In most human-dominated landscapes, including most military installations, native ecosystems have been fragmented and now occur as islands in seas of intensively impacted and managed lands. As mentioned above, this fragmentation harms species populations by restricting the movement of those pioneering individuals necessary to found new sub-populations and reinvigorate population sinks. Similarly, fragmentation changes how natural disturbance plays out on the landscape. Fires, for example, may be prevented from running across the landscape by the cutting of firebreaks. Thus, vegetation patches may persist for greater periods of time between fires, resulting in greater fuel accumulation, and subsequently more severe fires when they do occur.
The intensity and impact of any ecosystem process varies over time. Species and ecosystems respond to, and are organized around, these natural ranges of variation within these ecological processes. Thus, fires returning every five years will result in a very different community than when they return every hundred years. This is exemplified by both the longleaf pine forests of the southeast and ponderosa pine forests of the Rocky Mountain west. While there was, of course, variation in the frequency of naturally-ignited fires, typically any given patch would burn every ten years or so. This resulted in open forests, with relatively few large trees in a matrix of grasses and forbs. Both long-leaf and ponderosa pines have thick, fire-resistant bark and so the adult trees are not damaged by low-intensity ground fires. Active fire suppression over the past several decades has decreased the fire frequency and allowed other, less fire tolerant, species to get toeholds. Now, when fires do occur, the fire climbs into the canopy and the results are conflagrations that consume everything rather than the historically less intense ground fires that did not impact the trees.
Ecological processes that are impacted by military land uses include:
- fire, both in terms of frequency, seasonality, and intensity
- flooding, including frequency, duration, sediment movement
- disturbance of turf by rodents and large mammals in prairie systems
- sheet flow, and other water movement patterns in desert systems
Active ecosystem management by humans can mimic historic ecological processes and their effects; conservation managers can achieve both their conservation goals and meet the needs of the military. However, management with an eye toward variation is more challenging than managing for consistency. A large forest ecosystem will be very different if every management unit is burned on a 10-year cycle than if units were burned randomly on a 5- to 30-year pattern. The former is easier to plan and to implement, as managers can anticipate needs many years in advance. The latter is more complex structurally, and hence, harbors greater biological diversity.
Next Page: Ecological integrity
Bob Unnasch, Ph.D.
Sound Science LLC