Components of Biodiversity of Concern to Land Managers
The bogs and other freshwater wetlands on
theWarren Grove Air National Guard Range,
located in the Pinelands of southern New Jersey,
are areas of exceptional biological diversity.
(Photos: Douglas Ripley)
For one tasked with the conservation of biodiversity, the idea of planning to preserve
the "totality of genes, species, and ecosystems of a region" is daunting as
exhibited by the complexity of Figure 2.1. Attempting to implement the conservation
of biodiversity, as defined, is an overwhelming challenge. It is far too easy
to become stuck in the weeds of the details and to try to manage everything individually.
A land manager will justifiably ask, "How can I hope to manage for all species on my installation? How in the world do I manage for landscape function?
Where do I start?"
While it is important to keep all biological levels of organization in mind, one
does not need to plan, or manage, for each. In reality, planning for conservation
action leans most heavily on what is commonly called the coarse filter/fine filter
approach (Noss 1987). The coarse filter approach focuses on ecological systems
ecosystem management whereas the fine filter approach emphasizes individual
species management. Successful biodiversity management relies on both. In brief,
the reasoning supporting this paired approach is that most species are "captured"
by the coarse filter because of their association with specific ecosystem types.
Those species that are not captured in the coarse filter (e.g. wide ranging species)
need then be caught by the fine filter (Groves 2003).
While the concept of the coarse and fine filters was initially conceived to be independent
of spatial scale, in reality those species not captured by the coarse filter
tend to be intermediate, coarse, or regional scale species as defined by Figure
2.2 (Poiani et al. 2000). These tend to be larger, wide-ranging species that are often
dependent on a diversity of ecosystems during their lives.
While most resources managers, and many non-biologists, have an intrinsic
understanding of these levels of biological organization, it is always a good idea
to review these terms and concepts as their precise meanings are often different
from the perceived gestalt.
POPULATIONS. A population is typically defined as a group of interbreeding individuals of the
same species living within a defined area. The key to this definition is that individuals
within a population must, at the very least, have the potential to interbreed.
Thus, dispersal potential can drive the size of a population. Many wideranging
species, for example migratory birds, have huge populations that can span
thousands of square kilometers. More stationary species, for example, bog lemmings,
will have more restricted population sizes where the entire population exists
within a small peat bog.
META-POPULATIONS, NATURAL AND DERIVED. Between these two extremes, most
species exist as constellations of sub-populations where most individuals interfigure act within their small group, with the rare individual dispersing over greater distances.
So, these species are structured as a meta-population, or a population of
sub-populations. These sub-populations are distributed across a landscape (or a
military installation) as many "occurrences." Each occurrence has a low probability
of persisting over the long term, in isolation from other occurrences. Most
sub-populations are simply too small to be resilient to environmental variation,
or demographic or genetic bottlenecks. Neighboring occurrences are constantly
providing, at some low but critical rate, "new blood" into a given sub-population.
These neighboring occurrences also provide sets of "founder" genes that will recolonize
a vacant area.
Meta-populations can be envisioned, then, as a galaxy where each "star" is a
sub-population. These "stars" are winking off and on as sub-populations disappear,
and then reappear as the vacant areas are re-colonized. The space between
these stars the voids is not suitable habitat for these creatures, and so is simply
not available for colonization by this species.
A "bottleneck" occurs when a population
is dramatically reduced in size,
often by 90 percent or more. Bottlenecks
can result from any number of
impacts: droughts or other climatic
changes; epidemic disease; appearance
of an exotic competitor, or
human impacts. The consequences
of this decline are manifested both
in demographic and genetic realms.
The most severe demographic consequence
is, of course, extirpation.
The remaining individuals are too
dispersed to find each other, and
hence the reproductive rate drops
below replacement, and the population
slowly "winks out."
The genetic consequences of a bottleneck
event can be equally dramatic.
As the population size shrinks,
the genetic diversity also declines.
This lack of genetic diversity can
result in the expression of deleterious
genes that reduce the vigor of
the offspring of the remaining
individuals potentially leading to
SOURCES AND SINKS AND THEIR IMPORTANCE. Upon reflection, it becomes obvious
that all sub-populations are not the same. Some occur on tiny patches of acceptable
habitat, and never grow to more than a small number of individuals. These,
of course, never really escape the consequences of being in a "population bottleneck,"
and many have a low probability of persisting in isolation. Others occur
on large areas of acceptable habitat and, thus, tend to exist as large healthy populations.
These have greater demographic and genetic resilience, and hence a
greater probability of persistence. Simply because of their large size, these populations
tend to be the source of most of the dispersers that colonize vacant
patches, and reinvigorate the small sub-populations both by their numbers and
by their genetic diversity. These are thought of as "source" sub-populations,
whereas the smaller occurrences which tend to absorb migrants, but do not provide
dispersers, are considered "sinks."
The generalization that small populations tend to be sinks, and large populations
sources, is, like all generalizations, only true to a point. The key, which is
often difficult to measure, is whether the population produces significant numbers
of emigrants or not. Source populations do, sinks do not. In general, "sink"
sub-populations will not persist without continual immigration from "sources."
Thus, the destruction of a single "source" sub-population can result in the extirpation
of many surrounding "sinks" even if they are not directly impacted.
The Sikes Act and the Endangered Species Act require military installations to
prevent the loss of threatened and endangered species found within their boundaries.
Understanding the ecology of those species, and how their populations and
sub-populations are distributed, is key to meeting this requirement. Conserving
a wide-ranging species like the bald eagle might be accomplished simply by protecting
a limited number of nesting sites as only a small piece of a much larger
population exists on site. The Karner Blue butterfly, in contrast, exists as a metapopulation
where sub-populations exist in ephemeral patches of host plants. Conserving
this species requires an understanding of the disturbance dynamics creating
these patches of host plants, and the dispersal capabilities of the butterfly,
so as to manage the entire meta-population and not just a few occurrences, each
with a low probability of persistence in isolation. Understanding and managing
a meta-population often requires looking beyond an installation's borders to subpopulations
on neighboring lands.
COMMUNITIES AND ECOSYSTEMS
How are communities and ecosystems different than "habitat"? Managing for
species invariably means managing habitat. Habitat (which is Latin for "it inhabits")
is the place where a particular species lives and grows. It is essentially
the environment at least the physical environment that surrounds (influences
and is utilized by) the species population. The term was originally defined as the
physical conditions that surround a species population, or an assemblage of
species (Clements and Shelford 1939). Wildlife managers, in particular, tend to
focus on habitat management identifying and manipulating those environmental
factors limiting a targeted population's size (Leopold 1933, Yoakum and Dasmann
1971). Scientists often expand the concept of habitat to include an assemblage
of many species, living together in the same place. Thus, for example,
wildlife managers often work to improve shorebird habitat. The U.S. Fish and
Wildlife Service (USFWS) has spent many millions of dollars managing for breeding
habitat for migratory waterfowl in the prairie pothole region of North America.
Ecologists regard the habitat shared by many species to be a biotope a place
where a community of species lives.
The concept of habitat is not synonymous with that of the natural community
or ecosystem. A natural community is the assemblage of plants and animals sharing
the same habitat and interacting with each other. When one speaks of a natural
community, the focus is on the species and their interactions. The habitat, or
biotope, is the biophysical stage on which these species and their interactions occur.
Communities typically reoccur across a landscape as they track habitat conditions.
As such, communities do not occur at a single, specific spatial scale. Vegetation
communities are often perceived as the classic community, but one can
also describe the smaller community existing within a fallen log, or ephemeral
community within a vernal pool.
An ecosystem, then, can be thought of as the whole picture; the combination
of a natural community and its habitat (or biotope). As such, an ecosystem can
extend far beyond even a large military installation. But ecosystems are more than
just a community in its habitat. The concept of the ecosystem includes dynamic
ecological processes (see below) and the recognition that species composition (i.e.
the community) will change over time as well as over space. Every species within
a community responds to the environment differently from the others. Similarly,
each species interacts with different suites of other species. As conditions change,
as they certainly do within military installations as in other environmental settings,
some species become more abundant, while others become rarer.
Natural disturbances, ranging in size from gaps caused by fallen trees to massive
wildfires, all affect species abundance and distribution differently (Picket and
White 1986). Thus, ecosystems are neither static nor homogeneous. Rather, they
are composed of "patches" of various sizes and ages, and the relative abundance
and distribution of these patches is crucial to maintain the full suite of biodiversity
within an area. Maintaining ecological processes, such as fires, floods, and periodic
disease epidemics, is the keystone of successful ecosystem conservation. Indeed, the
core of the ecosystem-based management approach is the understanding that the
persistence of all biodiversity within an area is contingent on the persistence of this
crazy-quilt pattern of disturbed and recovering patches. Management, then, needs
to focus on the dynamic processes creating this pattern and not on maintaining a
static structure and condition. Military activities can mimic some natural disturbances,
and thus can often be integrated into a biodiversity management plan.
Dr. Walter Bien, Professor of Biology at
Drexel University, Philadelphia, explaining
his field work to graduate students and the
natural resources staff at the Warren Grove
Air National Guard Range, New Jersey.
Research by university and environmental
organization scientists has contributed
significantly to the DoD's understanding of
ecological processes on its lands. (Photo:
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 then 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, sediment movement
- Disturbance of turf 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.
From a biological perspective, a military installation is not an island, existing in
isolation. It lives within a larger landscape comprising both natural and anthropogenic
systems. A natural landscape can be thought of as the spatial scale at
which ecosystems reoccur (Forman 1995). Meta-populations often function at
this scale, with sub-populations occurring in ecosystem patches scattered throughout
the landscape. Many wide-ranging species are very sensitive to the landscape
pattern. These species often use, and require, two or more ecosystems for survival.
These ecosystems may often not be congruent, and the species must travel
through the landscape. Smaller installations may encompass only a small portion
of the landscape mosaic and, as a result, critical habitats and ecosystems may only
occur off-site. In these circumstances, it is very important to look beyond the installations
Alternatively, a large military installation can often be fruitfully managed as a
landscape unto itself or sometimes as a microcosm of a much larger landscape.
Natural buffer zones, impact areas, training areas, and other developed lands together
join to form a landscape mosaic. There is great opportunity to build upon
this existing mosaic, creating missing patches or systems, and enhancing others
to effect significant conservation results.
Proceed to Next Section: Learning to Think Like a Mountain: Tools for Conservation Practitioners