracks around the plants when it snows).

I’m really looking forward to next season’s butterfly transects.  I enjoyed reacquainting myself with butterflies last summer, and it was good for me to look at our prairie work through a different lens.  I’m sure we’ll learn more as we collect and analyze more data.  In the meantime, we (and our butterflies) will continue to enjoy and appreciate our weeds.

A big thank you to Dr. Ray Moranz for his assistance setting up this project.  Also, Mardell Jasnowski directed most of the data collection in 2010, with help from Nanette Whitten, Nelson Winkel, and Natalie Goergen.

Why Grassland Birds are Poor Indicators of Prairie Quality

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I presented this argument to a Nebraska symposium on grassland birds in 2008 and managed to escape relatively unscathed.  Now I’m testing my luck with a wider audience.  At least no one can throw things at me through the computer…

Let me start by saying that I’m a big fan of birds.  I really enjoyed working on my graduate research, which focused on grassland birds and their vulnerability to prairie fragmentation.  I also think birds are generally pretty and interesting. However, the truth is that prairie birds make up only a tiny percentage of the species in prairies (most of which are invertebrates, followed by plants).

Grassland birds make up a tiny percentage of the species living in a prairie - the vast majority of which are invertebrates and plants.

However, grassland birds are often held up as indicators of whether or not a prairie – or a prairie landscape – is “healthy” or “high quality.”  A common refrain in prairie conservation goes something like this; “If we have our full complement of grassland birds in this prairie and/or landscape, it’s a good bet that all the other species are also doing well.”  Unfortunately, while prairie birds are relatively easy to study and monitor, they may not do a good job of reflecting how the rest of the prairie is doing.  Let’s look at some of the most important attributes of prairies and some of their major threats – and consider how well birds correlate with them.

Species Needs – Survival, Reproduction, and Dispersal.

First and foremost, species have to survive and reproduce in order to persist in a prairie.  This applies to every species, from large vertebrates to tiny invertebrates and the entire suite of plants.  It’s important for us to know that grassland birds are surviving and reproducing, but can they tell us whether other species are doing the same?  You could argue that because they eat insects, grassland birds could have an impact on the survival of some insect species.  That’s true to a point, but grassland birds are generalist feeders – they tend to eat whatever insects are easiest to catch at any particular time – so while the abundance of grassland birds might impact the overall abundance of insects, you can’t really tie the presence of a particular grassland bird species to the survival of a particular insect species (or vice versa).  In other words, the plight of a rare leaf hopper or butterfly species is unlikely to be correlated with grassland birds.  Nor are grassland birds good predictors of plant species survival – the presence of meadowlarks or Henslow’s sparrows tell us nothing about whether or not compass plant or leadplant is thriving.  Grassland birds require certain habitat structure types (short vegetation, tall/dense vegetation, etc.) but they don’t much care whether that vegetation consists of smooth brome and sweet clover or a large diversity of native plants.

Henslow's sparrows are a bird of conservation concern and their presence in a prairie can be seen as a conservation success. However, although they tend to require fairly large prairies, and can indicate the presence of certain vegetation structure types, they don't indicate whether or not a prairie has a diverse plant or insect community.

In addition to basic survival, animal and plant species need to be able to move around the landscape in order to recolonize places where they have disappeared, and to maintain genetic interaction between populations (important for genetic diversity).  In landscapes where prairies exist as isolated remnants, moving between prairies becomes very difficult.  Corridors of prairie vegetation between prairies become important in those landscapes, and prairies near each other provide better opportunities for interaction within species than do more isolated prairies.  Because most grassland birds fly south at the end of each season and return the next year, (and the ones that don’t can still fly long distances between prairies) they don’t rely on those physical connections between prairies like most other animals and plants do.  Since grassland birds are pretty unique in terms of long-distance flying ability, they are a poor indicator of conditions that affect less mobile species.

Ecological Services and Ecological Function – Pollination, Seed Dispersal, etc.

Apart from the needs of individual species, prairies rely on certain processes to keep everything humming along.  Pollination and seed dispersal are two good examples.  Both affect the viability of prairie plant species, and neither has much to do with grassland birds.  Pollination primarily relies on plant diversity and bees – the most important pollinator group in prairies – and both plant diversity and bees are pretty disconnected from grassland birds.  We don’t know much about the role of prairie birds as seed dispersers, but it’s a good bet that they do very little seed dispersal during the summer when they’re primarily eating insects.  The role of migrating grassland birds as seed dispersers would be an interesting thing to study – but our use of grassland birds as indicators of prairie quality is always based on their presence during the breeding season.  If you were going to measure whether or not pollination and seed dispersal were functioning adequately, you’d likely evaluate the diversity of plants, the abundance of bees, and some of the potential obstacles to seed dispersal (tree lines, isolation of prairies, etc.), but I don’t think measuring grassland birds would tell you much.

Resilience – Redundancy and the Ability to Withstand Stresses and Invasive Species.

One way to think about the resilience of a prairie is as a measure of how well the prairie can bounce back from stresses.  For example, species diversity adds resilience to a prairie because when many species are present – especially when they overlap in the roles they play – the loss of an individual species can be a relatively minor blow.  If there are dozens of bee species pollinating flowers in a prairie, a disease that wipes out one or two species will probably not have a huge impact on seed production.  A diversity of plant species can also help to dampen the impacts of an event such as a severe drought or intensive grazing that temporarily weakens the vigor and growth of dominant plant species.  When there are lots of plant species present, the weakening of some leads to increased growth and abundance of others.  This helps maintain a stable supply of food for herbivores, and also helps prevent encroachment by invasive species that might otherwise take advantage of the weakened plant community.

Grassland birds may help bolster the resilience of a prairie in some ways.  They might, for example, help suppress an outbreak of grasshoppers by focusing their feeding on that easy-to-find prey species, and thus limit its abundance.  However, as discussed earlier, they don’t have much to do with pollination, nor would they help provide food for herbivores during a drought.

If you want pollination you need bees, not birds.

Threats to Prairies – Habitat Fragmentation, Invasive Species, Broadcast Herbicide Use, and Chronic Overgrazing.

One of the best arguments for grassland birds as an indicator of prairie health is that they are vulnerable to the loss and fragmentation of grassland habitat.  This is true.  A diverse and successfully reproducing community of grassland birds requires relatively large and unfragmented grassland.  In addition, because some grassland breeding birds need short vegetation and others need tall/dense vegetation, a diversity of birds can indicate a diversity of available habitat structure  – and that’s important to many other wildlife species as well.  Greater Prairie Chickens are often promoted as particularly good indicators because they are a single species that needs both large grasslands and a diversity of habitat structure.

However, there are a couple of other things to consider.  First, while some grassland bird species need large prairies, we don’t really know whether the minimum area required by grassland bird species is larger or smaller than that required by plant or insect species.  We know that many prairie plant species have survived for a very long time in tiny isolated prairies.  But because individual plants of many species can live almost indefinitely, due to their ability to generate new plants through rhizomes and other asexual means, the populations of plants in those tiny prairies could be in a death spiral due to the lack of genetic interaction with other populations.  You could make the argument that plants need much larger prairies than birds (thousands of acres, perhaps?) in order to maintain genetic fitness.  We simply don’t know.  And we know even less about the prairie size needs of insects.

Second, while grassland birds do require fairly large grasslands, most don’t actually require PRAIRIES.  Prairie chickens and many other grassland bird species have benefitted greatly from Conservation Reserve Program (CRP) fields that have added large numbers of acres of switchgrass, brome, and low-diversity grasslands to agricultural landscapes.  However, those same CRP fields have done very little for native wildflower populations or pollinator insects (or other insects that rely on diverse communities of native plants) so the increase in grassland birds in landscapes with CRP doesn’t really tell us much about the health of most other prairie species.

This smooth brome-dominated grassland is of very little to most prairie species, but would provide relatively good habitat for some grassland bird species (except for the trees in the background).

Because grassland birds can live comfortably in grasslands made up of a few native grasses, or even non-native grasses, they are poor indicators of the impacts of most invasive species – a major threat to prairies.  Similarly, broadcast herbicide use that greatly reduces the number of plant species in a prairie has little impact on the grassland birds nesting there.  Finally, a prairie that is being overgrazed would certainly have different bird species than one that is not being grazed at all, but a prairie that was chronically overgrazed for decades – and then managed well again – would have a pretty low number of prairie wildflower species but a very nice-looking grassland bird community.

One threat that grassland birds can be an excellent indicator for is tree encroachment on prairies.  Most grassland birds avoid nesting anywhere near even a solitary tree, let alone a grove of them, so a prairie with few birds could indicate a tree problem.  On the other hand, it’s pretty hard to miss a bunch of trees growing in a prairie…

Summary

Here’s the real point.  Grassland birds are an important component of prairies.  A prairie without all of its appropriate prairie bird species, or in which those species are not successfully raising broods, is missing something valuable.  Improving grassland bird success in prairie landscapes is an important and worthwhile objective.  At the same time, however, a prairie that has a full complement of successful grassland bird species doesn’t necessarily have diverse plant and insect communities, functioning ecological processes, or a low risk of invasive species or other threats.  In other words, grassland birds are an important component of high quality prairies, but their presence and/or success doesn’t necessarily mean a prairie is high quality.

Now that I’ve spent 1,500 words bashing prairie birds (I really do like birds…) the relevant question is, “What SHOULD we use as indicators of prairie conservation success?”  I wish I had a simple answer.  Part of the answer, of course, depends on how you visualize prairie quality (see my earlier post on this subject) because evaluation needs to reflect objectives.  But if our vision of a high-quality prairie includes species diversity, habitat heterogeneity, and other complexities, our evaluation methods will have to be complex as well.  Figuring out how to “take the pulse” of prairies may be the most important conservation challenge we face, because without that information we can’t design effective conservation strategies.

While we still have a lot to learn about how to take that pulse, it’s clear that we’ll have to do more than just count birds…