What’s the minimum effective size of a prairie?
For example, can a prairie be the size of a kitchen table? Let’s say someone converted a landscape full of prairie to an immense gravel parking lot, leaving only a round kitchen table-sized parcel of vegetation in the middle. Is that tiny isolated parcel a prairie?
The question might seem silly, but the question became a useful little thought experiment for me.
That little parcel certainly wouldn’t be big enough to meet the needs of most prairie animals. Birds, small mammals, snakes, and even smaller creatures like grasshoppers and bees would be unable to find enough food to survive within that small area. The loss of those animals would affect many of the ecological services and functions that make prairies work. Those services include pollination, nutrient cycling, herbivory and more.
Even small creatures like grasshoppers would have a hard time surviving in a patch of plants the size of a kitchen table.
Some tiny herbivorous invertebrates might be able to survive in that little parcel of vegetation, but probably not enough of them to support most predators that feed on them. The lack of predation would allow those invertebrate populations to grow much larger than they otherwise would, leading to significant damage, or even mortality, to the plants they feed on. Once their food is gone, the invertebrates would starve and die as well.
Plants that manage to survive invertebrate attacks and an absence of pollinators in our little parcel would still face major challenges. In the long-term, they would probably suffer from a huge genetic bottleneck because they don’t have other individuals of their species to cross breed with. In the meantime, it would take a lot of intensive and thoughtful management to keep them alive.
Smooth brome and other invaders can quickly dominate small prairie patches without constant vigilance and suppression.
Invasive species management would be a huge problem because it wouldn’t take long for an aggressive invader to quickly dominate that small area. Quick action would be needed to remove invasive plants as they arrive. Fire or mowing would also be needed to prevent a smothering thatch from accumulating as plants grow and die back each year. Unfortunately, every fire would kill most invertebrates aboveground at the time and destroy their food sources. We could try to burn only a portion of the parcel and save some of the insects, but with such small populations, we’d still probably lose most species eventually. Mowing and raking might be an alternative, but we’d still end up removing either the invertebrates or their food sources.
Ok, so we’d just have to live without most prairie animals, but we’d still have plants. Or at least a few of them. Some of those plants would be more competitive than others, especially in an animal-less environment, so it would take a lot of effort to keep them from pushing the less competitive plants out. And, of course, we’re assuming the mysterious belowground processes that allow plants to survive would still function in our tiny parcel – microbial relationships that allow plants to access and process water and nutrients, for example. If those are sufficiently intact, we’d have some plants.
Would that be a prairie?
I’m pretty sure no one would argue that a kitchen table-sized area containing few plants is a prairie. Even in the first moments after the parking lot was created, I would argue the remaining patch of vegetation had ceased to be a prairie, even though it still contained a reasonable diversity of plants and animals. It wasn’t really a prairie anymore, just a doomed fragment of its former self.
If we can agree that a kitchen tabled-size patch of land is too small, how big would we have to make that patch before we’d be willing to call it a prairie? What species and/or ecological processes should we use as criteria?
Can we agree a prairie needs to be big enough to support a healthy pollinator community? Does it need to be able to sustain viable populations of small mammals, snakes, leafhoppers, spiders, and other little creatures? Is it a prairie if it doesn’t have a full complement of grassland bird species? Does that requisite bird community include larger birds such prairie chickens or other grouse species? What about at least moderately-sized predators such as badgers and coyotes (or even bigger ones) or large ruminants like bison or elk? Which of those components are we willing to live without, and more importantly, which can a prairie live without and still sustain itself as an ecological system? A prairie without badgers, coyotes or bison is functionally different than one with those animals, but is it a non-prairie or just a different kind of prairie?
Bison herds need very large prairies, but we don’t know as much about the amount of land needed to sustain populations of bees, leafhoppers, jumping mice, or even genetically viable plant populations.
Even if we reach consensus on the key components of a prairie, we’re still hamstrung by our lack of information about how big a prairie needs to be to support each of them. We have decent data on the prairie size requirements for many grassland bird species, but beyond birds, we’re mostly just guessing. If we want the full complement of species, including bison and other large ruminants, we’re going to need thousands of acres, but how many thousands?
More importantly, what does this mean for the many remaining patches of prairie vegetation too small to support whatever we decide are the key components of a prairie? It certainly doesn’t make them worthless, but it might be important to make sure we’re viewing them realistically. What are the likely ramifications of the missing components? The absence of prairie chickens or upland sandpipers might be disappointing, but might not have the ripple effect that the absence of pollinators or coyotes might have. Can we identify and compensate for the absence of key prairie components by managing differently or more intensively? If not, how do we adjust our vision of the future for that prairie parcel, and how does that adjusted vision affect how much management effort we invest? (You can read more about the challenges of managing small prairies here.)
For many of today’s small prairie patches, the only chance of preserving their species and ecological functions is to make those small patches larger and/or more connected to others. Restoring adjacent land back to high-diversity prairie vegetation allows formerly landlocked populations to expand and interact with others, and creates enough habitat for larger animals to survive. Identifying potential restoration opportunities might be the highest priority conservation strategy for those of us working with small prairies.
Reasonable plant diversity and the presence of larval host plants like this prairie violet have so far allowed our family prairie to support a population of regal fritillary butterflies, but the small size and isolated nature of our prairie means if the butterflies have a bad year, they could easily disappear and never return.
Our family prairie is a little over 100 acres in size, is managed with large ruminants (cattle), and has regal fritillary butterflies, coyotes, badgers, upland sandpipers, and even an occasional prairie chicken. However, I’m certainly not comfortable that our 100 acre island within a sea of cropland will to sustain a prairie ecosystem indefinitely. This thought experiment has forced me to think more seriously about prospects for increasing the size of our prairie and building connectivity to other grasslands. I hope it’s useful to others as well.
The Prairie Ecologist 