Diversity, Redundancy, and Resilience

Grasslands face a long list of challenges.  In many regions, habitat loss and fragmentation top that list, leaving prairies to struggle for survival as tiny isolated patches of habitat.  In addition, invasive plants and animals keep finding new footholds within both fragmented and unfragmented prairies.  Many of those invaders are aided by nutrient pollution – increasing levels of nitrogen, for example, which help species like reed canarygrass and smooth brome monopolize formerly diverse plant communities.  Most of all, the climate continues to flail crazily about, ratcheting up the temperature and tossing out more and more extreme weather events.

How can grasslands possibly survive all of that?

I’m actually pretty optimistic about the future of prairies.  Prairies are inherently resilient, and if we do our jobs as land managers and supporters of conservation, we can help ensure their continued resilience and survival.  Resilience in prairies and other ecosystems is the capacity to absorb and adapt to whatever challenges are thrown at them, while sustaining their essential functions and processes.  That resilience is built largely upon two pillars: biological diversity and the size/connectivity of the habitats that biological diversity depends upon.

Plant diversity is a key component of ecological resilience, along with the other biological diversity associated with it.  Taberville Prairie, Missouri.

We’ve severely compromised the “habitat size/connectivity” pillar in many regions of North America, but even in little prairie fragments, there is an incredible diversity of organisms, providing the countless services needed to sustain life and productivity.  In a healthy and diverse prairie, not only are all the bases covered, there is considerable redundancy built in to the system because of the number of different species present.  If one plant, animal, or microbe is unable to do its job because of drought, fire, predation or disease, another can step up and fill the role. Diversity provides redundancy, and redundancy helps ensure that prairie systems stay healthy and productive, regardless of circumstances.

It’s not hard to find examples of this kind of built-in redundancy in prairies.  In fact, you can find it within some very recognizable groups of species.  Let’s start with sunflowers.

While most people know what a sunflower looks like, you might not realize how many different kinds there are.  Here in Nebraska, we have at least nine different sunflower species, plus a lot of other flower species that look and act much like sunflowers.  Two of our official sunflowers are annuals, often classified as weeds because of their ability to quickly colonize areas of bare or disturbed soil.  The other seven species are long-lived perennials, each with its own set of preferred habitat conditions.

Plains sunflower, an annual, is a rapid colonizer of exposed in sandy prairies around Nebraska. The Nature Conservancy’s Niobrara Valley Preserve.

All sunflowers are tremendously important providers of food and shelter to wildlife and invertebrates.  There’s a reason sunflower seeds are so prevalent in bird feeders – they pack an enormous amount of nutrition into a little package.  Because of that, a wide array of both vertebrate and invertebrate animals feed eagerly on sunflower seeds when they can find them.  Sunflowers also produce an abundance of pollen and nectar, and make it very accessible to pollinators and many other creatures by laying it out on a big open platter.  It’s rare to find a sunflower in full bloom that doesn’t have at least one little creature feeding on its nectar, pollen, or both.  Grazing animals can get a lot from sunflowers as well; the forage quality of sunflowers is very high, especially before they bloom.

During or after droughts, intensive grazing bouts, fires or other events that leave bare soil exposed, annual sunflowers thrive, and they can provide abundant resources at a time when many other plant species can’t.  We see this often in the Nebraska Sandhills, where plains sunflower (Helianthus petiolaris) turns the hills yellow during the summer after a spring fire or the year after a big drought.  Plains sunflower isn’t the only plant that flourishes under those conditions, but its presence in plant communities is a great example of the kind of built in redundancy that helps ensure there are plants for animals to eat, even when many normally-abundant prairie plants are scarce or weakened.

Nebraska’s perennial sunflowers span a wide range of habitats, from wet to dry and sunny to shady.  You can find a sunflower in just about any habitat type in Nebraska.  That’s another great example of built-in redundancy, and a reason for optimism about the future.  As climate change alters the growing conditions across much of Nebraska, it seems unlikely that any habitat will change so dramatically that it will become devoid of sunflowers.  Instead we’ll probably see changes in the relative abundance of each species from place to place.  In addition, remember that what we call a sunflower is a fairly arbitrary categorization; there are lots of other wildflowers that provide very similar resources/services, including plants like rosinweed (Silphium integrifolium), false sunflower (Heliopsis helianthoides), sneezeweed (Helenium autumnale), and many more.  Those sunflowerish plants also span a wide range of habitat preferences and growth strategies, making it likely that some of them will be blooming abundantly every year, no matter what drought, fire, or grazing conditions are thrown at them.

An illustration of the general habitat preferences of several perennial sunflowers found in Nebraska.  The variety among habitats used by these species makes it likely that some kind of perennial sunflower will persist in most locations, regardless of how climate and disturbance patterns change over time.

Milkweeds are another group of organisms that demonstrate the diversity and redundancy in prairie ecosystems.  There are 17 milkweed species here in Nebraska, along with several other related species (like dogbane) that produce the same kind of sticky white latex.  While that latex is toxic to most creatures, a number of invertebrates have figured out how to feed on milkweed plants without suffering harmful effects.  Many have actually turned the toxin into an advantage by ingesting the substance and making themselves toxic to potential predators.  The most famous of these critters, of course, is the monarch butterfly, which uses milkweeds as larval hosts.

A selection of milkweed species found in Nebraska, demonstrating the variety in flower colors and shapes among the group.

When you picture a monarch caterpillar on a milkweed plant, you probably envision a tall plant with a big pink flower.  In reality, monarchs can use many (maybe all?) milkweed species as larval hosts.  Because each species of milkweed has its own unique set of preferred habitat and growing conditions, the diversity of milkweed species in Nebraska should help monarchs find a place to lay eggs regardless of weather, disease outbreaks, or other events.

The spring of 2017 provided a compelling example of this.  In most years, monarchs overwintering in Mexico fly into the southern United States and lay eggs on milkweed plants there.  The subsequent generation than flies northward into Nebraska and other  nearby states.  For some reason, many monarchs broke from that pattern in 2017, and arrived in Nebraska much earlier than normal.  This caused a great deal of concern because the milkweed most commonly used for egg laying – common milkweed (Asclepias syriaca) wasn’t up yet, and just as it started emerging, a freeze knocked it back down.  Fortunately, common milkweed wasn’t the only option available to monarchs.  Whorled milkweed (Asclepias verticillata) is also fairly common, starts growing earlier in the year than common milkweed, and is more resistant to cold weather.  Monarchs seemed happy to lay their eggs on the skinny leaves of whorled milkweed, and those of us worried about monarchs breathed a sigh of relief.  Once again, diversity created redundancy, and monarchs found habitat for their babies, even though they arrived well ahead of schedule.

A monarch egg and caterpillar on whorled milkweed earlier this spring (April 27, 2017) in Nebraska.

A broader example of redundancy and resilience in prairies includes the interdependence between bees and plants.  If you’ve followed this blog for long, you’re surely aware that there are thousands of bee species in North America, and potentially 80-100 or more species in a single prairie.  Most of those bees can feed on the pollen and nectar from many kinds of wildflowers, though some are restricted by their size or tongue length from accessing certain species. Because most plants only bloom for a few weeks, and most bees need considerably longer than that to successfully raise a family, bees require more than one kind of wildflower near their nest.  In fact, in order to support a broad diversity of bee species, a prairie needs an equally diverse set of wildflower species.  That way, a bee can find sufficient food throughout the growing season, even if drought, grazing, or other events keep some plant species from blooming in a particular year.

On the flip side, most wildflowers rely on the diversity of bees and other pollinators to ensure successful pollination.  While some insect-pollinated plants are very selective about who they let in, most rely on the availability of many potential pollinators.  If some species of bees are suffering from a disease, or have a weather-related population crash, it’s awfully nice to know that there are other bees (along with butterflies, moths, wasps, and other insects) that will still be able to transfer pollen from one flower to another.  A diverse pollinator community relies on a diverse wildflower community, and vice versa.  Diversity, redundancy, and resilience.  No matter what happens, flowers make fruits and seeds – which, by the way, is pretty important all the various creatures that rely on those fruits and seeds for food.

Bees rely on plant diversity to ensure a consistent supply of pollen and nectar across the growing season. In this case, tall thistle, an important native wildflower, is supplying food to a bee in return for pollination services.

All of us have our favorite prairie species, whether we’re fans of flowers, butterflies, birds, or some other group of organisms.  It’s easy to focus our attention on those favorite species, and worry about whether they will survive all the challenges that face prairies today.  If we really care about prairies, however, we should probably focus more on (and celebrate) the richness of species that keep prairies humming along, no matter what gets thrown at them.  The variety of yellow-flowered sunflowerish plants, the broad array of latex-producing milkweed-like plants, the complexity of the plant-pollinator relationship, and countless other examples of diversity and redundancy help ensure the survival of prairies well into the future.  That resilience is why I remain optimistic about the future of prairies.

A Plot-Sized Biodiversity and Photography Project

Today, I’m beginning a new photography project aimed at exploring and celebrating the small scale diversity and complexity of prairies.   I’ve picked out a 1 x 1 meter plot in a small patch of restored prairie here in Aurora, Nebraska, and I will be photographing everything I can within that tiny area over the next year or so.  My objectives are to find and document as much beauty and diversity as I can and to show the dynamic nature of prairie life, even at a very small scale.

Lincoln Creek Prairie, with the yellow flags marking my square meter plot.

The plot sits in a narrow strip of land restored in the 1980’s by Bill Whitney and Prairie Plains Resource Institute.  I picked Lincoln Creek Prairie because it’s right across town from me, and therefore easy for me to get to frequently.  It’s also a great restored site that was planted with a diverse mixture of prairie species (over 100 species) and is well-established.  However, the prairie is small enough that it doesn’t host any grassland-nesting birds or other animal species that need relatively large and open prairie habitats, and suffers from all the other issues that come along with tiny prairies.  I anticipate that most or all of the organisms I photograph during the coming year will be plants and invertebrates, but I’m confident that I won’t find a shortage of subject matter.

A view of the 1 x 1 plot from above

I didn’t pick the small plot randomly, but I also didn’t try to find a spot with more diversity than any other nearby.  Instead, I looked for a place that would catch the light well during most of the year but was out of the way enough to not be disturbed by people hiking the nearby trail.  I freely admit that I chose the exact location of the 1 x 1 plot because it has a butterfly milkweed plant in it – it’s a nicely photogenic species.  This isn’t research, after all, and I don’t have to select my plot in a completely unbiased way!  However, I’m confident that the 1 x 1 plot I chose is representative of the rest of the prairie around it.

Switchgrass (Panicum virgatum) with a flower that never completely came out of its sheath last summer.

I began my photography journey within the plot yesterday, and took the photos you see here in this post.  I’ve already discovered one big challenge regarding my plans – it’s going to be difficult to avoid crushing or breaking the vegetation within the plot during my frequent visits.  I don’t see any way to avoid matting down the vegetation around the edge of the plot, but I’ll try not to do any more of that than necessary, and hopefully that won’t excessively impact what I see inside the plot.

The curled and dried leaf of stiff sunflower (Helianthus pauciflorus)

Right now, the plot is fairly uniformly brown, and perhaps drab looking from a distance.  However, I didn’t have much trouble finding interesting shapes and textures to photograph during the 10 minutes or so I spent there yesterday.  Even without green vegetation or crawling invertebrates, there was plenty to look at.  That bodes well for the coming year, I think!  Stay tuned…

Maximilian sunflower (Helianthus maximiliani) seed head, minus the seeds.  Birds, mammals, and/or other creatures likely picked it clean last fall.
A butterfly milkweed (Asclepias tuberosa) seed pod on a curled stem.

A Visual Illustration of Plant Diversity’s Importance

Last week, I took some photos that powerfully demonstrate the importance of plant diversity.

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Research plots at The Nature Conservancy’s Platte River Prairies.

Several years ago, we created some research plots to help us learn more about how plant diversity interacts with ecosystem function.  As you can see above, the plots include a grid of squares (3/4 acre in size), each planted with one of three seed mixtures: monoculture (big bluestem), low diversity (grasses and a few forbs harvested in the fall), and high diversity (100 species).  Working with academic partners, we have several research projects underway, including a couple that demonstrate the influence plant diversity has on the spread of invasive plant species.

Other researchers have found similar relationships between plant diversity and resistance to invasive species, but that is only one of many benefits from having a wide variety of plants in a prairie.  Both herbivores and pollinators benefit from having a broad selection of food choices available to them.  During extreme weather conditions (hot, dry, wet, cold), high diversity prairies always have plants that flourish under those conditions and help provide habitat and food conditions for animals.  Most importantly, because of these and other reasons, prairies with high plant diversity also have high total biological diversity, including more species of microbes, insects, other invertebrates and vertebrates.  That overall diversity is important for its own sake, but also because of the role each species plays in the functioning of the ecosystem.

The aerial photo I took from our small drone last week illustrates another benefit of plant diversity; prairies with high plant diversity have green vegetation for more of the growing season.  Every plant species starts and ends their growth period at different times.  Some start early, bloom, and are done before summer even starts.  Others bide their time and don’t bloom until late in the fall.  When you mix all those species together in one prairie, you end up with consistent, but ever-changing, availability of nutritious vegetation and flowers throughout the growing season.

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Labeled plots, showing the location of the various treatments.

Our monoculture research plots (big bluestem only) looked very different from our other plots last week because big bluestem was just starting to grow (on April 23).  Even after we burned all the plots this spring, allowing the soil to warm earlier than it otherwise would have, the most advanced big bluestem plants only had leaves of a few inches in length.  Our low diversity plots (mostly grasses) showed more green, but only due to the presence of one grass species, Canada wildrye (Elymus canadensis), a cool-season grass that begins its grown early in the year.  By contrast, the high-diversity plots had many plant species growing, with some close to blooming.

The monoculture plots were planted only with big bluestem,
The monoculture plots were planted only with big bluestem, which was just barely starting to grow last week.
Low diversity
The low diversity plots were slightly greener than the monoculture plots, mainly because Canada wildrye was growing quickly (most of the other grasses were just breaking dormancy).
high diversity
The high diversity plots had numerous species showing vigorous growth, even early in the season.

Although the visual differences between plots on April 23 are striking, they are ecologically significant as well.  Invertebrate and vertebrate herbivores (including cattle, if the plots were grazed) can find a variety of forage options in the high diversity plots right now – far more than in the low diversity or monoculture plots.  Within a week or so, the high diversity plots will also have several different kinds of wildflowers in bloom, providing resources for early-season bees and other pollinators.  The low diversity and monoculture plots will have far fewer resources for pollinators throughout the season.  Furthermore, invasive plants species trying to establish themselves within the high diversity plots face stiff competition from plants using wide range of growth strategies.  They will find a smaller amount of resistance in the other plots, where less diverse plant communities are not as efficient at taking up space and fully utilizing resources.

Plant diversity is incredibly important in natural systems for a variety of reasons, only a few of which are mentioned here.  We still have a lot to learn about how plant communities function, and how plant diversity plays into that.  However, we already know enough to recognize the value of having numerous players in the game.  It was fun to see a visual demonstration of that value last week.

 

Why Grassland Birds are Poor Indicators of Prairie Quality

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…