Prairie diversity

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By Chris Helzer

Restored prairie at The Nature Conservancy’s Derr House showing a burned patch of a patch-burn grazing system. (Chris Helzer/The Nature Conservancy)Some of my favorite people are insects. I also have a thing for plants. Fortunately, I spend a lot of my time hanging around in prairies where no one much mocks me about my preferences for companionship.

When I can’t be out in a prairie, one of my other favorite pastimes is talking to people (yes, real people) who haven’t spent much time in prairies, introducing them to the kinds of things they’re missing. Prairies contain an amazing array of insects, animals and plants, most of which are easy to miss if you’re not looking carefully. There’s the inchworm that camouflages itself with bits of the flowers it eats, the crab spider that can change color to match the flower it hunts on, and ants that manage herds of aphids in underground caverns and harvest sugary sap from their rear ends. Among birds—besides the recognizable meadowlark—there are many other species, including the bobolink—which looks like a blackbird after a lobotomy and sings like R2D2 from “Star Wars”; the pencil-necked upland sandpiper—a shorebird that sits on fence posts and gives a wolf-whistle call as it circles overhead; and the sedge wren—whose machine-gun song matches its aggressive territorial behavior toward other, bigger birds. These and thousands of other insect and animal species live among plant communities that contain hundreds of plant species. You probably wouldn’t recognize the names of most of those plants, but even among the more familiar names there is amazing diversity. A single prairie can hold five or more species of sunflower, eight to 10 species of milkweed and well over 30 species of grass.

Preserving biological diversity in prairies (and elsewhere) is important for the sake of the individual species, each of which has its own long history of adaptation and survival. But a wide diversity of species is also the key to survival of the entire prairie. Diverse communities are stronger and more resilient than those that are less rich in species. More overlap in the ecological roles filled by various species means that the temporary loss or decline of one species can be compensated for by others. During periods of drought, for example, some plant species wither and die or go dormant for long periods of the hot dry summer. But a diverse prairie will have plenty of drought-tolerant species that can quickly expand into those newly opened spaces—helping to ensure a constant food supply for herbivores and decreasing the vulnerability of the prairie to invasive species. When wetter times come again, the community resets itself, with those species best adapted to current conditions becoming more abundant accordingly.

Because of the complex interactions between species in a diverse community, the loss of one or a few species can have cascading impacts on the larger community. Checkerspot butterfly on Canada wild rye in restored tallgrass prairie at Ducks Unlimited's Verona Complex, a rainwater basin wetland site, near Sutton, Neb. (Chris Helzer/TNC)For example, many insect larvae feed only on particular plant species for food (and often need entirely different plant species as adults). A commonly used example in prairies is the monarch-sized regal fritillary butterfly, which feeds only on violets as a caterpillar and then relies on a variety of flower species as nectar sources when it becomes an adult. The loss of violets means the loss of the regal fritillary. The loss of too many insect species can mean a smaller and less dependable food source for predatory insects and animals, and so on.

Species that provide ecological services like pollination can be even more important. Native bees are a diverse group, but they all rely heavily on the consistent availability of pollen and nectar through the season. Since not all bees can harvest from all types of flower, a diverse mixture of flowers is necessary at all times to provide for the entire bee community. If there is a gap in flower availability during the summer, it can mean starvation for foraging bees—and the nests they’re providing for. And the loss of bees, of course, can mean the loss of reproductive ability for many of the flowers that depend on them.

Besides the moral obligation many of us feel toward the conservation of species and natural communities, there are also self-serving reasons for humans to be concerned about biological diversity. The most obvious of those reasons may be simple aesthetics. Who wouldn’t rather walk through a meadow filled with hundreds of flower species—with colorful butterflies floating all around—than a field filled only with brome grass? Or listen to the innumerable songs of birds, insects and frogs on a cool summer morning in a lowland prairie instead of the relative silence in a football field’s worth of bluegrass turf? But biological diversity provides other tangible benefits as well. Insect pollination is a well-recognized ecological service, and, as stated earlier, relies heavily on a diversity of both pollinators and plants to persist. Because diverse grasslands provide higher and more stable productivity, maintaining that diversity is of critical importance for our livestock industry and the people it feeds.

The need for management

Targeting biological diversity as the goal for prairie management is a relatively recent phenomenon. Throughout our history, we humans have managed prairies to attract game species, and, in more recent times, to maximize forage production for domestic livestock. And for as long as humans and prairies have coexisted, a myriad of other species has flourished in those human-influenced grasslands. In the face of fires, droughts and massive bison herds—and also because of them—thousands of plant, insect and animal species found suitable habitat and survived. When a species’ habitat became less favorable in one particular place, through human actions or otherwise, that species would either move somewhere else or hunker down and wait for better conditions to return.

Now, however, human management of prairie land has reached the point at which our most productive soils have been largely converted to row-crop agriculture. As a result, prairie species have fewer options for survival when local conditions don’t fit their needs. Moving across the landscape is much more difficult because it entails crossing large areas of unfriendly habitat, including roads, crop fields and woodland. And hunkering down to wait for better conditions to return is less effective than in the old days because the waves of fire, bison and drought no longer spin the wheel of grassland fortune. Today’s prairies tend to be static—easily categorized as hayed prairies, grazed prairies or prairies left idle to be consumed by opportunistic trees. If this year’s habitat conditions don’t suit a particular species, it’s unlikely that those conditions will be any different next year or for many years to come.

In addition to the challenge of surviving in isolated static grassland patches, today’s prairie species also have to contend with a raft of invasive and aggressive species. Some of these are nonnative species like smooth brome, Siberian elm and leafy spurge, but others, like eastern red cedar and dogwood, are natives that have been released from historic shackles (e.g., fire) that had kept them suppressed. While adding new species can sometimes increase plant diversity, these and many other invading species tend to greatly reduce plant diversity because their dominance is so great that many species are displaced. In isolated prairies, once a species is lost (plant or animal), it’s unlikely to recolonize from other prairies.

Into this reality wades the prairie manager. Because the prairie landscape now consists of isolated fragments, the scale of management needs to be that of each fragment. Retaining and rebuilding prairie size and connectivity through restoration is important, where possible. But thoughtful management that helps to maximize species diversity within each prairie fragment is critically important. Unfortunately, there has historically been little available information on appropriate techniques and strategies for prairie owners/managers—largely because diversity-focused prairie management is still a relatively new idea and there are various and often conflicting ideas about how best to do it.

However, while each prairie has its own set of challenges and needs, there are some broad guiding principles for prairie management that encourage biological diversity. First, prairie management can largely be boiled down to the manipulation of competition between plants.

Plants are always in competition with each other, but a good manager can regulate and tweak that competition so that all plant species have the periodic opportunity to thrive and reproduce—thus keeping themselves alive in the community. The most important process for manipulating competition is defoliation. Defoliation—the removal of leaves and stems—has an immediate impact on plants if they’re actively growing. The loss of leaves means a plant can no longer support its entire root system, so it abandons those roots it can’t maintain. That opens up root space for other plants to take advantage of—altering the competitive balance. Fire and mowing/haying are both available tools for defoliation, but the most selective and flexible is grazing because the timing, frequency and intensity of defoliation can all be controlled. While grazing doesn’t make sense for all prairies—especially very small ones—diversity-friendly systems like patch-burn grazing can provide the kind of fire/intense-grazing/rest-period regime that prairies are adapted to. By adjusting the grazing pressure, a manager can regulate the selectivity of cattle are as they graze. Lighter stocking rates often induce cattle to graze primarily grasses, opening up space for abundant wildflowers, and heavier stocking rates can suppress a larger proportion of the plant community, opening up opportunities for short-lived plants like black-eyed susans, common evening primroses and others that normally wither under strong competition from perennials.

The second guiding principle is that repetitive management leads to simplified natural communities. Every management action favors some plant species over others. Utilizing the same management treatments at the same time year after year means that those winners and losers never change places, and eventually the losers will disappear forever. Altering management so that all plant species have a chance to succeed now and then ensures their survival in the prairie.

If these principles appear to ignore the needs of wildlife species, there’s a good explanation. While prairie animals tend to have fairly specific habitat needs, particularly regarding the height, density and heterogeneity of vegetation structure, maintaining a diverse plant community is usually the best objective for a prairie manager. With few exceptions, the tools and techniques used to maintain that plant diversity will lead to the heterogeneous habitat needed by wildlife species as well. Sustaining a diverse community of plants will provide the required ecological foundation to support those other species.

The upshot

The world has changed dramatically for prairies and people over the last century or two, and this century may bring even more dramatic change than the last. Adapting to the new world has been hard, but so far we’ve both survived. Although many prairies are degraded and/or relegated to small isolated fragments, they’re still here.

I’m greatly encouraged by the growing awareness of prairies and their role in the history and future of humans. It wasn’t long ago that most schoolchildren in Nebraska knew more about rainforests than prairies. I think prairie conservation can be one of the greatest accomplishments of Nebraskans over the coming decades. We certainly have an abundance of prairies left to conserve, especially relative to our neighboring states to the east. Thoughtful management of individual prairies and some coordinated planning at a larger scale can help ensure their future. And it may turn out that their future is more strongly linked to our own than it might seem at first glance.

 

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