The pockets of land still harboring tallgrass are many. I started labeling on the map the various islands still lingering within the former lands of the prairie, pockets held by city, state, and federal governments, ranchers, and nongovernment organizations such as The Nature Conservancy. I got to about 80 before the names I was jotting down began to run together. In places, the map seemed full, especially in western Minnesota, Iowa, western Missouri, and Kansas. But I had to remind myself that the names I wrote took up much more space on the map than the actual acreage. The site might be only a few acres, while the one-word name ran on for 25 miles, according to the scale on the map.
So how could such a broken, piecemeal landscape ever be reconnected for a wildlife corridor? An effort, of sorts, was initiated in 1995 by the states of Minnesota, Iowa, Missouri, Kansas, Oklahoma, and Texas to designate a driving route connecting preserved pieces of prairie. Called “Prairie Passage,” the effort was intended to identify and protect prairie remnants, especially in highway rights-of-way, and connect them in a tallgrass corridor. Where restoration is necessary, these native remnants would serve as seed sources. Highway transportation departments were asked to preserve these roadside remnants by not mowing or using chemicals, allowing prairie plants to grow and flourish. An obvious adjunct was to promote tourism and local economies. This would have been a surprising example of taking roadways, the primary culprit in the fragmentation of wild lands, and using them as a tool in the effort to reestablish a prairie corridor in North America. Unfortunately, after an enthusiastic start in the late ‘90s and early 2000s, the project idled, the states seemingly having lost interest.
According to a National Geographic encyclopedic entry, “Wildlife conservation aims to protect plant and animal species as the human population encroaches on their resources.” Yes, plants, at least native ones, are “wild” and are certainly “life.” So connections among prairie remnants that enable plants to disperse would constitute a wildlife corridor and would, of course, include associated birds, small mammals, insects, and on remnants with adequate acreage, larger fauna such as white-tailed deer and bison.
But perhaps a better term for such a linkage is “ecological corridor,” which Jodi Hilty, President and Chief Scientist of the Yellowstone to Yukon Conservation Initiative, and others in Guidelines for Conserving Connectivity Through Ecological Networks and Corridors (published by the International Union for Conservation of Nature and Natural Resources) define as “A clearly defined geographical space that is governed and managed over the long term to maintain or restore effective ecological connectivity.” The Convention on the Conservation of Migratory Species of Wild Animals, a multi-state agreement under the United Nations, has endorsed a definition of “ecological connectivity” as the “unimpeded movement of species and the flow of natural processes that sustain life on Earth.”
The IUCN group proposes an even broader term for what is typically meant by a wildlife corridor: “ecological network for conservation.” They define an ecological network as a “system of core habitats … connected by ecological corridors …” The core habitats consist of protected areas, other effective area-based conservation measures (OECMs), and other intact natural areas. An OECM is described as a “geographically defined area, other than a protected area, which is governed and managed in ways that achieve positive and sustained long-term outcomes for the in situ conservation of biodiversity…”
With that meaning, a linkage between two core habitats would be an ecological corridor, while many preserved habitats linked by corridors would be an ecological network. And so, for examples, Yellowstone to Yukon and Algonquin to Adirondacks are ecological networks of preserved areas each linked by existing or proposed ecological corridors. The Wildlands Network’s Pacific, Western, and Eastern Wildways constitute even larger ecological networks.
The emphasis on expansive ecological networks derives from the need for preservation on a large scale, a need reflected in the current visions of 30 x 30 (to preserve 30% of the world’s lands and oceans by 2030) and Half Earth (to conserve 50% to stave off mass extinction of species). “With increasing human alteration of Earth, especially by rapid climate change,” the IUCN group says, “it is necessary to think and act at the larger spatial scales at which many species and processes actually operate.”
Dispersal of prairie plant species through an ecological network by wind, water, insects, birds, and the fur of mammals would be slow compared with migration of animals. Prairie grasses and forbs produce a new generation by dispersal of seeds, buds, suckers, spores to a new suitable location where plants grow and thrive, moving only inches, feet, perhaps a few miles to produce the next generation a step away.
Hilty and others in an earlier book, Corridor Ecology, Linking Landscapes for Biodiversity Conservation and Climate Adaptation, point out, “Species with limited dispersal capabilities will need corridors that provide conditions suitable for the species to live in and reproduce over generations …”—what is called “functional connectivity,” the degree to which linkages provide habitat in which wildlife can thrive as they pass through. (“Structural connectivity” refers to the degree to which the physical features of linkages enable or hinder migrating species.)
Climate change, they say, “has added a new dimension to connectivity conservation.” With warming temperatures, plants as well as animals will need to move north or upslope to maintain a climate suitable for their growth and continued existence. So protection or re-creation of a prairie corridor network must consider a divergent future, connecting what may become unsuitable habitat with habitat that will become suitable in the future. Over decades as the climate becomes hotter and drier, suitable tallgrass habitat will likely move north, farther into Manitoba, perhaps even vacating Texas in the distant future.
Inevitably, dispersal along a corridor is influenced by “edge effects,” impacts at the boundary between the corridor and the matrix landscape through which it passes. Along the edges, exotic vegetation may invade, replacing native vegetation. “It is generally acknowledged that invasive species are second only to habitat loss and fragmentation in causing the decline and extinction of species worldwide.” The smaller the corridor or prairie remnant, the greater the seed pressure from surrounding nonnative vegetation.
So corridors need to be wide whenever possible. The central pathway of a wider corridor will be more immune to edge effects. According to Hilty et al. in Corridor Ecology:
The results of a spatial simulator used to explore the effect of dimensions suggest that width has a much greater effect than length on the probability that plant propagules will be successful, although length remains a significant variable. … when the length of the corridor increases, so should the width; a corridor will generally need to be wider in landscapes that provide limited habitat or that are dominated by human use; and corridors that need to function for decades and are intended to facilitate range shifts should be wider.
A prairie corridor, as with most wildlife corridors, cannot be wide everywhere along its length. In the past, the prairie proved to possess such a fertile soil for growing crops and seemingly unlimited grass for ranching, most of the acreage within our map-defined Tallgrass Prairie is now privately owned. So a prairie corridor will necessarily be composed of preserved “blocks of natural landscapes” connected in places by narrow strips of prairie. Or where a physical connection is not possible, smaller landscape blocks will serve as steppingstones, where prairie species’ propagules (vegetative structures that can become detached from a plant and give rise to a new plant) can jump the gaps, carried by wind and animals from one steppingstone to the next.
Mark Anderson, TNC’s Director of Conservation Science, and others in Resilient and Connected Landscapes for Terrestrial Conservation, help make the case for the steppingstone idea:
Absolute contiguity of appropriate habitats may not be necessary and is in many cases impossible for most species, but proximity helps increase the odds of successful dispersal. The stepping stone concept makes sense. … after glaciation many specialist species with poor dispersal prospects somehow relocated to pockets of suitable substrate and climate. … we should not discount the importance of sites that are distant and seemingly disconnected from additional habitat if they are robust source areas for multiple species, and especially if they are source areas for uncommon habitat specialists.