Monday, January 14, 2008

Of Mice and Fire

Driving southwest out of the Ozarks, the landscape changes quickly. Leaving behind dolomite bluffs and limestone outcroppings, forests and woodlands transition into tallgrass prairie. But in between the deep, fern-rich forest and prairie are acres of open oak savannas where the dominant components of prairie and forest grow together. Historically, savannas occurred along the east and south edges of prairie and resulted from the invasion of fire into the woodlands, rather than trees moving into the prairie.

To revisit an old saw, in the absence of fire many of these historical savannas, once rich with prairie grasses and wildflowers, have turned into closed canopy forests with little herbaceous growth. In the past 25 years throughout the Ozarks, efforts have been made to restore this landscape using fire as the primary management tool. Vegetative response has been overwhelmingly positive; Chief calls savannas “nature’s food plots,” because of the rich diet they offer every animal, from mice to deer.

While the vegetative response to fire has been well-documented in these restored Ozark savannas, the associated faunal populations have only been tracked by a handful of graduate students. Out of this body of research comes an interesting discussion about mice populations in a fire-dependent landscape. Early research suggested that mouse populations are decimated after a burn. Some writers have even argued that while certain mouse species live in fire-dependent landscape, our current practice of implementing prescribed fire every three to five years may be excessive and therefore harmful. Populations just can’t recover that fast. The latest study on this topic proves that populations aren’t wiped out, they simply change.

As partial fulfillment of a Master’s Thesis from Central Missouri State University, Becky Erickson examined mice populations in several comparative habitats: among them, savannas managed with frequent, low intensity fires, closed canopy forests, and savannas with no fire management. The fire-managed savanna site provided 90.4% herbaceous cover and 40% canopy cover. On the flip side, herbaceous cover in the forest ranked 58% and canopy cover was 97.2%. White footed mice dominated the captures across all sites. Chipmunks, Eastern wood rats, and short tailed shrews only appeared in the sites managed with fire.

Erickson discovered that the lowest numbers of small mammals were trapped in the closed canopy forest. However, short-tailed shrews and chipmunks were aided by the deep leaf litter and rocky outcroppings in the forest. The largest number of animals appeared, incidentally, in the savannas with rich herbaceous cover. Not surprisingly, mice captured in the savannas, the habitat with a stable food source, typically weighed more than the ones caught in the forest.

Some of the animals she captured, namely hispid cotton rats and woodrats, thrive in the open canopy savanna, but only when there is plenty of grass for nest building and warmth. So, following a burn, they will be in the adjacent woodlands or forest, not in the savanna. Shrews were most often encountered in closed canopy systems where the abundance of leaf litter allowed for frequent feeding on invertebrates. The white footed mouse she describes as a habitat generalist, showing up in every habitat under all circumstances.

Because the fire regime tends to run on a 3 to 5 (or more) year cycle, the composition of small mammal populations changes with the landscape. This study revealed that intermediate and irregular fire disturbance benefits small mammal populations. The first year after a burn, white footed mice appear. As the herbaceous layer grows, hispid cotton mice and wood rats move in. Later, after four or five years without fire, the chipmunks and short-tailed shrews move in. But to maintain the open canopy and the herbaceous layer, fire must be applied. And so,the circle continues. Her conclusion that the most speciose and abundant populations were in the fire-controlled savannas was no surprise, really. When landscapes are managed for biodiversity rather than for an endangered plant, or a certain species of mouse, or deer, they respond in kind--with biodiversity at every level.

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