Fire and Bison Grazing in Grasslands Lead to Diversity

Biological Strategy

Great Plains mesic grasslands

Megan Schuknecht

Image: cassi saari / CC BY SA - Creative Commons Attribution + ShareAlike

Manage Disturbance in a Community

When environmental conditions change, they can disrupt an ecosystem’s equilibrium. Excessive rain can cause flooding and drought can cause forest fires. An ecosystem must be resilient to such disturbances. Disturbances are unpredictable in location, size, and intensity, so ecosystems must be able to regrow and must have a variety of duplicate forms, processes, or systems that are dispersed in location. For example, a forest ecosystem can recover from fire because diverse organisms play different roles in different ways and in different locations. Many organisms can resprout or grow from seeds triggered by fire, and their dispersed distribution ensures that an entire population isn’t decimated. Though the recovered ecosystem may look totally different from the pre-fire one, the ecosystem as a whole remains healthy.

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Maintain Biodiversity

The greater the amount of genetic and species variation in an ecosystem, the more resilient that ecosystem is to disturbances. Variation in ecosystems across the Earth also contributes to the Earth’s resilience to unpredictable changes. This variation is called biodiversity. Because living systems compete with each other for scarce resources, maintaining biodiversity involves creating conditions for a large variety of species to successfully co-occur and reproduce. For example, within a wetland, there are different vegetation types. This diversity results in a complex mosaic of microenvironments as the vegetation types alter air flows, light regimes, and water temperatures and chemistry. Because organisms vary in their ideal environmental conditions, these micro-environments increase the diversity of plants in the wetland. In turn, having a wetland in an otherwise dry area increases biodiversity at an even larger scale.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The interaction of two disturbances—bison grazing patterns and fire—increases biodiversity by creating a heterogeneous patchwork of plant communities in grasslands.

Introduction

The Great Plains are a huge expanse of grassland located west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. Millions of bison used to roam the tallgrass prairies that once covered the Great Plains, grazing in patches as they went. Fires caused by lightning or set intentionally by native peoples to renew the grasslands were also common. The relationship between fire and grazing patterns had a big impact on how plant communities developed in the tallgrass prairie.

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Bison prefer grazing recently burned grasslands, and their grazing patterns influence the size and intensity of fires.

Image: cassi saari / CC BY SA - Creative Commons Attribution + ShareAlike

This restored tallgrass priarie in Illinois shows a patchy mix of diverse plants.

The Strategy

Fires are important to , because bison prefer to graze patches of grassland that have been burned recently. Bison grazing patterns also influence the size and intensity of fires. The interactions between these two different types of disturbances— fires and grazing patterns—helps create a heterogenous, or mixed, patchwork of plant communities. For example, these interactions lead to different plant heights, density, and mixtures of species. This helps support more biodiversity in tallgrass prairie ecosystems.

Scientists think of biodiversity on three levels—genetic diversity (different genes and combinations within a species), species diversity (the number of different species within an ecosystem), and ecosystem diversity (how many different ecosystems are found in a region). Biodiversity helps organisms adapt to environmental changes, maintain food webs, and provide services that life depends on, such as water retention and waste decomposition.

Today, most of the land that made up the Great Plains is managed as rangeland for cattle production. Land managers usually only manage the number of cattle and where and when they are grazing on the land. Fire is rarely used anymore as a way to manage cattle grazing. Instead, intensive rotational grazing approaches, where animals are moved rapidly between heavily grazed plots, have become more popular as a management approach. This style of grazing allows the plants that the cattle most like to eat to recover between grazing sessions. However, it does not produce the patchwork of plants that leads to greater biodiversity.

Preserving the Tallgrass Prairie

This Oklahoma State University video takes viewers on a tour of TNC’s Tallgrass Prairie Preserve and talks about the impact of fire, grazing, and drought on the ecosystem:

The Potential

The Nature Conservancy has successfully used fires and continual cattle grazing to manage its Tallgrass Prairie Preserve in Oklahoma. This approach helps to promote biological diversity and increases agricultural productivity, and can help support native plant and animal species. There is even some evidence that this approach, where appropriate, could reduce the need for supplements in cattle during winter months. This would lead to lower management costs.

Canopies Enhance Plant Biodiversity

Forest canopies

Forest canopies create microclimates where tens of thousands of other plant species grow.

Last Updated June 17, 2020

References

“The interaction of grazing and fire is an important influence on diversity and spatial patterns of vegetation in mesic Great Plains grasslands (Biondini et al. 1989, Vinton et al. 1993, Steuter et al. 1995, Hartnett et al. 1996). Fire influences bison grazing patterns, and bison grazing determines the extent and intensity of fires. Bison prefer recently burned areas because of the high-quality regrowth after a fire (Coppedge and Shaw 1998). When only a portion of the area available to bison is burned, intense grazing of burned patches postpones grazing on unburned patches (Figure 4), which results in an accumulation of fuel and an increased probability of fire in unburned patches (Steuter 1986, Hobbs et al. 1991). This interactive model is complicated by the season in which a burn occurs, which influences the effects of fire (Ewing and Engle 1988, Biondini et al. 1989, Howe 1994a) and bison preferences for certain patches (Shaw and Carter 1990, Coppedge and Shaw 1998). Thus, the interaction of these two disturbances—fire and bison grazing patterns—is capable of producing a dynamic patch mosaic of plant communities within grazed grasslands (Steuter et al. 1995, Hamilton 1996).” (Fuhlendorf and Engle 2001:628)

Journal article

Restoring heterogeneity on rangelands: ecosystem management based on evolutionary grazing patterns

BioScience, Vol 51, Issue 8 | Fuhlendorf, SD; Engle, DM

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