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.

Compatibility of Cows, Conservation and Climate Change?

I’ve been asked a number of times why I advocate for cattle grazing in prairies when cattle are such strong contributors to greenhouse gas emissions and rapid climate change.  It’s a fair question, but also a complicated one.  I don’t have a definitive answer, but I can share some of what makes it a thought-provoking subject.  Rather than providing a lot of specific research citations, I’m aiming instead to provide some general information that highlights the complexity of the topic.  Feel free to contribute additional information and perspectives in the comments section below (as long as you keep it constructive and polite).

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Cattle graze among leadplant and prairie clover at Konza Prairie in the Kansas Flint Hills.  What are the ramifications of cattle grazing for greenhouse gas emissions and other contributing factors to climate change?

Cattle: The Downsides

First, here are some reasons people are concerned about the impact of cattle on climate change.  According to the EPA, agriculture is responsible for about 9% of the U.S. greenhouse gas emissions, and beef production makes the largest contribution to that category.  Most of the impact comes not from carbon emissions, but from methane and nitrous oxide, both of which influence climate change more strongly (pound for pound) than does carbon.  “Enteric methane” (cow burps) is a big part of that equation, but so is manure, urine, and application of fertilizers to pastures.  These emissions are bad enough, but there are other negative impacts from beef production as well, including emissions from growing corn and other feed for cattle, emissions from manure in feedlots, water consumption by cattle and feed production, and pollution from sedimentation and nutrient runoff of pastureland.  Reading a list of bad stuff like this, it’s easy to see how people might wonder why I keep talking about grazing like it’s a good thing.

Cattle

Cattle are sometimes their own worst enemy in terms of advocating for their own existence.  I mean, come on, man!  This is just not a good look.

Predecessors to Cattle

As I provide some counterpoints, I’m going to do so from the perspective of the central Great Plains – the area of the world I’m most familiar with.  Outside the Great Plains, the situation varies greatly; there are places in the world where grazing may not be compatible with local ecosystems, for example, and where forest or other land cover types are being converted to pasture.  Here in my part of the country, however, we are in the heart of the historic bison range.  Before Europeans entered the picture in the Great Plains, prairies here were being grazed by bison, elk, pronghorn, and other large animals.  There are many arguments about the size of those historic bison populations, fluctuations in herd size and geographic range over time, and when/where bison impacts were important for prairie ecology.  For the purposes of this discussion, the important point is that cattle (and their emissions) weren’t introduced into a landscape with no history of methane emissions.  Bison were here prior to cattle, and they burped too.

The most cited article I’ve seen on the issue of methane emissions from historic bison populations is by Francis Kelliher and Harry Clark.  They use a fairly standard estimate of 30 million bison across the Great Plains prior to European contact.  Based on their calculations, the methane (CH4) emissions from those bison (2.2 Tg CH4 year-1) are not hugely different from those of today’s 36.5 million cattle across the same geography (2.5 Tg CH4 year-1).  The exact numbers are less important than this basic idea: the prairie ecosystem was contributing large amounts of methane to the atmosphere before humans brought cattle to the Plains.

Of course, feedlots, fertilization, and forage production, along with all the greenhouse gas emissions and other concerns associated with them, were not part of the historic bison landscape.  We definitely have an obligation to examine those aspects of cattle production and do what we can to limit their negative impacts.  In addition, the fact that cattle on native rangeland are producing emissions similar to their bison predecessors doesn’t release us from the responsibility of trying to reduce those emissions where possible.  I’m hopeful that research over the next decade or so will provide us with more guidance on how we might do that.

Bison

Before there were cattle, bison roamed (and burped) across the Plains.

Get Rid of Cattle?

What if we just stopped grazing cattle on the Great Plains?  Well, since the vast majority of the Great Plains is privately owned, grassland still exists primarily because it produces income.  Without cattle production, much of that grassland would likely be converted to row crop agriculture – a scenario that would probably be worse for climate change and would certainly spell disaster for prairie ecosystems.  Some have argued that a majority of the Great Plains should be turned into public land that would support both wildlife and tourism.  There are way too many economic and social issues associated with that for me to deal with here, but from a climate change emissions standpoint, I’m not sure it would solve the problem.  Either cattle would be replaced by bison again (see previous paragraph) or, if bison were not reintroduced, prairies would suffer from the loss of grazing, a major component of ecosystem function (see next paragraphs).

Simply getting rid of cattle altogether is probably not a great strategy for conservation. Plus, how could you get rid of something this cute?

Simply getting rid of cattle altogether is probably not a great strategy for conservation. Besides, how could you get rid of something this cute?

Grazing as a Positive Force

Despite the fact that chronic overgrazing can cause degradation of prairies (loss of plant species and habitat, soil erosion, etc.), grasslands and large grazers evolved together and grazing is still an essential component of grassland ecosystems.  This is especially true in North America’s Great Plains where there are still grasslands large enough to support wide-ranging wildlife species such as grouse and pronghorn.  Grazing, along with fire and drought, is one of the three major forces that affects prairies and prairie species.  For example, large herbivore grazing helps keep grasses from being so competitive that they overwhelm and reduce the diversity of plant communities, something that leads to a cascade of negative and interconnected impacts on pollinators, productivity, wildlife/insect communities, and more.  In addition, grazing alters vegetation structure, creating a wide range of habitat conditions.  Ungrazed prairie provides fairly uniform vegetation structure, even if it is hayed or burned.  Grazed prairie (under the right management) is heterogeneous, with patches of tall/dense vegetation, patches of short/sparse vegetation, and many other habitat types in-between – allowing the widest possible spectrum of prairie wildlife and insect species to thrive.

Maintaining plant and animal diversity, ecosystem function, and ecological resilience within the historic range of American bison would be very difficult without some kind of large ruminant, and in the face of climate change, we need our grasslands to be as resilient as possible.  Resilient grasslands will better adapt and maintain their ecological functions as climate changes, and that means they’ll continue to pull carbon from the atmosphere and store it belowground – an incredibly important part of our global climate change strategy.  While the impact of grazing on carbon storage of grasslands is, in itself, a complex topic, the general scientific consensus seems to be that a moderate level of grazing facilitates more carbon storage than no grazing (and more than chronic overgrazing).

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Strategic cattle grazing can create a variety of wildlife habitat structure types and help sustain plant diversity and ecological diversity.  It can also help maximize carbon storage in grasslands.

The Upshot

In the Great Plains of North America, grazing is an essential part of grassland ecosystems – a component that maintains the ecological health and resilience of prairies.  Cattle have mostly replaced bison as the large ruminant on stage at the moment, but they are filling many of the same basic roles – regulating plant competition and creating wildlife habitat, and also pooping, peeing, and burping.  We absolutely need to find ways to minimize the impacts of today’s grazing on climate change.  Livestock confinement operations, pasture fertilization, forage production, and other related practices provide opportunities for continued improvement.  In addition, some rangeland grazing practices, such as chronic overgrazing, are known to be detrimental, and not just from a climate change standpoint, so that’s an obvious place to focus.  Beyond that, we need to figure out how best to limit methane and nitrous oxide emissions and increase carbon storage on rangeland.  That will likely mean changing techniques for managing cattle in pastures, but also dealing with issues related to pasture fertilization, forage production, forage and animal transportation, feeding operations, and more.

The topic of cattle grazing and climate change is incredibly complex.  There is much more involved than I could possibly cover here, and what I did include is plenty complicated.  I don’t pretend to fully understand all the facets of the issue, but for now, I feel comfortable in my stance that cattle (and/or bison) grazing can be compatible with responsible conservation of our prairies here in the Great Plains.

 

More Information and Acknowledgements

Several scientists from The Nature Conservancy wrote a really helpful piece on the beef supply-chain and its impacts on water, wildlife, and climate.  You can see a summary and get access to the full report here.

Special thanks to Jon Fisher and Joe Fargione, who both helped me refine and improve this post.  Any remaining errors are my fault, not theirs.