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Chapter 5
Modern Perspectives of Habitat
Wild species cannot exist in isolation from their habitats. Although scientists and lay persons
often speak of species as though they were independent entities, organisms and habitats are inextricably
linked. In his classic textbook, Elton (1927) recognized that the identification and naming of a species in
essence coded the species for its habitat and ecological role. This insight he encapsulated in the term
"niche." In a real sense, then, species and habitats are but elements of a single ecological system. This
insight is echoed in the concept of the ecosystem, which was introduced to describe the intimate linkage
of organisms with their habitats and the physical conditions, resources, and other interacting organisms
in those habitats. Thus, an ecosystem was viewed as the assemblage of organisms and the physical
environment with which they exchange energy and matter. That the system is fully linked is a subtle and
powerful concept that sometimes even ecologists fail to recognize or appreciate. The ecosystem concept
demands that we understand the reciprocal (not necessarily always beneficial or mutualistic)
relationships between organisms and their habitats. The Endangered Species Act encapsulates part of
this wisdom in its explicit statement of purpose "to conserve the ecosystems upon which threatened and
endangered species depend."
Recognizing that species and habitats are two components of a single system puts the language
of the ESA in a specific scientific light. Although a substantial portion of the language of the
Endangered Species Act focuses on the organismal part of the species-habitat system (harassing,
prohibitions against the killing or pursuit of listed species, etc.), the term "harm" biologically should
encompass damage to the entire system, including the physical components of the system, through
damage to any of its parts. Such is the nature of systems organization (e.g., von Bertalanffy, 1968;
Pattee, 1973; Kolasa and Pickett, 1989~. Therefore, harm in an ecological sense applies to damage to the
habitat of a species or curtailment of a species' access to a habitat.
Species and habitat conservation would seem to be about saving living things. However, the
issue is not so straightforward. Populations with identifiably distinct evolutionary and ecological
features that exist at any one time and across a certain space are not static entities. Thev are resort calf
~ ---- r--
ecological and evolutionary streams that stretch Into the past and have at least some biological potential
for continuing into the future. The evolutionary stream for one species can interact with other
evolutionary streams by sharing genetic material or by dividing to produce separate lineages. Viewed
from this dynamic perspective, conservation of species requires the conservation of ongoing evolutionary
processes and potential. Species, as diagnosed at any particular time, represent a sampling of a
continuous evolutionary process.
As members of communities and ecosystems, species take part in ecological processes. These
ecological processes can also be thought of as streams, including the dynamics of community succession,
the rhythm of natural disturbance, the waxing and waning of predator and prey populations, and the
cycling of soil nutrients. To protect species, their ecological streams must also be protected.
Habitats that support these evolutionary and ecological streams are heterogeneous, and that
· · . · . · .
(, ~ ,
heterogeneity is expressed in space and in time. Like the profound significance of habitat, this too is one
of the fundamental concepts of modern ecology (Wiens, 1976; Kolasa and Pickett, 19911. Habitats (or
habitat diversity) should be viewed both spatially and temporally if conservation planning is to be
successful. Spatial heterogeneity takes several common forms ranging from gradual to discrete (Kolasa
and Rollo, 199 l ). Gradients of heterogeneity are patterns established by gradual change in such factors
75
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Science and the Endangered Species Act
as moisture, nutrients and prey, temperature, topography, soil chemistry, exposure to the elements, cover
from preciators, and the like (Whittaker, 1975; Austin, 1985; Schoener, 1986~. Ecotones are coarse-scale
gradients representing the transitional boundaries of biomes or community types, and are governed by
spatial changes in climatic anti human lancl-use practices (Holland et al., 1991~. On finer scales, spatial
gradients can be generated at the edges of communities having contrasting structures or compositions
(Gosz, 19911.
In contrast with the pattern illustrated by gradients, habitat heterogeneity can be patchy in its
configuration. Patches are discrete spatial units that are cletectable on a certain scale. Patches may result
from precipitous changes in physical environment or biological composition through space (Forman and
Goclron, 1986; Forman, 1987~. For example, a pond is a patch recognizable on a topographical map. On
the same map, other patches, such as vegetated land, wetlands, human settlements, or hedgerows in an
agricultural matrix, might be discernable. Patches can arise from fixed spatial patterns of resources, soil
features, or topographic features. More mutable patches may reflect human land uses, biological
interactions, natural or human ciisturbances, ant! successions. The fact that biological interactions and
successions can form patches means that patches in an area can ciiffer in age as well as origin and can
change over time.
Patchiness can reflect the unclerlying physical characteristics of a site ant! the biological
environment that emerges from that physical environment ant! from the interactions of species. Thus, the
distribution, procluctivity, and assemblages of organisms are themselves sources of heterogeneity above
and beyond that which is caused clirectly by the physical environment (Schoener, 19861. Organisms in
one type of patch might depend in turn on organisms or processes that depend on contrasting patch types
(e.g., pollinators of certain tropical vines) (Gilbert, 19801. Each organism responds to the physical and
biotic heterogeneity it is exposed to in an indiviclualistic fashion, based on its unique combination of
resource requirements, physical tolerances, and capacities for biotic interaction (Austin, 1985; Shipley
and Kedfdy, 1987~.
The spatial pattern in the physical and biological environment and species' responses to them are
not static. Rather, change in the environment and, consequently, change in the distribution, growth.
abundance, and interaction of species are common. Habitat can shift in distribution or be unoccupied for
a time (Levins, 1970; Horn and MacArthur, 1972~. A landscape perspective highlights the fluxes
between patches and provides critical scientific information about the contexts on which species depend
(Pickett anti Thompson, 1978; Moss, 1987a, 1987b; Angelstam, 1992; Fiecller et al., 1993~. Organisms
might encounter habitat as shifting mosaics or dynamic arrays of patches for two reasons. First, habitat
can shift in location through time due to a variety of changes in environment; natural changes in climate
are a major source of habitat shifts (Neilson ant! WulIstein, 19831. In addition, habitat can be created or
clestroyed by episodic or rare events, such as fire or windstorms (Garwood et al., 1979; White, 1979~.
Habitat destruction means that a site is converted from an environment suitable to a species to an
environment that is adverse to that species. Furthermore, habitat destruction for one species might
constitute habitat enhancement for another. Episodic natural events that can alter habitat include
physical disturbances, such as fire, windthrow, mass movements, flooding, and outbreaks of (liseases or
herbivorous insects (Pickett and White, 1985~.
Second, habitat can be unoccupied for a time clue to migrations or movements of organisms in
response to seasonal cycles, reproductive behavior, localized resource clepletion or creation, and a search
for protection (Rotenberry and Wiens, 1980; Angelstam, 19921. Areas through which a species may
travel for any of the reasons above should be considered a part of its habitat as much as areas
continuously occupied by a species (Noss, 19911.
A lanclscape perspective can provi(le an encompassing view of the dynamics of habitat. A
lanclscape is a large area in which a certain array of ecosystem types is linked by natural disturbance
O - ~
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Modern Perspectives of Habitat
regime, pattern of human land use and disturbance, and distribution of land forms (Forman and Godron,
1986; Risser, 19871. Recovery planning for threatened and endangered species that reflects a landscape
view of habitat is more likely to be successful than planning that ignores it (Franklin, 19934.
LANDSCAPES AND POPULATIONS
Populations often occur as collections of relatively discrete demographic units distributed over a
landscape. Such subdivided populations, in which individual demographic units are connected through
dispersal or migration, are referred to as metapopulations (Levins, 19704. The concept has had its widest
use in evolutionary biology, where the movement of genes and the potentially differential effects of
natural selection among the populations are important mechanisms for evolutionary change or stasis
(Soule and Wilcox, 19801. Whether demographic units are genetically differentiated and the degree of
gene flow that occurs among the populations are important concerns in evolutionary metapopulation
dynamics.
Metapopulation concepts are applicable to ecological concerns as well as to evolution. In an
ecological context, concerns are with 1) the degree of landscape heterogeneity, 2) the degree to which
population structure reflects environmental heterogeneity, and 3) the demographic, community, and
ecosystem consequences of population subdivision (Hanski, 1982; Wiens, 19841. In addition,
environmental heterogeneity can change through time, and ~netapopulation dynamics might respond to
those changes. The local dynamics of a population can be determined in large part by influences from
adjacent patches in a landscape (Angelstam, 1992). As a consequence of all the interactions mentioned
above, metapopulation structure and dynamics might be best examined through an inclusive and dynast
view of habitat (Pulliam, 1988). Specific sites become more or less suitable as habitat, reflecting a wide
variety of factors. Furthermore, the environmental heterogeneity that affects the persistence and
performance of a species can change through time (Pickett, 1976; Pickett and Thompson, 1978). A
concept that recognizes environmental heterogeneit~that of source and sink habitats-should be
incorporated into conservation planning.
77
:1C
SOURCES AND SINKS
In natural populations, individuals reside in habitat patches of differing quality. Individuals in
highly productive habitats can be expected to be more successful in producing offspring than those in
poor habitats, which can be expected to suffer poor reproductive success or survival. The fate of a
population as a whole can depend on whether the reproductive success of the individuals in high value
habitats outweighs the failure of the inclividuals in the poor areas. This concept has its own
nomenclature. "Sources" are areas where local reproductive success is greater than local mortality.
Populations in source habitats produce an excess of individuals, which disperse outside their natal habitat
patch to find a place to settle and to breed. In contrast, "sinks" are habitat areas where local productivity
is less than local mortality; in the absence of immigration from source areas, populations in sink habitats
decline toward extinction.
Sources and sinks can be defined in reference to the finite rate of increase (~) for the population in
a given area. The finite rate of increase can change across either space or time as the survival rates and
or reproductive rates of the population vary. The geometric mean ~ of the rates for a sequence of t years
characterizes the mean growth rate. When the long-term mean growth rate (~) is less than 1.0, the
population will decline, but if ~ exceeds 1.0, the population will grow.
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Science and the Endangered Species Act
The finite rate of increase can also be used to describe spatial variation in population growth rates
by calculating ~ based on the birth and death rates that apply in a specific habitat or patch of habitat
(Pulliam, 19884. If patches of habitat are isolated from one another, the finite rate of increase for each
habitat patch describes the growth rate in that patch; however, when habitat patches are less isolated the
concept of habitat-specific growth rate is complicated by dispersal. When habitats are connected by
dispersal, different parts of the population are growing at different rates. The growth rate and the
habitat-specific hi gives the rate that the population in habitat patch i should grow in the absence of
immigration or emigration. The finite rate of increase of the entire interconnected population is given by
the weighted average of the As across all habitats Ski is weighted by niN, where ni is the population size
in the ith habitat, and N is the total population size over habitats).
Large numbers of inciivicluals live in sink habitats, despite the potential disappearance of the sink
population without immigration from more productive areas. To illustrate this point, assume that the
subpopulation in the source grows at the rate 1~ until it reaches a maximum size (id, ), which represents
the maximum number of breeding individuals that can be accommodated in the source. Once the source
has reached its maximum size (fin ~ individuals at the end of each season), only no can remain to
breed; the remaining no (~-1) are assumed to emigrate from the source habitat into sink habitat. In the
absence of immigration, the sink subpopulation would soon disappear. However, with a steady
immigration of individuals from the source habitat, the sink population will grow to an equilibrium
population of no = Hi (~-11/(1-12~. Clearly, if the per capita reproductive surplus (11-1) in the source
is much larger than the reproductive cleficit (1-12) in the sink, then the sink habitat will contain
substantially more individuals than the source habitat, despite the fact that the sink subpopulation is
dependent on emigration from the source for its very existence. This example illustrates a more general
conclusion that the majority of individuals in a local Population likely exist in habitat which is unsuitable
for them over the long term (Pulliam, 19884.
~A
It is important to remember that source habitat is cleaned by demographic characteristics-habitat-
specific reproductive success and survivorship and not by population density; therefore, population
density can be a misleading indicator of habitat quality (van Home, 1982~. Source habitats could easily
be overlooked if conservation efforts concentrate only on habitats where a species is most common,
rather than where it is most productive. If source habitats are not protected by conservation plans, an
entire metapopulation collie be threatened.
Environmental heterogeneity on the landscape scale can be represented as a mosaic. Changes in
the kinds or arrangement of patches in a landscape result in a shifting mosaic (Botkin and Sobel, 1975;
Bormann and Likens, 19791. Underlying such mosaics are the various mechanisms of patch dynamics,
which include natural disturbance, life history features of organisms, and succession (Pickett and White,
1985; Walker, 1989; Luken, 1990), as well as anthropogenic changes. Source-sink relations likely exist
in such mosaic lanclscapes. A step toward incoporating the landscape perspective of habitat is to broaden
the analysis to the concept of the metapopulation.
METAPOPULATIONS
Metapopu1lation is a more encompassing concept than that of source and sink dynamics, because
demographic rates in metapopulations might not be the same in different patches of habitat. Source and
sink dynamics are a special case of metapopulation dynamics in which some habitat patches (sources)
are substantially better than others (sinks).
Levins (1969) argues that the fraction of suitable habitat patches that are occupied at any time
represents a balance of the rates at which subpopulations go extinct in occupied patches and the rates of
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Modern Perspectives of Habitat
79
colonization of empty patches (see Hanski, 19891. The rate of local extinction depends on conditions
within a patch and the stochastic nature of the dynamics of small populations. The rate of colonization
of empty patches depends on the dispersal ability of the species and the location of suitable patches in the
landscape (Hansson et al., 19921.
Metapopulation models can be used to describe the structure and dynamics of populations that are
scattered across a landscape in spatially isolated patches. Such models are useful in the identification of
particular subpopulations, habitat patches, or links between patches that are critical to the maintenance of
the overall metapopulation. Beier's (1993) study of cougars (Fe1lis concolor) in the Santa Ana Mountains
of southern California provides an excellent example of this type of analysis. Beier used radiotelemetry
data to show that the California cougars exist in a collection of semi-isolated populations found mostly in
small mountain ranges linked by riparian corridors. He developed a metapopulation simulation mode}
and showed that the metapopulation of the region was heavily dependent on movement by individual
cougars through the corridors to colonize empty areas. Beier's analysis quantified how the loss of habitat
in this region and corresponding decrease in population size would affect the chance of extinction for the
entire metapopulation. For example, by examining the importance of specific patches and corridors in
maintaining the metapopulation, Beier showed that one corridor in the northern part of the study area
linked a 150 km2 patch (8% of the total area) with the rest of the region. Recognition of the roles of
habitat heterogeneity, habitat patchiness, the importance of suitable but temporarily unoccupied habitat,
and the distinctions between source and sink habitats are all critical to successful implementation of the
ESA. Recent advances in metapopulation theory and the modeling of demographic phenomena offer
managers the tools necessary to plan better for conservation of species and the habitats upon which they
depend. Spatially explicit models incorporate these critical concepts.
SPATIALLY EXPLICIT MODELS
Landscape ecology and conservation biology have made clear that the geometry of habitat patches
in a landscape can influence population trends and extinction probabilities. Metapopulation models have
generally ignored the complexities of dispersal behavior and habitat geometry by assuming that
individuals are equally likely to disperse to near and distant sites. The models are useful for gaining
general insights into population dynamics but not for ~nanaging particular species on real landscapes.
Such models should incorporate landscape patterns that determine spatial patterns in populations.
Disturbance opens new patches in a landscape. Fires or windstorms open communities, alter
resources, and kill existing organisms; newly arrived organisms or seeds and spores respond to the
disturbed sites (Grubb, 19771. Life-history phenomena, such as rates of growth, maximum longevity,
change in growth form with age, and onset and temporal patterns of reproduction, can all affect the
origin and disappearance of patches in landscapes (Thompson, 19821. Likewise, the interaction of
organisms that leads to successional change in community composition or structure through time alters
the distribution of patches in a landscape (Foster, 19801. Because various areas in a landscape can
undergo disturbance, planning must accommodate infrequent events (Pickett and Thompson, 1978;
Committee on Scientific ant! Technical Criteria for Federal Acquisition of Lands for Conservation,
1993~. Disturbances are either biotic, as with diseases, or abiotic, as with windstorms. If disturbance is
likely to obliterate or reduce significantly the density of a rare species in certain patches in a landscape,
other patches must remain occupied to permit recolonization of the disturbed patches (Shafer, 1990~.
Species distributions can vary dramatically through time due to patch dynamics and shifting
mosaics, and so species might require more sites over the long term than is apparent from their
distribution at any one time. A long-term perspective is required to understand habitat requirements
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Science and the Endangered Species Act
thoroughly. Dispersal patterns and mechanisms become critical aspects of species biology in situations
where movement among patches is man(latecl by patch (dynamics (McNaughton, 1989; Hansson et al.,
19921. Dispersal can be episodic or continuous and can require available sites for species to move
through sites required for dispersal are part of the habitat of a species. Such dispersal habitat can be
arrayed as stepping stones or unbroken corridors (McDonnell and Pickett, 19881. Stepping-stone
patterns are exemplifiecl by the clispersal habitat user! by migrating waterfowl or other bircts, and the
more readily recognized corridor patterns are exemplified by senate mammals (Merriam and Lanoue,
1990).
Patch dynamics involve another kind of habitat as well. In addition to habitat that organisms
occasionally or perioclically disperse through, refuge areas might be needed. During periods of physical
environmental stress or unusually intense or large (listurbances, organisms might be extirpated from their
usual or customary habitat (Pickett and Thompson, 19781. Unless organisms have areas in which they
can temporarily find shelter or in which seeds, larvae, or adults can persist through disturbances and
stresses, the long-term persistence of a species will be compromised. Refuges and recolonization sources
thus become an important aspect of habitat (As et al., 19921. The possibility that organisms would!
require clispersal patches or corridors, refuges, or recolonization sources demands that a habitat plan
explicitly include such areas. Likewise, organisms might depend on others (e.g., for food or clispersal)
that require acictitional habitats than those in which the interactions take place (Gilbert, 1980~.
Spatially explicit population models are well suited for encompassing realistic details of particular
species and landscapes into conservation plans. Spatially explicit moclels incorporate the actual location
of suitable patches of habitat and explicitly consider the movement of organisms among such patches.
For example, Mobile Animal Population is a class of spatially explicit population-simulation moclels
(Pulliam et al., 1992; Liu, 1992) that incorporates changes in land-use and habitat availability, habitat-
specific clemography, and the dispersal behavior of organisms in computer representations of real
landscapes. In MAP models, landscapes are represented as grids of cells and clusters of adjacent cells
that represent the size and location of habitat patches in mosaic landscapes. MAP moclels contain
subroutines that specify, for example, forest management practices, succession, ant! other aspects of
forest dynamics. MAP models can depict the current landscape structure and project that landscape
structure into the future based on a management plan specifying a harvest and replanting sche(lule.
Management activities, such as thinning or controllecl burning of stands, which might influence stand
suitability for particular species of interest, can be easily incorporated into MAP models. MAP models
can run on landscape maps generated by geographic information systems, which incorporate the actual
distribution of habitat patches in a region.
Although spatially explicit models are a new development, they are beginning to be used as lan(l-
management and planning tools. Spatially explicit population models developer! for the spotted owl have
proven useful in the Pacific Northwest and California (Verner et al., 1992; McKelvey et al., 1993~. An
analysis using the spatially explicit model for the spotted owl has identifier! specific owl populations i
the San Gabriel and San Bernardino Mountains as being critical to the viability of an entire southern
California metapopulation (Verner et al., 1992~. In another application of spatially explicit landscape
models, Turner et al. (1994) cieveloped a spatially explicit mode} for wintering herds of bison and elk in
Yellowstone National Park. That moclel has been used to explain how bison ant! elk have responcled to
the local patterns of habitat diversity causer! by the large-scale Yellowstone fires of 1988 and should be
useful in future land-use management and fire-control planning.
One of the best studies of patch dynamics for a plant species was carried out before the recent
development of spatially explicit models by Menges and coworkers on Furbish's lousewort (Pedficularis
furbishiae) (Merges, 1990; Menges et al., 1986~. This is an herbaceous perennial species endemic to the
Saint John River Valley in Northern Maine. Furbish's lousewort exists in very unstable habitat patches
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Modern Perspectives of Habitat
along the banks of the Saint John River. Menges describes this lousewort as inhabiting "a
ciisturbance/successional niche" clefined by hydrology and vegetation response. The species Is a poor
competitor and seems to clo best in low riverbank sites characterized by nonwoocly vegetation, frequent
flooding, and springtime ice scour. Menges anti his colleagues measurer! habitat-specific demographic
variables and concluder! that in the absence of catastrophic disturbance, only wetter, early successional
sites can maintain viable populations. The system is characterized by catastrophic events that lead to
local extinction, such as ice scour and bank slumping. Menges concluder! that local population
81
extinction probability was high even in the best of sites, stating that "individual P. f?vrbishiae populations
are temporary features of the riverine ecosystem" and that metapopulation viability clepencls on a positive
balance between new populations and extinction (Merges, 19901.
The above examples of models and studies are sufficiently well developed or defined to be used in
conservation and management. But for most species, the relevant cletails of population biology necessary
for conservation planning are not known, and years of concentrated field! work would be required to
parameterize the models. However, a wale array of new ecological concepts ant} information can be
applied to the conservation and recovery of endangered species. Since the passage of the ESA in 1973, a
variety of new ecological tools have been developed that can help plan and manage subclivided
populations in spatially heterogeneous and dynamic landscapes.
A SPATIAL PERSPECTIVE AND POPULATION VIABILITY ANALYSIS
Planning for habitat and population management must account for metapopulation structure, both
from a genetic and an ecological perspective. Several specific mechanisms are requires! to maintain
dispersed populations in a landscape. Spatial connections between populations must be permitted to
continue. Depending on the nature of the connections between different subpopulations or landscape
patches, contiguous habitat or dispersal must be allowecl or encouraged (Noss, 1983; Noss and Harris,
19861. Habitat-connecting corridors for subpopulations in a landscape might not be continuously
occupied.
Determining which habitats are sources and which are sinks requires detailed fecal studies and a
great deal of knowledge about the natural history of the organisms of concern. Simple measures of
densities can be inadequate to expose source-sink dynamics. A rigorous analysis of source-sink
dynamics requires information on birth ant! cleath rates of inclividuals in each habitat type and some
knowledge of dispersal behavior of the organism. And although studies to obtain such basic information
are critical for managing population viability, preliminary conservation strategies can be formulatecl
without cletailed estimates of needed cletails of organisms' biology, and models can be upciatect as more
information becomes available.
Consideration of source-sink dynamics is an important aspect of reserve design and habitat
protection. In some cases, adding ad(litional habitat to a reserve actually results in a smaller
metapopulation, if most of the aciclitional land is sink habitat (Pulliam and Danielson, 1991~. Individuals
dispersing within a reserve might settle in the unproductive sink patches if the available source patches
are too hard to find. Recent studies using the metapopulation mode! developed for spotted owls predict
such a problem with some reserve designs proposed for the species in the Pacific Northwest (McKeivey
et al., 19931.
Population Viability Analysis (PVA) provides an inclusive technique that can accommodate many
of the insights from the modern ecological view of habitat as a landscape phenomenon. PVA is
concerned! with how habitat loss, environmental uncertainty, demographic stochasticity, ant! genetic
factors interact to determine extinction probabilities for individual species (Soule, 1987; Shafer, 1990~.
-ark =
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82 Science and the Endangered Species Act
Though PVA is a relatively new approach, several excellent studies have clemonstrateci its usefulness
(e.g., Ehrlich and Murphy, 1987; Marcot anti Holthausen, 1987; Menges, 1990; Murphy et al., 1990;
Stacey and Taper, 1992~.
Many ecological factors that influence the likelihood of population extinction can be incorporated
into PVA. These include (1) demographic stochasticity, (2) environmental uncertainty, (3) natural
catastrophes, and (4) genetic uncertainty.
As a rough rule of thumb, genetic anti (demographic uncertainty are important factors only in small
populations, or populations that have low effective population sizes despite relatively high actual census
sizes (see Chanter 71. Environmental uncertainty and catastrophes can affect the viability of much larger
N~ 1
. - ~ . - . . · . . · ,~, . . . ·, . , . . · . ^^ .
populations. (conservation strategies and recovery planning often must cleat with tne como~necl effects of
all four factors, because many endangered species, especially large vertebrates, exist in small
populations. The recovery plans for endangered species should! usually employ two goals for promoting
viable populations: creation of multiple populations, so that a single catastrophic event cannot destroy
the whole species, ant! increasing the size of each population to a level where the threats of genetic,
demographic, and normal environmental uncertainties are diminished. Any attempt to determine
population viability must be clone with an unclerstanciing that predictions are made in a context of
uncertainty (see Chapter 71.
Most PVAs to date have combined feld studies on important demographic parameters and
simulation modeling on the possible effects of various extinction factors. Generally, the object of the
analyses is to generate a prediction of the probability that a population will become extinct in a given
number of years (e.g., a 95% probability of extinction within 100 years). Murphy et al. (1990) suggested
that species fall along a continuum between two extremes:
~ Organisms, such as most large vertebrates, with low population densities that are comparatively
wiclespread (most endangered large vertebrates, for example). PVAs for such species should focus on
the genetic and demographic factors that affect especially small populations. (This is the style of PVA
that has been clone most frequently.)
· Organisms, such as most invertebrates and small vertebrates, that are frequently restricted to
few habitat patches, but within those patches can reach high population densities. PVAs for those
species must emphasize environmental uncertainty and catastrophic factors.
Extinction clue to environmental and catastrophic stochasticity is more important in small
populations, so all factors need to be taken into account in such situations. This is not to say that some
factors will not be more important than others in specific cases.
Many human-induced anti other changes in landscape and ecosystem function can be slow to
become apparent, especially when long-lived species are involved. For example, the long-lived
razorback sucker (Xyrauchen texanus) remained common in impoundments of the lower Coloraclo River
for many years although no juvenile suckers were found there (Ono et al., 19831. As another example,
fire suppression can take a long time to produce effects in an ecosystem. This means that conclusions
from PVAs and management based on them should! be viewer! with caution.
CONCLUSIONS
· Assessing a conservation anti habitat plan must take a retrospective view in many situations. In
some cases, metapopulation dynamics in human-populated landscapes suffers from the absence of
processes that previously contributed to maintaining the species population (see Ehrlich and Murphy,
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Modern Perspectives of Habitat
83
1987~. Such processes might include succession, disturbance, predation, mutualism, and the like.
Processes that have been lost must be replaced, substituted, or compensates! for if the species is to be
maintained (Walker, 1989; Wagner and Kay, 19931.
· Management and planning for metapopulation dynamics in lanclscapes must be monitored to
determine their effectiveness, because the conditions in the landscape might change, or the management
may not be as effective as initially thought (Barrett, 1985; National Research Council, 1986; Schroeder
and Keller, 1990; Irwin and Wigley, 1993~. The status of the component populations must be assessed at
intervals. The monitoring interval will be determined by the longevity ant! generation time of the
organism of interest or the expected periodicity of rare events and episodic interactions in which the
species is involved. Monitoring must also assess the condition of the occupied habitat ant! the habitat
necessary for dispersal (Hansson, 1992~.
· Monitoring will indicate the effectiveness of a management strategy. If the management floes
not maintain an occupied or dispersal habitat in suitable condition for a species, then the tactics and
environmental components targeted! by the management can be adjusted (Schroecler ant! Keller, 19901.
This strategy of monitoring the results of management to assess the appropriateness ant! success of the
strategy and to adjust it if necessary is labeled adaptive management, a particularly appropriate term,
considering the environments of species can undergo many natural and anthropogenic changes. Such
changes can be rapid and unexpected or gradual and difficult to detect. In either event, the changes can
have untoward results for a target species, necessitating adjustments in conservation efforts.
· A second characteristic of successful planning for maintenance of species is inclu(ling
information on the interactions in which they engage. All species exist as parts of foot! webs and
interaction networks (McNaughton, 1989; Pimm, 19911. Interactions include those with prey and
resources, potential mates, consumers, competitors, pollinators ant! dispersers. Management without
attention to networks of interaction will fad! to maintain critical resources or constraining factors in the
species' environment (Holt and Talbot, 1978~. Management that accommodates the interaction networks
is labele~i ecosystem management (see chapters 9 ant! 101. Ecosystem management involves a turn from
the focus on management for commodities only (Jones, 1987; Hartshorn and Pariona-A, 1993) and
focuses instead on the ecosystem processes of population, community, and biogeochemical interactions
to maintain the condition anti function of a site as a whole (Likens, 1992; Society of American Foresters,
1993~.
FUEFEFUENCES
Angelstam, P. 1992. Conservation of communities -- the importance of edges, surroundings and
landscape mosaic structure. Pages 9-70 in L. Hansson, editor. Ecological principles of nature
conservation: applications in temperate and boreal environments. Elsevier Applied Science,
New York.
As, S., J. Bengtsson, and T. Eberhard. 1992. Archipelagoes and theories of insularity. Pages 201-251 in
L. Hansson, editor. Ecological principles of nature conservation: applications in temperate and
boreal environments. Elsevier Applied Science, New York.
Austin, M. P. 1985. Continuum concept, ordination methods and niche theory. Ann. Rev. Ecol. Syst.
16:39-61.
Barrett, G. W. 1985. A problem-solving approach to resource management. BioScience 35:423-427.
Beier, P. 1993. Determining minimum habitat areas ant! habitat corridors for cougars. Conservation
Biology 7:94- 108.
Bormann, F. H., and G. E. Likens. 1979. Catastrophic disturbance anti the steady-state in northern
OCR for page 84
84
Science and the Endangered Species Act
hardwood! forests. American Scientist 67:660-669.
Botkin, D. B., and M. I. Sobel. 1975. Stability in time-varying ecosystems. American Naturalist
109:625-646.
Committee on Scientific and Technical Criteria for Fecleral Acquisition of Lands for Conservation.
1993. Setting priorities for lan(1 conservation. National Research Council, Washington, DC.
Ehrlich. P. R., and D. D. Murphy. 1987. Conservation lessons from long-term studies of checkerspot
butterflies. Conservation Biology 1: 122-131.
Elton, C. 1927. Animal ecology. Seclgwick and Jackson, London.
Fie(ller, P. L., R. A. Leicly, R. D. Laven, N. Gershenz, and L. Saul. 1993. The contemporary paradigm
in ecology and its implications for endangered species conservation.
Forman, R. T. T. 1987. The ethics of isolation, the spread of disturbance, and landscape heterogeneity.
Pages 213-229 in M. G. Turner, editor. Landscape heterogeneity and clisturbance. Springer-
VerIag, New York.
Forman, R. T. T., and M. Goctron. 1986. Landscape ecology. John Wiley & Sons, New York.
Foster, R. B. 1980. Heterogeneity ant! disturbance in tropical vegetation. Pages 75-92 in M. E. Soule
and B. A. Wilcox, editors. Conservation biology: an evolutionary-ecological perspective. Sinauer
Associates, SunderIand, Massachusetts.
Franklin, J. F. 1993. Preserving biodiversity: species ecosystems, or landscapes. Ecological
Applications 3:202-205.
Garwood, N. C., D. P. lanos, and N. Brokaw. 1979. Earthquake caused landslides: A major disturbance
to tropical forest. Science 205:997-999.
Gilbert, L. E. 1980. Food web organization and the conservation of neotropical diversity. Pages 11-33
in M. E. Soule ant! B. A. Wilcox, editors. Conservation biology: an evolutionary-ecological
perspective. Sinauer Associates, Sunderiand, Massachusetts.
Gosz, I. R. 1991. Funciamental ecological characteristics of landscape boundaries. Pages 8-30 in M. M.
Holland, P. G. Risser and R. J. Naiman, editors. Ecotones: the role of changing landscape
boundaries in the management and restoration of changing environments. Chapman and Hall,
New York.
Grubb, P. J. 1977. The maintenance of species-richness in plant communities: the importance of the
regeneration niche. Biological Review 52:107-145.
Hanski, I. 1982. Dynamics of regional distribution: the core anti satellite species hypothesis. Oikos
38:210-221.
Hanski, I. 1989. Metapopulation dynamics: Does it help to have more of the same? Trends in Ecology
and Evcolution 4: 113- 114.
Hansson, L. 1992. Landscape ecology of boreal forests. Trends in Ecology and Evolution 7:229-302.
Hansson, L., L. Soclerstrom, and C. Solbreck. 1992. The ecology of dispersal in relation to conservation.
Pages 162-200 in L. Hansson, ed. Ecological Principles of Nature Conservation: Applications in
Temperate and Boreal Environments. Elsevier Applier! Science, New York.
Hartshorn, G. S., and W. Pariona-A. 1993. Ecologically sustainable forest management in the Peruvian
Amazon. Pages 151-166 in C. S. Potter, J. I. Cohen ant! D. Janczewski, editors. Perspectives on
bioctiversity: case studies of genetic resource conservation and clevelopment. American
Association for the Advancement of Science Press, Washington, D.C.
Holland, M. M., P. G. Risser, and R. I. Naiman, editors. 1991. Ecotones: the role of landscape
boundaries in the management and restoration of changing environments. Chapman and Hall,
New York.
Holt, S. J., and L. M. Talbot. 1978. New principles for the conservation of wilct living resources.
Volume 59. Wildlife Society, Louisville.
OCR for page 85
Modern Perspectives of Habitat
Horn, H. S., and R. H. MacArthur. 1972. Competition among fugitive species in a harlequin
environment. Ecology 53:749-752.
Irwin, L. L., and T. B. Wigley. 1993. Toward an experimental basis for protecting forest wildlife.
Ecological Applications 3:213-217.
Jones, G. E. 1987. The conservation of ecosystems and species. Croom Helm, New York.
Kolasa, J., and Pickett. 1989. Ecological systems and the concept of organization. Proceedings
National Academy of Sciences 86:8837-8841.
Kolasa, I., and S. T. A. Pickett, eclitors. 1991. Ecological heterogeneity. Springer-VerIag, New York.
Kolasa, J., and C. D. Rollo. 1991. Introduction: the heterogeneity of heterogeneity: a glossary. Pages 1-
23 in J. Kolasa and S. T. A. Pickett, editors. Ecological heterogeneity. Springer VerIag, New
York.
Levins, R. 1969. Some demographic an(l genetic consequences of environmental heterogeneity for
environmental control. Bulletin of the Entomological Scoiety of America 15:237-240.
Levins, R. 1970. Extinction. Pages 77-107 in M. Gerstenhaber, editor. Some mathematical questions in
biology. Volume II. American Mathematical Society, Providence, R.I.
Likens, G. E. 1992. Excellence in Ecology.3: The Ecosystem Approach: Its Use and Abuse. Ecology
Institute, Oldendorf/Luhe, Germany.
Liu, I. 1992. ECOLECON: A spatially explicit mocle} for ecological economics of species conservation
in complex forest landscapes. Ph.D. Dissertation, University of Georgia, Athens.
Luken, J. O. 1990. Directing Ecological Succession. Chapman and Hall, New York.
Marcot, B. G., and R. Holthausen. 1987. Analyzing population viability of the spotted owl in the Pacific
Northwest. Transactions of the North American Wildlife Natural Resources Conference 52:333-
347.
McDonnell, M. J., and S. T. A. Pickett. 1988. Connectivity and the theory of landscape ecology.
Munstersche Geographische Arbeiten 29:17-21.
McKelvey, K., B. R. Noon, and R. H. Lamberson. 1993. Conservation planning for species occupying
fragmented landscapes: the case of the northern spotter! owl, pp. 424--450. In P. M. Kareiva, J.
G. Kingsolver, and R. B. Huey. teds.) Biotic Interactions and Global Change. Sinauer Assocs.,
Inc. SunderIand, MA.
McNaughton, S. J. 1989. Ecosystems and conservation in the twenty-first century. Pages 109-120 in D.
Western and M. C. Pearl, editors. Conservation for the twenty-f~rst century. Oxford University
Press, New York.
Menges, E. 1990. Population viability analysis for an endangered plant. Conservation Biology 4:52-62.
Menges, E., D. M. Wailer, and S. C. Gawler. 1986. See set and seed predation in Raclicularis furbishiae,
a rare endemic of the St. Johns River, Maine. American Journal of Botany 73: 1 1 68- 1 177.
Merriam, G., and A. Lanoue. 1990. Corridor use by small mammals: field measurement for three
experimental types of Peromyscus [eucopus. Landscape Ecology 4:123-131.
Murphy, D. D., K. E. Freas, and S. B. Weiss. 1990. An environment-metapopulation approach to
population viability
National Research Council. 1986. Ecological Knowledge and Environmental Problem-Solving:
Concepts and Case Studies. National Academy Press, Washington, DC.
Neilson, R. P., and L. H. Wulistein. 1983. Biogeography of two southwest American oaks in relation to
atmospheric dynamics. Journal of Biogeography 10:275-297.
Noss, R. F. 1983. A regional landscape approach to maintain diversity. BioScience 33:700-706.
Noss, R. F. 1987a. From plant communities to landscapes in conservation inventories: A look at The
Nature Conservancy (USA). Biological Conservation 41:11-37.
Noss, R. F. 1987b. Protecting natural areas in fragmented landscapes. Natural Areas Journal 7: 2-13.
85
OCR for page 86
86
Science and the Endangered Species Act
Noss, R. F. 1991. Landscape connectivity: different functions at different scales. Pages 27-39 in W. E.
Hucison, editor. Landscape linkages and biocliversity. Island Press, Washington, DC.
Noss, R. F., anti L. D. Harris. 1986. Nocles, networks, and MUMs: preserving diversity at all scales.
Environmental Management 10:299-309.
Ono, R. D., J. D. Williams, and A. Wagner. 1983. Vanishing Fishes of North America. Stone Wall
Press, Washington, D.C.
Pattee, H. H. 1973. The physical basis and origin of hierarchical control. Pp. 71 - 108 in H. H. Pattee, ed.
Hierarchy Theory: The Challenge of Complexity. Braziller, New York.
Pickett, S. T. A. 1976. Succession: an evolutionary interpretation. American Naturalist 110: 107- 119.
Pickett, S. T. A., and J. N. Thompson. 1978. Patch dynamics and the design of nature reserves.
Biological Conservation 13 :27-37.
Pickett, S. T. A., and P. S. White, editors. 1985. Tile ecology of natural disturbance and patch dynamics.
Academic Press, Orlando, FL.
Pimm, S. L. 1991. The Balance of Nature? Ecological Issues in the Conservation of Species and
Communities. University of Chicago Press, Chicago.
Pulliam, H. R. 1988. Sources, sinks, end population regulation. Amer.Natur. 132:652-661.
Pulliam, H. R., ant! B. J. Danielson. 1991. Sources, sinks, anti habitat selection: A landscape
perspective on population dynamics. American Naturlaist 137:S50-S66.
Pulliam, H. R., J. B. Dunning, and J. Liu. 1992. Population dynamics in complex landscapes: a case
stucly. Ecological Applications 2: 165- 177.
Risser, P. G. 1987. Landscape ecology: state ofthe art. Pages 3-14 in M. G. Turner, editor. Landscape
heterogeneity and disturbance. Springer-Veriag, New York.
Rotenberry, J. T., and J. A. Wiens. 1980. Temporal variation in habitat structure and shrub steppe bir
dynamics. Oecologia 47: 1 -9.
Schoener, T. W. 1986. Overview: kinds of ecological communities -- ecology becomes pluralistic.
Pages 467-479 in J. Diamond and T. J. Case, editors. Community ecology. Harper anti Row,
New York.
Schroeder, R. L., and M. E. Keller. 1990. Setting objectives: a prerequisite of ecosystem management.
Pages 1-4 in R. S. Mitchell, C. J. Sheviak and D. J. Leopold, editors. Ecosystem management:
rare species and significant habitats. New York State Museum, Albany.
Shafer, C. L. 1990. Nature reserves: island theory and conservation practice. Smithsonian Institution
Press, Washington.
Shipley, W., and P. A. Kecl(ly. 1987. The inclividualistic and community-unit concepts an(l falsifiable
hypotheses. Vegetatio 69:47-55.
Society of American Foresters. 1993. Task force report on sustaining long-term forest health and
productivity. Society of American Foresters, Bethesda, Maryland.
Soule, M. E. eel. 1987. Viable populations for conservation. Cambridge University Press.
Soule, M. E., ant! B. A. Wilcox, editors. 1980. Conservation biology: an evolutionary-ecological
perspective. Sinauer, Sunderian(l, Mass.
Stacey, P. B., anti M. Taper. 1992. Environmental variation an(1 persistence of small populations.
Ecological Applications 2:18-29.
Thompson, J. N. 1982. Interaction and coevolution. Wiley, New York.
Turner, M. G., Y. Wu, L. L. Wallace, and W. H. Romme. 1994. Simulating winter interactions among
ungulates, vegetation, and fire in northern Yellowstone Park. Ecological Applications 4:472-
496.
van Home, 1982. Population density as a misleading indicator of habitat quality. J. Wildlife
Management 47: 893-901.
OCR for page 87
Moclern Perspectives of Habitat
Verner, J., K. S. McKeIvey, B. R. Noon, R. J. Gutierrez, G. I. GouIci, and J. W. Bock. 1992. The
California Spotted Owl: A Technical Assessment of its Current Status. USDA Firest Service
report PSW GJR-133. USDA Forest Service, Washington, DC.
von Bertelanffy, L. 1968. General System Theory: Foundations, Deveriopment, and Applications.
Revised Edition. George Braziller, New York.
Wagner, F. H., and C. E. Kay. 1993. "Natural" or "healthy" ecosystems: are U.S. national parks
providing them. Pages 257-270 in M. J. McDonnell ant! S. T. A. Pickett, editors. Humans as
components of ecosystems: the ecology of subtle human effects anti populated areas. Springer-
VerIag, New York.
Walker, B. 1989. Diversity anti stability in ecosystem conservation. Pages 121-130 in D. Western and
M. C. Pearl, editors. Conservation for tile twenty-first century. Oxford University Press, New
York.
White, P. S. 1979. Pattern, process, and natural disturbance in vegetation. Botanical Review 45:229-
299.
Whittaker, R. H. 1975. Communities and ecosystems, 2nd Edition. Macmillan, New York.
Wiens J. A. l 976. Population responses to patchy environments. Annual Reviews of Ecology and
87
Systematics 7:81 - 120.
Wiens, J. A. 1984. On understanding a non-equilibrium world: myth and reality in community patterns
and processes. Pages 439-458 in D. R. Strong, D. Simberioff, L. Abele anti A. B. Thistle, editors.
Ecological communities: conceptual issues and the evidence. Princeton University Press,
Princeton.
OCR for page 88
Representative terms from entire chapter:
spatially explicit
